Rewrite Tracing to allow for only one TraceT in the entire stack.

This PR is on top of #7789.

### Description

This PR entirely rewrites the API of the Tracing library, to make `interpTraceT` a thing of the past. Before this change, we ran traces by sticking a `TraceT` on top of whatever we were doing. This had several major drawbacks:
- we were carrying a bunch of `TraceT` across the codebase, and the entire codebase had to know about it
- we needed to carry a second class constraint around (`HasReporterM`) to be able to run all of those traces
- we kept having to do stack rewriting with `interpTraceT`, which went from inconvenient to horrible
- we had to declare several behavioral instances on `TraceT m`

This PR rewrite all of `Tracing` using a more conventional model: there is ONE `TraceT` at the bottom of the stack, and there is an associated class constraint `MonadTrace`: any part of the code that happens to satisfy `MonadTrace` is able to create new traces. We NEVER have to do stack rewriting, `interpTraceT` is gone, and `TraceT` and `Reporter` become  implementation details that 99% of the code is blissfully unaware of: code that needs to do tracing only needs to declare that the monad in which it operates implements `MonadTrace`.

In doing so, this PR revealed **several bugs in the codebase**: places where we were expecting to trace something, but due to the default instance of `HasReporterM IO` we would actually not do anything. This PR also splits the code of `Tracing` in more byte-sized modules, with the goal of potentially moving to `server/lib` down the line.

### Remaining work

This PR is a draft; what's left to do is:
- [x] make Pro compile; i haven't updated `HasuraPro/Main` yet
- [x] document Tracing by writing a note that explains how to use the library, and the meaning of "reporter", "trace" and "span", as well as the pitfalls
- [x] discuss some of the trade-offs in the implementation, which is why i'm opening this PR already despite it not fully building yet
- [x] it depends on #7789 being merged first

PR-URL: https://github.com/hasura/graphql-engine-mono/pull/7791
GitOrigin-RevId: cadd32d039134c93ddbf364599a2f4dd988adea8

server/graphql-engine.cabal

@@ -748,6 +748,15 @@ library
                      , Hasura.LogicalModel.Schema
                      , Hasura.LogicalModel.Types
 
+                     , Hasura.Tracing
+                     , Hasura.Tracing.Class
+                     , Hasura.Tracing.Context
+                     , Hasura.Tracing.Monad
+                     , Hasura.Tracing.Reporter
+                     , Hasura.Tracing.Sampling
+                     , Hasura.Tracing.TraceId
+                     , Hasura.Tracing.Utils
+
                      , Hasura.Server.Auth.WebHook
                      , Hasura.Server.Middleware
                      , Hasura.Server.Cors
@@ -988,8 +997,6 @@ library
                      , Hasura.SQL.Types
                      , Hasura.SQL.Value
                      , Hasura.SQL.WKT
-                     , Hasura.Tracing
-                     , Hasura.Tracing.TraceId
                      , Hasura.QueryTags
                      , Network.HTTP.Client.Transformable
                      , Network.HTTP.Client.DynamicTlsPermissions

server/lib/test-harness/src/Harness/GraphqlEngine.hs

@@ -374,10 +374,9 @@ runApp serveOptions = do
       pure (EKG.subset EKG.emptyOf store, serverMetrics)
   prometheusMetrics <- makeDummyPrometheusMetrics
   let managedServerCtx = App.initialiseContext env globalCtx serveOptions Nothing serverMetrics prometheusMetrics sampleAlways
-  runManagedT managedServerCtx \(appCtx, appEnv) -> do
-    flip App.runPGMetadataStorageAppT (appCtx, appEnv)
-      . lowerManagedT
-      $ do
+  runManagedT managedServerCtx \(appCtx, appEnv) ->
+    App.runPGMetadataStorageAppT (appCtx, appEnv) $
+      lowerManagedT $
         App.runHGEServer
           (const $ pure ())
           appCtx

server/src-exec/Main.hs

@@ -30,7 +30,7 @@ import Hasura.Server.Migrate (downgradeCatalog)
 import Hasura.Server.Prometheus (makeDummyPrometheusMetrics)
 import Hasura.Server.Version
 import Hasura.ShutdownLatch
-import Hasura.Tracing (sampleAlways)
+import Hasura.Tracing (ignoreTraceT, sampleAlways)
 import System.Environment (getEnvironment, lookupEnv, unsetEnv)
 import System.Exit qualified as Sys
 import System.Metrics qualified as EKG
@@ -55,7 +55,7 @@ main = maybeWithGhcDebug $ do
     clearEnvironment = getEnvironment >>= traverse_ \(v, _) -> unsetEnv v
 
 runApp :: Env.Environment -> HGEOptions (ServeOptions Hasura) -> IO ()
-runApp env (HGEOptions rci metadataDbUrl hgeCmd) = do
+runApp env (HGEOptions rci metadataDbUrl hgeCmd) = ignoreTraceT do
   initTime <- liftIO getCurrentTime
 
   case hgeCmd of
@@ -74,7 +74,7 @@ runApp env (HGEOptions rci metadataDbUrl hgeCmd) = do
 
         pure (EKG.subset EKG.emptyOf store, serverMetrics)
 
-      prometheusMetrics <- makeDummyPrometheusMetrics
+      prometheusMetrics <- lift makeDummyPrometheusMetrics
 
       -- It'd be nice if we didn't have to call runManagedT twice here, but
       -- there is a data dependency problem since the call to runPGMetadataStorageApp
@@ -93,10 +93,12 @@ runApp env (HGEOptions rci metadataDbUrl hgeCmd) = do
         let Loggers _ logger _ = appEnvLoggers appEnv
 
         _idleGCThread <-
+          lift $
             C.forkImmortal "ourIdleGC" logger $
               GC.ourIdleGC logger (seconds 0.3) (seconds 10) (seconds 60)
 
-        flip runPGMetadataStorageAppT (appCtx, appEnv) . lowerManagedT $ do
+        runPGMetadataStorageAppT (appCtx, appEnv) $
+          lowerManagedT $
             runHGEServer (const $ pure ()) appCtx appEnv initTime Nothing ekgStore
     HCExport -> do
       GlobalCtx {..} <- initGlobalCtx env metadataDbUrl rci

server/src-lib/Hasura/App.hs

@@ -13,7 +13,8 @@ module Hasura.App
     ExitException (ExitException),
     GlobalCtx (..),
     AppContext (..),
-    PGMetadataStorageAppT (runPGMetadataStorageAppT),
+    PGMetadataStorageAppT,
+    runPGMetadataStorageAppT,
     accessDeniedErrMsg,
     flushLogger,
     getCatalogStateTx,
@@ -154,7 +155,7 @@ import Hasura.Server.Version
 import Hasura.Services
 import Hasura.Session
 import Hasura.ShutdownLatch
-import Hasura.Tracing qualified as Tracing
+import Hasura.Tracing
 import Network.HTTP.Client qualified as HTTP
 import Network.HTTP.Client.Blocklisting (Blocklist)
 import Network.HTTP.Client.CreateManager (mkHttpManager)
@@ -279,8 +280,8 @@ initGlobalCtx env metadataDbUrl defaultPgConnInfo = do
       mkGlobalCtx mdConnInfo (Just (dbUrl, srcConnInfo))
 
 -- | An application with Postgres database as a metadata storage
-newtype PGMetadataStorageAppT m a = PGMetadataStorageAppT {runPGMetadataStorageAppT :: (AppContext, AppEnv) -> m a}
-  deriving
+newtype PGMetadataStorageAppT m a = PGMetadataStorageAppT (ReaderT (AppContext, AppEnv) (TraceT m) a)
+  deriving newtype
     ( Functor,
       Applicative,
       Monad,
@@ -289,20 +290,29 @@ newtype PGMetadataStorageAppT m a = PGMetadataStorageAppT {runPGMetadataStorageA
       MonadCatch,
       MonadThrow,
       MonadMask,
-      HasServerConfigCtx,
       MonadReader (AppContext, AppEnv),
       MonadBase b,
       MonadBaseControl b
     )
-    via (ReaderT (AppContext, AppEnv) m)
-  deriving
-    ( MonadTrans
-    )
-    via (ReaderT (AppContext, AppEnv))
+
+instance (MonadIO m, MonadBaseControl IO m) => MonadTrace (PGMetadataStorageAppT m) where
+  newTraceWith c p n (PGMetadataStorageAppT a) = PGMetadataStorageAppT $ newTraceWith c p n a
+  newSpanWith i n (PGMetadataStorageAppT a) = PGMetadataStorageAppT $ newSpanWith i n a
+  currentContext = PGMetadataStorageAppT currentContext
+  attachMetadata = PGMetadataStorageAppT . attachMetadata
+
+instance MonadTrans PGMetadataStorageAppT where
+  lift = PGMetadataStorageAppT . lift . lift
 
 instance Monad m => ProvidesNetwork (PGMetadataStorageAppT m) where
   askHTTPManager = appEnvManager <$> asks snd
 
+instance HasServerConfigCtx m => HasServerConfigCtx (PGMetadataStorageAppT m) where
+  askServerConfigCtx = lift askServerConfigCtx
+
+runPGMetadataStorageAppT :: (AppContext, AppEnv) -> PGMetadataStorageAppT m a -> m a
+runPGMetadataStorageAppT c (PGMetadataStorageAppT a) = ignoreTraceT $ runReaderT a c
+
 resolvePostgresConnInfo ::
   (MonadIO m) => Env.Environment -> UrlConf -> Maybe Int -> m PG.ConnInfo
 resolvePostgresConnInfo env dbUrlConf maybeRetries = do
@@ -314,7 +324,7 @@ resolvePostgresConnInfo env dbUrlConf maybeRetries = do
     retries = fromMaybe 1 maybeRetries
 
 initAuthMode ::
-  (C.ForkableMonadIO m, Tracing.HasReporter m) =>
+  (C.ForkableMonadIO m) =>
   HashSet AdminSecretHash ->
   Maybe AuthHook ->
   [JWTConfig] ->
@@ -337,7 +347,7 @@ initAuthMode adminSecret authHook jwtSecret unAuthRole httpManager logger = do
   -- forking a dedicated polling thread to dynamically get the latest JWK settings
   -- set by the user and update the JWK accordingly. This will help in applying the
   -- updates without restarting HGE.
-  _ <- C.forkImmortal "update JWK" logger $ updateJwkCtx authMode httpManager logger
+  void $ C.forkImmortal "update JWK" logger $ updateJwkCtx authMode httpManager logger
   return authMode
 
 initSubscriptionsState ::
@@ -414,7 +424,7 @@ initialiseContext ::
   Maybe ES.SubscriptionPostPollHook ->
   ServerMetrics ->
   PrometheusMetrics ->
-  Tracing.SamplingPolicy ->
+  SamplingPolicy ->
   ManagedT m (AppContext, AppEnv)
 initialiseContext env GlobalCtx {..} serveOptions@ServeOptions {..} liveQueryHook serverMetrics prometheusMetrics traceSamplingPolicy = do
   instanceId <- liftIO generateInstanceId
@@ -647,7 +657,7 @@ runHGEServer ::
     MonadMask m,
     MonadStateless IO m,
     LA.Forall (LA.Pure m),
-    UserAuthentication (Tracing.TraceT m),
+    UserAuthentication m,
     HttpLog m,
     ConsoleRenderer m,
     MonadVersionAPIWithExtraData m,
@@ -657,13 +667,13 @@ runHGEServer ::
     MonadQueryLog m,
     WS.MonadWSLog m,
     MonadExecuteQuery m,
-    Tracing.HasReporter m,
     HasResourceLimits m,
     MonadMetadataStorageQueryAPI m,
     MonadResolveSource m,
     EB.MonadQueryTags m,
     MonadEventLogCleanup m,
     ProvidesHasuraServices m,
+    MonadTrace m,
     MonadGetApiTimeLimit m
   ) =>
   (AppContext -> Spock.SpockT m ()) ->
@@ -738,7 +748,7 @@ mkHGEServer ::
     MonadMask m,
     MonadStateless IO m,
     LA.Forall (LA.Pure m),
-    UserAuthentication (Tracing.TraceT m),
+    UserAuthentication m,
     HttpLog m,
     ConsoleRenderer m,
     MonadVersionAPIWithExtraData m,
@@ -748,13 +758,13 @@ mkHGEServer ::
     MonadQueryLog m,
     WS.MonadWSLog m,
     MonadExecuteQuery m,
-    Tracing.HasReporter m,
     HasResourceLimits m,
     MonadMetadataStorageQueryAPI m,
     MonadResolveSource m,
     EB.MonadQueryTags m,
     MonadEventLogCleanup m,
     ProvidesHasuraServices m,
+    MonadTrace m,
     MonadGetApiTimeLimit m
   ) =>
   (AppContext -> Spock.SpockT m ()) ->
@@ -1089,8 +1099,6 @@ mkHGEServer setupHook appCtx@AppContext {..} appEnv@AppEnv {..} ekgStore = do
           (getSchemaCache cacheRef)
           lockedEventsCtx
 
-instance (Monad m) => Tracing.HasReporter (PGMetadataStorageAppT m)
-
 instance (Monad m) => HasResourceLimits (PGMetadataStorageAppT m) where
   askHTTPHandlerLimit = pure $ ResourceLimits id
   askGraphqlOperationLimit _ _ _ = pure $ ResourceLimits id
@@ -1113,10 +1121,10 @@ instance (MonadIO m) => HttpLog (PGMetadataStorageAppT m) where
         mkHttpAccessLogContext userInfoM loggingSettings reqId waiReq reqBody (BL.length response) compressedResponse qTime cType headers rb batchQueryOpLogs
 
 instance (Monad m) => MonadExecuteQuery (PGMetadataStorageAppT m) where
-  cacheLookup _ _ _ _ = pure ([], Nothing)
-  cacheStore _ _ _ = pure (Right CacheStoreSkipped)
+  cacheLookup _ _ _ _ = pure $ Right ([], Nothing)
+  cacheStore _ _ _ = pure $ Right (Right CacheStoreSkipped)
 
-instance (MonadIO m, MonadBaseControl IO m) => UserAuthentication (Tracing.TraceT (PGMetadataStorageAppT m)) where
+instance (MonadIO m, MonadBaseControl IO m) => UserAuthentication (PGMetadataStorageAppT m) where
   resolveUserInfo logger manager headers authMode reqs =
     runExceptT $ do
       (a, b, c) <- getUserInfoWithExpTime logger manager headers authMode reqs

server/src-lib/Hasura/Backends/DataConnector/Adapter/Metadata.hs

@@ -4,7 +4,7 @@
 module Hasura.Backends.DataConnector.Adapter.Metadata () where
 
 import Control.Arrow.Extended
-import Control.Monad.Trans.Control (MonadBaseControl)
+import Control.Monad.Trans.Control
 import Data.Aeson qualified as J
 import Data.Aeson.Key qualified as K
 import Data.Aeson.KeyMap qualified as KM
@@ -76,6 +76,7 @@ instance BackendMetadata 'DataConnector where
   supportsBeingRemoteRelationshipTarget = supportsBeingRemoteRelationshipTarget'
 
 resolveBackendInfo' ::
+  forall arr m.
   ( ArrowChoice arr,
     Inc.ArrowCache m arr,
     Inc.ArrowDistribute arr,
@@ -97,14 +98,6 @@ resolveBackendInfo' logger = proc (invalidationKeys, optionsMap) -> do
   returnA -< HashMap.catMaybes maybeDataConnectorCapabilities
   where
     getDataConnectorCapabilitiesIfNeeded ::
-      forall arr m.
-      ( ArrowChoice arr,
-        Inc.ArrowCache m arr,
-        ArrowWriter (Seq (Either InconsistentMetadata MetadataDependency)) arr,
-        MonadIO m,
-        MonadBaseControl IO m,
-        ProvidesNetwork m
-      ) =>
       (Inc.Dependency (Maybe (HashMap DC.DataConnectorName Inc.InvalidationKey)), DC.DataConnectorName, DC.DataConnectorOptions) `arr` Maybe DC.DataConnectorInfo
     getDataConnectorCapabilitiesIfNeeded = Inc.cache proc (invalidationKeys, dataConnectorName, dataConnectorOptions) -> do
       let metadataObj = MetadataObject (MODataConnectorAgent dataConnectorName) $ J.toJSON dataConnectorName
@@ -117,7 +110,6 @@ resolveBackendInfo' logger = proc (invalidationKeys, optionsMap) -> do
         |) metadataObj
 
     getDataConnectorCapabilities ::
-      (MonadIO m, MonadBaseControl IO m) =>
       DC.DataConnectorOptions ->
       HTTP.Manager ->
       m (Either QErr DC.DataConnectorInfo)
@@ -133,7 +125,9 @@ resolveBackendInfo' logger = proc (invalidationKeys, optionsMap) -> do
       capabilitiesCase defaultAction capabilitiesAction errorAction capabilitiesU
 
 resolveSourceConfig' ::
-  (MonadIO m, MonadBaseControl IO m) =>
+  ( MonadIO m,
+    MonadBaseControl IO m
+  ) =>
   Logger Hasura ->
   SourceName ->
   DC.ConnSourceConfig ->

server/src-lib/Hasura/Backends/DataConnector/Adapter/Transport.hs

@@ -57,7 +57,7 @@ runDBQuery' ::
 runDBQuery' requestId query fieldName _userInfo logger SourceConfig {..} action queryRequest _ = do
   void $ HGL.logQueryLog logger $ mkQueryLog query fieldName queryRequest requestId
   withElapsedTime
-    . Tracing.trace ("Data Connector backend query for root field " <>> fieldName)
+    . Tracing.newSpan ("Data Connector backend query for root field " <>> fieldName)
     . flip runAgentClientT (AgentClientContext logger _scEndpoint _scManager _scTimeoutMicroseconds)
     . runOnBaseMonad
     $ action
@@ -108,7 +108,7 @@ runDBMutation' ::
 runDBMutation' requestId query fieldName _userInfo logger SourceConfig {..} action queryRequest _ = do
   void $ HGL.logQueryLog logger $ mkQueryLog query fieldName queryRequest requestId
   withElapsedTime
-    . Tracing.trace ("Data Connector backend mutation for root field " <>> fieldName)
+    . Tracing.newSpan ("Data Connector backend mutation for root field " <>> fieldName)
     . flip runAgentClientT (AgentClientContext logger _scEndpoint _scManager _scTimeoutMicroseconds)
     . runOnBaseMonad
     $ action

server/src-lib/Hasura/Backends/DataConnector/Agent/Client.hs

@@ -14,7 +14,7 @@ import Hasura.Base.Error
 import Hasura.HTTP qualified
 import Hasura.Logging (Hasura, Logger)
 import Hasura.Prelude
-import Hasura.Tracing (MonadTrace, tracedHttpRequest)
+import Hasura.Tracing (MonadTrace, traceHTTPRequest)
 import Network.HTTP.Client (Manager)
 import Network.HTTP.Client qualified as HTTP
 import Network.HTTP.Client.Transformable qualified as TransformableHTTP
@@ -57,7 +57,8 @@ runRequestAcceptStatus' acceptStatus req = do
         transformableReq &~ do
           for _accResponseTimeout \x -> TransformableHTTP.timeout .= HTTP.responseTimeoutMicro x
 
-  (tracedReq, responseOrException) <- tracedHttpRequest transformableReq' (\tracedReq -> fmap (tracedReq,) . liftIO . try @HTTP.HttpException $ TransformableHTTP.performRequest tracedReq _accHttpManager)
+  (tracedReq, responseOrException) <- traceHTTPRequest transformableReq' \tracedReq ->
+    fmap (tracedReq,) . liftIO . try @HTTP.HttpException $ TransformableHTTP.performRequest tracedReq _accHttpManager
   logAgentRequest _accLogger tracedReq responseOrException
   case responseOrException of
     -- throwConnectionError is used here in order to avoid a metadata inconsistency error

server/src-lib/Hasura/Backends/DataConnector/Logging.hs

@@ -67,8 +67,8 @@ logAgentRequest (Logger writeLog) req responseOrError = do
         Right response -> Just . statusCode $ responseStatus response
         Left httpExn -> Hasura.HTTP.getHTTPExceptionStatus $ Hasura.HTTP.HttpException httpExn
       _aclError = either (Just . Hasura.HTTP.serializeHTTPExceptionMessageForDebugging) (const Nothing) responseOrError
-      _aclTraceId = bsToTxt $ traceIdToHex $ Tracing.tcCurrentTrace traceCtx
-      _aclSpanId = bsToTxt $ spanIdToHex $ Tracing.tcCurrentSpan traceCtx
+      _aclTraceId = maybe "" (bsToTxt . traceIdToHex . Tracing.tcCurrentTrace) traceCtx
+      _aclSpanId = maybe "" (bsToTxt . spanIdToHex . Tracing.tcCurrentSpan) traceCtx
   writeLog AgentCommunicationLog {..}
 
 extractRequestLogInfoFromClientRequest :: Request -> RequestLogInfo
@@ -88,8 +88,8 @@ logClientError (Logger writeLog) clientError = do
         _ -> Nothing
       _aclRequest = extractRequestLogInfoFromClientInfo clientError
       _aclError = Just $ Hasura.HTTP.serializeServantClientErrorMessageForDebugging clientError
-      _aclTraceId = bsToTxt $ traceIdToHex $ Tracing.tcCurrentTrace traceCtx
-      _aclSpanId = bsToTxt $ spanIdToHex $ Tracing.tcCurrentSpan traceCtx
+      _aclTraceId = maybe "" (bsToTxt . traceIdToHex . Tracing.tcCurrentTrace) traceCtx
+      _aclSpanId = maybe "" (bsToTxt . spanIdToHex . Tracing.tcCurrentSpan) traceCtx
   writeLog AgentCommunicationLog {..}
 
 extractRequestLogInfoFromClientInfo :: ClientError -> Maybe RequestLogInfo

server/src-lib/Hasura/Backends/MSSQL/DDL/EventTrigger.hs

@@ -112,7 +112,7 @@ insertManualEvent ::
   TriggerName ->
   J.Value ->
   UserInfo ->
-  Tracing.TraceContext ->
+  Maybe Tracing.TraceContext ->
   m EventId
 insertManualEvent sourceConfig tableName triggerName payload _userInfo _traceCtx =
   liftEitherM $

server/src-lib/Hasura/Backends/MSSQL/Instances/Transport.hs

@@ -74,7 +74,7 @@ runQuery ::
 runQuery reqId query fieldName _userInfo logger _sourceConfig tx genSql _ = do
   logQueryLog logger $ mkQueryLog query fieldName genSql reqId
   withElapsedTime $
-    trace ("MSSQL Query for root field " <>> fieldName) $
+    newSpan ("MSSQL Query for root field " <>> fieldName) $
       run tx
 
 runQueryExplain ::
@@ -109,7 +109,7 @@ runMutation ::
 runMutation reqId query fieldName _userInfo logger _sourceConfig tx _genSql _ = do
   logQueryLog logger $ mkQueryLog query fieldName Nothing reqId
   withElapsedTime $
-    trace ("MSSQL Mutation for root field " <>> fieldName) $
+    newSpan ("MSSQL Mutation for root field " <>> fieldName) $
       run tx
 
 runSubscription ::

server/src-lib/Hasura/Backends/MySQL/Instances/Transport.hs

@@ -49,7 +49,7 @@ runQuery ::
 runQuery reqId query fieldName _userInfo logger _sourceConfig tx genSql _ = do
   logQueryLog logger $ mkQueryLog query fieldName genSql reqId
   withElapsedTime $
-    trace ("MySQL Query for root field " <>> fieldName) $
+    newSpan ("MySQL Query for root field " <>> fieldName) $
       run tx
 
 runQueryExplain ::

server/src-lib/Hasura/Backends/Postgres/Connection/MonadTx.hs

@@ -137,19 +137,18 @@ sessionInfoJsonExp = S.SELit . encodeToStrictText
 withUserInfo :: (MonadIO m) => UserInfo -> PG.TxET QErr m a -> PG.TxET QErr m a
 withUserInfo uInfo tx = setHeadersTx (_uiSession uInfo) >> tx
 
-setTraceContextInTx :: (MonadIO m) => Tracing.TraceContext -> PG.TxET QErr m ()
-setTraceContextInTx traceCtx = PG.unitQE defaultTxErrorHandler sql () False
-  where
-    sql =
-      PG.fromText $
-        "SET LOCAL \"hasura.tracecontext\" = "
-          <> toSQLTxt (S.SELit . encodeToStrictText . Tracing.injectEventContext $ traceCtx)
+setTraceContextInTx :: (MonadIO m) => Maybe Tracing.TraceContext -> PG.TxET QErr m ()
+setTraceContextInTx = \case
+  Nothing -> pure ()
+  Just ctx -> do
+    let sql = PG.fromText $ "SET LOCAL \"hasura.tracecontext\" = " <> toSQLTxt (S.SELit . encodeToStrictText . toJSON $ ctx)
+    PG.unitQE defaultTxErrorHandler sql () False
 
 -- | Inject the trace context as a transaction-local variable,
 -- so that it can be picked up by any triggers (including event triggers).
 withTraceContext ::
   (MonadIO m) =>
-  Tracing.TraceContext ->
+  Maybe (Tracing.TraceContext) ->
   PG.TxET QErr m a ->
   PG.TxET QErr m a
 withTraceContext ctx tx = setTraceContextInTx ctx >> tx

server/src-lib/Hasura/Backends/Postgres/DDL/EventTrigger.hs

@@ -112,7 +112,7 @@ insertManualEvent ::
   TriggerName ->
   Value ->
   UserInfo ->
-  Tracing.TraceContext ->
+  Maybe Tracing.TraceContext ->
   m EventId
 insertManualEvent sourceConfig tableName triggerName payload userInfo traceCtx =
   -- NOTE: The methods `setTraceContextInTx` and `setHeadersTx` are being used

server/src-lib/Hasura/Backends/Postgres/Execute/Insert.hs

@@ -109,7 +109,7 @@ insertMultipleObjects multiObjIns additionalColumns userInfo mutationOutput plan
               mutationOutput
               columnInfos
           rowCount = tshow . length $ IR._aiInsertObject multiObjIns
-      Tracing.trace ("Insert (" <> rowCount <> ") " <> qualifiedObjectToText table) do
+      Tracing.newSpan ("Insert (" <> rowCount <> ") " <> qualifiedObjectToText table) do
         Tracing.attachMetadata [("count", rowCount)]
         PGE.execInsertQuery stringifyNum tCase userInfo (insertQuery, planVars)
 
@@ -146,7 +146,8 @@ insertObject ::
   Options.StringifyNumbers ->
   Maybe NamingCase ->
   m (Int, Maybe (ColumnValues ('Postgres pgKind) TxtEncodedVal))
-insertObject singleObjIns additionalColumns userInfo planVars stringifyNum tCase = Tracing.trace ("Insert " <> qualifiedObjectToText table) do
+insertObject singleObjIns additionalColumns userInfo planVars stringifyNum tCase =
+  Tracing.newSpan ("Insert " <> qualifiedObjectToText table) do
     validateInsert (Map.keys columns) (map IR._riRelationInfo objectRels) (Map.keys additionalColumns)
 
     -- insert all object relations and fetch this insert dependent column values

server/src-lib/Hasura/Backends/Postgres/Instances/Execute.hs

@@ -491,7 +491,7 @@ mkCurPlanTx userInfo ps@(PreparedSql q prepMap) =
       -- WARNING: this quietly assumes the intmap keys are contiguous
       prepArgs = fst <$> IntMap.elems args
    in (,Just ps) $ OnBaseMonad do
-        Tracing.trace "Postgres" $
+        Tracing.newSpan "Postgres" $
           runIdentity . PG.getRow
             <$> PG.rawQE dmlTxErrorHandler q prepArgs True
 

server/src-lib/Hasura/Backends/Postgres/Instances/Transport.hs

@@ -79,7 +79,7 @@ runPGQuery reqId query fieldName _userInfo logger sourceConfig tx genSql resolve
   -- log the generated SQL and the graphql query
   logQueryLog logger $ mkQueryLog query fieldName genSql reqId (resolvedConnectionTemplate <$ resolvedConnectionTemplate)
   withElapsedTime $
-    trace ("Postgres Query for root field " <>> fieldName) $
+    newSpan ("Postgres Query for root field " <>> fieldName) $
       runQueryTx (_pscExecCtx sourceConfig) (GraphQLQuery resolvedConnectionTemplate) $
         runOnBaseMonad tx
 
@@ -104,7 +104,7 @@ runPGMutation reqId query fieldName userInfo logger sourceConfig tx _genSql reso
   -- log the graphql query
   logQueryLog logger $ mkQueryLog query fieldName Nothing reqId (resolvedConnectionTemplate <$ resolvedConnectionTemplate)
   withElapsedTime $
-    trace ("Postgres Mutation for root field " <>> fieldName) $
+    newSpan ("Postgres Mutation for root field " <>> fieldName) $
       runTxWithCtxAndUserInfo userInfo (_pscExecCtx sourceConfig) (Tx PG.ReadWrite Nothing) (GraphQLQuery resolvedConnectionTemplate) $
         runOnBaseMonad tx
 
@@ -189,6 +189,6 @@ runPGMutationTransaction reqId query userInfo logger sourceConfig resolvedConnec
   withElapsedTime $
     runTxWithCtxAndUserInfo userInfo (_pscExecCtx sourceConfig) (Tx PG.ReadWrite Nothing) (GraphQLQuery resolvedConnectionTemplate) $
       flip OMap.traverseWithKey mutations \fieldName dbsi ->
-        trace ("Postgres Mutation for root field " <>> fieldName) $
+        newSpan ("Postgres Mutation for root field " <>> fieldName) $
           runOnBaseMonad $
             dbsiAction dbsi

server/src-lib/Hasura/Eventing/EventTrigger.hs

@@ -55,6 +55,7 @@ import Control.Monad.Trans.Control (MonadBaseControl)
 import Data.Aeson qualified as J
 import Data.Aeson.Key qualified as Key
 import Data.Aeson.KeyMap qualified as KeyMap
+import Data.Aeson.Lens qualified as JL
 import Data.Aeson.TH
 import Data.Has
 import Data.HashMap.Strict qualified as M
@@ -280,10 +281,10 @@ logFetchedEventsStatistics logger backendEvents =
 processEventQueue ::
   forall m.
   ( MonadIO m,
-    Tracing.HasReporter m,
     MonadBaseControl IO m,
     LA.Forall (LA.Pure m),
-    MonadMask m
+    MonadMask m,
+    Tracing.MonadTrace m
   ) =>
   L.Logger L.Hasura ->
   FetchedEventsStatsLogger ->
@@ -418,16 +419,31 @@ processEventQueue logger statsLogger httpMgr getSchemaCache EventEngineCtx {..}
                       "It looks like the events processor is keeping up again."
               return (eventsNext, 0, False)
 
+    -- \| Extract a trace context from an event trigger payload.
+    extractEventContext :: forall io. MonadIO io => J.Value -> io (Maybe Tracing.TraceContext)
+    extractEventContext e = do
+      let traceIdMaybe =
+            Tracing.traceIdFromHex . txtToBs
+              =<< e ^? JL.key "trace_context" . JL.key "trace_id" . JL._String
+      for traceIdMaybe $ \traceId -> do
+        freshSpanId <- Tracing.randomSpanId
+        let parentSpanId =
+              Tracing.spanIdFromHex . txtToBs
+                =<< e ^? JL.key "trace_context" . JL.key "span_id" . JL._String
+            samplingState =
+              Tracing.samplingStateFromHeader $
+                e ^? JL.key "trace_context" . JL.key "sampling_state" . JL._String
+        pure $ Tracing.TraceContext traceId freshSpanId parentSpanId samplingState
+
     processEvent ::
       forall io r b.
       ( MonadIO io,
         MonadReader r io,
         Has HTTP.Manager r,
         Has (L.Logger L.Hasura) r,
-        Tracing.HasReporter io,
         MonadMask io,
-        MonadBaseControl IO io,
-        BackendEventTrigger b
+        BackendEventTrigger b,
+        Tracing.MonadTrace io
       ) =>
       EventWithSource b ->
       io ()
@@ -441,11 +457,11 @@ processEventQueue logger statsLogger httpMgr getSchemaCache EventEngineCtx {..}
 
       cache <- liftIO getSchemaCache
 
-      tracingCtx <- liftIO (Tracing.extractEventContext (eEvent e))
+      trace <-
+        extractEventContext (eEvent e) <&> \case
+          Nothing -> Tracing.newTrace Tracing.sampleAlways
+          Just ctx -> Tracing.newTraceWith ctx Tracing.sampleAlways
       let spanName eti = "Event trigger: " <> unNonEmptyText (unTriggerName (etiName eti))
-          runTraceT =
-            (maybe Tracing.runTraceT Tracing.runTraceTInContext tracingCtx)
-              Tracing.sampleAlways
 
       maintenanceModeVersionEither :: Either QErr (MaintenanceMode MaintenanceModeVersion) <-
         case maintenanceMode of
@@ -468,7 +484,7 @@ processEventQueue logger statsLogger httpMgr getSchemaCache EventEngineCtx {..}
               -- For such an event, we unlock the event and retry after a minute
               runExceptT (setRetry sourceConfig e (addUTCTime 60 currentTime) maintenanceModeVersion)
                 >>= flip onLeft logQErr
-            Right eti -> runTraceT (spanName eti) do
+            Right eti -> trace (spanName eti) do
               eventExecutionStartTime <- liftIO getCurrentTime
               let webhook = wciCachedValue $ etiWebhookInfo eti
                   retryConf = etiRetryConf eti

server/src-lib/Hasura/Eventing/HTTP.hs

@@ -336,7 +336,7 @@ invokeRequest reqDetails@RequestDetails {..} respTransform' sessionVars logger =
       reqBody = fromMaybe J.Null $ view HTTP.body finalReq >>= J.decode @J.Value
   manager <- asks getter
   -- Perform the HTTP Request
-  eitherResp <- tracedHttpRequest finalReq $ runHTTP manager
+  eitherResp <- traceHTTPRequest finalReq $ runHTTP manager
   -- Log the result along with the pre/post transformation Request data
   logger eitherResp reqDetails
   resp <- eitherResp `onLeft` (throwError . HTTPError reqBody)

server/src-lib/Hasura/Eventing/ScheduledTrigger.hs

@@ -124,7 +124,6 @@ where
 import Control.Concurrent.Extended (Forever (..), sleep)
 import Control.Concurrent.STM
 import Control.Lens (view)
-import Control.Monad.Trans.Control (MonadBaseControl)
 import Data.Aeson qualified as J
 import Data.Environment qualified as Env
 import Data.Has
@@ -230,9 +229,8 @@ generateCronEventsFrom startTime CronTriggerInfo {..} =
 
 processCronEvents ::
   ( MonadIO m,
-    MonadBaseControl IO m,
-    Tracing.HasReporter m,
-    MonadMetadataStorage m
+    MonadMetadataStorage m,
+    Tracing.MonadTrace m
   ) =>
   L.Logger L.Hasura ->
   HTTP.Manager ->
@@ -284,8 +282,7 @@ processCronEvents logger httpMgr prometheusMetrics cronEvents getSC lockedCronEv
 
 processOneOffScheduledEvents ::
   ( MonadIO m,
-    MonadBaseControl IO m,
-    Tracing.HasReporter m,
+    Tracing.MonadTrace m,
     MonadMetadataStorage m
   ) =>
   Env.Environment ->
@@ -332,8 +329,7 @@ processOneOffScheduledEvents
 
 processScheduledTriggers ::
   ( MonadIO m,
-    MonadBaseControl IO m,
-    Tracing.HasReporter m,
+    Tracing.MonadTrace m,
     MonadMetadataStorage m
   ) =>
   Env.Environment ->
@@ -367,8 +363,7 @@ processScheduledEvent ::
     Has HTTP.Manager r,
     Has (L.Logger L.Hasura) r,
     MonadIO m,
-    MonadBaseControl IO m,
-    Tracing.HasReporter m,
+    Tracing.MonadTrace m,
     MonadMetadataStorage m,
     MonadError QErr m
   ) =>
@@ -381,7 +376,7 @@ processScheduledEvent ::
   ScheduledEventType ->
   m ()
 processScheduledEvent prometheusMetrics eventId eventHeaders retryCtx payload webhookUrl type' =
-  Tracing.runTraceT Tracing.sampleAlways traceNote do
+  Tracing.newTrace Tracing.sampleAlways traceNote do
     currentTime <- liftIO getCurrentTime
     let retryConf = _rctxConf retryCtx
         scheduledTime = sewpScheduledTime payload

server/src-lib/Hasura/GraphQL/Execute/Action.hs

@@ -432,9 +432,9 @@ asyncActionsProcessor ::
   ( MonadIO m,
     MonadBaseControl IO m,
     LA.Forall (LA.Pure m),
-    Tracing.HasReporter m,
     MonadMetadataStorage m,
-    ProvidesNetwork m
+    ProvidesNetwork m,
+    Tracing.MonadTrace m
   ) =>
   Env.Environment ->
   L.Logger L.Hasura ->
@@ -469,7 +469,8 @@ asyncActionsProcessor env logger getSCFromRef' lockedActionEvents prometheusMetr
         liftIO $ sleep $ milliseconds sleepTime
   where
     callHandler :: ActionCache -> ActionLogItem -> m ()
-    callHandler actionCache actionLogItem = Tracing.runTraceT Tracing.sampleAlways "async actions processor" do
+    callHandler actionCache actionLogItem =
+      Tracing.newTrace Tracing.sampleAlways "async actions processor" do
         httpManager <- askHTTPManager
         let ActionLogItem
               actionId
@@ -492,7 +493,6 @@ asyncActionsProcessor env logger getSCFromRef' lockedActionEvents prometheusMetr
                 metadataResponseTransform = _adResponseTransform definition
             eitherRes <-
               runExceptT $
-              -- TODO: do we need to add the logger as a reader? can't we just give it as an argument?
                 flip runReaderT logger $
                   callWebhook
                     env
@@ -593,7 +593,7 @@ callWebhook
         actualSize = fromMaybe requestBodySize transformedReqSize
 
     httpResponse <-
-      Tracing.tracedHttpRequest actualReq $ \request ->
+      Tracing.traceHTTPRequest actualReq $ \request ->
         liftIO . try $ HTTP.performRequest request manager
 
     let requestInfo = ActionRequestInfo webhookEnvName postPayload (confHeaders <> toHeadersConf clientHeaders) transformedReq

server/src-lib/Hasura/GraphQL/RemoteServer.hs

@@ -164,7 +164,7 @@ execRemoteGQ env userInfo reqHdrs rsdef gqlReq@GQLReq {..} = do
           & set HTTP.timeout (HTTP.responseTimeoutMicro (timeout * 1000000))
 
   manager <- askHTTPManager
-  Tracing.tracedHttpRequest req \req' -> do
+  Tracing.traceHTTPRequest req \req' -> do
     (time, res) <- withElapsedTime $ liftIO $ try $ HTTP.performRequest req' manager
     resp <- onLeft res (throwRemoteSchemaHttp webhookEnvRecord)
     pure (time, mkSetCookieHeaders resp, resp ^. Wreq.responseBody)

server/src-lib/Hasura/GraphQL/Transport/HTTP.hs

@@ -21,6 +21,7 @@ module Hasura.GraphQL.Transport.HTTP
     AnnotatedResponsePart (..),
     CacheStoreSuccess (..),
     CacheStoreFailure (..),
+    CacheStoreResponse,
     SessVarPred,
     filterVariablesFromQuery,
     runSessVarPred,
@@ -28,7 +29,6 @@ module Hasura.GraphQL.Transport.HTTP
 where
 
 import Control.Lens (Traversal', foldOf, to)
-import Control.Monad.Morph (hoist)
 import Control.Monad.Trans.Control (MonadBaseControl)
 import Data.Aeson qualified as J
 import Data.Aeson.Ordered qualified as JO
@@ -84,8 +84,7 @@ import Hasura.Server.Telemetry.Counters qualified as Telem
 import Hasura.Server.Types (RequestId)
 import Hasura.Services.Network
 import Hasura.Session
-import Hasura.Tracing (MonadTrace, TraceT, trace)
-import Hasura.Tracing qualified as Tracing
+import Hasura.Tracing (MonadTrace, TraceT, newSpan)
 import Language.GraphQL.Draft.Syntax qualified as G
 import Network.HTTP.Types qualified as HTTP
 import Network.Wai.Extended qualified as Wai
@@ -136,7 +135,7 @@ class Monad m => MonadExecuteQuery m where
     -- the client should store it locally.  The value ([], Just json) represents
     -- that the client should not store the response locally, but we do have a
     -- server-side cache value that can be used to avoid query execution.
-    TraceT (ExceptT QErr m) (HTTP.ResponseHeaders, Maybe EncJSON)
+    m (Either QErr (HTTP.ResponseHeaders, Maybe EncJSON))
 
   -- | Store a json response for a query that we've executed in the cache.  Note
   -- that, as part of this, 'cacheStore' has to decide whether the response is
@@ -152,7 +151,7 @@ class Monad m => MonadExecuteQuery m where
     -- | Result of a query execution
     EncJSON ->
     -- | Always succeeds
-    TraceT (ExceptT QErr m) CacheStoreResponse
+    m (Either QErr CacheStoreResponse)
 
   default cacheLookup ::
     (m ~ t n, MonadTrans t, MonadExecuteQuery n) =>
@@ -160,22 +159,22 @@ class Monad m => MonadExecuteQuery m where
     [ActionsInfo] ->
     QueryCacheKey ->
     Maybe CachedDirective ->
-    TraceT (ExceptT QErr m) (HTTP.ResponseHeaders, Maybe EncJSON)
-  cacheLookup a b c d = hoist (hoist lift) $ cacheLookup a b c d
+    m (Either QErr (HTTP.ResponseHeaders, Maybe EncJSON))
+  cacheLookup a b c d = lift $ cacheLookup a b c d
 
   default cacheStore ::
     (m ~ t n, MonadTrans t, MonadExecuteQuery n) =>
     QueryCacheKey ->
     Maybe CachedDirective ->
     EncJSON ->
-    TraceT (ExceptT QErr m) CacheStoreResponse
-  cacheStore a b c = hoist (hoist lift) $ cacheStore a b c
+    m (Either QErr CacheStoreResponse)
+  cacheStore a b c = lift $ cacheStore a b c
 
 instance MonadExecuteQuery m => MonadExecuteQuery (ReaderT r m)
 
-instance MonadExecuteQuery m => MonadExecuteQuery (ExceptT r m)
+instance MonadExecuteQuery m => MonadExecuteQuery (ExceptT e m)
 
-instance MonadExecuteQuery m => MonadExecuteQuery (TraceT m)
+instance (MonadExecuteQuery m, MonadIO m) => MonadExecuteQuery (TraceT m)
 
 -- | A partial response, e.g. from a remote schema call or postgres
 -- postgres query, which we'll assemble into the final response for
@@ -387,7 +386,7 @@ runGQ env logger reqId userInfo ipAddress reqHeaders queryType reqUnparsed = do
       E.ResolvedExecutionPlan ->
       m AnnotatedResponse
     executePlan reqParsed runLimits execPlan = case execPlan of
-      E.QueryExecutionPlan queryPlans asts dirMap -> trace "Query" $ do
+      E.QueryExecutionPlan queryPlans asts dirMap -> newSpan "Query" $ do
         -- Attempt to lookup a cached response in the query cache.
         -- 'keyedLookup' is a monadic action possibly returning a cache hit.
         -- 'keyedStore' is a function to write a new response to the cache.
@@ -574,10 +573,8 @@ runGQ env logger reqId userInfo ipAddress reqHeaders queryType reqUnparsed = do
                   queryPlans
           cacheKey = QueryCacheKey reqParsed (_uiRole userInfo) filteredSessionVars
           cachedDirective = runIdentity <$> DM.lookup cached dirMap
-       in ( Tracing.interpTraceT (liftEitherM . runExceptT) $
-              cacheLookup remoteSchemas actionsInfo cacheKey cachedDirective,
-            Tracing.interpTraceT (liftEitherM . runExceptT)
-              . cacheStore cacheKey cachedDirective
+       in ( liftEitherM $ cacheLookup remoteSchemas actionsInfo cacheKey cachedDirective,
+            liftEitherM . cacheStore cacheKey cachedDirective
           )
 
     recordTimings :: DiffTime -> AnnotatedResponse -> m ()

server/src-lib/Hasura/GraphQL/Transport/WSServerApp.hs

@@ -54,16 +54,16 @@ createWSServerApp ::
   ( MonadIO m,
     MC.MonadBaseControl IO m,
     LA.Forall (LA.Pure m),
-    UserAuthentication (Tracing.TraceT m),
+    UserAuthentication m,
     E.MonadGQLExecutionCheck m,
     WS.MonadWSLog m,
     MonadQueryLog m,
-    Tracing.HasReporter m,
     MonadExecuteQuery m,
     MonadMetadataStorage m,
     EB.MonadQueryTags m,
     HasResourceLimits m,
-    ProvidesNetwork m
+    ProvidesNetwork m,
+    Tracing.MonadTrace m
   ) =>
   Env.Environment ->
   HashSet (L.EngineLogType L.Hasura) ->

server/src-lib/Hasura/GraphQL/Transport/WebSocket.hs

@@ -478,7 +478,7 @@ onStart env enabledLogTypes serverEnv wsConn shouldCaptureVariables (StartMsg op
   (parameterizedQueryHash, execPlan) <- onLeft execPlanE (withComplete . preExecErr requestId (Just gqlOpType))
 
   case execPlan of
-    E.QueryExecutionPlan queryPlan asts dirMap -> Tracing.trace "Query" $ do
+    E.QueryExecutionPlan queryPlan asts dirMap -> Tracing.newSpan "Query" $ do
       let filteredSessionVars = runSessVarPred (filterVariablesFromQuery asts) (_uiSession userInfo)
           cacheKey = QueryCacheKey reqParsed (_uiRole userInfo) filteredSessionVars
           remoteSchemas =
@@ -499,7 +499,10 @@ onStart env enabledLogTypes serverEnv wsConn shouldCaptureVariables (StartMsg op
 
       -- We ignore the response headers (containing TTL information) because
       -- WebSockets don't support them.
-      (_responseHeaders, cachedValue) <- Tracing.interpTraceT (withExceptT mempty) $ cacheLookup remoteSchemas actionsInfo cacheKey cachedDirective
+      cachedValue <-
+        cacheLookup remoteSchemas actionsInfo cacheKey cachedDirective >>= \case
+          Right (_responseHeaders, cachedValue) -> pure cachedValue
+          Left _err -> throwError ()
       case cachedValue of
         Just cachedResponseData -> do
           logQueryLog logger $ QueryLog q Nothing requestId QueryLogKindCached
@@ -554,7 +557,6 @@ onStart env enabledLogTypes serverEnv wsConn shouldCaptureVariables (StartMsg op
               -- Note: The result of cacheStore is ignored here since we can't ensure that
               --       the WS client will respond correctly to multiple messages.
               void $
-                Tracing.interpTraceT (withExceptT mempty) $
                 cacheStore cacheKey cachedDirective $
                   encodeAnnotatedResponseParts results
 
@@ -1000,16 +1002,16 @@ onStart env enabledLogTypes serverEnv wsConn shouldCaptureVariables (StartMsg op
 
 onMessage ::
   ( MonadIO m,
-    UserAuthentication (Tracing.TraceT m),
+    UserAuthentication m,
     E.MonadGQLExecutionCheck m,
     MonadQueryLog m,
-    Tracing.HasReporter m,
     MonadExecuteQuery m,
     MC.MonadBaseControl IO m,
     MonadMetadataStorage m,
     EB.MonadQueryTags m,
     HasResourceLimits m,
-    ProvidesNetwork m
+    ProvidesNetwork m,
+    Tracing.MonadTrace m
   ) =>
   Env.Environment ->
   HashSet (L.EngineLogType L.Hasura) ->
@@ -1019,7 +1021,8 @@ onMessage ::
   LBS.ByteString ->
   WS.WSActions WSConnData ->
   m ()
-onMessage env enabledLogTypes authMode serverEnv wsConn msgRaw onMessageActions = Tracing.runTraceT (_wseTraceSamplingPolicy serverEnv) "websocket" do
+onMessage env enabledLogTypes authMode serverEnv wsConn msgRaw onMessageActions =
+  Tracing.newTrace (_wseTraceSamplingPolicy serverEnv) "websocket" do
     case J.eitherDecode msgRaw of
       Left e -> do
         let err = ConnErrMsg $ "parsing ClientMessage failed: " <> T.pack e
@@ -1096,7 +1099,7 @@ stopOperation serverEnv wsConn opId logWhenOpNotExist = do
     opDet n = OperationDetails opId Nothing n ODStopped Nothing Nothing
 
 onConnInit ::
-  (MonadIO m, UserAuthentication (Tracing.TraceT m)) =>
+  (MonadIO m, UserAuthentication m) =>
   L.Logger L.Hasura ->
   HTTP.Manager ->
   WSConn ->
@@ -1106,7 +1109,7 @@ onConnInit ::
   WS.WSOnErrorMessageAction WSConnData ->
   -- | this is the message handler for handling "keep-alive" messages to the client
   WS.WSKeepAliveMessageAction WSConnData ->
-  Tracing.TraceT m ()
+  m ()
 onConnInit logger manager wsConn authMode connParamsM onConnInitErrAction keepAliveMessageAction = do
   -- TODO(from master): what should be the behaviour of connection_init message when a
   -- connection is already iniatilized? Currently, we seem to be doing

server/src-lib/Hasura/RQL/DDL/DataConnector.hs

@@ -14,7 +14,7 @@ where
 --------------------------------------------------------------------------------
 
 import Control.Monad.Except
-import Control.Monad.Trans.Control (MonadBaseControl)
+import Control.Monad.Trans.Control
 import Data.Aeson (FromJSON, ToJSON, (.!=), (.:), (.:?), (.=))
 import Data.Aeson qualified as Aeson
 import Data.Has
@@ -85,9 +85,9 @@ runAddDataConnectorAgent ::
     SC.Build.CacheRWM m,
     Has (L.Logger L.Hasura) r,
     MonadReader r m,
-    MonadBaseControl IO m,
     MonadError Error.QErr m,
-    MonadIO m
+    MonadIO m,
+    MonadBaseControl IO m
   ) =>
   DCAddAgent ->
   m EncJSON

server/src-lib/Hasura/RQL/DDL/Schema/Source.hs

@@ -458,7 +458,10 @@ lookupDataConnectorOptions dcName bmap =
         `onNothing` (Error.throw400 Error.DataConnectorError ("Data connector named " <> Text.E.toTxt dcName <> " was not found in the data connector backend config"))
 
 querySourceSchema ::
-  (MonadIO m, MonadBaseControl IO m, MonadError QErr m) =>
+  ( MonadIO m,
+    MonadBaseControl IO m,
+    MonadError QErr m
+  ) =>
   L.Logger L.Hasura ->
   HTTP.Manager ->
   Maybe DC.Types.SourceTimeout ->

server/src-lib/Hasura/RQL/Types/Eventing/Backend.hs

@@ -38,7 +38,7 @@ class Backend b => BackendEventTrigger (b :: BackendType) where
     TriggerName ->
     Value ->
     UserInfo ->
-    Tracing.TraceContext ->
+    Maybe Tracing.TraceContext ->
     m EventId
 
   -- | @fetchUndeliveredEvents@ fetches the undelivered events from the source

server/src-lib/Hasura/RemoteSchema/SchemaCache/Build.hs

@@ -8,7 +8,7 @@ where
 
 import Control.Arrow.Extended
 import Control.Arrow.Interpret
-import Control.Monad.Trans.Control (MonadBaseControl)
+import Control.Monad.Trans.Control
 import Data.Aeson
 import Data.ByteString.Lazy qualified as BL
 import Data.Environment qualified as Env
@@ -93,7 +93,8 @@ buildRemoteSchemas env =
 
     -- TODO continue propagating MonadTrace up calls so that we can get tracing
     -- for remote schema introspection. This will require modifying CacheBuild.
-    noopTrace = Tracing.runTraceTWithReporter Tracing.noReporter Tracing.sampleNever "buildSchemaCacheRule"
+    -- TODO(Antoine): do this when changing CacheBuild to be on top of the app's m.
+    noopTrace = Tracing.ignoreTraceT
 
     mkRemoteSchemaMetadataObject remoteSchema =
       MetadataObject (MORemoteSchema (_rsmName remoteSchema)) (toJSON remoteSchema)

server/src-lib/Hasura/Server/API/Metadata.hs

@@ -399,7 +399,7 @@ runMetadataQuery ::
   m (EncJSON, RebuildableSchemaCache)
 runMetadataQuery env logger instanceId userInfo serverConfigCtx schemaCacheRef RQLMetadata {..} = do
   schemaCache <- liftIO $ fst <$> readSchemaCacheRef schemaCacheRef
-  (metadata, currentResourceVersion) <- Tracing.trace "fetchMetadata" $ liftEitherM fetchMetadata
+  (metadata, currentResourceVersion) <- Tracing.newSpan "fetchMetadata" $ liftEitherM fetchMetadata
   let exportsMetadata = \case
         RMV1 (RMExportMetadata _) -> True
         RMV2 (RMV2ExportMetadata _) -> True
@@ -439,7 +439,7 @@ runMetadataQuery env logger instanceId userInfo serverConfigCtx schemaCacheRef R
             String $
               "Attempting to put new metadata in storage"
         newResourceVersion <-
-          Tracing.trace "setMetadata" $
+          Tracing.newSpan "setMetadata" $
             liftEitherM $
               setMetadata (fromMaybe currentResourceVersion _rqlMetadataResourceVersion) modMetadata
         L.unLogger logger $
@@ -448,7 +448,7 @@ runMetadataQuery env logger instanceId userInfo serverConfigCtx schemaCacheRef R
               "Put new metadata in storage, received new resource version " <> tshow newResourceVersion
 
         -- notify schema cache sync
-        Tracing.trace "notifySchemaCacheSync" $
+        Tracing.newSpan "notifySchemaCacheSync" $
           liftEitherM $
             notifySchemaCacheSync newResourceVersion instanceId cacheInvalidations
         L.unLogger logger $
@@ -457,7 +457,7 @@ runMetadataQuery env logger instanceId userInfo serverConfigCtx schemaCacheRef R
               "Sent schema cache sync notification at resource version " <> tshow newResourceVersion
 
         (_, modSchemaCache', _) <-
-          Tracing.trace "setMetadataResourceVersionInSchemaCache" $
+          Tracing.newSpan "setMetadataResourceVersionInSchemaCache" $
             setMetadataResourceVersionInSchemaCache newResourceVersion
               & runCacheRWT modSchemaCache
               & peelRun (RunCtx userInfo serverConfigCtx)
@@ -617,10 +617,10 @@ runMetadataQueryM env currentResourceVersion =
     -- NOTE: This is a good place to install tracing, since it's involved in
     -- the recursive case via "bulk":
     RMV1 q ->
-      Tracing.trace ("v1 " <> T.pack (constrName q)) $
+      Tracing.newSpan ("v1 " <> T.pack (constrName q)) $
         runMetadataQueryV1M env currentResourceVersion q
     RMV2 q ->
-      Tracing.trace ("v2 " <> T.pack (constrName q)) $
+      Tracing.newSpan ("v2 " <> T.pack (constrName q)) $
         runMetadataQueryV2M currentResourceVersion q
 
 runMetadataQueryV1M ::

server/src-lib/Hasura/Server/API/V2Query.hs

@@ -122,7 +122,7 @@ runQuery env instanceId userInfo schemaCache serverConfigCtx rqlQuery = do
   when ((_sccReadOnlyMode serverConfigCtx == ReadOnlyModeEnabled) && queryModifiesUserDB rqlQuery) $
     throw400 NotSupported "Cannot run write queries when read-only mode is enabled"
 
-  (metadata, currentResourceVersion) <- Tracing.trace "fetchMetadata" $ liftEitherM fetchMetadata
+  (metadata, currentResourceVersion) <- Tracing.newSpan "fetchMetadata" $ liftEitherM fetchMetadata
   result <-
     runQueryM env rqlQuery & \x -> do
       ((js, meta), rsc, ci) <-
@@ -142,11 +142,11 @@ runQuery env instanceId userInfo schemaCache serverConfigCtx rqlQuery = do
           MaintenanceModeDisabled -> do
             -- set modified metadata in storage
             newResourceVersion <-
-              Tracing.trace "setMetadata" $
+              Tracing.newSpan "setMetadata" $
                 liftEitherM $
                   setMetadata currentResourceVersion updatedMetadata
             -- notify schema cache sync
-            Tracing.trace "notifySchemaCacheSync" $
+            Tracing.newSpan "notifySchemaCacheSync" $
               liftEitherM $
                 notifySchemaCacheSync newResourceVersion instanceId invalidations
           MaintenanceModeEnabled () ->
@@ -185,7 +185,7 @@ runQueryM ::
   Env.Environment ->
   RQLQuery ->
   m EncJSON
-runQueryM env rq = Tracing.trace (T.pack $ constrName rq) $ case rq of
+runQueryM env rq = Tracing.newSpan (T.pack $ constrName rq) $ case rq of
   RQInsert q -> runInsert q
   RQSelect q -> runSelect q
   RQUpdate q -> runUpdate q

server/src-lib/Hasura/Server/App.hs

@@ -35,6 +35,7 @@ import Data.Aeson.KeyMap qualified as KM
 import Data.Aeson.Types qualified as J
 import Data.ByteString.Builder qualified as BB
 import Data.ByteString.Char8 qualified as B8
+import Data.ByteString.Char8 qualified as Char8
 import Data.ByteString.Lazy qualified as BL
 import Data.CaseInsensitive qualified as CI
 import Data.HashMap.Strict qualified as M
@@ -96,6 +97,7 @@ import Hasura.Server.Utils
 import Hasura.Server.Version
 import Hasura.Services
 import Hasura.Session
+import Hasura.Tracing (MonadTrace)
 import Hasura.Tracing qualified as Tracing
 import Network.HTTP.Types qualified as HTTP
 import Network.Mime (defaultMimeLookup)
@@ -129,8 +131,7 @@ newtype Handler m a = Handler (ReaderT HandlerCtx (ExceptT QErr m) a)
       MonadBaseControl b,
       MonadReader HandlerCtx,
       MonadError QErr,
-      -- Tracing.HasReporter,
-      Tracing.MonadTrace,
+      MonadTrace,
       HasResourceLimits,
       MonadResolveSource,
       HasServerConfigCtx,
@@ -271,10 +272,10 @@ mkSpockAction ::
   ( MonadIO m,
     MonadBaseControl IO m,
     FromJSON a,
-    UserAuthentication (Tracing.TraceT m),
+    UserAuthentication m,
     HttpLog m,
-    Tracing.HasReporter m,
-    HasResourceLimits m
+    HasResourceLimits m,
+    MonadTrace m
   ) =>
   AppContext ->
   AppEnv ->
@@ -282,7 +283,7 @@ mkSpockAction ::
   (Bool -> QErr -> Value) ->
   -- | `QErr` modifier
   (QErr -> QErr) ->
-  APIHandler (Tracing.TraceT m) a ->
+  APIHandler m a ->
   Spock.ActionT m ()
 mkSpockAction appCtx@AppContext {..} appEnv@AppEnv {..} qErrEncoder qErrModifier apiHandler = do
   req <- Spock.request
@@ -294,19 +295,35 @@ mkSpockAction appCtx@AppContext {..} appEnv@AppEnv {..} qErrEncoder qErrModifier
   (ioWaitTime, reqBody) <- withElapsedTime $ liftIO $ Wai.strictRequestBody req
 
   (requestId, headers) <- getRequestId origHeaders
-  tracingCtx <- liftIO $ Tracing.extractB3HttpContext headers
+  tracingCtx <- liftIO do
+    -- B3 TraceIds can have a length of either 64 bits (16 hex chars) or 128 bits
+    -- (32 hex chars). For 64-bit TraceIds, we pad them with zeros on the left to
+    -- make them 128 bits long.
+    let traceIdMaybe =
+          lookup "X-B3-TraceId" headers >>= \rawTraceId ->
+            if
+                | Char8.length rawTraceId == 32 ->
+                    Tracing.traceIdFromHex rawTraceId
+                | Char8.length rawTraceId == 16 ->
+                    Tracing.traceIdFromHex $ Char8.replicate 16 '0' <> rawTraceId
+                | otherwise ->
+                    Nothing
+    for traceIdMaybe $ \traceId -> do
+      freshSpanId <- Tracing.randomSpanId
+      let parentSpanId = Tracing.spanIdFromHex =<< lookup "X-B3-SpanId" headers
+          samplingState = Tracing.samplingStateFromHeader $ lookup "X-B3-Sampled" headers
+      pure $ Tracing.TraceContext traceId freshSpanId parentSpanId samplingState
 
-  let runTraceT ::
+  let runTrace ::
         forall m1 a1.
-        (MonadIO m1, MonadBaseControl IO m1, Tracing.HasReporter m1) =>
-        Tracing.TraceT m1 a1 ->
+        (MonadIO m1, MonadTrace m1) =>
+        m1 a1 ->
         m1 a1
-      runTraceT = do
-        (maybe Tracing.runTraceT Tracing.runTraceTInContext tracingCtx)
-          appEnvTraceSamplingPolicy
-          (fromString (B8.unpack pathInfo))
+      runTrace = case tracingCtx of
+        Nothing -> Tracing.newTrace appEnvTraceSamplingPolicy (fromString (B8.unpack pathInfo))
+        Just ctx -> Tracing.newTraceWith ctx appEnvTraceSamplingPolicy (fromString (B8.unpack pathInfo))
 
-      getInfo parsedRequest = do
+  let getInfo parsedRequest = do
         authenticationResp <- lift (resolveUserInfo (_lsLogger appEnvLoggers) appEnvManager headers acAuthMode parsedRequest)
         authInfo <- onLeft authenticationResp (logErrorAndResp Nothing requestId req (reqBody, Nothing) False origHeaders (ExtraUserInfo Nothing) . qErrModifier)
         let (userInfo, _, authHeaders, extraUserInfo) = authInfo
@@ -318,7 +335,7 @@ mkSpockAction appCtx@AppContext {..} appEnv@AppEnv {..} qErrEncoder qErrModifier
             extraUserInfo
           )
 
-  mapActionT runTraceT $ do
+  mapActionT runTrace do
     -- Add the request ID to the tracing metadata so that we
     -- can correlate requests and traces
     lift $ Tracing.attachMetadata [("request_id", unRequestId requestId)]
@@ -400,7 +417,7 @@ v1QueryHandler ::
     MonadError QErr m,
     MonadBaseControl IO m,
     MonadMetadataApiAuthorization m,
-    Tracing.MonadTrace m,
+    MonadTrace m,
     MonadReader HandlerCtx m,
     MonadMetadataStorageQueryAPI m,
     MonadResolveSource m,
@@ -453,7 +470,7 @@ v1MetadataHandler ::
     MonadError QErr m,
     MonadBaseControl IO m,
     MonadReader HandlerCtx m,
-    Tracing.MonadTrace m,
+    MonadTrace m,
     MonadMetadataStorageQueryAPI m,
     MonadResolveSource m,
     MonadMetadataApiAuthorization m,
@@ -463,7 +480,7 @@ v1MetadataHandler ::
   ) =>
   RQLMetadata ->
   m (HttpResponse EncJSON)
-v1MetadataHandler query = Tracing.trace "Metadata" $ do
+v1MetadataHandler query = Tracing.newSpan "Metadata" $ do
   (liftEitherM . authorizeV1MetadataApi query) =<< ask
   userInfo <- asks hcUser
   AppContext {..} <- asks hcAppContext
@@ -505,7 +522,7 @@ v2QueryHandler ::
     MonadError QErr m,
     MonadBaseControl IO m,
     MonadMetadataApiAuthorization m,
-    Tracing.MonadTrace m,
+    MonadTrace m,
     MonadReader HandlerCtx m,
     MonadMetadataStorage m,
     MonadResolveSource m,
@@ -514,7 +531,7 @@ v2QueryHandler ::
   ) =>
   V2Q.RQLQuery ->
   m (HttpResponse EncJSON)
-v2QueryHandler query = Tracing.trace "v2 Query" $ do
+v2QueryHandler query = Tracing.newSpan "v2 Query" $ do
   (liftEitherM . authorizeV2QueryApi query) =<< ask
   scRef <- asks (acCacheRef . hcAppContext)
   logger <- asks (_lsLogger . appEnvLoggers . hcAppEnv)
@@ -553,7 +570,7 @@ v1Alpha1GQHandler ::
     MonadBaseControl IO m,
     E.MonadGQLExecutionCheck m,
     MonadQueryLog m,
-    Tracing.MonadTrace m,
+    MonadTrace m,
     GH.MonadExecuteQuery m,
     MonadError QErr m,
     MonadReader HandlerCtx m,
@@ -595,7 +612,7 @@ v1GQHandler ::
     MonadBaseControl IO m,
     E.MonadGQLExecutionCheck m,
     MonadQueryLog m,
-    Tracing.MonadTrace m,
+    MonadTrace m,
     GH.MonadExecuteQuery m,
     MonadError QErr m,
     MonadReader HandlerCtx m,
@@ -613,7 +630,7 @@ v1GQRelayHandler ::
     MonadBaseControl IO m,
     E.MonadGQLExecutionCheck m,
     MonadQueryLog m,
-    Tracing.MonadTrace m,
+    MonadTrace m,
     GH.MonadExecuteQuery m,
     MonadError QErr m,
     MonadReader HandlerCtx m,
@@ -634,7 +651,7 @@ gqlExplainHandler ::
     MonadReader HandlerCtx m,
     MonadMetadataStorage m,
     EB.MonadQueryTags m,
-    Tracing.MonadTrace m
+    MonadTrace m
   ) =>
   GE.GQLExplain ->
   m (HttpResponse EncJSON)
@@ -712,7 +729,13 @@ renderHtmlTemplate template jVal =
 -- | Default implementation of the 'MonadConfigApiHandler'
 configApiGetHandler ::
   forall m.
-  (MonadIO m, MonadBaseControl IO m, UserAuthentication (Tracing.TraceT m), HttpLog m, Tracing.HasReporter m, HasResourceLimits m) =>
+  ( MonadIO m,
+    MonadBaseControl IO m,
+    UserAuthentication m,
+    HttpLog m,
+    HasResourceLimits m,
+    MonadTrace m
+  ) =>
   AppContext ->
   AppEnv ->
   Spock.SpockCtxT () m ()
@@ -751,13 +774,13 @@ mkWaiApp ::
     ConsoleRenderer m,
     MonadVersionAPIWithExtraData m,
     HttpLog m,
-    UserAuthentication (Tracing.TraceT m),
+    UserAuthentication m,
     MonadMetadataApiAuthorization m,
     E.MonadGQLExecutionCheck m,
     MonadConfigApiHandler m,
     MonadQueryLog m,
     WS.MonadWSLog m,
-    Tracing.HasReporter m,
+    MonadTrace m,
     GH.MonadExecuteQuery m,
     HasResourceLimits m,
     MonadMetadataStorageQueryAPI m,
@@ -818,12 +841,12 @@ httpApp ::
     ConsoleRenderer m,
     MonadVersionAPIWithExtraData m,
     HttpLog m,
-    UserAuthentication (Tracing.TraceT m),
+    UserAuthentication m,
     MonadMetadataApiAuthorization m,
     E.MonadGQLExecutionCheck m,
     MonadConfigApiHandler m,
     MonadQueryLog m,
-    Tracing.HasReporter m,
+    MonadTrace m,
     GH.MonadExecuteQuery m,
     MonadMetadataStorageQueryAPI m,
     HasResourceLimits m,
@@ -908,10 +931,11 @@ httpApp setupHook appCtx@AppContext {..} appEnv@AppEnv {..} ekgStore = do
           MonadMetadataStorage n,
           EB.MonadQueryTags n,
           HasResourceLimits n,
-          ProvidesNetwork n
+          ProvidesNetwork n,
+          MonadTrace n
         ) =>
         RestRequest Spock.SpockMethod ->
-        Handler (Tracing.TraceT n) (HttpLogGraphQLInfo, APIResp)
+        Handler n (HttpLogGraphQLInfo, APIResp)
       customEndpointHandler restReq = do
         endpoints <- liftIO $ scEndpoints <$> getSchemaCache acCacheRef
         execCtx <- mkExecutionContext
@@ -1077,14 +1101,14 @@ httpApp setupHook appCtx@AppContext {..} appEnv@AppEnv {..} ekgStore = do
       ( FromJSON a,
         MonadIO n,
         MonadBaseControl IO n,
-        UserAuthentication (Tracing.TraceT n),
+        UserAuthentication n,
         HttpLog n,
-        Tracing.HasReporter n,
+        MonadTrace n,
         HasResourceLimits n
       ) =>
       (Bool -> QErr -> Value) ->
       (QErr -> QErr) ->
-      APIHandler (Tracing.TraceT n) a ->
+      APIHandler n a ->
       Spock.ActionT n ()
     spockAction qErrEncoder qErrModifier apiHandler = mkSpockAction appCtx appEnv qErrEncoder qErrModifier apiHandler
 

server/src-lib/Hasura/Server/Auth.hs

@@ -45,7 +45,6 @@ import Hasura.Server.Auth.JWT hiding (processJwt_)
 import Hasura.Server.Auth.WebHook
 import Hasura.Server.Utils
 import Hasura.Session
-import Hasura.Tracing qualified as Tracing
 import Network.HTTP.Client qualified as HTTP
 import Network.HTTP.Types qualified as HTTP
 
@@ -106,9 +105,9 @@ data AuthMode
 --
 -- This must only be run once, on launch.
 setupAuthMode ::
-  ( Tracing.HasReporter m,
-    MonadError Text m,
-    MonadIO m
+  ( MonadError Text m,
+    MonadIO m,
+    MonadBaseControl IO m
   ) =>
   Set.HashSet AdminSecretHash ->
   Maybe AuthHook ->
@@ -147,7 +146,7 @@ setupAuthMode adminSecretHashSet mWebHook mJwtSecrets mUnAuthRole logger httpMan
       " requires --admin-secret (HASURA_GRAPHQL_ADMIN_SECRET) or "
         <> " --access-key (HASURA_GRAPHQL_ACCESS_KEY) to be set"
 
-    mkJwtCtx :: (MonadIO m, MonadError Text m) => JWTConfig -> m JWTCtx
+    mkJwtCtx :: (MonadIO m, MonadBaseControl IO m, MonadError Text m) => JWTConfig -> m JWTCtx
     mkJwtCtx JWTConfig {..} = do
       (jwkUri, jwkKeyConfig) <- case jcKeyOrUrl of
         Left jwk -> do
@@ -157,16 +156,15 @@ setupAuthMode adminSecretHashSet mWebHook mJwtSecrets mUnAuthRole logger httpMan
         -- which will be populated by the 'updateJWKCtx' poller thread
         Right uri -> do
           -- fetch JWK initially and throw error if it fails
-          void $ liftEitherM $ liftIO $ runExceptT $ withJwkError $ Tracing.runTraceT Tracing.sampleAlways "jwk init" $ fetchJwk logger httpManager uri
+          void $ withJwkError $ fetchJwk logger httpManager uri
           jwkRef <- liftIO $ newIORef (JWKSet [], Nothing)
           return (Just uri, jwkRef)
       let jwtHeader = fromMaybe JHAuthorization jcHeader
       return $ JWTCtx jwkUri jwkKeyConfig jcAudience jcIssuer jcClaims jcAllowedSkew jwtHeader
 
-    withJwkError :: ExceptT JwkFetchError IO (JWKSet, HTTP.ResponseHeaders) -> ExceptT Text IO (JWKSet, HTTP.ResponseHeaders)
-    withJwkError act = do
-      res <- lift $ runExceptT act
-      onLeft res $ \case
+    withJwkError a = do
+      res <- runExceptT a
+      onLeft res \case
         -- when fetching JWK initially, except expiry parsing error, all errors are critical
         JFEHttpException _ msg -> throwError msg
         JFEHttpError _ _ _ e -> throwError e
@@ -176,7 +174,8 @@ setupAuthMode adminSecretHashSet mWebHook mJwtSecrets mUnAuthRole logger httpMan
 -- | Core logic to fork a poller thread to update the JWK based on the
 -- expiry time specified in @Expires@ header or @Cache-Control@ header
 updateJwkCtx ::
-  (MonadIO m, Tracing.HasReporter m) =>
+  forall m.
+  (MonadIO m, MonadBaseControl IO m) =>
   AuthMode ->
   HTTP.Manager ->
   Logger Hasura ->
@@ -187,10 +186,7 @@ updateJwkCtx authMode httpManager logger = forever $ do
     _ -> pure ()
   liftIO $ sleep $ seconds 1
   where
-    updateJwkFromUrl ::
-      (Tracing.HasReporter m, MonadIO m) =>
-      JWTCtx ->
-      m ()
+    updateJwkFromUrl :: JWTCtx -> m ()
     updateJwkFromUrl (JWTCtx url ref _ _ _ _ _) =
       for_ url \uri -> do
         (jwkSet, jwkExpiry) <- liftIO $ readIORef ref
@@ -208,7 +204,7 @@ updateJwkCtx authMode httpManager logger = forever $ do
 -- | Authenticate the request using the headers and the configured 'AuthMode'.
 getUserInfoWithExpTime ::
   forall m.
-  (MonadIO m, MonadBaseControl IO m, MonadError QErr m, Tracing.MonadTrace m) =>
+  (MonadIO m, MonadBaseControl IO m, MonadError QErr m) =>
   Logger Hasura ->
   HTTP.Manager ->
   [HTTP.Header] ->

server/src-lib/Hasura/Server/Auth/JWT.hs

@@ -99,7 +99,6 @@ import Hasura.Server.Utils
     userRoleHeader,
   )
 import Hasura.Session
-import Hasura.Tracing qualified as Tracing
 import Network.HTTP.Client.Transformable qualified as HTTP
 import Network.HTTP.Types as N
 import Network.URI (URI)
@@ -310,18 +309,14 @@ $(J.deriveJSON hasuraJSON ''HasuraClaims)
 -- | An action that fetches the JWKs and updates the expiry time and JWKs in the
 -- IORef
 fetchAndUpdateJWKs ::
-  (MonadIO m) =>
+  (MonadIO m, MonadBaseControl IO m) =>
   Logger Hasura ->
   HTTP.Manager ->
   URI ->
   IORef (Jose.JWKSet, Maybe UTCTime) ->
   m ()
 fetchAndUpdateJWKs logger httpManager url jwkRef = do
-  res <-
-    liftIO $
-      runExceptT $
-        Tracing.runTraceT Tracing.sampleAlways "jwk fetch" $
-          fetchJwk logger httpManager url
+  res <- runExceptT $ fetchJwk logger httpManager url
   case res of
     -- As this 'fetchJwk' is going to happen always in background thread, we are
     -- not going to throw fatal error(s). If there is any error fetching JWK -
@@ -352,8 +347,7 @@ fetchAndUpdateJWKs logger httpManager url jwkRef = do
 fetchJwk ::
   ( MonadIO m,
     MonadBaseControl IO m,
-    MonadError JwkFetchError m,
-    Tracing.MonadTrace m
+    MonadError JwkFetchError m
   ) =>
   Logger Hasura ->
   HTTP.Manager ->
@@ -366,9 +360,7 @@ fetchJwk (Logger logger) manager url = do
   res <- try $ do
     req <- liftIO $ HTTP.mkRequestThrow $ tshow url
     let req' = req & over HTTP.headers addDefaultHeaders
-
-    Tracing.tracedHttpRequest req' \req'' -> do
-      liftIO $ HTTP.performRequest req'' manager
+    liftIO $ HTTP.performRequest req' manager
   resp <- onLeft res logAndThrowHttp
   let status = resp ^. Wreq.responseStatus
       respBody = resp ^. Wreq.responseBody

server/src-lib/Hasura/Server/Auth/WebHook.hs

@@ -24,7 +24,6 @@ import Hasura.Prelude
 import Hasura.Server.Logging
 import Hasura.Server.Utils
 import Hasura.Session
-import Hasura.Tracing qualified as Tracing
 import Network.HTTP.Client.Transformable qualified as HTTP
 import Network.Wreq qualified as Wreq
 
@@ -54,7 +53,7 @@ hookMethod authHook = case ahType authHook of
 --   for finer-grained auth. (#2666)
 userInfoFromAuthHook ::
   forall m.
-  (MonadIO m, MonadBaseControl IO m, MonadError QErr m, Tracing.MonadTrace m) =>
+  (MonadIO m, MonadBaseControl IO m, MonadError QErr m) =>
   Logger Hasura ->
   HTTP.Manager ->
   AuthHook ->
@@ -73,22 +72,22 @@ userInfoFromAuthHook logger manager hook reqHeaders reqs = do
     performHTTPRequest = do
       let url = T.unpack $ ahUrl hook
       req <- liftIO $ HTTP.mkRequestThrow $ T.pack url
-      Tracing.tracedHttpRequest req \req' -> liftIO do
+      liftIO do
         case ahType hook of
           AHTGet -> do
             let isCommonHeader = (`elem` commonClientHeadersIgnored)
                 filteredHeaders = filter (not . isCommonHeader . fst) reqHeaders
-                req'' = req' & set HTTP.headers (addDefaultHeaders filteredHeaders)
-            HTTP.performRequest req'' manager
+                req' = req & set HTTP.headers (addDefaultHeaders filteredHeaders)
+            HTTP.performRequest req' manager
           AHTPost -> do
             let contentType = ("Content-Type", "application/json")
                 headersPayload = J.toJSON $ Map.fromList $ hdrsToText reqHeaders
-                req'' =
+                req' =
                   req
                     & set HTTP.method "POST"
                     & set HTTP.headers (addDefaultHeaders [contentType])
                     & set HTTP.body (Just $ J.encode $ object ["headers" J..= headersPayload, "request" J..= reqs])
-            HTTP.performRequest req'' manager
+            HTTP.performRequest req' manager
 
     logAndThrow :: HTTP.HttpException -> m a
     logAndThrow err = do

server/src-lib/Hasura/Tracing.hs

@@ -1,498 +1,83 @@
-{-# LANGUAGE UndecidableInstances #-}
+module Hasura.Tracing (module Tracing) where
 
-module Hasura.Tracing
-  ( MonadTrace (..),
-    TraceT,
-    runTraceT,
-    runTraceTWith,
-    runTraceTWithReporter,
-    runTraceTInContext,
-    ignoreTraceT,
-    interpTraceT,
-    TraceContext (..),
-    Reporter (..),
-    noReporter,
-    HasReporter (..),
-    SamplingPolicy,
-    sampleNever,
-    sampleAlways,
-    sampleRandomly,
-    sampleOneInN,
-    TracingMetadata,
-    extractB3HttpContext,
-    tracedHttpRequest,
-    injectEventContext,
-    extractEventContext,
-  )
-where
+import Hasura.Tracing.Class as Tracing
+import Hasura.Tracing.Context as Tracing
+import Hasura.Tracing.Monad as Tracing
+import Hasura.Tracing.Reporter as Tracing
+import Hasura.Tracing.Sampling as Tracing
+import Hasura.Tracing.TraceId as Tracing
+import Hasura.Tracing.Utils as Tracing
 
-import Control.Lens (over, view, (^?))
-import Control.Monad.Catch (MonadCatch, MonadMask, MonadThrow)
-import Control.Monad.Morph
-import Control.Monad.Trans.Control
-import Data.Aeson qualified as J
-import Data.Aeson.Lens qualified as JL
-import Data.ByteString.Char8 qualified as Char8
-import Data.IORef
-import Data.String (fromString)
-import Hasura.Prelude
-import Hasura.Tracing.TraceId
-  ( SpanId,
-    TraceId,
-    randomSpanId,
-    randomTraceId,
-    spanIdFromHex,
-    spanIdToHex,
-    traceIdFromHex,
-    traceIdToHex,
-  )
-import Network.HTTP.Client.Transformable qualified as HTTP
-import Refined (Positive, Refined, unrefine)
-import System.Random.Stateful qualified as Random
+{- Note [Tracing]
 
--- | Any additional human-readable key-value pairs relevant
--- to the execution of a block of code.
-type TracingMetadata = [(Text, Text)]
+## Usage
 
-newtype Reporter = Reporter
-  { runReporter ::
-      forall io a.
-      (MonadIO io, MonadBaseControl IO io) =>
-      TraceContext ->
-      -- the current trace context
-      Text ->
-      -- human-readable name for this block of code
-      IO TracingMetadata ->
-      -- an IO action that gets all of the metadata logged so far by the action
-      -- being traced
-      io a ->
-      -- the action we want to trace
-      io a
-  }
+The Tracing library allows us to trace arbitrary pieces of our code, providing
+that the current monad implements 'MonadTrace'.
 
-noReporter :: Reporter
-noReporter = Reporter \_ _ _ -> id
+    newTrace "request" do
+      userInfo    <- newSpan "authentication" retrieveUserInfo
+      parsedQuery <- newSpan "parsing" $ parseQuery q
+      result      <- newSpan "execution" $ runQuery parsedQuery userInfo
+      pure result
 
--- | A type class for monads which support some way to report execution traces.
---
--- See @instance Tracing.HasReporter (AppM impl)@ in @HasuraPro.App@.
-class Monad m => HasReporter m where
-  -- | Get the current tracer
-  askReporter :: m Reporter
-  default askReporter :: m Reporter
-  askReporter = pure noReporter
+## Trace and span
 
-instance HasReporter m => HasReporter (ReaderT r m) where
-  askReporter = lift askReporter
+Each _trace_ is distinct, and is composed of one or more _spans_. Spans are
+organized as a tree: the root span covers the entire trace, and each sub span
+keeps track of its parent.
 
-instance HasReporter m => HasReporter (ExceptT e m) where
-  askReporter = lift askReporter
+We report each span individually, and to each of them we associate a
+'TraceContext', that contains:
+  - a trace id, common to all the spans of that trace
+  - a unique span id, generated randomly
+  - the span id of the parent span, if any
+  - whether that trace was sampled (see "Sampling").
 
-instance HasReporter IO
+All of this can be retrieved for the current span with 'currentContext'.
 
--- | A trace context records the current active trace,
--- the active span within that trace, and the span's parent,
--- unless the current span is the root.
-data TraceContext = TraceContext
-  { -- | TODO what is this exactly? The topmost span id?
-    tcCurrentTrace :: !TraceId,
-    tcCurrentSpan :: !SpanId,
-    tcCurrentParent :: !(Maybe SpanId),
-    tcSamplingState :: !SamplingState
-  }
+Starting a new trace masks the previous one; in the following example, "span2"
+is associated to "trace2" and "span1" is associated to "trace1"; the two trees
+are distinct:
 
--- | B3 propagation sampling state.
---
--- Debug sampling state not represented.
-data SamplingState = SamplingDefer | SamplingDeny | SamplingAccept
+    newTrace "trace1" $
+      newSpan "span1" $
+        newTrace "trace2" $
+          newSpan "span2"
 
--- | Convert a sampling state to a value for the X-B3-Sampled header. A return
--- value of Nothing indicates that the header should not be set.
-samplingStateToHeader :: IsString s => SamplingState -> Maybe s
-samplingStateToHeader = \case
-  SamplingDefer -> Nothing
-  SamplingDeny -> Just "0"
-  SamplingAccept -> Just "1"
+Lastly, a span that is started outside of a root trace is, for now, silently
+ignored, as it has no trace id to attach to. This is a design decision we may
+revisit.
 
--- | Convert a X-B3-Sampled header value to a sampling state. An input of
--- Nothing indicates that the header was not set.
-samplingStateFromHeader :: (IsString s, Eq s) => Maybe s -> SamplingState
-samplingStateFromHeader = \case
-  Nothing -> SamplingDefer
-  Just "0" -> SamplingDeny
-  Just "1" -> SamplingAccept
-  Just _ -> SamplingDefer
+## Metadata
 
-data TraceTEnv = TraceTEnv
-  { tteTraceContext :: TraceContext,
-    tteReporter :: Reporter,
-    tteMetadataRef :: IORef TracingMetadata,
-    tteSamplingDecision :: SamplingDecision
-  }
+Metadata can be attached to the current trace with 'attachMetadata', as a list
+of pair of text key and text values.
 
--- | A local decision about whether or not to sample spans.
-data SamplingDecision = SampleNever | SampleAlways
+## Reporters
 
--- | An IO action for deciding whether or not to sample a trace.
---
--- Currently restricted to deny access to the B3 sampling state, but we may
--- want to be more flexible in the future.
-type SamplingPolicy = IO SamplingDecision
+'TraceT' is the de-facto implementation of 'MonadTrace'; but, in practice, it
+only does half the job: once a span finishes, 'TraceT' delegates the job of
+actually reporting / exporting all relevant information to a 'Reporter'. Said
+reporter must be provided to 'runTraceT', and is a wrapper around a function in
+IO that processes the span.
 
--- Helper for consistently deciding whether or not to sample a trace based on
--- trace context and sampling policy.
-decideSampling :: SamplingState -> SamplingPolicy -> IO SamplingDecision
-decideSampling samplingState samplingPolicy =
-  case samplingState of
-    SamplingDefer -> samplingPolicy
-    SamplingDeny -> pure SampleNever
-    SamplingAccept -> pure SampleAlways
+In practice, 'TraceT' is only a reader that keeps track of the reporter, the
+default sampling policy, and the current trace.
 
--- Helper for consistently updating the sampling state when a sampling decision
--- is made.
-updateSamplingState :: SamplingDecision -> SamplingState -> SamplingState
-updateSamplingState samplingDecision = \case
-  SamplingDefer ->
-    case samplingDecision of
-      SampleNever -> SamplingDefer
-      SampleAlways -> SamplingAccept
-  SamplingDeny -> SamplingDeny
-  SamplingAccept -> SamplingAccept
+## Sampling
 
-sampleNever :: SamplingPolicy
-sampleNever = pure SampleNever
+To run 'TraceT', you must also provide a 'SamplingPolicy': an IO action that,
+when evaluated, will decide whether an arbitrary trace should be reporter or
+not. This decision is only made once per trace: every span within a trace will
+use the same result: they're either all reporter, or none of them are.
 
-sampleAlways :: SamplingPolicy
-sampleAlways = pure SampleAlways
+When starting a trace, the default sampling policy can be overriden. You can for
+instance run 'TraceT' with an action that, by default, only reports one out of
+every ten traces, but use 'newTraceWithPolicy sampleAlways' when sending
+critical requests to your authentication service.
 
--- @sampleRandomly p@ returns `SampleAlways` with probability @p@ and
--- `SampleNever` with probability @1 - p@.
-sampleRandomly :: Double -> SamplingPolicy
-sampleRandomly samplingProbability
-  | samplingProbability <= 0 = pure SampleNever
-  | samplingProbability >= 1 = pure SampleAlways
-  | otherwise = do
-      x <- Random.uniformRM (0, 1) Random.globalStdGen
-      pure $ if x < samplingProbability then SampleAlways else SampleNever
+Note that sampling and reporting are distinct: using 'sampleAlways' simply
+guarantees that the 'Reporter' you provided will be called.
 
--- Like @sampleRandomly@, but with the probability expressed as the denominator
--- N of the fraction 1/N.
-sampleOneInN :: Refined Positive Int -> SamplingPolicy
-sampleOneInN denominator
-  | n == 1 = pure SampleAlways
-  | otherwise = do
-      x <- Random.uniformRM (0, n - 1) Random.globalStdGen
-      pure $ if x == 0 then SampleAlways else SampleNever
-  where
-    n = unrefine denominator
-
--- | The 'TraceT' monad transformer adds the ability to keep track of
--- the current trace context.
-newtype TraceT m a = TraceT {unTraceT :: ReaderT TraceTEnv m a}
-  deriving
-    ( Functor,
-      Applicative,
-      Monad,
-      MonadIO,
-      MonadFix,
-      MonadMask,
-      MonadCatch,
-      MonadThrow,
-      MonadBase b,
-      MonadBaseControl b
-    )
-
-instance MonadTrans TraceT where
-  lift = TraceT . lift
-
-instance MFunctor TraceT where
-  hoist f (TraceT rwma) = TraceT (hoist f rwma)
-
-instance MonadError e m => MonadError e (TraceT m) where
-  throwError = lift . throwError
-  catchError (TraceT m) f = TraceT (catchError m (unTraceT . f))
-
-instance MonadReader r m => MonadReader r (TraceT m) where
-  ask = TraceT $ lift ask
-  local f m = TraceT $ mapReaderT (local f) (unTraceT m)
-
--- | Run an action in the 'TraceT' monad transformer.
--- 'runTraceT' delimits a new trace with its root span, and the arguments
--- specify a name and metadata for that span.
-runTraceT ::
-  (HasReporter m, MonadIO m, MonadBaseControl IO m) =>
-  SamplingPolicy ->
-  Text ->
-  TraceT m a ->
-  m a
-runTraceT policy name tma = do
-  rep <- askReporter
-  runTraceTWithReporter rep policy name tma
-
-runTraceTWith ::
-  (MonadIO m, MonadBaseControl IO m) =>
-  TraceContext ->
-  Reporter ->
-  SamplingPolicy ->
-  Text ->
-  TraceT m a ->
-  m a
-runTraceTWith ctx rep policy name tma = do
-  samplingDecision <- liftIO $ decideSampling (tcSamplingState ctx) policy
-  metadataRef <- liftIO $ newIORef []
-  let subCtx =
-        ctx
-          { tcSamplingState =
-              updateSamplingState samplingDecision (tcSamplingState ctx)
-          }
-      report =
-        case samplingDecision of
-          SampleNever -> id
-          SampleAlways -> do
-            runReporter rep ctx name (readIORef metadataRef)
-  report $
-    runReaderT (unTraceT tma) (TraceTEnv subCtx rep metadataRef samplingDecision)
-
--- | Run an action in the 'TraceT' monad transformer in an
--- existing context.
-runTraceTInContext ::
-  (MonadIO m, MonadBaseControl IO m, HasReporter m) =>
-  TraceContext ->
-  SamplingPolicy ->
-  Text ->
-  TraceT m a ->
-  m a
-runTraceTInContext ctx policy name tma = do
-  rep <- askReporter
-  runTraceTWith ctx rep policy name tma
-
--- | Run an action in the 'TraceT' monad transformer in an
--- existing context.
-runTraceTWithReporter ::
-  (MonadIO m, MonadBaseControl IO m) =>
-  Reporter ->
-  SamplingPolicy ->
-  Text ->
-  TraceT m a ->
-  m a
-runTraceTWithReporter rep policy name tma = do
-  ctx <-
-    TraceContext
-      <$> liftIO randomTraceId
-      <*> liftIO randomSpanId
-      <*> pure Nothing
-      <*> pure SamplingDefer
-  runTraceTWith ctx rep policy name tma
-
--- | Run an action in the 'TraceT' monad transformer while suppressing all
--- tracing-related side-effects.
-ignoreTraceT :: (MonadIO m, MonadBaseControl IO m) => TraceT m a -> m a
-ignoreTraceT = runTraceTWithReporter noReporter sampleNever ""
-
--- | Monads which support tracing. 'TraceT' is the standard example.
-class Monad m => MonadTrace m where
-  -- | Trace the execution of a block of code, attaching a human-readable name.
-  trace :: Text -> m a -> m a
-
-  -- | Ask for the current tracing context, so that we can provide it to any
-  -- downstream services, e.g. in HTTP headers.
-  currentContext :: m TraceContext
-
-  -- | Ask for the current tracing reporter
-  currentReporter :: m Reporter
-
-  -- | Ask for the current handle on the tracing metadata
-  currentMetadataRef :: m (IORef TracingMetadata)
-
-  -- | Ask for the current sampling decision
-  currentSamplingDecision :: m SamplingDecision
-
-  -- | Log some metadata to be attached to the current span
-  attachMetadata :: TracingMetadata -> m ()
-
--- | Reinterpret a 'TraceT' action in another 'MonadTrace'.
--- This can be useful when you need to reorganize a monad transformer stack, for
--- example, to embed an action in some monadic computation, while preserving tracing
--- metadata and context.
---
--- For example, we use this function in various places in 'BackendExecute',
--- where we receive an action to execute in some concrete monad transformer stack.
--- See the various implementations of 'runQuery' for examples.
--- Ideally, the input computation's type would be sufficiently polymorphic that
--- we would not need to reorder monads inthe transformer stack. However, the monad
--- transformer stacks must be concrete, because their types are defined by
--- an associated type family 'ExecutionMonad'. Hence, we need to use this function
--- to peel off the outermost 'TraceT' constructor, and embed the computation in some
--- other 'MonadTrace'.
---
--- A second example is related to caching. The 'cacheLookup' function returns an
--- action in a concrete transformer stack, again because we are constrained by the
--- usage of a type class. We need to reinterpret the 'TraceT' component of this
--- concrete stack in some other abstract monad transformer stack, using this function.
---
--- Laws:
---
--- > interpTraceT id (hoist f (TraceT x)) = interpTraceT f (TraceT x)
-interpTraceT :: MonadTrace n => (m a -> n b) -> TraceT m a -> n b
-interpTraceT f (TraceT rma) = do
-  ctx <- currentContext
-  rep <- currentReporter
-  metadataRef <- currentMetadataRef
-  samplingDecision <- currentSamplingDecision
-  f (runReaderT rma (TraceTEnv ctx rep metadataRef samplingDecision))
-
--- | If the underlying monad can report trace data, then 'TraceT' will
--- collect it and hand it off to that reporter.
-instance (MonadIO m, MonadBaseControl IO m) => MonadTrace (TraceT m) where
-  -- Note: this implementation is so awkward because we don't want to give the
-  -- derived MonadReader/Writer instances to TraceT
-  trace name ma =
-    TraceT $
-      ReaderT $ \env@(TraceTEnv ctx rep _ samplingDecision) -> do
-        case samplingDecision of
-          SampleNever -> runReaderT (unTraceT ma) env
-          SampleAlways -> do
-            spanId <- liftIO randomSpanId
-            let subCtx =
-                  ctx
-                    { tcCurrentSpan = spanId,
-                      tcCurrentParent = Just (tcCurrentSpan ctx)
-                    }
-            metadataRef <- liftIO $ newIORef []
-            runReporter rep subCtx name (readIORef metadataRef) $
-              runReaderT
-                (unTraceT ma)
-                (TraceTEnv subCtx rep metadataRef samplingDecision)
-
-  currentContext = TraceT (asks tteTraceContext)
-
-  currentReporter = TraceT (asks tteReporter)
-
-  currentMetadataRef = TraceT (asks tteMetadataRef)
-
-  currentSamplingDecision = TraceT (asks tteSamplingDecision)
-
-  attachMetadata metadata =
-    TraceT $
-      ReaderT $ \env ->
-        liftIO $ modifyIORef' (tteMetadataRef env) (metadata ++)
-
-instance MonadTrace m => MonadTrace (ReaderT r m) where
-  trace = mapReaderT . trace
-  currentContext = lift currentContext
-  currentReporter = lift currentReporter
-  currentMetadataRef = lift currentMetadataRef
-  currentSamplingDecision = lift currentSamplingDecision
-  attachMetadata = lift . attachMetadata
-
-instance MonadTrace m => MonadTrace (StateT e m) where
-  trace = mapStateT . trace
-  currentContext = lift currentContext
-  currentReporter = lift currentReporter
-  currentMetadataRef = lift currentMetadataRef
-  currentSamplingDecision = lift currentSamplingDecision
-  attachMetadata = lift . attachMetadata
-
-instance MonadTrace m => MonadTrace (ExceptT e m) where
-  trace = mapExceptT . trace
-  currentContext = lift currentContext
-  currentReporter = lift currentReporter
-  currentMetadataRef = lift currentMetadataRef
-  currentSamplingDecision = lift currentSamplingDecision
-  attachMetadata = lift . attachMetadata
-
--- | Inject the trace context as a set of HTTP headers.
-injectB3HttpContext :: TraceContext -> [HTTP.Header]
-injectB3HttpContext TraceContext {..} =
-  let traceId = (b3HeaderTraceId, traceIdToHex tcCurrentTrace)
-      spanId = (b3HeaderSpanId, spanIdToHex tcCurrentSpan)
-      parentSpanIdMaybe =
-        (,) b3HeaderParentSpanId . spanIdToHex <$> tcCurrentParent
-      samplingStateMaybe =
-        (,) b3HeaderSampled <$> samplingStateToHeader tcSamplingState
-   in traceId : spanId : catMaybes [parentSpanIdMaybe, samplingStateMaybe]
-
--- | Extract the trace and parent span headers from a HTTP request
--- and create a new 'TraceContext'. The new context will contain
--- a fresh span ID, and the provided span ID will be assigned as
--- the immediate parent span.
-extractB3HttpContext :: [HTTP.Header] -> IO (Maybe TraceContext)
-extractB3HttpContext hdrs = do
-  -- B3 TraceIds can have a length of either 64 bits (16 hex chars) or 128 bits
-  -- (32 hex chars). For 64-bit TraceIds, we pad them with zeros on the left to
-  -- make them 128 bits long.
-  let traceIdMaybe =
-        lookup b3HeaderTraceId hdrs >>= \rawTraceId ->
-          if
-              | Char8.length rawTraceId == 32 ->
-                  traceIdFromHex rawTraceId
-              | Char8.length rawTraceId == 16 ->
-                  traceIdFromHex $ Char8.replicate 16 '0' <> rawTraceId
-              | otherwise ->
-                  Nothing
-  for traceIdMaybe $ \traceId -> do
-    freshSpanId <- liftIO randomSpanId
-    let parentSpanId = spanIdFromHex =<< lookup b3HeaderSpanId hdrs
-        samplingState = samplingStateFromHeader $ lookup b3HeaderSampled hdrs
-    pure $ TraceContext traceId freshSpanId parentSpanId samplingState
-
-b3HeaderTraceId, b3HeaderSpanId, b3HeaderParentSpanId, b3HeaderSampled :: IsString s => s
-b3HeaderTraceId = "X-B3-TraceId"
-b3HeaderSpanId = "X-B3-SpanId"
-b3HeaderParentSpanId = "X-B3-ParentSpanId"
-b3HeaderSampled = "X-B3-Sampled"
-
--- | Inject the trace context as a JSON value, appropriate for
--- storing in (e.g.) an event trigger payload.
-injectEventContext :: TraceContext -> J.Value
-injectEventContext TraceContext {..} =
-  let idFields =
-        [ eventKeyTraceId J..= bsToTxt (traceIdToHex tcCurrentTrace),
-          eventKeySpanId J..= bsToTxt (spanIdToHex tcCurrentSpan)
-        ]
-      samplingFieldMaybe =
-        (J..=) eventKeySamplingState <$> samplingStateToHeader @Text tcSamplingState
-   in J.object $ idFields ++ maybeToList samplingFieldMaybe
-
--- | Extract a trace context from an event trigger payload.
-extractEventContext :: J.Value -> IO (Maybe TraceContext)
-extractEventContext e = do
-  let traceIdMaybe =
-        traceIdFromHex . txtToBs
-          =<< e ^? JL.key "trace_context" . JL.key eventKeyTraceId . JL._String
-  for traceIdMaybe $ \traceId -> do
-    freshSpanId <- randomSpanId
-    let parentSpanId =
-          spanIdFromHex . txtToBs
-            =<< e ^? JL.key "trace_context" . JL.key eventKeySpanId . JL._String
-        samplingState =
-          samplingStateFromHeader $
-            e ^? JL.key "trace_context" . JL.key eventKeySamplingState . JL._String
-    pure $ TraceContext traceId freshSpanId parentSpanId samplingState
-
-eventKeyTraceId, eventKeySpanId, eventKeySamplingState :: J.Key
-eventKeyTraceId = "trace_id"
-eventKeySpanId = "span_id"
-eventKeySamplingState = "sampling_state"
-
--- | Perform HTTP request which supports Trace headers using a
--- HTTP.Request value
---
--- TODO REFACTOR:
---   - inline 'HTTP.performRequest' so that we can be sure a trace is always logged
---   - Inline 'try' here since we always use that at call sites
-tracedHttpRequest ::
-  MonadTrace m =>
-  -- | http request that needs to be made
-  HTTP.Request ->
-  -- | a function that takes the traced request and executes it
-  (HTTP.Request -> m a) ->
-  m a
-tracedHttpRequest req f = do
-  let method = bsToTxt (view HTTP.method req)
-      uri = view HTTP.url req
-  trace (method <> " " <> uri) do
-    let reqBytes = HTTP.getReqSize req
-    attachMetadata [("request_body_bytes", fromString (show reqBytes))]
-    ctx <- currentContext
-    f $ over HTTP.headers (injectB3HttpContext ctx <>) req
+-}

server/src-lib/Hasura/Tracing/Class.hs

@@ -0,0 +1,91 @@
+-- | Defines the Tracing API.
+--
+-- The 'MonadTrace' class defines the "public API" of this component.
+module Hasura.Tracing.Class
+  ( MonadTrace (..),
+    newTrace,
+    newSpan,
+  )
+where
+
+import Control.Monad.Morph
+import Control.Monad.Trans.Maybe
+import Hasura.Prelude
+import Hasura.Tracing.Context
+import Hasura.Tracing.Sampling
+import Hasura.Tracing.TraceId
+
+--------------------------------------------------------------------------------
+-- MonadTrace
+
+class Monad m => MonadTrace m where
+  -- | Trace the execution of a block of code, attaching a human-readable
+  -- name. This starts a new trace and its corresponding root span, to which
+  -- subsequent spans will be attached.
+  newTraceWith ::
+    TraceContext ->
+    SamplingPolicy ->
+    Text ->
+    m a ->
+    m a
+
+  -- | Starts a new span within the current trace. No-op if there's no current
+  -- trace.
+  --
+  -- TODO: we could rewrite this to start a new trace if there isn't one, using
+  -- the default reporter and policy? This would guarantee that no span is ever
+  -- lost, but would also risk reporting undesired spans.
+  newSpanWith ::
+    SpanId ->
+    Text ->
+    m a ->
+    m a
+
+  -- | Ask for the current tracing context, so that we can provide it to any
+  -- downstream services, e.g. in HTTP headers. Returns 'Nothing' if we're not
+  -- currently tracing anything.
+  currentContext :: m (Maybe TraceContext)
+
+  -- | Log some arbitrary metadata to be attached to the current span, if any.
+  attachMetadata :: TraceMetadata -> m ()
+
+instance MonadTrace m => MonadTrace (ReaderT r m) where
+  newTraceWith c p n = mapReaderT (newTraceWith c p n)
+  newSpanWith i n = mapReaderT (newSpanWith i n)
+  currentContext = lift currentContext
+  attachMetadata = lift . attachMetadata
+
+instance MonadTrace m => MonadTrace (StateT e m) where
+  newTraceWith c p n = mapStateT (newTraceWith c p n)
+  newSpanWith i n = mapStateT (newSpanWith i n)
+  currentContext = lift currentContext
+  attachMetadata = lift . attachMetadata
+
+instance MonadTrace m => MonadTrace (ExceptT e m) where
+  newTraceWith c p n = mapExceptT (newTraceWith c p n)
+  newSpanWith i n = mapExceptT (newSpanWith i n)
+  currentContext = lift currentContext
+  attachMetadata = lift . attachMetadata
+
+instance MonadTrace m => MonadTrace (MaybeT m) where
+  newTraceWith c p n = mapMaybeT (newTraceWith c p n)
+  newSpanWith i n = mapMaybeT (newSpanWith i n)
+  currentContext = lift currentContext
+  attachMetadata = lift . attachMetadata
+
+--------------------------------------------------------------------------------
+-- Trace helpers
+
+-- | Create a new trace using a randomly-generated context.
+newTrace :: (MonadIO m, MonadTrace m) => SamplingPolicy -> Text -> m a -> m a
+newTrace policy name body = do
+  traceId <- randomTraceId
+  spanId <- randomSpanId
+  let context = TraceContext traceId spanId Nothing SamplingDefer
+  newTraceWith context policy name body
+
+-- | Create a new span with a randomly-generated id.
+newSpan :: (MonadIO m, MonadTrace m) => Text -> m a -> m a
+newSpan name body = do
+  spanId <- randomSpanId
+  newSpanWith spanId name body

server/src-lib/Hasura/Tracing/Context.hs

@@ -0,0 +1,37 @@
+module Hasura.Tracing.Context
+  ( TraceContext (..),
+    TraceMetadata,
+  )
+where
+
+import Data.Aeson ((.=))
+import Data.Aeson qualified as J
+import Hasura.Prelude
+import Hasura.Tracing.Sampling
+import Hasura.Tracing.TraceId
+
+-- | Any additional human-readable key-value pairs relevant
+-- to the execution of a block of code.
+type TraceMetadata = [(Text, Text)]
+
+-- | A trace context records the current active trace, the active span
+-- within that trace, and the span's parent, unless the current span
+-- is the root.
+data TraceContext = TraceContext
+  { tcCurrentTrace :: TraceId,
+    tcCurrentSpan :: SpanId,
+    tcCurrentParent :: Maybe SpanId,
+    tcSamplingState :: SamplingState
+  }
+
+-- Should this be here? This implicitly ties Tracing to the name of fields in HTTP headers.
+instance J.ToJSON TraceContext where
+  toJSON TraceContext {..} =
+    let idFields =
+          [ "trace_id" .= bsToTxt (traceIdToHex tcCurrentTrace),
+            "span_id" .= bsToTxt (spanIdToHex tcCurrentSpan)
+          ]
+        samplingFieldMaybe =
+          samplingStateToHeader @Text tcSamplingState <&> \t ->
+            "sampling_state" .= t
+     in J.object $ idFields ++ maybeToList samplingFieldMaybe

server/src-lib/Hasura/Tracing/Monad.hs

@@ -0,0 +1,141 @@
+{-# LANGUAGE UndecidableInstances #-}
+
+module Hasura.Tracing.Monad
+  ( TraceT (..),
+    runTraceT,
+    ignoreTraceT,
+  )
+where
+
+import Control.Lens
+import Control.Monad.Catch (MonadCatch, MonadMask, MonadThrow)
+import Control.Monad.Morph
+import Control.Monad.Trans.Control
+import Data.IORef
+import Hasura.Prelude
+import Hasura.Tracing.Class
+import Hasura.Tracing.Context
+import Hasura.Tracing.Reporter
+import Hasura.Tracing.Sampling
+
+--------------------------------------------------------------------------------
+-- TraceT
+
+-- | TraceT is the standard implementation of 'MonadTrace'. Via a 'Reader', it
+-- keeps track of the default policy and reporter to use thoughout the stack, as
+-- well as the current trace.
+newtype TraceT m a = TraceT (ReaderT (Reporter, Maybe TraceEnv) m a)
+  deriving
+    ( Functor,
+      Applicative,
+      Monad,
+      MonadIO,
+      MonadFix,
+      MonadMask,
+      MonadCatch,
+      MonadThrow,
+      MonadState s,
+      MonadError e,
+      MonadBase b,
+      MonadBaseControl b
+    )
+
+-- | Runs the 'TraceT' monad, by providing the default reporter. This does NOT
+-- start a trace.
+--
+-- TODO: we could change this to always start a trace with a default name? This
+-- would allow us to guarantee that there is always a current trace, but this
+-- might not always be the correct behaviour: in practice, we would end up
+-- generating one that spans the entire lifetime of the engine if 'runTraceT'
+-- were to be used from 'main'.
+runTraceT :: Reporter -> TraceT m a -> m a
+runTraceT reporter (TraceT m) = runReaderT m (reporter, Nothing)
+
+-- | Run the 'TraceT' monad, but without actually tracing anything: no report
+-- will be emitted, even if calls to 'newTraceWith' force the trace to be
+-- sampled.
+ignoreTraceT :: TraceT m a -> m a
+ignoreTraceT = runTraceT noReporter
+
+instance MonadTrans TraceT where
+  lift = TraceT . lift
+
+-- | Hides the fact that TraceT is a reader to the rest of the stack.
+instance MonadReader r m => MonadReader r (TraceT m) where
+  ask = lift ask
+  local f (TraceT m) = TraceT $ mapReaderT (local f) m
+
+instance (MonadIO m, MonadBaseControl IO m) => MonadTrace (TraceT m) where
+  newTraceWith context policy name (TraceT body) = TraceT do
+    reporter <- asks fst
+    samplingDecision <- decideSampling (tcSamplingState context) policy
+    metadataRef <- liftIO $ newIORef []
+    let report = case samplingDecision of
+          SampleNever -> id
+          SampleAlways -> runReporter reporter context name (readIORef metadataRef)
+        updatedContext =
+          context
+            { tcSamplingState = updateSamplingState samplingDecision (tcSamplingState context)
+            }
+        traceEnv = TraceEnv updatedContext metadataRef samplingDecision
+    report $ local (_2 .~ Just traceEnv) body
+
+  newSpanWith spanId name (TraceT body) = TraceT do
+    (reporter, traceEnv) <- ask
+    case traceEnv of
+      -- we are not currently in a trace: ignore this span
+      Nothing -> body
+      Just env -> case teSamplingDecision env of
+        -- this trace is not sampled: ignore this span
+        SampleNever -> body
+        SampleAlways -> do
+          metadataRef <- liftIO $ newIORef []
+          let subContext =
+                (teTraceContext env)
+                  { tcCurrentSpan = spanId,
+                    tcCurrentParent = Just (tcCurrentSpan $ teTraceContext env)
+                  }
+              subTraceEnv =
+                env
+                  { teTraceContext = subContext,
+                    teMetadataRef = metadataRef
+                  }
+          runReporter reporter subContext name (readIORef metadataRef) $
+            local (_2 .~ Just subTraceEnv) body
+
+  currentContext = TraceT $ asks $ fmap teTraceContext . snd
+
+  attachMetadata metadata = TraceT do
+    asks (fmap teMetadataRef . snd) >>= \case
+      Nothing -> pure ()
+      Just ref -> liftIO $ modifyIORef' ref (metadata ++)
+
+--------------------------------------------------------------------------------
+-- Internal
+
+-- | Information about the current trace and span.
+data TraceEnv = TraceEnv
+  { teTraceContext :: TraceContext,
+    teMetadataRef :: IORef TraceMetadata,
+    teSamplingDecision :: SamplingDecision
+  }
+
+-- Helper for consistently deciding whether or not to sample a trace based on
+-- trace context and sampling policy.
+decideSampling :: MonadIO m => SamplingState -> SamplingPolicy -> m SamplingDecision
+decideSampling samplingState samplingPolicy =
+  case samplingState of
+    SamplingDefer -> liftIO samplingPolicy
+    SamplingDeny -> pure SampleNever
+    SamplingAccept -> pure SampleAlways
+
+-- Helper for consistently updating the sampling state when a sampling decision
+-- is made.
+updateSamplingState :: SamplingDecision -> SamplingState -> SamplingState
+updateSamplingState samplingDecision = \case
+  SamplingDefer ->
+    case samplingDecision of
+      SampleNever -> SamplingDefer
+      SampleAlways -> SamplingAccept
+  SamplingDeny -> SamplingDeny
+  SamplingAccept -> SamplingAccept

server/src-lib/Hasura/Tracing/Reporter.hs

@@ -0,0 +1,28 @@
+module Hasura.Tracing.Reporter
+  ( Reporter (..),
+    noReporter,
+  )
+where
+
+import Control.Monad.Trans.Control
+import Hasura.Prelude
+import Hasura.Tracing.Context
+
+newtype Reporter = Reporter
+  { runReporter ::
+      forall m a.
+      (MonadIO m, MonadBaseControl IO m) =>
+      -- \| Current trace context, providing the trace id and span info.
+      TraceContext ->
+      -- \| Human readable name of this span.
+      Text ->
+      -- \| IO action that retrieves the metadata associated with the
+      -- current span.
+      IO TraceMetadata ->
+      -- \| The monadic action to report
+      m a ->
+      m a
+  }
+
+noReporter :: Reporter
+noReporter = Reporter \_ _ _ -> id

server/src-lib/Hasura/Tracing/Sampling.hs

@@ -0,0 +1,88 @@
+module Hasura.Tracing.Sampling
+  ( -- * SamplingState
+    SamplingState (..),
+    samplingStateToHeader,
+    samplingStateFromHeader,
+
+    -- * SamplingDecision
+    SamplingDecision (..),
+
+    -- * SamplingPolicy
+    SamplingPolicy,
+    sampleNever,
+    sampleAlways,
+    sampleRandomly,
+    sampleOneInN,
+  )
+where
+
+import Hasura.Prelude
+import Refined (Positive, Refined, unrefine)
+import System.Random.Stateful qualified as Random
+
+--------------------------------------------------------------------------------
+-- SamplingState
+
+-- | B3 propagation sampling state.
+--
+-- Debug sampling state not represented.
+data SamplingState = SamplingDefer | SamplingDeny | SamplingAccept
+
+-- | Convert a sampling state to a value for the X-B3-Sampled header. A return
+-- value of Nothing indicates that the header should not be set.
+samplingStateToHeader :: IsString s => SamplingState -> Maybe s
+samplingStateToHeader = \case
+  SamplingDefer -> Nothing
+  SamplingDeny -> Just "0"
+  SamplingAccept -> Just "1"
+
+-- | Convert a X-B3-Sampled header value to a sampling state. An input of
+-- Nothing indicates that the header was not set.
+samplingStateFromHeader :: (IsString s, Eq s) => Maybe s -> SamplingState
+samplingStateFromHeader = \case
+  Nothing -> SamplingDefer
+  Just "0" -> SamplingDeny
+  Just "1" -> SamplingAccept
+  Just _ -> SamplingDefer
+
+--------------------------------------------------------------------------------
+-- SamplingDecision
+
+-- | A local decision about whether or not to sample spans.
+data SamplingDecision = SampleNever | SampleAlways
+
+--------------------------------------------------------------------------------
+-- SamplingPolicy
+
+-- | An IO action for deciding whether or not to sample a trace.
+--
+-- Currently restricted to deny access to the B3 sampling state, but we may
+-- want to be more flexible in the future.
+type SamplingPolicy = IO SamplingDecision
+
+sampleNever :: SamplingPolicy
+sampleNever = pure SampleNever
+
+sampleAlways :: SamplingPolicy
+sampleAlways = pure SampleAlways
+
+-- @sampleRandomly p@ returns `SampleAlways` with probability @p@ and
+-- `SampleNever` with probability @1 - p@.
+sampleRandomly :: Double -> SamplingPolicy
+sampleRandomly samplingProbability
+  | samplingProbability <= 0 = pure SampleNever
+  | samplingProbability >= 1 = pure SampleAlways
+  | otherwise = do
+      x <- Random.uniformRM (0, 1) Random.globalStdGen
+      pure $ if x < samplingProbability then SampleAlways else SampleNever
+
+-- Like @sampleRandomly@, but with the probability expressed as the denominator
+-- N of the fraction 1/N.
+sampleOneInN :: Refined Positive Int -> SamplingPolicy
+sampleOneInN denominator
+  | n == 1 = pure SampleAlways
+  | otherwise = do
+      x <- Random.uniformRM (0, n - 1) Random.globalStdGen
+      pure $ if x == 0 then SampleAlways else SampleNever
+  where
+    n = unrefine denominator

server/src-lib/Hasura/Tracing/TraceId.hs

@@ -26,8 +26,7 @@ import Hasura.Prelude
 import System.Random.Stateful qualified as Random
 
 --------------------------------------------------------------------------------
-
--- * TraceId
+-- TraceId
 
 -- | 128-bit trace identifiers.
 --
@@ -42,8 +41,8 @@ data TraceId
 traceIdBytes :: Int
 traceIdBytes = 16
 
-randomTraceId :: IO TraceId
-randomTraceId = do
+randomTraceId :: MonadIO m => m TraceId
+randomTraceId = liftIO do
   (w1, w2) <-
     flip Random.applyAtomicGen Random.globalStdGen $ \gen0 ->
       let (!w1, !gen1) = Random.random gen0
@@ -84,8 +83,7 @@ traceIdToHex :: TraceId -> ByteString
 traceIdToHex = Base16.encode . traceIdToBytes
 
 --------------------------------------------------------------------------------
-
----- * SpanId
+-- SpanId
 
 -- | 64-bit span identifiers
 --
@@ -97,8 +95,8 @@ newtype SpanId = SpanId Word64
 spanIdBytes :: Int
 spanIdBytes = 8
 
-randomSpanId :: IO SpanId
-randomSpanId = do
+randomSpanId :: MonadIO m => m SpanId
+randomSpanId = liftIO do
   w <- Random.uniformM Random.globalStdGen
   if w == 0
     then randomSpanId

server/src-lib/Hasura/Tracing/Utils.hs

@@ -0,0 +1,49 @@
+-- | This module contains a collection of utility functions we use with tracing
+-- throughout the codebase, but that are not a core part of the library. If we
+-- were to move tracing to a separate library, those functions should be kept
+-- here in the core engine code.
+module Hasura.Tracing.Utils
+  ( traceHTTPRequest,
+  )
+where
+
+import Control.Lens
+import Data.String
+import Hasura.Prelude
+import Hasura.Tracing.Class
+import Hasura.Tracing.Context
+import Hasura.Tracing.Sampling
+import Hasura.Tracing.TraceId
+import Network.HTTP.Client.Transformable qualified as HTTP
+
+-- | Wrap the execution of an HTTP request in a span in the current
+-- trace. Despite its name, this function does not start a new trace, and the
+-- span will therefore not be recorded if the surrounding context isn't traced
+-- (see 'spanWith').
+--
+-- Additionally, this function adds metadata regarding the request to the
+-- created span, and injects the trace context into the HTTP header.
+traceHTTPRequest ::
+  (MonadIO m, MonadTrace m) =>
+  -- | http request that needs to be made
+  HTTP.Request ->
+  -- | a function that takes the traced request and executes it
+  (HTTP.Request -> m a) ->
+  m a
+traceHTTPRequest req f = do
+  let method = bsToTxt (view HTTP.method req)
+      uri = view HTTP.url req
+  newSpan (method <> " " <> uri) do
+    let reqBytes = HTTP.getReqSize req
+    attachMetadata [("request_body_bytes", fromString (show reqBytes))]
+    headers <- fmap (maybe [] toHeaders) currentContext
+    f $ over HTTP.headers (headers <>) req
+  where
+    toHeaders :: TraceContext -> [HTTP.Header]
+    toHeaders TraceContext {..} =
+      catMaybes
+        [ Just ("X-B3-TraceId", traceIdToHex tcCurrentTrace),
+          Just ("X-B3-SpanId", spanIdToHex tcCurrentSpan),
+          ("X-B3-ParentSpanId",) . spanIdToHex <$> tcCurrentParent,
+          ("X-B3-Sampled",) <$> samplingStateToHeader tcSamplingState
+        ]

server/src-test/Hasura/Server/AuthSpec.hs

@@ -4,8 +4,6 @@ module Hasura.Server.AuthSpec (spec) where
 
 import Control.Concurrent.Extended (ForkableMonadIO)
 import Control.Lens hiding ((.=))
-import Control.Monad.Trans.Control
-import Control.Monad.Trans.Managed
 import Crypto.JOSE.JWK qualified as Jose
 import Crypto.JWT qualified as JWT
 import Data.Aeson ((.=))
@@ -24,7 +22,6 @@ import Hasura.Server.Auth hiding (getUserInfoWithExpTime, processJwt)
 import Hasura.Server.Auth.JWT hiding (processJwt)
 import Hasura.Server.Utils
 import Hasura.Session
-import Hasura.Tracing qualified as Tracing
 import Network.HTTP.Client qualified as HTTP
 import Network.HTTP.Types qualified as HTTP
 import Test.Hspec
@@ -626,16 +623,8 @@ mkRoleNameE = fromMaybe (error "fixme") . mkRoleName
 mkJSONPathE :: Text -> J.JSONPath
 mkJSONPathE = either (error . T.unpack) id . parseJSONPath
 
-newtype NoReporter a = NoReporter {runNoReporter :: IO a}
-  deriving newtype (Functor, Applicative, Monad, MonadIO, MonadBase IO, MonadBaseControl IO)
-
-instance Tracing.HasReporter NoReporter
-
-instance Tracing.HasReporter (ManagedT NoReporter)
-
 setupAuthMode' ::
-  ( Tracing.HasReporter m,
-    ForkableMonadIO m
+  ( ForkableMonadIO m
   ) =>
   Maybe (HashSet AdminSecretHash) ->
   Maybe AuthHook ->
@@ -644,11 +633,7 @@ setupAuthMode' ::
   m (Either () AuthMode)
 setupAuthMode' mAdminSecretHash mWebHook jwtSecrets mUnAuthRole = do
   httpManager <- liftIO $ HTTP.newManager HTTP.defaultManagerSettings
-  -- just throw away the error message for ease of testing:
-  fmap (either (const $ Left ()) Right) $
-    liftIO $
-      runNoReporter $
-        lowerManagedT $
+  fmap (mapLeft $ const ()) $
     runExceptT $
       setupAuthMode
         (fromMaybe Set.empty mAdminSecretHash)

server/test-postgres/Main.hs

@@ -15,11 +15,12 @@ import Data.Time.Clock (getCurrentTime)
 import Data.URL.Template
 import Database.PG.Query qualified as PG
 import Hasura.App
-  ( PGMetadataStorageAppT (..),
+  ( PGMetadataStorageAppT,
     initGlobalCtx,
     initialiseContext,
     mkMSSQLSourceResolver,
     mkPgSourceResolver,
+    runPGMetadataStorageAppT,
   )
 import Hasura.Backends.Postgres.Connection.Settings
 import Hasura.Backends.Postgres.Execute.Types
@@ -132,7 +133,7 @@ main = do
         let run :: ExceptT QErr (PGMetadataStorageAppT CacheBuild) a -> IO a
             run =
               runExceptT
-                >>> flip runPGMetadataStorageAppT (appCtx, appEnv)
+                >>> runPGMetadataStorageAppT (appCtx, appEnv)
                 >>> runCacheBuild cacheBuildParams
                 >>> runExceptT
                 >=> flip onLeft printErrJExit