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Vamshi Surabhi a01d1188f2 scaffolding for remote-schemas module 
The main aim of the PR is:

1. To set up a module structure for 'remote-schemas' package.
2. Move parts by the remote schema codebase into the new module structure to validate it.

## Notes to the reviewer

Why a PR with large-ish diff?

1. We've been making progress on the MM project but we don't yet know long it is going to take us to get to the first milestone. To understand this better, we need to figure out the unknowns as soon as possible. Hence I've taken a stab at the first two items in the [end-state](https://gist.github.com/0x777/ca2bdc4284d21c3eec153b51dea255c9) document to figure out the unknowns. Unsurprisingly, there are a bunch of issues that we haven't discussed earlier. These are documented in the 'open questions' section.

1. The diff is large but that is only code moved around and I've added a section that documents how things are moved. In addition, there are fair number of PR comments to help with the review process.

## Changes in the PR

### Module structure

Sets up the module structure as follows:

```
Hasura/
  RemoteSchema/
    Metadata/
      Types.hs
    SchemaCache/
      Types.hs
      Permission.hs
      RemoteRelationship.hs
      Build.hs
    MetadataAPI/
      Types.hs
      Execute.hs
```

### 1. Types representing metadata are moved

Types that capture metadata information (currently scattered across several RQL modules) are moved into `Hasura.RemoteSchema.Metadata.Types`.

- This new module only depends on very 'core' modules such as
  `Hasura.Session` for the notion of roles and `Hasura.Incremental` for `Cacheable` typeclass.

- The requirement on database modules is avoided by generalizing the remote schemas metadata to accept an arbitrary 'r' for a remote relationship
  definition.

### 2. SchemaCache related types and build logic have been moved

Types that represent remote schemas information in SchemaCache are moved into `Hasura.RemoteSchema.SchemaCache.Types`.

Similar to `H.RS.Metadata.Types`, this module depends on 'core' modules except for `Hasura.GraphQL.Parser.Variable`. It has something to do with remote relationships but I haven't spent time looking into it. The validation of 'remote relationships to remote schema' is also something that needs to be looked at.

Rips out the logic that builds remote schema's SchemaCache information from the monolithic `buildSchemaCacheRule` and moves it into `Hasura.RemoteSchema.SchemaCache.Build`. Further, the `.SchemaCache.Permission` and `.SchemaCache.RemoteRelationship` have been created from existing modules that capture schema cache building logic for those two components.

This was a fair amount of work. On main, currently remote schema's SchemaCache information is built in two phases - in the first phase, 'permissions' and 'remote relationships' are ignored and in the second phase they are filled in.

While remote relationships can only be resolved after partially resolving sources and other remote schemas, the same isn't true for permissions. Further, most of the work that is done to resolve remote relationships can be moved to the first phase so that the second phase can be a very simple traversal.

This is the approach that was taken - resolve permissions and as much as remote relationships information in the first phase.

### 3. Metadata APIs related types and build logic have been moved

The types that represent remote schema related metadata APIs and the execution logic have been moved to `Hasura.RemoteSchema.MetadataAPI.Types` and `.Execute` modules respectively.

## Open questions:

1. `Hasura.RemoteSchema.Metadata.Types` is so called because I was hoping that all of the metadata related APIs of remote schema can be brought in at `Hasura.RemoteSchema.Metadata.API`. However, as metadata APIs depended on functions from `SchemaCache` module (see [1](ceba6d6226/server/src-lib/Hasura/RQL/DDL/RemoteSchema.hs (L55)) and [2](ceba6d6226/server/src-lib/Hasura/RQL/DDL/RemoteSchema.hs (L91)), it made more sense to create a separate top-level module for `MetadataAPI`s.

   Maybe we can just have `Hasura.RemoteSchema.Metadata` and get rid of the extra nesting or have `Hasura.RemoteSchema.Metadata.{Core,Permission,RemoteRelationship}` if we want to break them down further.

1. `buildRemoteSchemas` in `H.RS.SchemaCache.Build` has the following type:

   ```haskell
   buildRemoteSchemas ::
     ( ArrowChoice arr,
       Inc.ArrowDistribute arr,
       ArrowWriter (Seq CollectedInfo) arr,
       Inc.ArrowCache m arr,
       MonadIO m,
       HasHttpManagerM m,
       Inc.Cacheable remoteRelationshipDefinition,
       ToJSON remoteRelationshipDefinition,
       MonadError QErr m
     ) =>
     Env.Environment ->
     ( (Inc.Dependency (HashMap RemoteSchemaName Inc.InvalidationKey), OrderedRoles),
       [RemoteSchemaMetadataG remoteRelationshipDefinition]
     )
       `arr` HashMap RemoteSchemaName (PartiallyResolvedRemoteSchemaCtxG remoteRelationshipDefinition, MetadataObject)
   ```

   Note the dependence on `CollectedInfo` which is defined as

   ```haskell
   data CollectedInfo
     = CIInconsistency InconsistentMetadata
     | CIDependency
         MetadataObject
         -- ^ for error reporting on missing dependencies
         SchemaObjId
         SchemaDependency
     deriving (Eq)
   ```

   this pretty much means that remote schemas is dependent on types from databases, actions, ....

   How do we fix this? Maybe introduce a typeclass such as `ArrowCollectRemoteSchemaDependencies` which is defined in `Hasura.RemoteSchema` and then implemented in graphql-engine?

1. The dependency on `buildSchemaCacheFor` in `.MetadataAPI.Execute` which has the following signature:

   ```haskell
   buildSchemaCacheFor ::
     (QErrM m, CacheRWM m, MetadataM m) =>
     MetadataObjId ->
     MetadataModifier ->
   ```

   This can be easily resolved if we restrict what the metadata APIs are allowed to do. Currently, they operate in an unfettered access to modify SchemaCache (the `CacheRWM` constraint):

   ```haskell
   runAddRemoteSchema ::
     ( QErrM m,
       CacheRWM m,
       MonadIO m,
       HasHttpManagerM m,
       MetadataM m,
       Tracing.MonadTrace m
     ) =>
     Env.Environment ->
     AddRemoteSchemaQuery ->
     m EncJSON
   ```

   This should instead be changed to restrict remote schema APIs to only modify remote schema metadata (but has access to the remote schemas part of the schema cache), this dependency is completely removed.

   ```haskell
   runAddRemoteSchema ::
     ( QErrM m,
       MonadIO m,
       HasHttpManagerM m,
       MonadReader RemoteSchemasSchemaCache m,
       MonadState RemoteSchemaMetadata m,
       Tracing.MonadTrace m
     ) =>
     Env.Environment ->
     AddRemoteSchemaQuery ->
     m RemoteSchemeMetadataObjId
   ```

   The idea is that the core graphql-engine would call these functions and then call
   `buildSchemaCacheFor`.

PR-URL: https://github.com/hasura/graphql-engine-mono/pull/6291
GitOrigin-RevId: 51357148c6404afe70219afa71bd1d59bdf4ffc6
2022-10-21 03:15:04 +00:00
..
bench-wrk server/tests-py: Reduce the number of locations we check the status code. 2022-07-05 18:01:07 +00:00
benchmarks ci/benchmarks: fix copypaste bug affecting regression report highlighting in #5942 2022-09-22 16:24:15 +00:00
documentation server/postgres: test identifier uniqueness rewrite 2022-06-17 10:44:18 +00:00
lib tests: remove unused imports 2022-10-20 13:14:00 +00:00
src-emit-metadata-openapi server: initial set of DTO types for metadata 2022-06-27 16:33:31 +00:00
src-exec Import pg-client-hs as PG 2022-09-20 19:55:51 +00:00
src-lib scaffolding for remote-schemas module 2022-10-21 03:15:04 +00:00
src-rsr Mention all src-rsr files in 'extra-source-files' section 2022-10-13 13:58:20 +00:00
src-test scaffolding for remote-schemas module 2022-10-21 03:15:04 +00:00
tests-py server/tests-py: Start webhook.py inside the test harness. 2022-10-20 19:00:01 +00:00
.dockerignore update packages (#251) 2018-08-08 13:10:13 +05:30
.gitignore server: Clean up Cabal project files 2022-04-07 07:16:12 +00:00
commit_diff.txt Rewrite GraphQL schema generation and query parsing (close #2801) (#4111) 2020-08-21 12:27:01 -05:00
COMPILING-ON-MACOS.md server: remove ghc libs from freeze file, update guides 2022-10-06 12:27:09 +00:00
CONTRIBUTING.md server: Update the contributing guide. 2022-10-18 09:50:46 +00:00
graphql-engine.cabal scaffolding for remote-schemas module 2022-10-21 03:15:04 +00:00
Makefile ci: misc. cleanup, naming 2022-01-03 06:26:39 +00:00
STYLE.md Nix: Use nixpkgs-fmt to format everything. 2022-06-20 13:02:45 +00:00
testing-guidelines.md Extract the test harness from the Hspec tests 2022-10-04 08:31:26 +00:00
troubleshooting.md Create troubleshooting.md 2022-05-30 09:18:44 +00:00
VERSIONS.json Upgrade HLint to v3.4.1. 2022-10-12 19:14:56 +00:00