From 8acd35c2e71b52233ae2cbc41a22427842afc339 Mon Sep 17 00:00:00 2001
From: Karthikeyan Chinnakonda <karthikeyan@hasura.io>
Date: Wed, 20 Oct 2021 18:02:19 +0530
Subject: [PATCH] rfc: log DB stats when the catalog is migrated

PR-URL: https://github.com/hasura/graphql-engine-mono/pull/2637
GitOrigin-RevId: c605d93172ae3cbde669bcac76583fe3903dec34
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 rfcs/catalog-migration-db-stats-logging.md | 74 ++++++++++++++++++++++
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 create mode 100644 rfcs/catalog-migration-db-stats-logging.md

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+# Logging database stats during source catalog migration
+
+## Metadata
+
+```
+---
+authors:
+  Karthikeyan Chinnakonda <karthikeyan@hasura.io>
+state: In review
+---
+```
+## Description
+
+Sometimes we need to change the alter some structure in the source catalog, like adding a new column, transforming an existing column's type to another etc. The problem many users have faced during migrations is that sometimes the DDL
+queries run during the migration lock the database and as a result, the graphql-engine doesn't start up. When this happens
+there are no logs outputted by the graphql-engine and there's no way for the user to know what's happening and this is frustrating for the user.
+
+## Proposed solutions
+
+1. When the source catalog migration is being done, we can log the output of the following SQL query every five seconds till the migration completes:
+
+```sql
+SELECT json_agg(json_build_object('query', psa.query, 'lock_granted', pl.granted, 'lock_mode', pl.mode, 'transaction_start_time', psa.xact_start, 'query_start_time', psa.query_start, 'wait_event_type', psa.wait_event_type, 'blocking_query', SUBSTRING(blocking.query, 1, 20) ) order BY psa.query_start)
+FROM     pg_stat_activity psa
+JOIN     pg_stat_activity blocking ON blocking.pid = ANY(pg_blocking_pids(psa.pid))
+LEFT JOIN pg_locks pl ON psa.pid = pl.pid
+WHERE    psa.query LIKE '%hdb_catalog%' AND psa.wait_event_type IS NOT NULL AND psa.query ilike any (array ['%create%', '%drop%', '%alter%']);
+```
+
+The above query only looks for DDL changes made in the `hdb_catalog` schema. Only 20 characters of the blocking query is outputted because it may contain
+sensitive data.
+
+The output of the above query looks like the following:
+
+```json
+[
+  {
+    "query": "INSERT INTO authors (name) values ('boo');",
+    "lock_granted": true,
+    "lock_mode": "ExclusiveLock",
+    "transaction_start_time": "2021-10-19T12:08:14.903342+00:00",
+    "query_start_time": "2021-10-19T12:08:14.903342+00:00",
+    "wait_event_type": "Lock",
+    "blocking_query": "LOCK authors IN ACCESS EXCLUSIVE MODE;"
+  },
+  {
+    "query": "INSERT INTO authors (name) values ('boo');",
+    "lock_granted": false,
+    "lock_mode": "RowExclusiveLock",
+    "transaction_start_time": "2021-10-19T12:08:14.903342+00:00",
+    "query_start_time": "2021-10-19T12:08:14.903342+00:00",
+    "wait_event_type": "Lock",
+    "blocking_query": "LOCK authors IN ACCESS EXCLUSIVE MODE;"
+  }
+]
+```
+
+Doing this will let the user know that some of the queries that are running on the database are in a locked state.
+
+##  Implementation details
+
+1. We'll create a new function `logPGSourceCatalogStats` which will infinitely query in fixed intervals of five seconds, the database using the above mentioned SQL query. The type signature of the function will look like:
+
+`logPGSourceCatalogStats :: forall pgKind m . (MonadIO m, MonadTx m) => Logger Hasura -> SourceConfig '(Postgres pgKind) -> m Void`
+
+2. The `logPGSourceCatalogStats` function will be run in a separate thread so as to not block the migrations. This can be done by  calling `logPGSourceCatalogStats` using the `forkManagedT` function.
+
+3. To avoid implementing this feature for metadata DB catalog migrations, we need to make the 40_to_41 migration a no-op and move the logic to
+   be a source catalog migration. Which means to make the `40_to_41.sql` a noop migration and move the migration to `0_to_1.sql` in `pg_source_migrations`. The `41_to_40.sql` migration should change accordingly, it has to check first whether the `locked` column is a `timestamp with time zone` and iff alter it to the boolean type otherwise do nothing.
+
+
+## Open questions
+
+1. What will the user do from the above info to resolve the locking? Is the logging merely informational?