server/test-manual: A way to start PostgreSQL with a read replica.

This is for local testing. Details are in the README.

[DSF-218]: https://hasurahq.atlassian.net/browse/DSF-218?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

PR-URL: https://github.com/hasura/graphql-engine-mono/pull/8449
GitOrigin-RevId: 758aa9600fe79ed5c2917def3c86ac185267259e

server/test-manual/postgres-replicas/README.md

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+# Testing PostgreSQL read replication
+
+This provides a Docker Compose file that starts up a PostgreSQL primary server and a single read replica.
+
+We can use this to test how the HGE Server and Console behave when communicating with a read replica.
+
+## Starting and stopping
+
+To start them, run the following from this directory:
+
+    docker compose up --wait
+
+To stop them, run:
+
+    docker compose down
+
+(Add `-v` to delete the data too.)
+
+## Connecting
+
+### Ports
+
+To connect to the databases, you will need to get the ports.
+
+The primary port can be found by running:
+
+    docker compose port postgres-primary 5432
+
+The replica port can be found by running:
+
+    docker compose port postgres-replica 5432
+
+Note that these ports may change if you restart the server.
+
+### Credentials
+
+The database username is `postgres`, and the password is `password`, for both the primary and the replica.
+
+## Experimenting with the Hasura GraphQL Engine
+
+Run HGE as follows:
+
+    cabal run graphql-engine:exe:graphql-engine -- \
+      --metadata-database-url="postgresql://postgres:password@$(docker compose --project-directory=server/test-manual/postgres-replicas port postgres-primary 5432)" \
+      serve \
+      --enable-console \
+      --console-assets-dir=$PWD/frontend/dist/apps/server-assets-console
+
+Then run the following to get the replica database connection URL, and add it as the "default" data source.
+
+    echo "postgresql://postgres:password@$(docker compose --project-directory=server/test-manual/postgres-replicas port postgres-primary 5432)"

server/test-manual/postgres-replicas/docker-compose.yaml

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+version: "3"
+
+name: hge-postgres-replicas
+
+services:
+  # The primary server, which supports reads and writes.
+  postgres-primary:
+    extends:
+      service: postgres
+      file: ../../../docker-compose/databases.yaml
+    command:
+      # for some reason this is necessary, even if we don't actually use an archive
+      - "--archive_mode=on"
+    volumes:
+      - primary-data:/var/lib/postgresql/data
+      # this script enables replication for all users
+      - ./init-primary.sh:/docker-entrypoint-initdb.d/init-primary.sh:ro
+
+  # Initializes the replica by creating a backup of the primary into the
+  # replica's data directory, then marking it as "standby" by creating the
+  # "standby.signal" file.
+  init-replica:
+    extends:
+      service: postgres
+      file: ../../../docker-compose/databases.yaml
+    command: 
+      - su
+      - postgres
+      - -c
+      - |
+        set -ex
+        PGPASSWORD=password pg_basebackup --host=postgres-primary --username=postgres --no-password --pgdata=/var/lib/postgresql/data
+        touch /var/lib/postgresql/data/standby.signal
+    volumes:
+      - replica-data:/var/lib/postgresql/data
+    depends_on:
+      postgres-primary:
+        condition: service_healthy
+
+  # The replica server, which streams writes from the primary, and supports reads.
+  postgres-replica:
+    extends:
+      service: postgres
+      file: ../../../docker-compose/databases.yaml
+    command:
+      - "--primary_conninfo=host=postgres-primary port=5432 user=postgres password=password options='-c wal_sender_timeout=5000'"
+      - "--hot_standby=on"
+    volumes:
+      - replica-data:/var/lib/postgresql/data
+    depends_on:
+      postgres-primary:
+        condition: service_started
+      init-replica:
+        condition: service_completed_successfully
+
+volumes:
+  primary-data:
+  replica-data:

server/test-manual/postgres-replicas/init-primary.sh

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+#!/usr/bin/env bash
+
+set -e
+set -x
+
+# Allow all users to replicate the database.
+# This is very insecure.
+
+auth="$(postgres -C password_encryption)"
+echo "host replication all all ${auth}" >> "$PGDATA/pg_hba.conf"