2. Configuration

1. Primary Postgres Configuration:

Now, you'll likely want this at the bottom of your postgresql.conf to make sure it's ready to replicate:

hot_standby = on
archive_mode = off
wal_level = replica
max_wal_senders = 3
wal_keep_size = 1024

It'll need to be restarted for these changes to take effect, which would be safest done now before copying the data:

docker compose down db && docker compose up db -d

1. Preparing Replica Data:

Postgres replication involves streaming updates as they're made to the database, so to start we'll need to create a duplicate of the current database to use for the replica.

You can create a copy of your entire database like this, just substitute the container name and user as required:

docker exec -it synapse-db-1 pg_basebackup -h /sockets -U synapse -D /tmp/pgreplica

The data is initially written to /tmp/ inside the container as it's safest for permissions. We can then move it to /var/lib/postgresql/data/ so we can more easily access it from the host OS:

docker exec -it synapse-db-1 mv /tmp/pgreplica /var/lib/postgresql/data/

You can hopefully now reach the data and move it to a new directory for your replica, updating the ownership to match your existing Postgres data directory:

mv ./pgsql/pgreplica ./
chown -R 70:1000 ./pgreplica

1. Replica Postgres Configuration:

Now for the replica's postgresql.conf, add this to the bottom to tell it that it's a secondary and scale back its resource usage as it won't be actively serving clients:

port = 5433
hot_standby = on
checkpoint_timeout = 30min
shared_buffers = 512MB
effective_cache_size = 1GB
maintenance_work_mem = 128MB
work_mem = 4MB
max_wal_size = 2GB
max_parallel_workers_per_gather = 1
max_parallel_workers = 1
max_parallel_maintenance_workers = 1

1. Primary Postgres Replication

This will instruct the primary to allow replication:

# Enable replication for the user
docker exec -it your_primary_container_name psql -U synapse -c "ALTER USER synapse WITH REPLICATION;"

# Create a replication slot
docker exec -it your_primary_container_name psql -U synapse -c "SELECT * FROM pg_create_physical_replication_slot('replica_slot_name');"