Database Cross-Region Replication Patterns#

Cross-region replication exists because regions fail. AWS us-east-1 has had multiple multi-hour outages. If your database runs in a single region, a regional failure takes your application down entirely. Cross-region replication gives you a copy of the data somewhere else so you can recover.

The fundamental problem is physics. Light through fiber between US East and US West takes about 30ms one way. Every replication strategy is a different answer to the question: do you wait for the remote region to confirm it has the data before telling the client the write succeeded?

PostgreSQL Replication#

Streaming replication gives you a full binary copy for high availability and read scaling. Logical replication gives you selective table-level syncing between databases that can run different PostgreSQL versions.

Streaming Replication Setup#

Configure the Primary#

# postgresql.conf
wal_level = replica
max_wal_senders = 5
wal_keep_size = 1GB

Create a replication role and allow connections:

CREATE ROLE replicator WITH REPLICATION LOGIN PASSWORD 'repl-secret';

# pg_hba.conf
host  replication  replicator  10.0.0.0/8  scram-sha-256

Initialize the Standby#

sudo systemctl stop postgresql-16
sudo rm -rf /var/lib/postgresql/16/main/*
pg_basebackup -h primary-host -U replicator -D /var/lib/postgresql/16/main \
  --checkpoint=fast --wal-method=stream -R -P
sudo chown -R postgres:postgres /var/lib/postgresql/16/main
sudo systemctl start postgresql-16

The -R flag creates standby.signal and writes connection info to postgresql.auto.conf. The standby now continuously receives and replays WAL from the primary, accepting read-only queries by default.