PostgreSQL 19 beta primary + streaming replica lab for the WAIT FOR LSN post

wait_for_lsn_lab.sh

#!/bin/sh
# PostgreSQL 19 beta primary + streaming replica in Docker.
# Companion lab for https://rednafi.com/system/wait-for-lsn/
#
# The replica streams from the primary but applies every commit a fixed time
# late (recovery_min_apply_delay), so replication lag is real and reproducible.
# Pass the delay as the first argument; it defaults to 10s.
#
#   curl -fsSL .../wait_for_lsn_lab.sh | sh        # 10s delay
#   curl -fsSL .../wait_for_lsn_lab.sh | sh -s 30s # 30s delay
#
# Warning: removes any existing pg-primary / pg-replica containers
# and the pgnet network before starting fresh.
set -eu

DELAY="${1:-10s}"

docker rm -f pg-primary pg-replica 2>/dev/null || true
docker network rm pgnet 2>/dev/null || true

docker network create pgnet

docker run -d --name pg-primary --network pgnet \
    -e POSTGRES_PASSWORD=secret postgres:19beta1

# The image runs a temporary server on the unix socket during initdb, so probe
# over TCP instead; the temp server has listen_addresses='' and never answers it.
until docker exec pg-primary pg_isready -h 127.0.0.1 -U postgres >/dev/null 2>&1; do
    sleep 1
done

# Allow replication connections on the primary
docker exec pg-primary psql -U postgres \
    -c "CREATE USER replicator WITH REPLICATION PASSWORD 'replpass';"
docker exec pg-primary bash -c \
    'echo "host replication replicator all scram-sha-256" >> "$PGDATA/pg_hba.conf"'
docker exec pg-primary psql -U postgres -c "SELECT pg_reload_conf();"

# Clone the primary with pg_basebackup and start the replica with the apply delay
docker run -d --name pg-replica --network pgnet -u postgres \
    -e PGPASSWORD=replpass -e APPLY_DELAY="$DELAY" --entrypoint bash postgres:19beta1 -c '
    until pg_basebackup -h pg-primary -U replicator \
            -D /var/lib/postgresql/replica -R -X stream; do
        sleep 1
    done
    exec postgres -D /var/lib/postgresql/replica -c recovery_min_apply_delay="$APPLY_DELAY"'
until docker exec pg-replica pg_isready -h 127.0.0.1 -U postgres >/dev/null 2>&1; do
    sleep 1
done

echo ""
echo "Primary: docker exec -it pg-primary psql -U postgres"
echo "Replica: docker exec -it pg-replica psql -U postgres  (applies commits ${DELAY} late)"
echo "Teardown: docker rm -f pg-primary pg-replica && docker network rm pgnet"