d. Run

Rebuild all images and start the full stack:

docker compose up -d --build

Verify that three gateway replicas and one lb (Nginx) instance are running alongside the rest of the services:

docker compose ps -a

Expected output:

NAME                STATUS     PORTS
store-lb-1          running    0.0.0.0:8080->80/tcp
store-gateway-1     running
store-gateway-2     running
store-gateway-3     running
store-auth-1        running
store-account-1     running
store-db-1          running

Note that only lb publishes a port: the gateway containers (and the other services) are only reachable through Nginx on the internal Docker network.

Verify Load Distribution

Send a burst of requests through Nginx. The gateway's /health-check answers with the hostname of the container that served it, so the output cycles through the three replicas; the gateway logs show the same:

for i in {1..9}; do curl -s http://localhost:8080/health-check; echo; done

docker compose logs gateway --tail 30

Each of the three containers should show request entries. If all requests land on a single replica, Nginx may have started before the gateway replicas were fully ready. Reload Nginx to re-resolve the gateway DNS record:

docker compose restart lb

Scale Up at Runtime

Add replicas without rebuilding images:

docker compose up -d --scale gateway=5

After scaling, reload Nginx so it re-queries Docker DNS and picks up the new replica IPs:

docker compose exec lb nginx -s reload

Confirm the new replica count:

docker compose ps gateway

Scale Down

docker compose up -d --scale gateway=1

Nginx automatically stops routing to containers that are no longer running after the next reload.

Done! Nginx now distributes all external traffic across multiple gateway replicas. If one replica crashes, Docker restarts it and Nginx continues routing requests to the healthy instances — the failure is transparent to clients.