Two Containers Behind a Load Balancer

• Start two app containers from the same image.
• Send multiple requests to the load balancer at localhost:8080 and observe how requests are distributed.
• Stop one backend container while requests are being served and observe failover to the remaining container.

​

What you’ll observe

• Requests to localhost:8080 are proxied by NGINX to one of the two app containers.
• NGINX performs simple load distribution between the backends (round-robin or similar, depending on config).
• If one backend stops, NGINX continues to forward requests to the remaining healthy container, keeping the service available.

​

Quick step-by-step

1. Launch two app containers (they both use the same image).
2. Confirm each container is running.
3. Send multiple curl requests to the load balancer and observe responses from each backend.
4. Stop one backend and continue sending requests to observe failover.

​

Commands and examples

# Start two app containers (both use the same image)
docker run -d --name app1 photoapp:v1
docker run -d --name app2 photoapp:v1
# Example output: b7e2d1...

# Send requests to the load balancer
curl localhost:8080
curl localhost:8080
# Example response: Hello from app2

# Stop one backend container
docker stop app1

# Requests should continue and be routed to the remaining container
curl localhost:8080
# Example response: Hello from app2

​

Commands reference

​

Troubleshooting tips

• If curl localhost:8080 returns a connection refused error, validate that NGINX is running and listening on port 8080:
  • ss -ltnp | grep 8080 or docker ps (if NGINX is containerized).
• If responses always come from the same backend, inspect the NGINX upstream configuration to confirm the load-balancing method.
• Use docker logs <container> to see app-specific output if you don’t get the expected Hello from ... responses.

​

Links and references

• Introduction to NGINX
• Docker documentation: https://docs.docker.com/
• NGINX upstream module reference: https://nginx.org/en/docs/http/ngx_http_upstream_module.html