During automated CI/CD pipelines executing docker compose up -d, newly deployed application instances frequently return 502 Bad Gateway errors to end-users. While the container status reports as running, the load balancer (or reverse proxy like Nginx/HAProxy) is already forwarding traffic to the new instance before it has completed its internal initialization sequence (JIT warmup, database connection pool establishment, or static asset loading).
Typical Reverse Proxy Log:
2023-10-27T14:02:01Z error upstream prematurely closed connection while reading response header from upstream
2023-10-27T14:02:01Z crit *142 connect() to 172.18.0.5:8080 failed (111: Connection refused) while connecting to upstream