This is one of the most common reasons applications suddenly become unavailable. In most cases, the server itself isn't the problem. The health check is. The Gateway may be checking the wrong URL, expecting an incorrect HTTP status code, using the wrong port, or timing out before the application has finished starting. Before investigating the application, verify your health check configuration. A small mistake here can remove every backend server from rotation, making a healthy application appear completely offline.

A 502 Bad Gateway usually means the Gateway successfully received your request but couldn't get a valid response from the backend application. Common causes include the application not running, the backend listening on the wrong port, SSL being misconfigured, the application crashing after the health check passed, or reverse proxy settings not matching the backend. The Gateway is often only reporting the symptom. The root cause almost always exists behind it.

If requests never reach your application, don't assume the Gateway is broken. The issue is usually a security group or firewall rule blocking traffic, an incorrect backend IP address or port, DNS pointing to the wrong endpoint, SSL certificates preventing successful connections, or backend servers not being registered correctly. Start by confirming whether requests reach the Gateway at all. Then trace the request one hop at a time instead of changing multiple components together.

Adding a Gateway doesn't automatically give you zero-downtime deployments. During deployments, new application instances need time to start before they should receive traffic. If old servers are removed too early or new servers are marked healthy too soon, users experience failed requests. Rolling deployments, connection draining, readiness probes, and properly configured health checks are essential for achieving true zero-downtime releases.

This usually means Kubernetes is waiting for a cloud provider to provision an external load balancer. Possible causes include no cloud load balancer integration, a missing cloud controller manager, an unsupported Kubernetes environment, incorrect Service configuration, or cloud resource quota limits. On managed Kubernetes platforms, this is normally resolved automatically. On self-managed clusters, additional networking components are often required.

A Gateway distributes traffic. It doesn't make slow applications fast. Performance problems are often caused by slow database queries, CPU or memory bottlenecks, high application latency, network congestion, or insufficient backend capacity. If every backend responds slowly, distributing traffic simply spreads the problem across more servers.

Multiple servers improve resilience only when they fail independently. If every server depends on the same DNS service, storage platform, authentication system, or configuration service, one shared failure can take down every backend simultaneously. This is exactly what happened during several major cloud outages, including incidents affecting AWS and Cloudflare. Redundancy works best when critical dependencies are also redundant.

The best time to test failover is before you need it. Try taking one backend server offline during normal operating hours and confirm that traffic automatically shifts to healthy instances. Repeat the test after deployments, infrastructure changes, or scaling events. Many teams discover configuration issues only during real outages because failover has never been tested under realistic conditions.