Question 1

Why does my API return 502 Bad Gateway?

Accepted Answer

A 502 is reported by a proxy or load balancer, not by your application. It means the intermediary tried to forward the request to an upstream server and received an invalid response, or none at all. That distinction is the most useful thing to know, because it tells you where to look: the proxy is working well enough to answer, so the problem lies between it and your application.

The common causes divide cleanly.

- The application is not running, or not listening where the proxy expects. A crash on startup, an exited container, or a port mismatch after a configuration change all present identically.
- The application is listening on the loopback interface only, so it accepts connections from its own machine but not from a proxy running elsewhere, such as in another container. Binding to all interfaces resolves it.
- The application crashed mid-request, closing the connection before sending a complete response. Here the proxy is accurately reporting that it received something invalid, and the real error is in your application logs.
- The upstream took longer than the proxy timeout. Depending on the proxy this surfaces as 502 or 504, and the fix is either making the request faster or raising the timeout deliberately.

Because the status comes from the proxy, your application logs may show nothing at all for a failed request, which is often the first confusing detail. Reading the proxy error log is the fastest way forward, since it usually names the upstream address and the specific failure, whether a refused connection, a reset, or a timeout.

If the errors are intermittent under load, examine the upstream capacity and connection limits before adjusting the proxy, because the proxy is reporting a symptom rather than causing one.

Question 2

How do I troubleshoot Node.js connection issues?

Accepted Answer

Troubleshooting Node.js connection issues involves checking network connectivity, inspecting logs, validating configurations, using debugging tools, testing in different environments, and monitoring resource usage.

Question 3

Why does my database connection pool run out of connections?

Accepted Answer

Pool exhaustion means connections are being taken out and not returned as fast as they are requested. The symptom is a request that hangs and then times out waiting to acquire one, and it typically

Question 4

Why does my Node app crash with an unhandled promise rejection?

Accepted Answer

A promise that rejects with nothing attached to handle it produces an unhandled rejection, and modern Node treats that as fatal by default. The process exits. This changed from earlier versions where

Question 5

Why can my Docker container not connect to a service on localhost?

Accepted Answer

Inside a container, localhost means the container itself, not the machine running it. Each container has its own network namespace and its own loopback interface, so a connection to localhost from

Question 6

Why does my Docker container exit immediately after starting?

Accepted Answer

A container lives exactly as long as its main process. When the process started by the image's CMD or ENTRYPOINT finishes, the container stops, regardless of anything else running inside it. So an

Question 7

Why does my Node process fail with EADDRINUSE address already in use?

Accepted Answer

The error means something is already listening on the port your process tried to bind. The operating system allows only one listener per port and address combination, so the second attempt is

Related Questions