502 errors are often caused due to your application code being too busy to respond to nginx (the webserver that sits in front of your app that accepts and routes requests to and from your app).
Nginx will return a 502 back to the load balancer which is returned back to the client if it is not able to communicate with your app. It is therefore recommended to ensure that you are properly coding for Node-js and that you are not blocking the Node-js Event Loop <https://nodejs.org/en/docs/guides/dont-block-the-event-loop/> in order to always respond to nginx. If your application becomes too busy, and doesn't make time to accept and respond to health checks from nginx, nginx will stop communicating with your application and render it as unhealthy (and attempt to restart it, in turn stopping all requests it was handling). A busy application will also force new incoming requests to wait until previous requests have completed. This will cause increased latency for each new incoming request as they each wait in the pending queue for room on your instance. You can use the App Engine Dashboard <http://console.cloud.google.com/appengine/instances>to view the resource usage, latency, and traffic patterns of your app, and pinpoint a timestamp with high latency. Then use the Stackdriver Log viewer <http://console.cloud.google.com/logs/viewer> to see what happened in your code at that timestamp. Just ensure you choose the 'nginx.error', 'nginx.request', 'stderr', and 'stdout' log options to see all incoming requests, errors, and debug output from your application. If a request receives a 502, this means it never made it to your application (since nginx couldn't reach it), and the logs will therefore be seen at the loadbalancer level (by selecting the 'Cloud HTTP Load Balancer' logs and not 'GAE Application' logs). - Note: Google Groups is reserved for general product discussions and not for technical support. For further technical support it is recommended to post your detailed questions <https://stackoverflow.com/help/how-to-ask> to Stack Exchange <https://cloud.google.com/support/docs/stackexchange>using the supported Cloud tags. -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/google-appengine. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/25ba5413-6cac-47fc-be4c-87c9dbe86c6e%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
