Hey Vinay Chitlangia,Thanks for some preliminary troubleshooting and linking this interesting article. App Engine runs Nginx processes to handle routes to your application's handlers. Handlers serving static assets for instance are handled by this Nginx process and the resources are served directly, thus bypassing the application altogether to save on precious application resources.The Nginx process will often serve a 502 if the application raises an exception, an internal API call raises an exception or if the request simply takes too long. As such, the status code by itself does not tell us much.Looking at the GAE logs for your application, I found the 502s you mentioned. One thing I noticed is that they all occur from the /read endpoint. From the naming, I assume this endpoint is reading some data from BigTable. Investigating further, perhaps you could provide some additional information:
- What exactly is happening at the /read endpoint? A code sample would be ideal if that's not too sensitive.
- What kind of error handling exists in said endpoint if the BigTable API returns non-success responses?
- Can you log various steps in the /read endpoint? This might help identify the progress the request reaches before the 502 is served. It would also help in confirming that your application is actually even getting the request as I can't currently confirm that from the logs.
- If said endpoint does in fact read from BigTable, what API and java library are you using?
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I realize that we've already begun investigating this here but I think this would be most appropriate for the App Engine public issue tracker. The issue is leading to an increasingly specific situation and I suspect will require some exchange of code/project to reproduce the behavior you've described. We monitor that issue tracker closely.When filing a new issue on the tracker, please link back to this thread for context while posting a link to the issue here so that others in the community can see the whole picture.
- Be sure to include the latest logs for related to the 502s. When viewing the logs in Stackdriver Logging for instance, include All logs rather than just request_log as nginx.error, stderr, stdout and vm.* logs may reveal clues as to a root cause.
- Mention if your are using any middleware like servlet filters that may receive request before that actual handler
- Lastly, include what the CPU and/or memory usage looks like on the instance(s) at the time of the 502s. Screenshots of Utilization and Memory Usage graphs from the Developers Console will likely be sufficient
I look forward to this issue report.
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