Hi,

We've got a Node Express app on a GAE standard instance. It calls a PHP 
service on another GAE standard instance - through a server-to-server http 
request. 

Most of the time it works well, but since our first production deployment 
we had a few (32 times) 502 Bad Gateway errors every day on that request.

By looking to the trace logs:

   - I can see the NodeJS errors (getting a 502 response, throwing an 
   exception etc...), 
   - I can see a first log at 22:59:52.974 http_load_balancer WARN log 
   because a 502 error has been returned.  jsonPayload.statusDetail value is 
   "failed_to_connect_to_backend"
   - I can see a second log at 22:59:52.979 http_load_balancer WARN log 
   because a 502 error has been returned. jsonPayload.statusDetail value is 
   "response_sent_by_backend"

But I can't see any trace on the PHP service. I checked the health calls of 
the same timeframe for the PHP service, all the healthchecks return a 200 
OK.

My guess is a network error from Google Cloud - or maybe a latency matter, 
but is there a way I can diagnose and fix that ? What can be the causes of 
a load balancer returning a Bad Gateway whereas the GAE instance is up and 
running ?

Max

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