The "failed_to_connect_to_backend" is similar to "failed_to_pick_backend" response and this often occurs when the load balancer knows where it wants to send traffic but cannot successfully route its request or find a healthy or available instance. Reviewing the instance usage and health checks <https://cloud.google.com/load-balancing/docs/https/#http_load_balancing>, might be appropriate to ensure these responses are correlated.
This error message may also be returned whenever there are a low number of instances serving your service <https://issuetracker.google.com/71840041#comment25>. For example, this error may be returned whenever an unexpected spikes in traffic occur and there are not enough instances to handle the received requests. In this type of situations, the load balancer attempts to route your application requests but it does not have instances to serve those request, hence, it will produce this "failed_to_connect_to_backend" or 'failed_to_pick_backend' error. Perhaps you can add a screenshot of your app.yaml file of your AppEngine application, this should display the configurations that apply to your instance presently, and of course, the number of instances you configured. -- 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/b40b5a57-a322-434e-a5a5-299a7f7e7889%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
