What is a possible reason for a 502 Bad Gateway error when an application cannot connect to Memorystore?

An application encountering a 502 Bad Gateway error when unable to connect to Memorystore can signify that the application server acting as a gateway or proxy received an invalid response from the upstream server, which in this case is the Memorystore. The chosen reason indicates a probable misconfiguration regarding the geographic regions of the services involved.

When the VPC Access connector is configured in a different region than App Engine, it leads to cross-region communication issues. App Engine, when initialized in one region, expects to communicate with services within its own region or through properly configured networking elements. If the VPC Access connector is not in the same region, it can't successfully route requests to Memorystore, resulting in the application being unable to connect and hence triggering a 502 error.

Having the VPC Access connector and App Engine in different regions creates a mismatch, leading to connection timeouts or failures. This underscores the importance of ensuring that all components that require low-latency communication are deployed within the same geographical region.

The other reasons focus on different aspects, such as deployment configuration or firewall rules, which also could lead to connectivity issues, but the regional mismatch specifically targets the root cause of why the application cannot reach Memorystore, thus resulting in a 502 Bad Gateway error