I have two microservices "frontend" and "/users". They communicate with REST API When client makes a request to "frontend", it internally requests "/users" microservice.
What kind of status code should return frontend service in case when /users answers with 500-error?
500 Internal Error is good here: https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#5xx_server_errors
If you don't know what to do in such situation (if your service can't work without underlying service), 500 is dedicated for that.