What Are the Differences Between Layer-7 Status Codes and Backend Status Codes in ELB Metrics?

HTTP or HTTPS listeners terminate TCP connections. In other words, there are two TCP connections between the client and a backend server, one between the client and load balancer, and the other between the load balancer and backend server. The communication between the client and the backend server is divided into two parts. After receiving an HTTP request, the load balancer parses the request and routes the parsed request to the backend server for processing. The backend server returns a response to the load balancer after receiving the request. The load balancer then parses the response and returns the parsed response to the client. Therefore, there are two types of status codes: backend status codes returned by the backend server to the load balancer and Layer-7 status codes returned by the load balancer to the client.

You may encounter the following situations:

• The backend server returns a status code, and the load balancer directly transmits the status code to the client. In this case, the Layer-7 status code is the same as the backend status code.
• If the connection between the load balancer and backend server is abnormal or times out, the load balancer returns HTTP 502 or 504 to the client.
• If the listener configuration or the request format or content is incorrect, the load balancer directly returns an HTTP 4xx status code or 502 to the client, and does not route the request to the backend server. In this case, there will be only a Layer-7 status code, but no backend status code.