HTTP 503: Service unavailable is the correct response when an Application Load Balancer receives a request but the configured target group has no registered or no healthy targets to forward the request to.

The load balancer cannot route the request to any backend when there are no targets, and the ALB returns HTTP 503 to indicate the service is unavailable for that request. This response is generated before any backend connection is established and it is distinct from errors that require a reachable target to exist.

HTTP 504: Gateway timeout indicates a backend accepted the connection but did not respond within the timeout window and it therefore implies at least one reachable target was involved.

HTTP 502: Bad gateway usually means the load balancer received an invalid or malformed response from a target or could not establish a proper response chain and it typically presumes a target attempted to answer.

HTTP 500: Internal server error reflects an application fault on a responding target and it is not produced simply because the target group is empty.

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