5xx

5xx is the class of HTTP status codes that indicate a server error. These responses show that the server failed to fulfill a valid request due to a problem on the server side. The 5xx family covers codes from 500 through 599 and is distinct from the client error 4xx range, which signals problems on the client side.

Common 5xx codes include:

500 Internal Server Error, a generic catch‑all when the server encounters an unexpected condition; 501 Not Implemented, when the server does not support the requested method or feature; 502 Bad Gateway, when a gateway or proxy receives an invalid response from an upstream server; 503 Service Unavailable, indicating the server is temporarily unable to handle requests (often due to overload or maintenance); 504 Gateway Timeout, when a gateway or proxy times out waiting for a response; 505 HTTP Version Not Supported; 507 Insufficient Storage; 508 Loop Detected; 511 Network Authentication Required. These codes signal server-side problems rather than client mistakes.

Causes of 5xx errors vary and can include misconfigurations, software bugs, resource exhaustion, failing dependencies, or ongoing maintenance. Some errors are transient and resolve themselves, while others require server-side investigation and remediation. Clients should avoid aggressive retrying and implement backoff strategies, while operators should check logs, monitor service health, verify upstream dependencies, and apply fixes without exposing sensitive details to users.

Handling and best practices emphasize clear error reporting and resilience. In addition to proper logging, services often use health checks, circuit breakers, and rate limiting. Caching behavior generally treats 5xx responses as non‑cacheable by default, though some responses may be cached when explicitly permitted and accompanied by appropriate headers, such as Retry-After for 503 responses.