What causes a 503 Service Unavailable error?

503 is caused by temporary server overload, planned maintenance, dependency failures (database, cache, external API down), or an open circuit breaker. Unlike 500, it explicitly signals a temporary condition.

How should clients handle 503 responses?

Clients should treat 503 as retriable. Implement exponential backoff and respect the Retry-After header if present. Never treat 503 as a permanent failure — the service is expected to recover.

What is the Retry-After header?

Retry-After tells clients how long to wait before retrying. It can be a number of seconds (e.g. 'Retry-After: 120') or an HTTP date (e.g. 'Retry-After: Wed, 21 Oct 2025 07:28:00 GMT'). Servers should include it with 503 responses when possible.

When should I return 503 vs 500?

Return 503 when the failure is temporary and expected to resolve (overload, maintenance, dependency down). Return 500 when something unexpected went wrong in your code. 503 tells clients 'try again later' while 500 signals 'something is broken'.

How do search engine crawlers handle 503?

Crawlers treat 503 as temporary and will retry later without penalizing rankings. This is why 503 is the correct status during maintenance — using 500 or other errors can negatively impact SEO if they persist.

503 Service Unavailable: Strategies and Recovery