Status codes · one code503 — Service Unavailable
What a 503 Service Unavailable response means, who is at fault, what a visitor should do and what the server owner should do.
Open the reference on this codeWhat to do
If it is someone else's site
Come back later — the server is running and says itself that it is temporarily unavailable. If the response carries Retry-After, it states when to return; hammering refresh will not speed anything up.
If it is your server
Take a site down for maintenance with a 503 and a Retry-After header — not with a stub page returning 200 and not with a redirect to a «back soon» page: in those cases a search engine treats the stub as real content and may drop pages from the index, and recovery takes weeks. If the 503 comes from exhausted workers rather than from you, do not raise the limit blindly: work out how much memory one process takes, or the server will start swapping and get slower than it was.
Easily confused with
the application crashed on a request rather than being taken out of service deliberately.
500 →a limit aimed at you personally rather than the service being unavailable as a whole.
429 →the upstream is unreachable at all; here the server answers and states the reason.
502 →How to check
the code and the return time at once: without Retry-After a maintenance window is incomplete
whether the pool is hitting its process limit — the second cause of a 503
The reference holds sixty-four codes searchable by number, name and meaning — plus a breakdown of confusing pairs such as 401 and 403.
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