HTTP 503 Service Unavailable
The server is temporarily overloaded or down.
What HTTP 503 Service Unavailable means
503 Service Unavailable means the server is temporarily unable to handle the request — usually because it's overloaded or down for maintenance. It's meant to be transient, and a `Retry-After` header may say when to come back.
Which category is it?
503 is a 5xx code — Server error. 5xx means the server failed to fulfil an apparently valid request. It's the server's responsibility, not the caller's.
Common causes
- The server is overloaded or out of capacity
- Planned maintenance
- A dependency the app needs is unavailable
How to fix it
- Retry after the `Retry-After` interval
- Scale up capacity or add load shedding
- Check that required dependencies are healthy
Example 503 response
HTTP/1.1 503 Service Unavailable
Retry-After: 120
Content-Type: text/html; charset=utf-8
<h1>Down for maintenance</h1>How to diagnose a 503
- Find out whether every request fails or only some. Intermittent 503s usually mean overload or one bad node behind a load balancer; constant 503s point to a stopped service or maintenance mode.
- Look at saturation: CPU, memory, worker or thread pools, database connection pools and request queue depth around the time the errors started.
- Check the load balancer or reverse proxy health checks. When every backend is marked unhealthy, the proxy itself answers 503.
- Read the application logs just before the first 503 for crashes, out-of-memory kills, or a failing dependency such as the database, cache or an upstream API.
503 compared with similar codes
- 502: 502 means a proxy received an invalid response from the server behind it. 503 means the server (or proxy) is saying it cannot take requests right now.
- 504: 504 means the upstream did not answer in time. A 503 is returned immediately because the service knows it cannot serve the request.
- 500: 500 is an unexpected failure while handling a request. 503 is a deliberate "not available" signal and may carry Retry-After.
503 Service Unavailable: common questions
Is a 503 error the visitor's fault or the website's?
It is the server's side. A 503 means the server could not handle the request at that moment. As a visitor you can wait and retry; as the site owner, treat it as a capacity or availability problem.
Does a 503 hurt SEO?
Short 503 periods are treated as temporary: crawlers slow down and come back later. If URLs keep returning 503 for a prolonged time, they can drop out of the index. For planned maintenance, return 503 with a Retry-After header instead of a 200 or 404 error page.
What should Retry-After contain?
Either a number of seconds (Retry-After: 120) or an HTTP date. Well-behaved clients and crawlers wait at least that long before retrying.