HTTP 503 — Service Unavailable

What 503 actually means

The server is up and answering, but it is deliberately declining work right now. That is the difference from a 500: nothing is broken, the capacity or the willingness is temporarily absent. It is the correct code for planned maintenance and for load shedding, and it should carry a `Retry-After` header saying when to come back — which also tells search engines to hold off rather than deindex the page.

Common causes

• Planned maintenance with a maintenance page enabled
• The server overloaded and shedding requests
• An autoscaling group with no healthy instances
• A worker pool or connection pool exhausted
• A dependency the app needs — a database or cache — being unreachable, with the app failing fast
• A traffic spike, whether organic or a denial-of-service attack

If you're just trying to view the page

1. Wait and try again — this one really is temporary by definition
2. Check the site's status page or social accounts for a maintenance notice
3. Avoid rapid refreshing; if the site is overloaded, retries make it worse
4. Nothing on your side needs changing

If it's your site

1. Always send `Retry-After` with a 503 — it keeps search engines from treating the outage as removal and tells clients when to return
2. Check whether it is genuine overload or a failing dependency; the fixes are opposite
3. Look at connection and worker pool saturation, and at queue depth, before adding capacity blindly
4. Serve a static maintenance page from the edge so the origin is not carrying the load of its own error page
5. Set up autoscaling and load shedding so overload degrades gracefully instead of collapsing

Reference

Status code

503

Reason phrase

Service Unavailable

Category

5xx — Server Error

Cacheable by default

No

Defined in

RFC 9110 §15.6.4