HTTP 5xx
503 Service Unavailable
The server is temporarily unable to handle the request, usually because it is overloaded or down for maintenance.
What causes it
- Traffic spikes, an overloaded server, a maintenance window, or a crashed process.
How to fix it
Scale up or shed load if it is traffic, or wait out the maintenance window. Persistent 503s mean the service needs more capacity or has crashed.
Example response
A typical raw HTTP response a client receives for a 503, with the headers this code is defined around.
HTTP/1.1 503 Service Unavailable
Retry-After: 120
Content-Type: text/html; charset=utf-8Related codes
500Common
Internal Server ErrorA generic server-side error. Something failed on the server and it could not complete the request.501
Not ImplementedThe server does not support the functionality required to fulfill the request.502Common
Bad GatewayA server acting as a gateway or proxy got an invalid response from the upstream server it was trying to reach.504Common
Gateway TimeoutA gateway or proxy did not get a timely response from the upstream server.505
HTTP Version Not SupportedThe server does not support the HTTP protocol version used in the request.507Common
Insufficient StorageThe server cannot store what is needed to complete the request, often a full disk.Know before your visitors do.
Sentinel checks your site around the clock from multiple regions and alerts you the moment it starts returning a 5xx error, so you can fix it before it costs you.