- Source
- RFC 9110, section 15.6.4
A 503 means the server can't handle your request right now because it's overloaded or down for maintenance, and that it should pass. It can suggest when to come back with Retry-After, the same header the quartermaster uses in Room 429, except this time the problem is on our side.
The spec also points out that nothing makes a server use this code. Some overloaded servers just refuse the connection, so seeing a 503 at all means something was still healthy enough to complain. The gauge and the six-second recovery are invented for the room, and any chief engineer will tell you they padded the estimate.