What a 503 error means and what to do about it
503 Service Unavailable means your server was reached, but it could not handle the request at that moment. It is usually temporary, and it usually has one of a few causes.
What the number tells you
Error codes that start with 5 mean the problem is on the server's side, not the visitor's. A 503 in particular says "not right now": the server is overloaded, is being restarted or is in maintenance, or the program behind it is not running.
- 500 means the application hit an error while building the page.
- 502 means a server in front got a bad answer from the one behind it.
- 503 means the server cannot take the request at the moment.
- 504 means a server in front waited too long for the one behind it.
Common causes
- More visitors or more background work than the server can handle.
- The application crashed or is restarting after an update.
- The host put the site into maintenance mode, or a maintenance mode was left switched on.
- The database or another service the site depends on is not answering.
- The host is limiting your account because it used more than its share of resources.
What to check first
- Reload after a minute. A restart often clears it without any help.
- Look at your host's status page and your account's resource usage.
- Ask what changed just before it started: an update, a plugin, a deployment.
- Read the server's error log around the time of the first failure.
If 503 errors come and go, write down the times. A pattern, such as every night at the same hour, usually points straight at the cause.
Catch it when it happens
503 errors are easy to miss, because the site often works again by the time you look. KeenCrow checks every minute. When three checks in a row fail, it emails you and records an incident with the status code, the start time and how long it lasted.