503 Service Unavailable is an HTTP status code that signifies a temporary limitation in ability of otherwise healthy sites to serve standard requests due to a capacity bottleneck or platform impairment.
Here are some specific reasons why a 503 Service Unavailable HTTP status code may be returned:
- Upstream dependencies like databases, DNS, payment providers, etc. that the application relies on are unreachable or are sending errors/timeouts.
- There is an issue with the web server software itself causing it to be unable to continue handling requests properly such as from a software update or configuration change.
- Cloud services like app platforms, serverless functions, and object storage that back the application are hitting usage quotas or other limits that prevent serving requests.
- The site or service is undergoing scheduled downtime, maintenance, or is experiencing a partial outage making it unable to serve some requests.
- The web server or application is temporarily overloaded and does not have the capacity to handle additional requests at the current volume or velocity.
- Autoscaling is not set up on the web server, or is set up incorrectly, and due to large traffic surges, functionality is temporarily disabled.
In essence, the 503 status signifies a temporary limitation in ability of otherwise healthy sites to serve standard requests due to a capacity bottleneck or platform impairment.