503
HTTPSERVER_ERRORCritical5xx Server ErrorHIGH confidence
Service Unavailable
Production Risk
High. The service is currently unavailable to users. If it is due to overload, it indicates a scaling problem that needs to be addressed.
What this means
The server is not ready to handle the request. Common causes are a server that is down for maintenance or that is overloaded. This condition is generally temporary. A 'Retry-After' header might be sent with this response.
Why it happens
- 1The server is temporarily overloaded with traffic and cannot accept new connections.
- 2The application is being deployed, and the service is briefly offline.
- 3The server is down for scheduled maintenance.
- 4A critical backend resource, like a database, has reached its maximum connection limit.
How to reproduce
A website is taken down for a planned 30-minute maintenance window.
trigger — this will error
trigger — this will error
GET /index.html HTTP/1.1 Host: example.com
expected output
HTTP/1.1 503 Service Unavailable Retry-After: 1800
Fix 1
Wait and Retry
WHEN You are the client.
Wait and Retry
If a 'Retry-After' header is present, wait that amount of time before trying again. Otherwise, wait a reasonable period.
Why this works
Client-Side Logic
Fix 2
Increase Server Capacity
WHEN The server is frequently overloaded.
Increase Server Capacity
Add more server instances or increase the resources (CPU, RAM) of existing servers.
Why this works
Scaling Infrastructure
Sources
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