5xx Server Error

HTTP 503 Service Unavailable

The server is not ready to handle the request. Common causes include a server being down for maintenance or being overloaded. The response may include a Retry-After header indicating when to try again.

Common Causes

  • Server is under maintenance or being deployed
  • Server is overloaded with too many requests
  • Dependent service (database, cache) is unavailable
  • Auto-scaling hasn't yet provisioned enough instances

How to Fix

  1. 1Wait and retry after the period specified in Retry-After
  2. 2Check the service's status page for outage information
  3. 3Scale up server resources if capacity is the issue
  4. 4Implement health checks and automatic recovery

Example

HTTP 503 Service Unavailable
GET /api/data HTTP/1.1

→ 503 Service Unavailable
→ Retry-After: 300
→ "We're performing scheduled maintenance"

Related Server Error Codes

500

Internal Server Error

501

Not Implemented

502

Bad Gateway

504

Gateway Timeout

505

HTTP Version Not Supported

506

Variant Also Negotiates

507

Insufficient Storage

508

Loop Detected

510

Not Extended

511

Network Authentication Required