HTTP
502 Bad Gateway
The server, acting as a gateway or proxy, received an invalid response from the upstream server.
When You See This Code
When Nginx/Apache can't reach the application server (e.g., Gunicorn is down, upstream timeout).
How to Fix
Check if the upstream server is running. Verify proxy configuration. Check for upstream timeouts.
Specification
View RFC / Specification →Related Codes
Cross-Protocol Equivalents
Comparisons
Guides
Understanding HTTP Status Codes: A Complete Guide
HTTP Fundamentals
HTTP Redirects: 301 vs 302 vs 307 vs 308
HTTP Fundamentals
HTTP Caching: Cache-Control, ETags, and 304 Responses
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Choosing the Right HTTP Status Codes for REST APIs
API Design
gRPC Status Codes: A Practical Guide
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Debugging 502 Bad Gateway Errors
Debugging & Troubleshooting
Debugging CORS Errors: A Developer's Guide
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Debugging Timeout Errors: 408 and 504
Debugging & Troubleshooting
401 Unauthorized vs 403 Forbidden: When to Use Each
Security & Authentication
Essential HTTP Security Headers
Security & Authentication
Complete Guide to HTTP Methods: GET, POST, PUT, PATCH, DELETE
HTTP Fundamentals
Essential HTTP Headers Every Developer Should Know
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HTTP/2 vs HTTP/3: What Changed and Why
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How to Implement and Handle Rate Limiting (429)
API Design
API Versioning Strategies: URL, Header, and Query Param
API Design
Designing Consistent API Error Responses
API Design
Debugging 503 Service Unavailable Errors in Production
Debugging & Troubleshooting
TLS/SSL Certificate Troubleshooting Guide
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OAuth 2.0 Flows Explained: Authorization Code, Client Credentials, PKCE
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JSON Web Tokens: Structure, Signing, and Common Pitfalls
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API Keys vs OAuth vs JWT: Choosing the Right Auth Method
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