HTTP 101 Switching Protocols vs 502 Bad Gateway
HTTP 101 (Switching Protocols) is a 1xx Informational response, while 502 (Bad Gateway) is a 5xx Server Error response. 101 indicates that the server understands the Upgrade header field request and indicates which protocol it is switching to. In contrast, 502 means that the server, acting as a gateway or proxy, received an invalid response from the upstream server.
Description
The server understands the Upgrade header field request and indicates which protocol it is switching to.
When You See It
When upgrading from HTTP/1.1 to WebSocket, or to HTTP/2.
How to Fix
This is normal behavior during protocol upgrades. Ensure your client supports the target protocol.
Description
The server, acting as a gateway or proxy, received an invalid response from the upstream server.
When You See It
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.
Key Differences
101 is a 1xx Informational response, while 502 is a 5xx Server Error response.
HTTP 101: The server understands the Upgrade header field request and indicates which protocol it is switching to.
HTTP 502: The server, acting as a gateway or proxy, received an invalid response from the upstream server.
You encounter 101 when when upgrading from HTTP/1.1 to WebSocket, or to HTTP/2.
You encounter 502 when when Nginx/Apache can't reach the application server (e.g., Gunicorn is down, upstream timeout).
When to Use Which
For 101 (Switching Protocols): This is normal behavior during protocol upgrades. Ensure your client supports the target protocol. For 502 (Bad Gateway): Check if the upstream server is running. Verify proxy configuration. Check for upstream timeouts.