Troubleshooting

FastAPI returns 404 behind Nginx

Confirm that the external path still includes the configured PREFIX when it reaches the ASGI server. An Nginx proxy_pass URI suffix can strip /api.

Django session works on the website but not the API

Compare the WSGI and ASGI settings and environment:

• session backend and endpoint;
• SECRET_KEY;
• cookie name, domain, path, Secure, and SameSite;
• deployed application version;
• request host and scheme.

Local-memory sessions cannot be shared between processes or replicas.

CSRF fails only in production

Check the forwarded scheme, trusted origins, cookie domain/path, secure-cookie settings, and whether the client sends both the cookie and X-CSRFToken. Inspect names and status codes, never log token values.

SynchronousOnlyOperation

A synchronous Django API was called in an event-loop thread. Use the async ORM, change the FastAPI operation to def, or move the complete synchronous workflow into one sync_to_async(thread_sensitive=True) call.

Do not use DJANGO_ALLOW_ASYNC_UNSAFE as a production fix.

Streaming arrives all at once

Disable proxy buffering for the specific SSE or streaming location and extend its read timeout. Leave buffering enabled for ordinary JSON endpoints.

Database connections are exhausted after adding ASGI

Count pools across both WSGI and ASGI replicas, workers, background jobs, and deploy overlap. Reduce worker/pool sizes or introduce a correctly sized backend pooler before increasing database limits blindly.

OpenAPI has the wrong URL prefix

Preserve the mount path at the proxy and configure forwarded headers/root path consistently. Test /api/openapi.json through the public load balancer, not only against a container port.