How to configure Nginx to try two upstreams before 404ing? - nginx

Question

Given an Nginx configuration roughly like this:
upstream A {
server aa:8080;
}
upstream B {
server bb:8080;
}
server {
listen 80;
location #backendA {
proxy_pass http://A/;
}
location #backendB {
proxy_pass http://B/;
}
location / {
# This doesn't work. :)
try_files #backendA #backendB =404;
}
}
Basically, I would like Nginx to try upstream A, and if A returns a 404, then try upstream B instead, and failing that, return a 404 to the client. try_files does this for filesystem locations, then can fall back to a named location, but it doesn't work for multiple named locations. Is there something that will work?
Background: I have a Django web application (upstream A) and an Apache/Wordpress instance (upstream B) that I would like to coexist in the same URL namespace for simpler Wordpress URLs: mysite.com/hello-world/ instead of mysite.com/blog/hello-world/.
I could duplicate my Django URLs in the Nginx locations and use wordpress as a catch-all:
location /something-django-handles/ {
proxy_pass http://A/;
}
location /something-else-django-handles/ {
proxy_pass http://A/;
}
location / {
proxy_pass http://B/;
}
But this violates the DRY principle, so I'd like to avoid it if possible. :) Is there a solution?

Answer 1

After further googling, I came upon this solution:
location / {
# Send 404s to B
error_page 404 = #backendB;
proxy_intercept_errors on;
log_not_found off;
# Try the proxy like normal
proxy_set_header X-Forwarded-For $remote_addr;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_pass http://A;
}
location #backendB {
# If A didn't work, let's try B.
proxy_set_header X-Forwarded-For $remote_addr;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_pass http://B;
# Any 404s here are handled normally.
}