When a resource is cached, how do you know ahead of time how long to cache it for? If you set X-Accel-Expires to 5 minutes, but the resource is edited 3 minutes later, how do you evict the item from the nginx cache?

You can figure out where the item is in the nginx cache directory and delete it. But that is complicated by the fact that your app and nginx run as different users. Or you can send a specially crafted HTTP request to nginx assuming you have some kind of backdoor proxy_cache_bypass setup. But that's ugly too. You either have a race condition, or you have to hang your app's response until the invalidation request completes.

If there's another way to evict an item from the cache, I'm all ears.