Keystone still caches data in process memory after configuring caching backend to dogpile.cache.null

[Xiaojun Lin]

I have a keystone cluster running on 3 controller nodes. On each node, the process is running under mod_wsgi daemon mode. Keystone version is 16.0.1

I am experiencing a problem which is caused by inconsistency between in-process memory cache data and the database.
Here is the steps to reproduce:
1. create a project with "enabled" attribute set to False
2. assign an user and a role to the project
3. call get_project for 50 times.(in order to fill process caches of this project with "enabled" as False)
4. set the project's "enabled" attribute to True
5. get a token with scope as this project
6. use this token to call any api.
Step 6 will return a 404 error with an error message saying the project is disabled and the token is invalid, while in fact the project is already enabled.

My deployment does not have a caching backend service so I just leave the caching configuration as default, which is as follows:
[caching]
enabled = true
backend = dogpile.cache.null

[resource]
caching = true

...

According to the documentation, configuring caching.backend as dogpile.cache.null should "effectively disables all cache operations", but the actual behavior is that there's still an in-memory cache layer(common.cache._context_cache._ResponseCacheProxy) wrapped around the configured backend, which does not sync between processes and nodes.One can only disable this cache by setting the caching switch to false.

Here is my concerns:
1. The documentaion is misleading, either the document or the code should be modified to keep them consistent.
2. Will keystone still suffer from such data inconsistency problem even after a proper caching backend is configured(memcache for example)? Because _ResponseCacheProxy.get() tries to get value from "local cache" before querying from the caching backend:

def get(self, key):
    value = self._get_local_cache(key)
    if value is api.NO_VALUE:
        value = self.proxied.get(key)
        if value is not api.NO_VALUE:
            self._set_local_cache(key, value)
    return value