I have learned that keystone can store token data in cache. I am wondering whether keystone is going to clear token data from cache. If not, how to clear them?

As my effort to solve this problem, I use strace command to observe keystone process, I found that the commands sent by keystone to memcached(I used memcached as cache) didn't contain TTL information. Thus my question.

I later found that there is a cache_time config option in keystone.conf, which defaults to <none>. This rather confused me: Does keystone recommend that token data, when cached, shall not be cleared?