Performance issue when validating tokens

[Jose Castro Leon]

The revoke_token model when validating a token, it always does a lookup in the database for the possible revocation events that it may be affected with.

In our production setup with 10k requests per minute and 12k revocation events stored in the database, this takes quite some time in this operation while consuming CPU resources.

Many of our programatic clients reuse the token to do subsequent operations, and even if we have caching configured in all our APIs, quite of those requests arrive to our keystone servers producing CPU load.

We did an initial investigation and it seems that just by introducing caching on [1], it allows us to cut up to 100ms on our average response time. It also benefits the distribution of the response time, P99 goes from 3.55 to 1.55 over a day of gathering metrics.

[1] https://opendev.org/openstack/keystone/src/branch/master/keystone/revoke/core.py#L125