@Override
          public void run() {
            if (LOG.isDebugEnabled()) {
              LOG.debug("Cleaning up cache due to retention timeout of {} milliseconds",
                  units.toMillis(retentionDuration));
            }
            cache.cleanUp();
          }
Constructs a new InMemoryCredentialStore with a specified credential timeout

@param retentionDuration the time in some units to keep stored credentials, from the time they are added
@param units             the units for the retention duration (minutes, seconds, etc...)
@param activelyPurge     true to actively purge credentials after the retention time has expired;
                         otherwise false, to passively purge credentials after the retention time has expired
ambari-server/src/main/java/org/apache/ambari/server/security/encryption/InMemoryCredentialStore.java