code: private void startPurgeTimer() {
          if (null != this.future || null == this.scheduler) {
              return;
          }
          if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled()) {
              Tr.debug(tc, "Creating repeating purge event");
          }
          this.future = this.scheduler.schedule(SessionEventHandler.PURGE_EVENT, this.purgeInterval, this.purgeInterval, TimeUnit.SECONDS);
      }
docs: Once sessions are actively being used, this is used to start the
  background invalidation timer that periodically scans for expired
  sessions that should be purged from storage.