@Override
    public boolean cancel() {
      LOG.trace("Cancelled a callback: {} at {} for triggerKey {}", new Object[] {callback, scheduledTimeMs, triggerKey});
      return pendingCallbacks.remove(this);
    }
State corresponding to pending timer callbacks scheduled by various triggers.
samza-core/src/main/java/org/apache/samza/operators/impl/TriggerScheduler.java