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1. Where should the code live in? Should it live with operators, state backends, or both? We may also need something in operators or state backends (e.g. operators/StreamingRuntimeContext/KeyedStateStore) to have access to TimerService
   
   We may need to experiment and prototype before finalizing this decision.
   
   

2. How to distinguish TTL timers from user timers so that

   they don't interfere with each other?
   we

   user timers are always invoked before TTL timers?
   
   Upon discussion, we believe that the user timer should always be invoked first. We need a good strategy to ensure that

   the TTL timers do not interfere with user-configured timers, specifically, what happens if a user configures a timer

   , when a user timer and a TTL timer are configured for the same time

   as a Ttl timer? Upon discussion

   ,

   we believe that the

   user

   -configured

   timer

   should

   is always

   be invoked we distinguish user and TTL timers?

   triggered first.

   But how can 

   Currently, timer service does not support timer tags

   ,

   . We have two kinds of timers now and we

   may

   need

   to find

   a better

   of achieving the purpose, either adding support for timer tags or some other ways

   way of maintaining both of them. 
   
   Option1: We can add a tag for each timer to InternalTimer
   
   Option2: let InternalTimerService maintains user timers and TTL timers separately, by adding to it the following new APIs：

   Code Block
   language java
   public void registerTtlProcessingTimeTimer(N namespace, long time)； public void registerTtlEventTimeTimer(N namespace, long time)；

    And implementation classes of InternalTimerService should add two new sets of timers,  e.g. processingTtlTimeTimersQueue and eventTtlTimeTimersQueue for HeapInternalTimerService. Upon onProcessingTime() and advanceWatermark(), they will first iterate through ProcessingTtlTimeTimer and EventTtlTimeTimer before iterating the ProcessingTimeTimers and EventTimeTimer.

   The biggest advantage, compared to option 1, is that it doesn't impact existing timer-related checkpoint/savepoint and migrations.

Test Plan

Normal unit, integration, and end-to-end tests

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