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IoT Use Case, Problem and Thoughts

Fabian, Stephan, All,

I started a discussion a while back around having a form of event-based checkpointing policy that will help us in some of our high volume data pipelines. Here is an effort to put this in front of community and understand what capabilities can support these type of use cases, how much others feel the same need and potentially a feature that can make it to a user story.

Use Case Summary:
- Extremely high volume of data (events from consumer devices with customer base of over 100M)
- Multiple events need to be combined using a windowing streaming app grouped by keys (something like 5 min floor of timestamp and unique identifiers for customer devices)
- "Most" events by a group/key arrive in few seconds if not milliseconds however events can sometimes delay or get lost in transport (so delayed event handling and timeouts will be needed)
- Extremely low (pretty vague but hopefully details below clarify it more) data loss is acceptable
- Because of the volume and transient nature of source, checkpointing is turned off (saves on writes to persistence as states/sessions are active for only few seconds during processing)

Problem Summary:
Of course, none of the above is out of the norm for Flink and as a matter of factor we already have a Flink app doing this. The issue arises when it comes to graceful shutdowns and on operator failures (eg: Kafka timeouts etc.) On operator failures, entire job graph restarts which essentially flushes out in-memory states/sessions. I think there is a feature in works (not sure if it made it to 1.5) to perform selective restarts which will control the damage but still will result in data loss. Also, it doesn't help when application restarts are needed. We did try going savepoint route for explicit restart needs but I think MemoryBackedState ran into issues for larger states or something along those line(not certain). We obviously cannot recover an operator that actually fails because it's own state could be unrecoverable. However, it feels like Flink already has a lot of plumbing to help with overall problem of allowing some sort of recoverable state to handle graceful shutdowns and restarts with minimal data loss.

Some in community commented on my last email with decent ideas like having an event-based checkpointing trigger (on shutdown, on restart etc) or life-cycle hooks (onCancel, onRestart etc) in Functions that can be implemented if this type of behavior is needed etc. 

Appreciate feedback from community on how useful this might be for others and from core contributors on their thoughts as well.

Thanks in advance, Ashish