[jira] [Updated] (FLINK-5544) Implement Internal Timer Service in RocksDB

["Aljoscha Krettek (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/FLINK-5544?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Aljoscha Krettek updated FLINK-5544:
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    Component/s:     (was: Streaming)
                 State Backends, Checkpointing

> Implement Internal Timer Service in RocksDB
> -------------------------------------------
>
>                 Key: FLINK-5544
>                 URL: https://issues.apache.org/jira/browse/FLINK-5544
>             Project: Flink
>          Issue Type: New Feature
>          Components: State Backends, Checkpointing
>            Reporter: Xiaogang Shi
>            Assignee: Xiaogang Shi
>
> Now the only implementation of internal timer service is HeapInternalTimerService which stores all timers in memory. In the cases where the number of keys is very large, the timer service will cost too much memory. A implementation which stores timers in RocksDB seems good to deal with these cases.
> It might be a little challenging to implement a RocksDB timer service because the timers are accessed in different ways. When timers are triggered, we need to access timers in the order of timestamp. But when performing checkpoints, we must have a method to obtain all timers of a given key group.
> A good implementation, as suggested by [~StephanEwen], follows the idea of merge sorting. We can store timers in RocksDB with the format {{KEY_GROUP#TIMER#KEY}}. In this way, the timers under a key group are put together and are sorted. 
> Then we can deploy an in-memory heap which keeps the first timer of each key group to get the next timer to trigger. When a key group's first timer is updated, we can efficiently update the heap.



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