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From Stefan Richter <s.rich...@data-artisans.com>
Subject Re: Missing MapState when Timer fires after restored state
Date Tue, 15 May 2018 07:39:43 GMT
Hi,

I agree, this looks like a bug. Can you tell us your exact configuration of the state backend,
e.g. if you are using incremental checkpoints or not. Are you using the local recovery feature?
Are you restarting the job from a checkpoint or a savepoint? Can you provide logs for both
the job that failed and the restarted job?

Best,
Stefan

> Am 14.05.2018 um 13:00 schrieb Juho Autio <juho.autio@rovio.com>:
> 
> We have a Flink streaming job (1.5-SNAPSHOT) that uses timers to clear old state. After
restoring state from a checkpoint, it seems like a timer had been restored, but not the data
that was expected to be in a related MapState if such timer has been added.
> 
> The way I see this is that there's a bug, either of these:
> - The writing of timers & map states to Flink state is not synchronized (or maybe
there are no such guarantees by design?)
> - Flink may restore a checkpoint that is actually corrupted/incomplete
> 
> Our code (simplified):
> 
>     private MapState<String, String> mapState;
> 
>     public void processElement(..) {
>             mapState.put("lastUpdated", ctx.timestamp().toString());
>             ctx.timerService().registerEventTimeTimer(ctx.timestamp() + stateRetentionMillis);
>     }
> 
>     public void onTimer(long timestamp, OnTimerContext ctx, ..) {
>         long lastUpdated = Long.parseLong(mapState.get("lastUpdated"));
>         if (timestamp >= lastUpdated + stateRetentionMillis) {
>             mapState.clear();
>         }
>     }
> 
> Normally this "just works". As you can see, it shouldn't be possible that "lastUpdated"
doesn't exist in state if timer was registered and onTimer gets called.
> 
> However, after restoring state from a checkpoint, the job kept failing with this error:
> 
> Caused by: java.lang.NumberFormatException: null
> at java.lang.Long.parseLong(Long.java:552)
> at java.lang.Long.parseLong(Long.java:631)
> at ..EnrichEventProcessFunction.onTimer(EnrichEventProcessFunction.java:136)
> ..
> 
> So apparently onTimer was called but lastUpdated wasn't found in the MapState.
> 
> The background for restoring state in this case is not entirely clean. There was an OS
level issue "Too many open files" after running a job for ~11 days. To fix that, we replaced
the cluster with a new one and launched the Flink job again. State was successfully restored
from the latest checkpoint that had been created by the "problematic execution". Now, I'm
assuming that if the state wouldn't have been created successfully, restoring wouldn't succeed
either – correct? This is just to rule out that the issue with state didn't happen because
the checkpoint files were somehow corrupted due to the Too many open files problem.
> 
> Thank you all for your continued support!
> 
> P.S. I would be very much interested to hear if there's some cleaner way to achieve this
kind of TTL for keyed state in Flink.


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