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From Bowen Li <bowenl...@gmail.com>
Subject Re: Missing MapState when Timer fires after restored state
Date Mon, 14 May 2018 17:48:00 GMT
Hi Juho,

You are right, there's no transactional guarantee on timers and state in
processElement(). They may end up with inconsistency if your job was
cancelled in the middle of processing an element.

To avoid the situation, the best programming practice is to always check if
the state you're trying to get is null or not.

I've also created https://issues.apache.org/jira/browse/FLINK-9362 to
document this.


On Mon, May 14, 2018 at 4:00 AM, Juho Autio <juho.autio@rovio.com> wrote:

> 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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