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Missing MapState when Timer fires after restored state


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.