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Re: Best way to clean-up states in memory


Hi Ashish,

I had a look at your Trigger and couldn't spot anything that would explain leaking state.
You're properly cleaning up in clear().

However, I might have found the problem for the increasing state size.
A window is only completely deleted when the time passes its end timestamp (Window.maxTimestamp()).
Deletion of a window includes deletion of the window object, purging the window content (same as TriggerResult.PURGE), and calling Trigger.clear().

The max timestamp of a GlobalWindow is Long.MAX_VALUE. Hence, a GlobalWindow is never really deleted (the window content is gone if you PURGE  the window).
If you are running the window operator on a key with an evolving domain, this means you are accumulating state forever.

One way to solve the issue is to implement a custom window assigner with a conservative max timestamp.
Or you keep your ProcessFunction implementation, which is probably a lot easier to maintain than a custom window assigner, trigger, and window function.

Best, Fabian

Btw. you don't need to implement the merging logic of a Trigger, if the window assigner does not support merging (GlobalWindows does not).

2018-05-15 2:55 GMT+02:00 ashish pok <ashishpok@xxxxxxxxx>:
Thanks Fabian, Kostas,

Here is what I had in the Trigger - idea is to run bitwise OR until a threshold is reached or a timeout is reached (nothing too fancy here). Let me know what you guys think. Like I said, I moved this logic to Process Function and I haven't seen the same issue I was with this. 


@PublicEvolving
public class BitwiseOrTrigger<W extends Window> extends Trigger<FactoredEvent, W> {
private static final long serialVersionUID = 1L;
private final int threshold;
private final long epocDelta;
private final ReducingStateDescriptor<Tuple2<Integer, Long>> stateDesc = 
new ReducingStateDescriptor<>("bitwiseOr", new BitwiseOr(), TypeInformation.of(new TypeHint<Tuple2<Integer,Long>>() {}));


private BitwiseOrTrigger(int threshold, long allowedLateness) {
this.threshold = threshold;
this.epocDelta = allowedLateness;
}

@Override
public TriggerResult onElement(FactoredEvent event, long timestamp, W window, TriggerContext ctx) throws Exception {
ReducingState<Tuple2<Integer,Long>> currState = ctx.getPartitionedState(stateDesc);
if (this.epocDelta>0) {
ctx.registerProcessingTimeTimer(System.currentTimeMillis() + this.epocDelta);
}
currState.add(new Tuple2<Integer,Long>(event.getFactor(), this.epocDelta));
if (currState.get().f0 >= threshold) {
currState.clear();
return TriggerResult.FIRE_AND_PURGE;
}
return TriggerResult.CONTINUE;
}

@Override
public TriggerResult onEventTime(long time, W window, TriggerContext ctx) {
return TriggerResult.FIRE_AND_PURGE;
}

@Override
public TriggerResult onProcessingTime(long time, W window, TriggerContext ctx) throws Exception {
return TriggerResult.FIRE_AND_PURGE;
}

@Override
public void clear(W window, TriggerContext ctx) throws Exception {
ctx.getPartitionedState(stateDesc).clear();
}

@Override
public boolean canMerge() {
return true;
}

@Override
public void onMerge(W window, OnMergeContext ctx) throws Exception {
ctx.mergePartitionedState(stateDesc);
}

@Override
public String toString() {
return "BitwiseOrTrigger(" +  threshold + ")";
}

public static <W extends Window> BitwiseOrTrigger<W> of(int threshold, long expirationEpoc) {
return new BitwiseOrTrigger<>(threshold, expirationEpoc);
}

private static class BitwiseOr implements ReduceFunction<Tuple2<Integer, Long>> {
private static final long serialVersionUID = 1L;

@Override
public Tuple2<Integer, Long> reduce(Tuple2<Integer, Long> tup1, Tuple2<Integer, Long> tup2) throws Exception {
Tuple2<Integer, Long> retTup = tup1;
retTup.f0 = tup1.f0 | tup2.f0;
return retTup;
}

}
}


On Monday, May 14, 2018, 6:00:11 AM EDT, Fabian Hueske <fhueske@xxxxxxxxx> wrote:


Hi Ashish,

Did you use per-window state (also called partitioned state) in your Trigger?
If yes, you need to make sure that it is completely removed in the clear() method because processing time timers won't fire once a window was purged.
So you cannot (fully) rely on timers to clean up per-window state.

Best, Fabian

2018-05-14 9:34 GMT+02:00 Kostas Kloudas <k.kloudas@xxxxxxxxxxxxxxxxx>:
Hi Ashish,

It would be helpful to share the code of your custom trigger for the first case.
Without that, we cannot tell what state you create and how/when you update/clear it.

Cheers,
Kostas

On May 14, 2018, at 1:04 AM, ashish pok <ashishpok@xxxxxxxxx> wrote:

Hi Till,

Thanks for getting back. I am sure that will fix the issue but I feel like that would potentially mask an issue. I have been going back and forth with Fabian on a use case where for some of our highly transient datasets, it might make sense to just use memory based state (except of course data loss becomes an issue when apps occasionally hit a problem and whole job restarts or app has to be taken down etc - ie. handling graceful shutdowns / restarts better essentially). I was on the hook to create a business case and post it back to this forum (which I am hoping I can get around to at some point soon). Long story short, this is one of those datasets. 

States in this case are either fired and cleared normally or on processing timeout. So technically, unless there is a memory leak in app code, memory usage should plateau out at a high-point. What I was noticing was memory would start to creep up ever so slowly. 

I couldn't tell exactly why heap utilization kept on growing (ever so slowly but it had upward trend for sure) because the states should technically be cleared if not as part of a reducing function then on timeout. App after running for couple of days would then run into Java Heap issues. So changing to RocksDB probably will fix the issue but not necessarily leak of states that should be cleared IMO. Interestingly, I switched my app from using something like this:

WindowedStream<BasicFactTuple, String, GlobalWindow> windowedStats = statsStream
        .keyBy(BasicFactTuple::getKey)
        .window(GlobalWindows.create() )
        .trigger(BitwiseOrTrigger.of( 60, AppConfigs.getWindowSize(5*60* 1000)))
        ;

To 

 DataStream<PlatformEvent> processStats = pipStatsStream
        .keyBy(BasicFactTuple::getKey)
        .process(new IfStatsReduceProcessFn( AppConfigs.getWindowSize(5*60* 1000), 60))

I basically moved logic of trigger to process function over the weekend. Once I did that, heap is completely stabilized. In trigger implementation, I was using FIRE_AND_PURGE on trigger condition or onProcessingTime and in process implementation I am using .clear() method for same. 

I seem to have solved the problem by using process but I'd be interested to understand the cause of why heap would creep up in trigger scenario. 

Hope this makes sense,

Ashish

On Sunday, May 13, 2018, 4:06:59 PM EDT, Till Rohrmann <till.rohrmann@xxxxxxxxx> wrote:


Hi Ashish,

have you tried using Flink's RocksDBStateBackend? If your job accumulates state exceeding the available main memory, then you have to use a state backend which can spill to disk. The RocksDBStateBackend offers you exactly this functionality.

Cheers,
Till

On Mon, Apr 30, 2018 at 3:54 PM, ashish pok <ashishpok@xxxxxxxxx> wrote:
All,

I am using noticing heap utilization creeping up slowly in couple of apps which eventually lead to OOM issue. Apps only have 1 process function that cache state. I did make sure I have a clear method invoked when events are collected normally, on exception and on timeout.

Are any other best practices others follow for memory backed states?

Thanks,

-- Ashish