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From Alessandro Tagliapietra <tagliapietra.alessan...@gmail.com>
Subject Re: Reducing streams startup bandwidth usage
Date Wed, 04 Dec 2019 04:22:58 GMT
Thanks John for the explanation,

I thought that with EOS enabled (which we have) it would in the worst case
find a valid checkpoint and start the restore from there until it reached
the last committed status, not completely from scratch. What you say
definitely makes sense now.
Since we don't really need old time windows and we ensure data is ordered
when processed I think I"ll just write a custom transformer to keep only
the last window, store intermediate aggregation results in the store and
emit a new value only when we receive data belonging to a new window.
That with a compact only changelog topic should keep the rebuild data to
the minimum as it would have only the last value for each key.

Hope that makes sense

Thanks again

--
Alessandro Tagliapietra


On Tue, Dec 3, 2019 at 3:04 PM John Roesler <vvcephei@apache.org> wrote:

> Hi Alessandro,
>
> To take a stab at your question, maybe it first doesn't find it, but then
> restores some data, writes the checkpoint, and then later on, it has to
> re-initialize the task for some reason, and that's why it does find a
> checkpoint then?
>
> More to the heart of the issue, if you have EOS enabled, Streams _only_
> records the checkpoint when the store is in a known-consistent state. For
> example, if you have a graceful shutdown, Streams will flush all the
> stores, commit all the transactions, and then write the checkpoint file.
> Then, on re-start, it will pick up from that checkpoint.
>
> But as soon as it starts processing records, it removes the checkpoint
> file, so if it crashes while it was processing, there is no checkpoint file
> there, and it will have to restore from the beginning of the changelog.
>
> This design is there on purpose, because otherwise we cannot actually
> guarantee correctness... For example, if you are maintaining a count
> operation, and we process an input record i, increment the count and write
> it to the state store, and to the changelog topic. But we crash before we
> commit that transaction. Then, the write to the changelog would be aborted,
> and we would re-process record i . However, we've already updated the local
> state store, so when we increment it again, it results in double-counting
> i. The key point here is that there's no way to do an atomic operation
> across two different systems (local state store and the changelog topic).
> Since we can't guarantee that we roll back the incremented count when the
> changelog transaction is aborted, we can't keep the local store consistent
> with the changelog.
>
> After a crash, the only way to ensure the local store is consistent with
> the changelog is to discard the entire thing and rebuild it. This is why we
> have an invariant that the checkpoint file only exists when we _know_ that
> the local store is consistent with the changelog, and this is why you're
> seeing so much bandwidth when re-starting from an unclean shutdown.
>
> Note that it's definitely possible to do better than this, and we would
> very much like to improve it in the future.
>
> Thanks,
> -John
>
> On Tue, Dec 3, 2019, at 16:16, Alessandro Tagliapietra wrote:
> > Hi John,
> >
> > thanks a lot for helping, regarding your message:
> >  - no we only have 1 instance of the stream application, and it always
> > re-uses the same state folder
> >  - yes we're seeing most issues when restarting not gracefully due
> exception
> >
> > I've enabled trace logging and filtering by a single state store the
> > StoreChangelogReader messages are:
> >
> > Added restorer for changelog sensors-stream-aggregate-store-changelog-0
> > Added restorer for changelog sensors-stream-aggregate-store-changelog-1
> > Added restorer for changelog sensors-stream-aggregate-store-changelog-2
> > Did not find checkpoint from changelog
> > sensors-stream-aggregate-store-changelog-2 for store aggregate-store,
> > rewinding to beginning.
> > Did not find checkpoint from changelog
> > sensors-stream-aggregate-store-changelog-1 for store aggregate-store,
> > rewinding to beginning.
> > Did not find checkpoint from changelog
> > sensors-stream-aggregate-store-changelog-0 for store aggregate-store,
> > rewinding to beginning.
> > No checkpoint found for task 0_2 state store aggregate-store changelog
> > sensors-stream-aggregate-store-changelog-2 with EOS turned on.
> > Reinitializing the task and restore its state from the beginning.
> > No checkpoint found for task 0_1 state store aggregate-store changelog
> > sensors-stream-aggregate-store-changelog-1 with EOS turned on.
> > Reinitializing the task and restore its state from the beginning.
> > No checkpoint found for task 0_0 state store aggregate-store changelog
> > sensors-stream-aggregate-store-changelog-0 with EOS turned on.
> > Reinitializing the task and restore its state from the beginning.
> > Found checkpoint 709937 from changelog
> > sensors-stream-aggregate-store-changelog-2 for store aggregate-store.
> > Restoring partition sensors-stream-aggregate-store-changelog-2 from
> offset
> > 709937 to endOffset 742799
> > Found checkpoint 3024234 from changelog
> > sensors-stream-aggregate-store-changelog-1 for store aggregate-store.
> > Restoring partition sensors-stream-aggregate-store-changelog-1 from
> offset
> > 3024234 to endOffset 3131513
> > Found checkpoint 14514072 from changelog
> > sensors-stream-aggregate-store-changelog-0 for store aggregate-store.
> > Restoring partition sensors-stream-aggregate-store-changelog-0 from
> offset
> > 14514072 to endOffset 17116574
> > Restored from sensors-stream-aggregate-store-changelog-2 to
> aggregate-store
> > with 966 records, ending offset is 711432, next starting position is
> 711434
> > Restored from sensors-stream-aggregate-store-changelog-2 to
> aggregate-store
> > with 914 records, ending offset is 712711, next starting position is
> 712713
> > Restored from sensors-stream-aggregate-store-changelog-1 to
> aggregate-store
> > with 18 records, ending offset is 3024261, next starting position is
> 3024262
> >
> >
> > why it first says it didn't find the checkpoint and then it does find it?
> > It seems it loaded about  2.7M records (sum of offset difference in the
> > "restorting partition ...." messages) right?
> > Maybe should I try to reduce the checkpoint interval?
> >
> > Regards
> >
> > --
> > Alessandro Tagliapietra
> >
> >
> > On Mon, Dec 2, 2019 at 9:18 AM John Roesler <vvcephei@apache.org> wrote:
> >
> > > Hi Alessandro,
> > >
> > > I'm sorry to hear that.
> > >
> > > The restore process only takes one factor into account: the current
> offset
> > > position of the changelog topic is stored in a local file alongside the
> > > state stores. On startup, the app checks if the recorded position lags
> the
> > > latest offset in the changelog. If so, then it reads the missing
> changelog
> > > records before starting processing.
> > >
> > > Thus, it would not restore any old window data.
> > >
> > > There might be a few different things going on to explain your
> observation:
> > > * if there is more than one instance in your Streams cluster, maybe the
> > > task is "flopping" between instances, so each instance still has to
> recover
> > > state, since it wasn't the last one actively processing it.
> > > * if the application isn't stopped gracefully, it might not get a
> chance
> > > to record its offset in that local file, so on restart it has to
> restore
> > > some or all of the state store from changelog.
> > >
> > > Or it could be something else; that's just what comes to mind.
> > >
> > > If you want to get to the bottom of it, you can take a look at the
> logs,
> > > paying close attention to which tasks are assigned to which instances
> after
> > > each restart. You can also look into the logs from
> > > `org.apache.kafka.streams.processor.internals.StoreChangelogReader`
> (might
> > > want to set it to DEBUG or TRACE level to really see what's happening).
> > >
> > > I hope this helps!
> > > -John
> > >
> > > On Sun, Dec 1, 2019, at 21:25, Alessandro Tagliapietra wrote:
> > > > Hello everyone,
> > > >
> > > > we're having a problem with bandwidth usage on streams application
> > > startup,
> > > > our current setup does this:
> > > >
> > > > ...
> > > > .groupByKey()
> > > > .windowedBy<TimeWindow>(TimeWindows.of(Duration.ofMinutes(1)))
> > > > .aggregate(
> > > >         { MetricSequenceList(ArrayList()) },
> > > >         { key, value, aggregate ->
> > > >             aggregate.getRecords().add(value)
> > > >             aggregate
> > > >         },
> > > >         Materialized.`as`<String, MetricSequenceList,
> WindowStore<Bytes,
> > > >
> > >
> ByteArray>>("aggregate-store").withKeySerde(Serdes.String()).withValueSerde(Settings.getValueSpecificavroSerde())
> > > > )
> > > > .toStream()
> > > > .flatTransform(TransformerSupplier {
> > > > ...
> > > >
> > > > basically in each window we append the new values and then do some
> other
> > > > logic with the array of windowed values.
> > > > The aggregate-store changelog topic configuration  uses
> compact,delete as
> > > > cleanup policy and has 12 hours of retention.
> > > >
> > > > What we've seen is that on application startup it takes a couple
> minutes
> > > to
> > > > rebuild the state store, even if the state store directory is
> persisted
> > > > across restarts. That along with an exception that caused the docker
> > > > container to be restarted a couple hundreds times caused a big
> confluent
> > > > cloud bill compared to what we usually spend (1/4 of a full month in
> 1
> > > day).
> > > >
> > > > What I think is happening is that the topic is keeping all the
> previous
> > > > windows even with the compacting policy because each key is the
> original
> > > > key + the timestamp not just the key. Since we don't care about
> previous
> > > > windows as the flatTransform after the toStream() makes sure that we
> > > don't
> > > > process old windows (a custom suppressor basically) is there a way to
> > > only
> > > > keep the last window so that the store rebuilding goes faster and
> without
> > > > rebuilding old windows too? Or should I create a custom window using
> the
> > > > original key as key so that the compaction keeps only the last window
> > > data?
> > > >
> > > > Thank you
> > > >
> > > > --
> > > > Alessandro Tagliapietra
> > > >
> > >
> >
>

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