[
https://issues.apache.org/jira/browse/KAFKA-9921?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17102908#comment-17102908
]
Sophie Blee-Goldman commented on KAFKA-9921:
--------------------------------------------
Fair enough. I added the clarification to WindowStore#put for now – I was
thinking that ultimately it might make sense for the window store with
duplicates to use an entirely different interface, in which case the comment on
WindowStore#put would not really make sense, but we can always move things
around later.
I think refactoring out the retainDuplicates flag into a separate interface
might make sense to do as part of the fix for KAFKA-9923 although it'll require
a KIP if we change or add to any of the supplier methods in Stores. It should
be fairly straightforward though. If you have any strong opinions on what the
duplicates API should look like we'd welcome a KIP/PR.
> Caching is not working properly with WindowStateStore when retaining
> duplicates
> -------------------------------------------------------------------------------
>
> Key: KAFKA-9921
> URL: https://issues.apache.org/jira/browse/KAFKA-9921
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Affects Versions: 2.5.0
> Reporter: Georgi Petkov
> Assignee: Sophie Blee-Goldman
> Priority: Major
> Fix For: 2.6.0, 2.5.1
>
>
> I'm using the current latest version 2.5.0 but this is not something new.
> I have _WindowStateStore_ configured as following (where _true_ stands for
> the _retainDuplicates_ paramter):
> _builder.addStateStore(windowStoreBuilder(persistentWindowStore(name,
> retentionPeriod, windowSize, *true*), keySerde,
> valueSerde)*.withCachingEnabled()*)_
> If I put 4 key-value pairs with the same key and values *1, 2, 3, 4* in that
> order when reading them through the iterator I'll get the values *4, 2, 3, 4*.
> I've done a bit of investigation myself and the problem is that *the whole
> caching feature is written without consideration of the case where duplicates
> are retained*.
> The observed behavior is due to having the last value in the cache (and it
> can have only one since it's not aware of the retain duplicates option) and
> it is read first (while skipping the first from the RocksDB iterator even
> though the values are different). This can be observed (for version 2.5.0) in
> _AbstractMergedSortedCacheStoreIterator#next()_ lines 95-97. Then the next 3
> values are read from the RocksDB iterator so they are as expected.
> As I said, the whole feature is not considering the _retainDuplicates_ option
> so there are other examples of incorrect behavior like in
> _AbstractMergedSortedCacheStoreIterator__#peekNextKey()_ - for each call, you
> would skip one duplicate entry in the RocksDB iterator for the given key.
> In my use case, I want to persist a list of values for a given key without
> increasing the complexity to linear for a single event (which would be the
> case if I was always reading the current list appending one value and writing
> it back). So I go for _List<KeyValuePair<K, V>>_ instead of _KeyValuePair<K,
> List<V>>_. The whole use case is more complex than that so I use
> _#transformValues_ and state stores.
> So as an impact I can't use caching on my state stores. For others - they'll
> have incorrect behavior that may take a lot of time to be discovered and even
> more time to fix the results.
--
This message was sent by Atlassian Jira
(v8.3.4#803005)
