Georgi Petkov created KAFKA-9921:
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Summary: Caching is not working properly with WindowStateStore
when rataining 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
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 we read the
next 3 values from the RocksDB iterator and 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'm currently not able to use caching on my state stores.
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