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https://issues.apache.org/jira/browse/KAFKA-4212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16876553#comment-16876553
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James Ritt commented on KAFKA-4212:
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Thanks [~ableegoldman]!
I definitely should have included more context in my comment above: the
situation we're looking at is we have an underlying topic with a
cleanup.policy=delete & delete.retention.ms set. We then use Streams API create
a GlobalKTable over that topic. So in my understanding, the topic will get
cleaned out automatically, but without setting a TTL for the persistent KV
topic cache, the underlying rocksdb will grow unbounded, thus this PR (see also
https://stackoverflow.com/questions/48080721/kafka-streams-ktable-from-topic-with-retention-policy?rq=1).
But please LMK if I'm wrong!
W.r.t. strictness, thanks for the heads up: in our particular case we're fine
with the lower bound behavior as it's fine semantically if the values stay in
our cache longer than in the topic.
> Add a key-value store that is a TTL persistent cache
> ----------------------------------------------------
>
> Key: KAFKA-4212
> URL: https://issues.apache.org/jira/browse/KAFKA-4212
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Affects Versions: 0.10.0.1
> Reporter: Elias Levy
> Priority: Major
> Labels: api
>
> Some jobs needs to maintain as state a large set of key-values for some
> period of time. I.e. they need to maintain a TTL cache of values potentially
> larger than memory.
> Currently Kafka Streams provides non-windowed and windowed key-value stores.
> Neither is an exact fit to this use case.
> The {{RocksDBStore}}, a {{KeyValueStore}}, stores one value per key as
> required, but does not support expiration. The TTL option of RocksDB is
> explicitly not used.
> The {{RocksDBWindowsStore}}, a {{WindowsStore}}, can expire items via segment
> dropping, but it stores multiple items per key, based on their timestamp.
> But this store can be repurposed as a cache by fetching the items in reverse
> chronological order and returning the first item found.
> KAFKA-2594 introduced a fixed-capacity in-memory LRU caching store, but here
> we desire a variable-capacity memory-overflowing TTL caching store.
> Although {{RocksDBWindowsStore}} can be repurposed as a cache, it would be
> useful to have an official and proper TTL cache API and implementation.
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