[jira] [Comment Edited] (KAFKA-4212) Add a key-value store that is a TTL persistent cache

["Elias Levy (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-4212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15521287#comment-15521287 ] 

Elias Levy edited comment on KAFKA-4212 at 9/25/16 7:17 PM:
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It should be noted that a variable-capacity memory-overflowing TTL caching store is semantically equivalent to a KTable that expires entries via a TTL.  Such a KTable may be a viable alternative or at least a useful additional abstraction.


was (Author: elevy):
I should be noted that a variable-capacity memory-overflowing TTL caching store is semantically equivalent to a KTable that expires entries via a TTL.  Such a KTable may be a viable alternative or at least a useful additional abstraction.

> 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
>            Assignee: Guozhang Wang
>
> 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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