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https://issues.apache.org/jira/browse/CASSANDRA-19918?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17946423#comment-17946423
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Chris Lohfink commented on CASSANDRA-19918:
-------------------------------------------
+1
> Automated Repair Inside Cassandra
> ---------------------------------
>
> Key: CASSANDRA-19918
> URL: https://issues.apache.org/jira/browse/CASSANDRA-19918
> Project: Apache Cassandra
> Issue Type: Epic
> Components: Consistency/Repair
> Reporter: Jaydeepkumar Chovatia
> Assignee: Jaydeepkumar Chovatia
> Priority: Normal
> Attachments: trunk_cep_37_2025_04_06-1_ci_summary.html,
> trunk_cep_37_post_accord.html
>
>
> h1. Motivation
> Anti-entropy (Apache Cassandra repairs) is essential for every Apache
> Cassandra cluster to fix data inconsistencies. Frequent data deletions and
> downed nodes are common causes of data inconsistency. A few open-source
> orchestration solutions that trigger repair externally are available, as many
> large users have needed to figure out a scalable repair solution. However,
> multiple custom solutions have led to a lot of confusion in the community.
> Therefore, the repair activity, like Compaction, should be an integral part
> of Cassandra to call it a complete solution.
>
> The proposal is to align one solution among the existing solutions and make
> it part of the core Cassandra. Here is the design for one of the solutions:
>
> Inside Cassandra, there are multiple repairs we would have to schedule:
> 1) Full repair
> 2) Incremental Repair
> 3) Paxos repair
>
> The design of the scheduler should be capable of extending multiple repair
> categories with a minimal code change, and all repair types should progress
> automatically with minimal manual intervention.
> Migrating[[1|https://stackoverflow.com/questions/42182984/how-do-i-enable-incremental-repair-on-cassandra-2-1-13]]
> (and rollback) to/from incremental repair has been extremely challenging,
> especially in a large fleet. One of the design principles is to make it
> almost touchless from the operator’s point of view.
> h1. The Scheduler
> Keeping the above motivation in mind, this design embarks on our journey to
> have the repair orchestration inside Cassandra itself, which will repair the
> entire ring.
> A dedicated thread pool is assigned to the repair scheduler at a higher
> level. The repair scheduler inside Cassandra maintains a new replicated table
> under a distributed _system_distributed_ keyspace. This table maintains the
> repair history for all the nodes, such as when it was repaired the last time,
> etc. The scheduler will pick the node(s) that run the repair first and
> continue orchestration to ensure Every table and all of their token ranges
> are repaired. The algorithm can also run repairs simultaneously on multiple
> nodes and splits the token range into subranges with the necessary retry to
> handle transient failures. Over the period, the automatic repair has become
> so reliable that it runs as soon as we start a Cassandra cluster, like
> Compaction, and does not require manual intervention.
> Due to this fully automated repair scheduler inside Cassandra, there is no
> dependency on the control plane, significantly reducing our operational
> overhead.
> *CEP:*
> [https://cwiki.apache.org/confluence/display/CASSANDRA/CEP-37+Apache+Cassandra+Unified+Repair+Solution]
> h2. Detailed Design Doc
> [Automated Repair in
> Cassandra|https://docs.google.com/document/d/1CJWxjEi-mBABPMZ3VWJ9w5KavWfJETAGxfUpsViPcPo/edit#heading=h.r112r46toau0]
> h2. PR (on 4.1.6) (Last active: Sep 2024)
> Many folks currently are using 4.1.6 in production. Hence, the following PR
> on 4.1.6 will make it easier for everybody to review the code, test, etc. If
> the community decides to merge this CEP, then it will land on the _trunk_ as
> opposed to {_}4.1{_}.
> [https://github.com/apache/cassandra/pull/3367/]
> h2. PR (on {_}trunk{_}) (Last active: Sep 2024)
> [https://github.com/apache/cassandra/pull/3598]
>
> h2. Discussion over Slack
> [[1]|https://the-asf.slack.com/archives/CK23JSY2K/p1690225062383619]
> [[2]|http://cassandra-repair-scheduling-cep37/]
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