[jira] [Updated] (CASSANDRA-19436) When transitioning to Accord migration it's not safe to read immediately using Accord due to concurrent non-serial writes

["Ariel Weisberg (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-19436?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Ariel Weisberg updated CASSANDRA-19436:
---------------------------------------
    Status: Ready to Commit  (was: Review In Progress)

> When transitioning to Accord migration it's not safe to read immediately using Accord due to concurrent non-serial writes
> -------------------------------------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-19436
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-19436
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Accord, Consistency/Coordination
>            Reporter: Ariel Weisberg
>            Assignee: Ariel Weisberg
>            Priority: Normal
>             Fix For: 5.x
>
>          Time Spent: 3h
>  Remaining Estimate: 0h
>
> Concurrent writes at the same time that migration starts make it unsafe to read from Accord because txn recovery will not be deterministic in the presences of writes not done through Accord.
> Migration to Accord needs to be split into two phases, in the first phase we write through Accord and always respect the consistency level and do synchronous commit. This allows Paxos and non-serial writes to continue while Accord's metadata covers everything needed for future reads. Paxos continues to operate as normal since it has enough metadata to allow key migration in the second phase and we need to stay online so something needs to handle LWTs.
> Then repair runs and makes it possible for Accord to read any data written non-transactionally and we can then do key migration and route all updates (conditional or blind) through Accord while Paxos repair runs so we can stop doing key migration.



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