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From "ASF GitHub Bot (Jira)" <server-...@james.apache.org>
Subject [jira] [Commented] (JAMES-3435) Relaxing LWT usage: domain, users
Date Fri, 23 Oct 2020 04:38:00 GMT

    [ https://issues.apache.org/jira/browse/JAMES-3435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17219453#comment-17219453

ASF GitHub Bot commented on JAMES-3435:

chibenwa opened a new pull request #255:
URL: https://github.com/apache/james-project/pull/255


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> Relaxing LWT usage: domain, users
> ---------------------------------
>                 Key: JAMES-3435
>                 URL: https://issues.apache.org/jira/browse/JAMES-3435
>             Project: James Server
>          Issue Type: Improvement
>          Components: cassandra
>    Affects Versions: master
>            Reporter: Benoit Tellier
>            Priority: Major
>             Fix For: master
> https://www.mail-archive.com/server-dev@james.apache.org/msg68713.html
> {code:java}
> Cassandra is an eventually consistent datastore, that can be used in a
> consistant fashion. To do so, we rely on a mechanism called "LightWeight
> Transactions (LWT)". Lightweight transactions relies on the PAXOS
> distributed consensus algorithm to enforce a condition upon data
> mutation. A table, system.paxos, is used to track the state of
> transactions. Furthermore, upon writes, several round-trips (two) are
> needed to ensure data integrity accross replica(minimum round trips to
> achieve consensus) and the system.paxos table is read / written to in
> addition to the applicative table.
> All of this causes LWT to be significantly slower than their lower
> consistency counterparts. On some Linagora owned production instances,
> regular reads takes 2ms while reads on tables relying on LWT takes 6ms.
> Similar figures are found for writes. We also noticed some high
> compaction throughtput on the paxos table, leading to many back-ground
> writes.
> Given the massive impact of LWT usage on performance, and given the lack
> of debate upon LWT adoption, I would like to re-challenge their usage...
> Here are the places we rely on LWT for the Distributed Server:
>  - IMAP UID generation (monotic integer) - strong consistency is
> strictly required to not loose data as overwriting a uid means
> overwriting a message.
>  - IMAP ModSeq generation (monotic integer) - strong consistency is
> required, as modseq overwrites can lead to some data not being well
> synchronised.
>  - Domain and users - we rely on LWT to return an error when deleting a
> user that do not exist, or creating an already existing user. It sounds
> unecessary.
>  - Message flags relies on LWT to ensure updates are not overwritten. As
> an often read metadata, the impact is high, for limited criticity for
> the end user. After all, no data is lost, only a user action like
> marking a message as Seen, an action that he can very well perform again
>  - Mailbox path registration, ACL - required to prevent data races
> My proposal would be:
>  - Keep using LWT for UID and modseq generation, as well as Mailbox path
> registration.
>  - Make the use of LWT for message flags update configurable - as an
> admin I can choose to disable it.
>  - I am also fine with completly removing LWT usage for message flags
> update.
>  - No longer use LWT on domain or users. Instead use idempotent create /
> delete. The contract test will thus need to be relaxed.
>  - On the long term, relying on a CRDT to represent ACLs at the
> Cassandra level, instead of serialized JSON, would enable to get rid of
> LWT usage on the ACL table.
> {code}
> Let's start relaxing LWT transaction for users & domains.

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