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https://issues.apache.org/jira/browse/JAMES-3435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17219546#comment-17219546
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ASF GitHub Bot commented on JAMES-3435:
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chibenwa commented on pull request #255:
URL: https://github.com/apache/james-project/pull/255#issuecomment-715151761
> Managing domain and users is not a very recurring operation
Reading them is.
And SERIAL consistency is required in such cases, implying a read of paxos
system table.
That's why I do you expect some performance improvement by relaxing these
constraints.
That being said, it looks like SERIAL consistency level was not even
specified there (!!!) giving a false feeling of safety (we likely had untested
concurrency issues with this validation behavior).
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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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