Major Description
https://www.mail-archive.com/server-dev@james.apache.org/msg68713.html
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.
Let's start relaxing LWT transaction for users & domains.
