Benoit Tellier created JAMES-3428:
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Summary: Read repairs - Mailbox counters
Key: JAMES-3428
URL: https://issues.apache.org/jira/browse/JAMES-3428
Project: James Server
Issue Type: Improvement
Components: cassandra, mailbox
Reporter: Benoit Tellier
= Why?
Mailbox conters inconsistencies requires manual intervention of the admin (via
the solve-inconsistency webadmin endpoint) to be resolved.
Having such task to ensure a correct denormalisation state is desirable, but
optimally we should not have to rely on the admin to remember running it.
We would prefer an auto-healing solution.
Mailbox counters consistency had been a blocker for some of our existing
deployments.
= Context
Read repair is a classic mechanism in eventual consistent databases (like
cassandra) to piggy back consistency healing operation upon reads.
A fraction of the reads will have a wider reach, read more data, and ensure it
is correctly replicated.
Cassandra eventual consistency is all about "replication", but
"denormalization" consistency needs to be handled at the applicative layer. In
the past we did set up "Solve inconsistency" tasks that can be assimilated to
Cassandra repairs. In order to achieve denormalization auto-healing, we thus
needs to implement "applicative read repairs".
= Technically speaking
Upon reads, for each mailbox, the mailbox mapper have a configurable random
chance to trigger read repairs.
In this case we reaqd the counters, and iterate counters metadata, and the
"solve inconsistency" mechanism is applied, if needed.
Write unit tests demonstrating this "solve inconsistency" behavior.
Given that reading "all messages metadata" can be an expensive operation, we
want to be running the repair asynchronously, in the background.
= Definition Of Done
Record a video : In a webmail ...
- Given a read repair chance of 0.2 (20%)
- There is a mailbox counter inconsistency inconsistency (10 mails unread in
the mailbox but the counter display 26) - (use cqlsh to alter the value of the
mailbox counter to something invalid)
- The user does several reads - after some time, the inconsistency is fixed:
The count of unread mails is 10!
As an admin, I should be able to configure (or disable) the read-repair-chance
for the mailbox entity
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