chibenwa commented on pull request #570:
URL: https://github.com/apache/james-project/pull/570#issuecomment-894576161
# Why
James POP3 implementation is backed by the IMAP UID, a monotic counter.
Cassandra implementation uses LightWeight transactions to back a compare and
swap.
This implementation have expensive run time costs, especially in a multi
data center setup (LWTs requires 4 round trips accross replica even for reads).
# How
We should contribute to apache/james-project an alternative implementation
of the POP3 server not
leveraging UIDs but instead using messageIds, enabling safe, easy to
configure multi-datacenter POP3 setup for the Distributed Server.
Use a dedicated view in order to list the messageIds within a mailbox and
the size of the messages and use
the messageIdManager to retrieve the given emails.
Offer a configuration option to choose between the classic 'uid based'
implementation or the 'messageId based' implementation via the mechanism of
module-choosing.
# Consequences
By implementing this we will have more options in the face of bad
Lightweight Transaction performances:
- Use of `-Dcassandra.unsafe.disable-serial-reads-linearizability=true`
option with Cassandra would be acceptable for POP3 workload but it would lead
to data loss in IMAP.
- More aggressivley the tailor made server could include a UID/ModSeq
allocation compare and swap mechanism not being backed by LWTs. Trivialy
exposed to data races, it
simulates in the absence of concurrency a correct behaviour without the
costs of LightWeight transactions and could timely be setted up. It would lead
to data loss in IMAP.
Also POP3 workload would not need a failover script to increment UIDs upon
failover. However not playing it timely would result in data loss with IMAP.
All places currently relying on IMAP UID as an identifier will not be able
to be relied upon. Impact studies shows that, for some tasks like mail
re-indexing will be impacted (as they are backed by IMAP UID), which is likely
non critical for a pure POP3 usage.
# Tests
The following changeset had been tested with Thunderbird (for the Cassandra
set up)
We had been contributing extensive integration tests for the POP3 servers.
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