Francesco Nigro created ARTEMIS-3340:
----------------------------------------
Summary: Replicated Journal quorum-based logical timestamp
Key: ARTEMIS-3340
URL: https://issues.apache.org/jira/browse/ARTEMIS-3340
Project: ActiveMQ Artemis
Issue Type: Improvement
Reporter: Francesco Nigro
Shared-nothing replication, both classic and using pluggable quorum vote, can
cause journal misalignment despite no split-brain events.
Scenario without network partitions/outages:
# Master/Primary start as live, clients connect to it
# Backup become an in-sync replica
# User stop live and backup failover to it
# Backup serve clients, modifying its journal
# User stop backup
# User start master/primary: it become live with a journal misaligned with the
most up-to-date one
The main cause of this scenario is because we allow a single broker to server
clients, despite configured with HA.
A secondary cause (for other journal misalignment cases) is that the quorum
service (embedded on classic, pluggable on
https://issues.apache.org/jira/browse/ARTEMIS-2716)
just take care of mutual exclusive presence of broker for the live role,
without considering ordering
For a backup broker, with no primary/master around, makes sense, but this can
cause bad restart/retry ordering to let a broker with a stale journal to win
the race to become live.
A possible solution is to leverage on
https://issues.apache.org/jira/browse/ARTEMIS-2716 and store a "logical
timestamp" that mark the age of the journal of a broker in order to allow only
the one with the most up-to-date one to become a proper live.
It means that in case of quorum service restart/outage, the admin must have
some command/configuration to let a broker to ignore the age of its journal and
just force it to start.
In addition must be exposed some new journal CLI commands to inspect the age of
a broker journal, for troubleshooting reasons.
Its very important to capture every possible event that cause the journal age
to change
eg
# live broker send its journal file to a not yet in sync replica backup, along
with its "journal age"
# backup is now ready to failover in any moment
# a network partition happen
# backup try to become live for vote-retries times
# live detect replication disconnection but is "lucky" that can reach the
quorum and continue serving clients
# live increment the age of its journal
# an outage cause live to die
# network partition is restored
# backup detect that journal age is no longer matching its own journal: it stop
trying to become live
The key parts related to journal age/version are:
* only who's live can change journal version (with a monotonic increment)
* every breaking point event must cause journal age/version to change eg
starting as live, loosing its backup, etc etc
--
This message was sent by Atlassian Jira
(v8.3.4#803005)
