Base class for consistent Local and Coordinator sessions
There are 4 stages to a consistent incremental repair.
Repair prepare
First, the normal
ActiveRepairService.prepareForRepair(TimeUUID, InetAddressAndPort, Set, RepairOption, boolean, List) stuff
happens, which sends out
PrepareMessage and creates a
ActiveRepairService.ParentRepairSession
on the coordinator and each of the neighbors.
Consistent prepare
The consistent prepare step promotes the parent repair session to a consistent session, and isolates the sstables
being repaired from other sstables. First, the coordinator sends a
PrepareConsistentRequest message to each repair
participant (including itself). When received, the node creates a
LocalSession instance, sets it's state to
PREPARING, persists it, and begins a preparing the tables for incremental repair, which segregates the data
being repaired from the rest of the table data. When the preparation completes, the session state is set to
PREPARED, and a
PrepareConsistentResponse is sent to the coordinator indicating success or failure.
If the pending anti-compaction fails, the local session state is set to
FAILED.
(see
LocalSessions.handlePrepareMessage(org.apache.cassandra.net.Message)
Once the coordinator recieves positive
PrepareConsistentResponse messages from all the participants, the
coordinator begins the normal repair process.
(see
CoordinatorSession#handlePrepareResponse(InetAddressAndPort, boolean)
Repair
The coordinator runs the normal data repair process against the sstables segregated in the previous step. When a
node recieves a
ValidationRequest, it sets it's local session state to
REPAIRING.
If all of the RepairSessions complete successfully, the coordinator begins the
Finalization process. Otherwise,
it begins the
Failure process.
Finalization
The finalization step finishes the session and promotes the sstables to repaired. The coordinator begins by sending
FinalizePropose messages to each of the participants. Each participant will set it's state to
FINALIZE_PROMISED
and respond with a
FinalizePromise message. Once the coordinator has received promise messages from all participants,
it will send a
FinalizeCommit message to all of them, ending the coordinator session. When a node receives the
FinalizeCommit message, it will set it's sessions state to
FINALIZED, completing the
LocalSession.
For the sake of simplicity, finalization does not immediately mark pending repair sstables repaired because of potential
conflicts with in progress compactions. The sstables will be marked repaired as part of the normal compaction process.
On the coordinator side, see
CoordinatorSession.finalizePropose(),
CoordinatorSession#handleFinalizePromise(InetAddressAndPort, boolean),
&
CoordinatorSession.finalizeCommit()
On the local session side, see
LocalSessions#handleFinalizeProposeMessage(InetAddressAndPort, FinalizePropose)
&
LocalSessions#handleFinalizeCommitMessage(InetAddressAndPort, FinalizeCommit)
Failure
If there are any failures or problems during the process above, the session will be failed. When a session is failed,
the coordinator will send
FailSession messages to each of the participants. In some cases (basically those not
including Validation and Sync) errors are reported back to the coordinator by the local session, at which point, it
will send
FailSession messages out.
Just as with finalization, sstables aren't immediately moved back to unrepaired, but will be demoted as part of the
normal compaction process.
See
LocalSessions#failSession(UUID, boolean) and
CoordinatorSession.fail()
Failure Recovery & Session Cleanup
There are a few scenarios where sessions can get stuck. If a node fails mid session, or it misses a
FailSession
or
FinalizeCommit message, it will never finish. To address this, there is a cleanup task that runs every
10 minutes that attempts to complete idle sessions.
If a session is not completed (not
FINALIZED or
FAILED) and there's been no activity on the session for
over an hour, the cleanup task will attempt to finish the session by learning the session state of the other participants.
To do this, it sends a
StatusRequest message to the other session participants. The participants respond with a
StatusResponse message, notifying the sender of their state. If the sender receives a
FAILED response
from any of the participants, it fails the session locally. If it receives a
FINALIZED response from any of the
participants, it will set it's state to
FINALIZED as well. Since the coordinator won't finalize sessions until
it's received
FinalizePromise messages from
all participants, this is safe.
If a session is not completed, and hasn't had any activity for over a day, the session is auto-failed.
Once a session has been completed for over 2 days, it's deleted.
Operators can also manually fail sessions with
nodetool repair_admin --cancel
See
LocalSessions.cleanup() and
RepairAdmin