Ivan Bessonov created IGNITE-17083:
--------------------------------------
Summary: Universal full rebalance procedure for MV storage
Key: IGNITE-17083
URL: https://issues.apache.org/jira/browse/IGNITE-17083
Project: Ignite
Issue Type: Improvement
Reporter: Ivan Bessonov
Canonical way to make "full rebalance" in RAFT is to have a persisted snapshots
of data. This is not always a good idea. First of all, for persistent data is
already stored somewhere and can be read at any time. Second, for volatile
storage this requirement is just absurd.
So, a "rebalance snapshot" should be streamed from one node to another instead
of being written to a storage. What's good is that this approach can be
implemented independently from the storage engine (with few adjustments to
storage API, of course).
h2. General idea
Once a "rebalance snapshot" operation is triggered, we open a special type of
cursor from the partition storage, that is able to give us all versioned chains
in {_}some fixed order{_}. Every time the next chain has been read, it's
remembered as the last read (let's call it{{ lastRowId}} for now). Then all
versions for the specific row id should be sent to receiver node in "Oldest to
Newest" order to simplify insertion.
This works fine without concurrent load. To account for that we need to have a
additional collection of row ids, associated with a snapshot. Let's call it
{{{}overwrittenRowIds{}}}.
With this in mind, every write command should look similar to this:
{noformat}
for (var rebalanceSnaphot : ongoingRebalanceSnapshots) {
try (var lock = rebalanceSnaphot.lock()) {
if (rowId <= rebalanceSnaphot.lastRowId())
continue;
if (!rebalanceSnaphot.overwrittenRowIds().put(rowId))
continue;
rebalanceSnapshot.sendRowToReceiver(rowId);
}
}
// Now modification can be freely performed.
// Snapshot itself will skip everything from the "overwrittenRowIds"
collection.{noformat}
NOTE: rebalance snapshot scan must also return uncommitted write intentions.
Their commit will be replicated later from the RAFT log.
NOTE: receiving side will have to rebuild indexes during the rebalancing. Just
like it works in Ignite 2.x.
NOTE: Technically it is possible to have several nodes entering the cluster
that require a full rebalance. So, while triggering a rebalance snapshot
cursor, we could wait for other nodes that might want to read the same data and
process all of them with a single scan. This is an optimization, obviously.
h2. Implementation
The implementation will have to be split into several parts, because we need:
* Support for snapshot streaming in RAFT state machine.
* Storage API for this type of scan.
* Every storage must implement the new scan method.
* Streamer itself should be implemented, along with a specific logic in write
commands.
--
This message was sent by Atlassian Jira
(v8.20.7#820007)
