ConsistentCut is a distributed algorithm that splits WAL on 2 global areas - Before
and After
. It guarantees that every transaction committed Before
also will be committed Before
on every other node participated in the transaction. It means that an Ignite nodes can safely recover themself to the consistent Before
state without any coordination with each other.
The border between Before
and After
areas consists of two WAL records - ConsistentCutStartRecord
and ConsistentCutFinishRecord
. It guarantees that the Before
consists of:
ConsistentCutStartRecord
AND weren't included into ConsistentCutFinishRecord#after().
ConsistentCutStartRecord
and ConsistentCutFinishRecord
AND were included into ConsistentCutFinishRecord#before()
.On the picture below the Before
area consist of transactions colored to yellow, while After
is green.
/** */ public class ConsistentCutStartRecord extends WALRecord { /** Consistent Cut ID. */ private final UUID cutId; } /** */ public class ConsistentCutFinishRecord extends WALRecord { /** Consistent Cut ID. */ private final UUID cutId; /** * Collections of transactions committed BEFORE. */ private final Set<GridCacheVersion> before; /** * Collections of transactions committed AFTER. */ private final Set<GridCacheVersion> after; }
Picture bellow illustrates steps of the algorithm on single node:
lastFinishedCutId
holds previous ConsistentCutId
, or null.SnapshotOperationRequest
that holds new ConsistentCutId
(goal is to notify every node in a cluster about running incremental snapshot). SnapshotOperationRequest#ConsistentCutId
by DiscoverySPI (by the DistributedProcess).ConsistentCutAwareMessage#ConsistentCutId
by CommunicationSPI (by transaction messages - Prepare, Finish).ConsistentCutId
it starts local ConsistentCut: ConsistentCut
!= null) or finished (lastFinishedCutId
== id) for this id, skip if it has.ConsistentCut
future.removedActiveTxs
(This collection doesn't remove transactions unlike IgniteTxManager#activeTx
does).ConsistentCutStartRecord
to WAL with the received ConsistentCutId
.IgniteTxManager#activeTx
. Set listeners on those tx#finishFuture
.tx#status == ACTIVE
. It's guaranteed that such transactions belongs After side.removedActiveTxs
(contains transactions that are might be cleaned from IgniteTxManager#activeTx
). Set listeners on those tx#finishFuture
.removedActiveTxs
to null. We don't care of txs concurrently added to removedActiveTxs
, they just don't land into "before" or "after" set and will be excluded from recovery.removedActiveTxs
if ConsistentCut != null
and removedActiveTxs != null
:removedActiveTxs
right before it is removed from IgniteTxManager#activeTx
.tx#txCutId
equals to local, then put transaction into after, otherwise put into before.ConsistentCutFinishRecord
into WAL with the collections ( before, after ). ConsistentCut
future.ConsistentCutAwareMessage#topVer
with local node order:ConsistentCut
future.ConsistentCut != null
wraps outgoing messages to ConsistentCutAwareMessage
. It contains info:ConsistentCutId
(to start ConsistentCut on remote node, if not yet).Before
Consistent Cut started, otherwise After
.ConsistentCutAwareMessage
that make transaction committed (FinishRequest for 2PC, PrepareResponse for 1PC) Ignite set tx#txCutId = message#txCutId
before it starts handling the received message.ConsistentCut
future becomes null.Consistent Cut is such cut that correctly finished on all baseline nodes - ConsistentCutStartRecord
and ConsistentCutFinishRecord
are written.
"Inconsistent" Cut is such a cut when one or more baseline nodes hasn't wrote ConsistentCutFinishRecord
. It's possible in cases:
tx.finalizationStatus
== RECOVERY_FINISH).Ignite transaction protocol includes multiple messages. But only some of them affects meaningful (relating to the algorithm) that change state of transactions (PREPARED, COMMITTED):
GridNearTxPrepareRequest / GridDhtTxPrepareRequest
GridNearTxPrepareResponse / GridDhtTxPrepareResponse
GridNearTxFinishRequest / GridDhtTxFinishRequest
Those messages are wrapped in ConsistentCutAwareMessage
that is prepared right before sending message on other node. They used the current ConsistentCutId
.
Also some messages require to be combine with additional ConsistentCutId
to check it them on primary/backup node:
GridNearTxFinishRequest / GridDhtTxFinishRequest
GridNearTxPrepareResponse / GridDhtTxPrepareResponse
(for 1PC algorithm).Those messages are filled with txCutId
that is prepared right before transaction starts committing on first committing node. They used the current ConsistentCutId
for this setting. If current ConsistentCutId
is not null, then transaction starts committing after ConsistentCut started and it means that this transaction belongs the After
side.
class ConsistentCutAwareMessage { /** Original transaction message. */ Message msg; /** Consistent Cut ID. */ UUID cutId; /** Consistent Cut ID after which transaction committed. */ @Nullable UUID txCutId; /** Cluster topology version on which Consistent Cut started. */ long topVer; }