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.

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1. Transactions committed before ConsistentCutStartRecord and AND weren't included into ConsistentCutFinishRecord#after().
2. Transactions committed between ConsistentCutStartRecord and ConsistentCutFinishRecord and AND were included into ConsistentCutFinishRecord#before().

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/** */
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;
 }

Algorithm

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Picture bellow illustrates steps of the algorithm on single node:

Image AddedImage Added

1. Initial state:
   
   1. No concurrent ConsistentCut process is running.
   2. lastFinishedCutId lastFinishedCutId holds previous ConsistentCutId, or null.
2. User starts a command for creating new incremental snapshot:
   1. Ignite node inits a DistributedProcess with special message a DistributedProcess with discovery message SnapshotOperationRequest that holds new ConsistentCutId  (goal is to notify every node in a cluster about running incremental snapshot). 
   2. DistributedProcess fix store the topology version topVer on which ConsistentCut started.
3. Process of creation of incremental snapshot consistent cut can be started by two events (what will happen earlier):
   1. Receive the ConsistentCutId by discoverythe SnapshotOperationRequest#ConsistentCutId by DiscoverySPI (by the DistributedProcess).
   2. Receive the ConsistentCutId by communication ( ConsistentCutAwareMessage#ConsistentCutId by CommunicationSPI (by transaction messages - Prepare, Finish).
4. On receiving the ConsistentCutId , every node: it starts local ConsistentCut: 
   1. There are 2 roles that node might play:
      1. ROLE#1 - wraps outgoing messages - for all Ignite nodes: client, baseline, non-baseline server nodes.
      2. ROLE#2 - prepares data to be written in WAL - for baseline nodes only.
   2. Before start check:
      1. Whether
   3. Checks whether
      1. ConsistentCut has already started (ConsistentCut != null) or finished (lastFinishedCutId == id) for this
      ID
      1. id, skip if it has.
      If received form CommunicationSPI then compare the topVer with mapped topology version of received transaction message. Skip ConsistentCut initialization if the received version is higher.
      1. On non-baseline nodes In case ConsistentCut is inited by CommunicationSPI then compare the ConsistentCutAwareMessage#topVer with local node order:
         1. Local node order equals to new topVer on the moment when node joined to a cluster.
         2. If the order is higher than ConsistentCut topVer it means the node joined after ConsistentCut started. Skip start ConsistentCut on this node.
   4. ROLE#1:
      1. creates new ConsistentCut future.
         1. If node is  non-baseline (client, non-baseline servers) - complete it right after creation, and notify a node-initiator about local procedure has finished (by DistributedProcess protocol).
      2. While ConsistentCut != null wraps outgoing messages to ConsistentCutAwareMessage. It contains info:
         
         1. ConsistentCutId (to start ConsistentCut  on remote node, if not yet).
         2. Messages contain additional field txCutId. It is originally set on the nodes that commit first:
            1. For 2PC it is an originated node.
            2. For 1PC it is a backup node.
         3. If txCutId equals to null then transaction starts committing Before Consistent Cut started, otherwise After.
      3. On receive ConsistentCutAwareMessage that makes transaction committed (FinishRequest for 2PC, PrepareResponse for 1PC) sets tx#txCutId = message#txCutId.
   5. ROLE#2 - for baseline nodes only:
      1. In the message thread atomically inits ConsistentCut:
         1. creates new ConsistentCut
          future
         1. future.
         2. creates
         committingTx (goal is to track COMMITTING transactions, that aren't part of IgniteTxManager#activeTx)starts signing outgoing messages with the ConsistentCutId
         1. empty collection removedActiveTxs (This collection doesn't remove transactions unlike IgniteTxManager#activeTx does).
      2. In the background thread:
         
         1. Writes a ConsistentCutStartRecord  to WAL with the received ConsistentCutId.
         2. Creates a copy
         of 
         1. (weakly-consistent) of IgniteTxManager#activeTx.
          Set
         1. Set listeners on
         those 
         1. those tx#finishFuture.
            Writes a ConsistentCutStartRecord  to WAL with the received ConsistentCutId
            1. 1. For optimization it's safely exclude transactions that tx#status == ACTIVE. It's guaranteed that such transactions belongs After side.
            2. Creates a copy
            of committingTxs. Set listeners on those 
            1. of removedActiveTxs (contains transactions that are might be cleaned from IgniteTxManager#activeTx). Set listeners on those tx#finishFuture.
         2. Set committingTxs to null.
   6. While the DistributedProcess  is running every node is signing output transaction messages:
      1. Prepare and Finish messages are signed with the ConsistentCutId (to trigger ConsistentCut  on remote node, if not yet).
      2. Finish messages are signed additionally with txCutId on the node that commits first (if it's not null then transaction starts committing After Consistent Cut):
         1. For 2PC it is an originated node.
         2. For 1PC it is a backup node.
   7. 1. 1. 1. Set 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.
         2. In transaction threads fills removedActiveTxs if ConsistentCut != null and removedActiveTxs != null:
            1. Every transaction is added into removedActiveTxsright before it is removed from IgniteTxManager#activeTx
      For every receiving FinishMessage  puts the transaction into committingTxs, and marks the transaction with extracted from the message txCutId
      1. 1. 1. .
         2. For every listening transaction,
      the callback
      1. 1. the callback is called when transaction finished:
            
            1. check If transaction state is UNKNOWN or status is RECOVERY_FINISH, then complete ConsistentCut with exception.
            2. If transaction mapped to a higher topology version than
         ConsistentCut 
         1. 1. ConsistentCut topVer, then put it into after.
            2. if
         tx#txCutId equals
         1. 1. tx#txCutId equals to local, then put transaction
         into 
         1. 1. into after, otherwise put into before.
         2. After every listening transaction finished:
            1. Writes a ConsistentCutFinishRecord  into WAL with
         the collections 
         1. 1. the collections ( before, after ). 
            2. Completes ConsistentCut  future.
      2. Note, that it continues to sign messages even after local ConsistentCut finish.
      After ConsistentCut finish, DistributeProcess automatically notifies
      1. 1. Notify a node-initiator about local procedure has finished (by DistributedProcess protocol).
   8. After all nodes finished ConsistentCut,  on on every node:
      
      1. Updates Updates lastFinishedCutId with the current id.
      2. ConsistentCut  future becomes future becomes null.
      3. Stops signing outgoing transaction messages.
   9. Node initiator checks that every node completes correctly.
      1. If any node complete exceptionally - complete Incremental Snapshot with exception.

   Consistent and inconsistent Cuts

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   1. any errors appeared during processing local Cut.
   2. if a transaction is recovered with transaction recovery protocol (tx.finalizationStatus == RECOVERY_FINISH).
   3. if transaction finished in UNKNOWN state.
   4. topology change.

   Signing messages

   On the picture below on left side is a diagram of sending transaction messages. Before sending message it checks whether cut is running. If it is then wraps the message, otherwise send ordinary message (PrepareRequest in example).

   Image Removed

   Wrapping messages

   Ignite transaction protocol includes multiple messages. But only some of them affects meaningful (relating to the algorithm) that change state of transactions (PREPARED, COMMITTED):

   ...

   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:

   1. GridNearTxFinishRequest / GridDhtTxFinishRequest
   2. 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. 

   
   

   Code Block
   language java title ConsistentCutAwareMessage
   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; }

   Also some messages require to be signed with additional ConsistentCutId to check it them on primary/backup node.

   1. GridNearTxFinishRequest / GridDhtTxFinishRequest
   2. GridNearTxPrepareResponse / GridDhtTxPrepareResponse (for 1PC algorithm).

   ...

   
   

   Transaction

   A new field added to IgniteInternalTx

   Code Block
   language java title IgniteInternalTx
   class IgniteInternalTx { /** * @param ID of {@link ConsistentCut} AFTER which this transaction was committed, {@code null} if transaction * committed BEFORE. */ public void cutId(@Nullable UUID id); }

   Consistent Cut Classes

   Code Block
   language java title ConsistentCutManager
   // Class is responsible for managing all stuff related to Consistent Cut. It's an entrypoint for transaction threads to check running consistent cut. class ConsistentCutManager extends GridCacheSharedManagerAdapter {           // Current Consistent Cut. All transactions threads wraps outgoing messages if this field is not null. */ volatile @Nullable ConsistentCut cut; // Entrypoint for handling received new Consistent Cut ID. void handleConsistentCutId(UUID id); }

   
   

   Code Block
   language java title ConsistentCutAwareTxFinishMessageConsistentCut
   class ConsistentCutAwareTransactionFinishMessageConsistentCut extends ConsistentCutAwareMessageGridFutureAdapter<WALPointer> { /** Consistent  Cut  ID  after which transaction committed. */ Set<GridCacheVersion> beforeCut; @Nullable UUID txCutIdSet<GridCacheVersion> afterCut; Set<IgniteInternalFuture<IgniteInternalTx>> removedActive; }

   Links

   1. ON DISTRIBUTED SNAPSHOTS, Ten H. LAI and Tao H. YANG, 29 May 1987