ConsistentCut is a distributed algorithm that splits timeline WAL on 2 global areas - BEFORE Before and AFTER After. It guarantees that every transaction committed BEFORE committed Before also will be committed BEFORE Before on every other node participated in the transaction. It means that an Ignite node nodes can safely recover itself to this point themself to the consistent Before state without any coordination with other nodes.

Algorithm

each other.

The border between Before and After areas consists of two WAL records - ConsistentCutStartRecord and ConsistentCutFinishRecord. It guarantees that the Before consists of:

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

Image Added

/** */
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

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 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 ConsistentCutMarker ConsistentCutId (goal is to deliver the marker to notify every node in a cluster about running incremental snapshot). 
   2. DistributedProcess 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 ConsistentCutMarker by discoverythe SnapshotOperationRequest#ConsistentCutId by DiscoverySPI (by the DistributedProcess).
   2. Receive the ConsistentCutMarker by transaction message ( ConsistentCutAwareMessage#ConsistentCutId by CommunicationSPI (by transaction messages - Prepare, Finish).
4. On receiving the marker, every node: the ConsistentCutId 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 ConsistentCut has already started (ConsistentCut != null) or finished (lastFinishedCutId == id) for this id, skip if it has.
      2. 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.
   3. 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.
   4. ROLE#2 - for baseline nodes only:
      1. In the message thread atomically inits ConsistentCut
   5. In message thread atomically
      1. :
         1. creates new ConsistentCut
          future
      2. creates committingTx, goal is to track COMMITTING+ transactions, that aren't part of IgniteTxManager#activeTx
      3. starts signing outgoing messages with the ConsistentCutMarker.
      In
      1. 1. future.
         2. creates 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
         1. with the received
         ConsistentCutMarker 
         1. ConsistentCutId.
      3. Collects active transactions - concat of IgniteTxManager#activeTx and committingTxs .
5. While the DistributedProcess  is alive every node signs output transaction messages:
   1. Prepare messages signed with the ConsistentCutMarker  (to trigger ConsistentCut  on remote node, if not yet).
   2. Finish messages signed with the ConsistentCutMarker  (to trigger...) and transaction ConsistentCutMarker  (to notify nodes which side of cut this transaction belongs to).
   3. Finish messages is signed with transaction ConsistentCutMarker on node that commits first.
6. For every collected active transaction, node waits for Finish message, to extract the ConsistentCutMarker  and prepares before , after  collections:
   1. if received marker is null or differs from local, then transaction on before  side
   2. if received color equals to local, then transaction on after  side
7. 1. 1. 1. Creates a copy (weakly-consistent) of IgniteTxManager#activeTx. Set listeners on those tx#finishFuture.
            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 removedActiveTxs (contains transactions that are might be cleaned from IgniteTxManager#activeTx). Set listeners on those tx#finishFuture.
         3. 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.
      3. For every listening transaction, 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 topVer, then put it into after.
         3. if tx#txCutId equals to local, then put transaction into after, otherwise put into before.
      4. After every listening transaction finished:
         1. Writes a ConsistentCutFinishRecord  into WAL with the collections
   After all transactions finished:
   1. Writes a ConsistentCutFinishRecord  into WAL with the collections 
      1. 1. ( before, after ). 
   2. Stops filling committingTxs .
   3. 1. 1. Completes ConsistentCut  future
      , and notifies
      1. 1. .
      2. Notify a node-initiator about
      finishing
      1. local procedure has finished (
      with
      1. by DistributedProcess
       
      1. protocol).
8. After all nodes finished ConsistentCut, on every node:
   
   1. Updates lastFinishedCutId with the current id.
   2. ConsistentCut 
   :
   1. future becomes null.
   2. Stops
   3. every node stops signing outgoing transaction messages
   4. ConsistentCut  future becomes null.
   5. .
9. Node initiator checks that every node completes correctly.
   1. If any node complete exceptionally - complete Incremental Snapshot with exceptionIgnite node now in the initial state again.

Consistent and inconsistent Cuts

...

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. baseline topology change, Ignite nodes finishes local Cuts running in this moment, making them inconsistent.

ConsistentCutMarker

Every ignite nodes tracks current ConsistentCutMarker :

class ConsistentCutMarker {
	UUID id;
}

id is just a unique ConsistentCut  ID (is assigned on the node initiator).

1. .

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):

1. GridNearTxPrepareRequest / GridDhtTxPrepareRequest
2. GridNearTxPrepareResponse / GridDhtTxPrepareResponse

3. 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 signed with ConsistentCutMarkercombine with additional ConsistentCutId to check it them on primary/backup node:

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

WAL records

There are 2 records: ConsistentCutStartRecord  for Start event and ConsistentCutFinishRecord for Finish event. 

• ConsistentCutStartRecord: record is written to WAL in moment when CC starts on a local node. It helps to limit amout of active transactions to check. But there is no strict guarantee for all transactions belonged to the BEFORE side to be physically committed before ConsistentCutStartRecord, and vice versa. This is the reason for having ConsistentCutFinishRecord.
• ConsistentCutFinishRecord: This record is written to WAL after Consistent Cut stopped analyzing transactions and storing them in a particular bucket (BEFORE or AFTER).

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. */
public class ConsistentCutStartRecord extends WALRecord {
	Message msg;

	/** Consistent Cut ID. */
	UUID cutId;

 	/** Marker that inits Consistent CutConsistent Cut ID after which transaction committed. */
	private final ConsistentCutMarker marker;
}


/**    @Nullable UUID txCutId;

	/** Cluster topology version on which Consistent Cut started. */
	long topVer;
}

Transaction

A new field added to IgniteInternalTx

class IgniteInternalTx {     public class ConsistentCutFinishRecord extends WALRecord {
    /**
         * @param CollectionsID of TXs{@link committedConsistentCut} BEFOREAFTER thewhich ConsistentCutthis (senttransaction - received).
     */
    private final Set<GridCacheVersion> before;

was committed, {@code null} if transaction
     /**
     * Collections of TXs committed AFTER the ConsistentCut (exclude).
    *           committed BEFORE.
     */
    private final Set<GridCacheVersion> after;
 }

Unstable topology

There are some cases to handle for unstable topology:

1. Client or non-baseline server node leaves – no need to handle.
2. Server node leaves:
   1. all nodes finish local running ConsistentCut, making them in-consistent
3. Server node joins:
   1. all nodes finish local running ConsistentCut, making them in-consistent
   2. new node checks whether rebalance was required for recovering. If it is required, then handle it TBD

TBD: Which ways to use to avoid inconsistency between data and WAL after rebalance. There are options:

...

    public void cutId(@Nullable UUID id);
}

Consistent Cut Classes

// 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);
}

class ConsistentCut extends GridFutureAdapter<WALPointer> {          
	Set<GridCacheVersion> beforeCut;

    Set<GridCacheVersion> afterCut;

    Set<IgniteInternalFuture<IgniteInternalTx>> removedActive;
}

...

Links

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