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GlobalSessionTracker.java
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GlobalSessionTracker.java
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/*
* Copyright (c) 2002-2016 "Neo Technology,"
* Network Engine for Objects in Lund AB [http://neotechnology.com]
*
* This file is part of Neo4j.
*
* Neo4j is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as
* published by the Free Software Foundation, either version 3 of the
* License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package org.neo4j.coreedge.raft.replication.session;
import java.util.HashMap;
import java.util.Map;
import java.util.UUID;
import org.neo4j.coreedge.server.CoreMember;
/**
* Each instance has a global session as well as several local sessions. Each local session
* tracks its operation by assigning a unique sequence number to each operation. This allows
* an operation originating from an instance to be uniquely identified and duplicate attempts
* at performing that operation can be filtered out.
*
* The session tracker defines the strategy for which local operations are allowed to be performed
* and the strategy is to only allow operations to occur in strict order, that is with no gaps,
* starting with sequence number zero. This is done for reasons of efficiency and creates a very
* direct coupling between session tracking and operation validation. This class is in charge
* of both.
*/
public class GlobalSessionTracker
{
class LocalSessionTracker
{
UUID globalSessionId;
Map<Long,Long> lastSequenceNumberPerSession = new HashMap<>(); /* localSessionId -> lastSequenceNumber */
LocalSessionTracker( UUID globalSessionId )
{
this.globalSessionId = globalSessionId;
}
boolean validateAndTrackOperation( LocalOperationId operationId )
{
if ( !isValidOperation( operationId ) )
{
return false;
}
lastSequenceNumberPerSession.put( operationId.localSessionId(), operationId.sequenceNumber() );
return true;
}
/**
* The sequence numbers under a single local session must come strictly in order
* and are only valid exactly once.*/
private boolean isValidOperation( LocalOperationId operationId )
{
Long lastSequenceNumber = lastSequenceNumberPerSession.get( operationId.localSessionId() );
if ( lastSequenceNumber == null )
{
if ( operationId.sequenceNumber() != 0 )
{
return false;
}
}
else if ( operationId.sequenceNumber() != lastSequenceNumber + 1 )
{
return false;
}
return true;
}
}
/** Each owner can only have one local session tracker, identified by the unique global session ID. */
Map<CoreMember,LocalSessionTracker> sessionTrackers = new HashMap<>();
/** Tracks the operation and returns whether or not this operation should be allowed. */
public boolean validateAndTrackOperation( GlobalSession globalSession, LocalOperationId localOperationId )
{
LocalSessionTracker localSessionTracker = validateGlobalSessionAndGetLocalSessionTracker( globalSession );
return localSessionTracker.validateAndTrackOperation( localOperationId );
}
private LocalSessionTracker validateGlobalSessionAndGetLocalSessionTracker( GlobalSession globalSession )
{
LocalSessionTracker localSessionTracker = sessionTrackers.get( globalSession.owner() );
if( localSessionTracker == null )
{
localSessionTracker = new LocalSessionTracker( globalSession.sessionId() );
sessionTrackers.put( globalSession.owner(), localSessionTracker );
}
else if ( !localSessionTracker.globalSessionId.equals( globalSession.sessionId() ) )
{
localSessionTracker = new LocalSessionTracker( globalSession.sessionId() );
sessionTrackers.put( globalSession.owner(), localSessionTracker );
}
return localSessionTracker;
}
}