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Followers.java
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Followers.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;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
import org.neo4j.coreedge.raft.state.follower.FollowerStates;
public class Followers
{
// TODO: This method is inefficient.. we should not have to update this state by a complete
// TODO: iteration each time. Instead it should be updated as a direct response to each
// TODO: append response.
public static <MEMBER> long quorumAppendIndex( Set<MEMBER> votingMembers, FollowerStates<MEMBER> states )
{
/*
* Build up a map of tx id -> number of instances that have appended,
* sorted by tx id.
*
* This allows us to then iterate backwards over the values in the map,
* adding up a total count of how many have appended, until we reach a majority.
* Once we do, the tx id at the current entry in the map will be the highest one
* with a majority appended.
*/
TreeMap</* txId */Long, /* numAppended */Integer> appendedCounts = new TreeMap<>();
for ( MEMBER member : votingMembers )
{
long txId = states.get( member ).getMatchIndex();
Integer currentCount = appendedCounts.get( txId );
if ( currentCount == null )
{
appendedCounts.put( txId, 1 );
}
else
{
appendedCounts.put( txId, currentCount + 1 );
}
}
// Iterate over it until we find a majority
int total = 0;
for ( Map.Entry<Long, Integer> entry : appendedCounts.descendingMap().entrySet() )
{
total += entry.getValue();
if ( MajorityIncludingSelfQuorum.isQuorum( votingMembers.size(), total ) )
{
return entry.getKey();
}
}
// No majority for any appended entry
return -1;
}
}