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This component provides the means to set up a cluster of Neo4j instances that act together as a cluster, providing Master-Slave replication and other features.

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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 .
 */
package org.neo4j.kernel.ha.id;

import java.time.Clock;

import org.neo4j.kernel.ha.cluster.member.ClusterMembers;
import org.neo4j.kernel.ha.cluster.modeswitch.HighAvailabilityModeSwitcher;
import org.neo4j.kernel.impl.api.KernelTransactionsSnapshot;
import org.neo4j.kernel.impl.store.id.IdReuseEligibility;

/**
 * This {@link IdReuseEligibility} only buffer ids for reuse when we're the master.
 * This is mostly an optimization since when in slave role the ids are thrown away anyway.
 */
public class HaIdReuseEligibility implements IdReuseEligibility
{
    private final ClusterMembers members;
    private final Clock clock;
    private final long idReuseSafeZone;

    public HaIdReuseEligibility( ClusterMembers members, Clock clock, long idReuseSafeZone )
    {
        this.members = members;
        this.clock = clock;
        this.idReuseSafeZone = idReuseSafeZone;
    }

    @Override
    public boolean isEligible( KernelTransactionsSnapshot snapshot )
    {
        switch ( members.getCurrentMemberRole() )
        {
        case HighAvailabilityModeSwitcher.SLAVE:
            // If we're slave right now then just release them because the id generators in slave mode
            // will throw them away anyway, no need to keep them in memory. The architecture around
            // how buffering is done isn't a 100% fit for HA since the wrapping if IdGeneratorFactory
            // where the buffering takes place is done in a place which is oblivious to HA and roles
            // which means that buffering will always take place. For now we'll have to live with
            // always buffering and only just release them as soon as possible when slave.
            return true;
        case HighAvailabilityModeSwitcher.MASTER:
            // If we're master then we have to keep these ids around during the configured safe zone time
            // so that slaves have a chance to read consistently as well (slaves will know and compensate
            // for falling outside of safe zone).
            return clock.millis() - snapshot.snapshotTime() >= idReuseSafeZone;
        default:
            // If we're anything other than slave, i.e. also pending then retain the ids since we're
            // not quite sure what state we're in at the moment and we clear the id buffers anyway
            // during state switch.
            return false;
        }
    }
}