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//@file balance.h
/**
* Copyright (C) 2008 10gen Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* 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/>.
*/
#pragma once
#include "../pch.h"
#include "../util/background.h"
#include "../client/dbclient.h"
#include "balancer_policy.h"
namespace mongo {
/**
* The balancer is a background task that tries to keep the number of chunks across all servers of the cluster even. Although
* every mongos will have one balancer running, only one of them will be active at the any given point in time. The balancer
* uses a 'DistributedLock' for that coordination.
*
* The balancer does act continuously but in "rounds". At a given round, it would decide if there is an imbalance by
* checking the difference in chunks between the most and least loaded shards. It would issue a request for a chunk
* migration per round, if it found so.
*/
class Balancer : public BackgroundJob {
public:
Balancer();
virtual ~Balancer();
// BackgroundJob methods
virtual void run();
virtual string name() const { return "Balancer"; }
private:
typedef BalancerPolicy::ChunkInfo CandidateChunk;
typedef shared_ptr<CandidateChunk> CandidateChunkPtr;
// hostname:port of my mongos
string _myid;
// time the Balancer started running
time_t _started;
// number of moved chunks in last round
int _balancedLastTime;
// decide which chunks to move; owned here.
BalancerPolicy* _policy;
/**
* Checks that the balancer can connect to all servers it needs to do its job.
*
* @return true if balancing can be started
*
* This method throws on a network exception
*/
bool _init();
/**
* Gathers all the necessary information about shards and chunks, and decides whether there are candidate chunks to
* be moved.
*
* @param conn is the connection with the config server(s)
* @param candidateChunks (IN/OUT) filled with candidate chunks, one per collection, that could possibly be moved
*/
void _doBalanceRound( DBClientBase& conn, vector<CandidateChunkPtr>* candidateChunks );
/**
* Issues chunk migration request, one at a time.
*
* @param candidateChunks possible chunks to move
* @return number of chunks effectively moved
*/
int _moveChunks( const vector<CandidateChunkPtr>* candidateChunks );
/**
* Marks this balancer as being live on the config server(s).
*
* @param conn is the connection with the config server(s)
*/
void _ping( DBClientBase& conn );
/**
* @return true if all the servers listed in configdb as being shards are reachable and are distinct processes
*/
bool _checkOIDs();
};
extern Balancer balancer;
}
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