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diff --git a/src/mongo/s/balance.h b/src/mongo/s/balance.h
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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.
+        scoped_ptr<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;
+}