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/**
* Copyright (C) 2016 MongoDB 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/>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the GNU Affero General Public License in all respects
* for all of the code used other than as permitted herein. If you modify
* file(s) with this exception, you may extend this exception to your
* version of the file(s), but you are not obligated to do so. If you do not
* wish to do so, delete this exception statement from your version. If you
* delete this exception statement from all source files in the program,
* then also delete it in the license file.
*/
#pragma once
#include <string>
#include <vector>
#include "mongo/s/balancer/balancer_chunk_selection_policy.h"
#include "mongo/util/background.h"
#include "mongo/util/timer.h"
namespace mongo {
class ChunkType;
class ClusterStatistics;
class MigrationSecondaryThrottleOptions;
class OperationContext;
template <typename T>
class StatusWith;
/**
* 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 distributed lock
* 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();
~Balancer();
/**
* Retrieves the per-service instance of the Balancer.
*/
static Balancer* get(OperationContext* operationContext);
/**
* Blocking call, which requests the balancer to move a single chunk to a more appropriate
* shard, in accordance with the active balancer policy. It is not guaranteed that the chunk
* will actually move because it may already be at the best shard. An error will be returned if
* the attempt to find a better shard or the actual migration fail for any reason.
*/
Status rebalanceSingleChunk(OperationContext* txn, const ChunkType& chunk);
private:
// BackgroundJob methods implementation
void run() override;
std::string name() const override {
return "Balancer";
}
/**
* 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(OperationContext* txn);
/**
* Marks this balancer as being live on the config server(s).
*/
void _ping(OperationContext* txn, bool waiting);
/**
* Returns true if all the servers listed in configdb as being shards are reachable and are
* distinct processes (no hostname mixup).
*/
bool _checkOIDs(OperationContext* txn);
/**
* Iterates through all chunks in all collections and ensures that no chunks straddle tag
* boundary. If any do, they will be split.
*/
Status _enforceTagRanges(OperationContext* txn);
/**
* Issues chunk migration request, one at a time.
*
* @param candidateChunks possible chunks to move
* @param writeConcern detailed write concern. NULL means the default write concern.
* @param waitForDelete wait for deletes to complete after each chunk move
* @return number of chunks effectively moved
*/
int _moveChunks(OperationContext* txn,
const BalancerChunkSelectionPolicy::MigrateInfoVector& candidateChunks,
const MigrationSecondaryThrottleOptions& secondaryThrottle,
bool waitForDelete);
// hostname:port of my mongos
std::string _myid;
// Time the Balancer started running
Timer _timer;
// number of moved chunks in last round
int _balancedLastTime;
// Balancer policy
std::unique_ptr<BalancerChunkSelectionPolicy> _chunkSelectionPolicy;
// Source for cluster statistics
std::unique_ptr<ClusterStatistics> _clusterStats;
};
} // namespace mongo
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