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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 "mongo/base/disallow_copying.h"
#include "mongo/s/balancer/balancer_chunk_selection_policy.h"
#include "mongo/stdx/condition_variable.h"
#include "mongo/stdx/mutex.h"
#include "mongo/stdx/thread.h"
namespace mongo {
class ChunkType;
class ClusterStatistics;
class MigrationSecondaryThrottleOptions;
class OperationContext;
class ServiceContext;
class Status;
/**
* 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 {
MONGO_DISALLOW_COPYING(Balancer);
public:
Balancer();
~Balancer();
/**
* Instantiates an instance of the balancer and installs it on the specified service context.
* This method is not thread-safe and must be called only once when the service is starting.
*/
static void create(ServiceContext* serviceContext);
/**
* Retrieves the per-service instance of the Balancer.
*/
static Balancer* get(ServiceContext* serviceContext);
static Balancer* get(OperationContext* operationContext);
/**
* Starts the main balancer thread and returns immediately. If the thread was successfully
* started (or if it was already running), returns an OK status. Otherwise, an error is
* returned.
*
* Known errors include:
* ConflictingOperationInProgress - if the balancer is being shut down
*/
Status startThread(OperationContext* txn);
/**
* If the main balancer thread is running, requests it to stop and returns immediately without
* waiting for it to terminate. The join method must be called afterwards in order to wait for
* the thread to complete.
*/
void stopThread();
/**
* Must always be called after stop has been called and only then. Ensures that the balancer
* thread has terminated.
*/
void joinThread();
/**
* Potentially blocking method, which will return immediately if the balancer is not running a
* balancer round and will block until the current round completes otherwise.
*/
void joinCurrentRound(OperationContext* txn);
/**
* 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);
/**
* Blocking call, which requests the balancer to move a single chunk to the specified location
* in accordance with the active balancer policy. An error will be returned if the attempt to
* move fails for any reason.
*
* NOTE: This call disregards the balancer enabled/disabled status and will proceed with the
* move regardless. If should be used only for user-initiated moves.
*/
Status moveSingleChunk(OperationContext* txn,
const ChunkType& chunk,
const ShardId& newShardId,
uint64_t maxChunkSizeBytes,
const MigrationSecondaryThrottleOptions& secondaryThrottle,
bool waitForDelete);
/**
* Appends the runtime state of the balancer instance to the specified builder.
*/
void report(OperationContext* txn, BSONObjBuilder* builder);
private:
/**
* Possible runtime states of the balancer. The comments indicate the allowed next state.
*/
enum State {
kStopped, // kRunning
kRunning, // kStopping
kStopping, // kStopped
};
/**
* The main balancer loop, which runs in a separate thread.
*/
void _mainThread();
/**
* Checks whether the balancer main thread has been requested to stop.
*/
bool _stopRequested();
/**
* Signals the beginning and end of a balancing round.
*/
void _beginRound(OperationContext* txn);
void _endRound(OperationContext* txn, Seconds waitTimeout);
/**
* Blocks the caller for the specified timeout or until the balancer condition variable is
* signaled, whichever comes first.
*/
void _sleepFor(OperationContext* txn, Seconds waitTimeout);
/**
* 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);
/**
* Schedules migrations for the specified set of chunks and returns how many chunks were
* successfully processed.
*/
int _moveChunks(OperationContext* txn,
const BalancerChunkSelectionPolicy::MigrateInfoVector& candidateChunks);
/**
* Performs a split on the chunk with min value "minKey". If the split fails, it is marked as
* jumbo.
*/
void _splitOrMarkJumbo(OperationContext* txn,
const NamespaceString& nss,
const BSONObj& minKey);
// The main balancer thread
stdx::thread _thread;
// Protects the state below
stdx::mutex _mutex;
// Indicates the current state of the balancer
State _state{kStopped};
// Indicates whether the balancer is currently executing a balancer round
bool _inBalancerRound{false};
// Counts the number of balancing rounds performed since the balancer thread was first activated
int64_t _numBalancerRounds{0};
// Condition variable, which is signalled every time the above runtime state of the balancer
// changes (in particular, state/balancer round and number of balancer rounds).
stdx::condition_variable _condVar;
// 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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