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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* 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
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* 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 Server Side 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 <set>
#include <vector>
#include "mongo/base/disallow_copying.h"
#include "mongo/bson/bsonobj.h"
#include "mongo/bson/simple_bsonobj_comparator.h"
#include "mongo/db/namespace_string.h"
#include "mongo/db/s/balancer/cluster_statistics.h"
#include "mongo/s/catalog/type_chunk.h"
#include "mongo/s/shard_id.h"
namespace mongo {
struct ZoneRange {
ZoneRange(const BSONObj& a_min, const BSONObj& a_max, const std::string& _zone);
std::string toString() const;
BSONObj min;
BSONObj max;
std::string zone;
};
struct MigrateInfo {
MigrateInfo(const ShardId& a_to, const ChunkType& a_chunk);
std::string getName() const;
std::string toString() const;
NamespaceString nss;
ShardId to;
ShardId from;
BSONObj minKey;
BSONObj maxKey;
ChunkVersion version;
};
typedef std::vector<ClusterStatistics::ShardStatistics> ShardStatisticsVector;
typedef std::map<ShardId, std::vector<ChunkType>> ShardToChunksMap;
/**
* This class constitutes a cache of the chunk distribution across the entire cluster along with the
* zone boundaries imposed on it. This information is stored in format, which makes it efficient to
* query utilization statististics and to decide what to balance.
*/
class DistributionStatus {
MONGO_DISALLOW_COPYING(DistributionStatus);
public:
DistributionStatus(NamespaceString nss, ShardToChunksMap shardToChunksMap);
DistributionStatus(DistributionStatus&&) = default;
/**
* Returns the namespace for which this balance status applies.
*/
const NamespaceString& nss() const {
return _nss;
}
/**
* Appends the specified range to the set of ranges tracked for this collection and checks if
* it overlaps with existing ranges.
*/
Status addRangeToZone(const ZoneRange& range);
/**
* Returns total number of chunks across all shards.
*/
size_t totalChunks() const;
/**
* Returns the total number of chunks across all shards, which fall into the specified zone's
* range.
*/
size_t totalChunksWithTag(const std::string& tag) const;
/**
* Returns number of chunks in the specified shard.
*/
size_t numberOfChunksInShard(const ShardId& shardId) const;
/**
* Returns number of chunks in the specified shard, which have the given tag.
*/
size_t numberOfChunksInShardWithTag(const ShardId& shardId, const std::string& tag) const;
/**
* Returns all chunks for the specified shard.
*/
const std::vector<ChunkType>& getChunks(const ShardId& shardId) const;
/**
* Returns all tag ranges defined for the collection.
*/
const BSONObjIndexedMap<ZoneRange>& tagRanges() const {
return _zoneRanges;
}
/**
* Returns all tags defined for the collection.
*/
const std::set<std::string>& tags() const {
return _allTags;
}
/**
* Using the set of tags defined for the collection, returns what tag corresponds to the
* specified chunk. If the chunk doesn't fall into any tag returns the empty string.
*/
std::string getTagForChunk(const ChunkType& chunk) const;
/**
* Returns a BSON/string representation of this distribution status.
*/
void report(BSONObjBuilder* builder) const;
std::string toString() const;
private:
// Namespace for which this distribution applies
NamespaceString _nss;
// Map of what chunks are owned by each shard
ShardToChunksMap _shardChunks;
// Map of zone max key to the zone description
BSONObjIndexedMap<ZoneRange> _zoneRanges;
// Set of all zones defined for this collection
std::set<std::string> _allTags;
};
class BalancerPolicy {
public:
/**
* Determines whether a shard with the specified utilization statistics would be able to accept
* a chunk with the specified tag. According to the policy a shard cannot accept chunks if its
* size is maxed out and if the chunk's tag conflicts with the tag of the shard.
*/
static Status isShardSuitableReceiver(const ClusterStatistics::ShardStatistics& stat,
const std::string& chunkTag);
/**
* Returns a suggested set of chunks to move whithin a collection's shards, given the specified
* state of the shards (draining, max size reached, etc) and the number of chunks for that
* collection. If the policy doesn't recommend anything to move, it returns an empty vector. The
* entries in the vector do are all for separate source/destination shards and as such do not
* need to be done serially and can be scheduled in parallel.
*
* The balancing logic calculates the optimum number of chunks per shard for each zone and if
* any of the shards have chunks, which are sufficiently higher than this number, suggests
* moving chunks to shards, which are under this number.
*
* The usedShards parameter is in/out and it contains the set of shards, which have already been
* used for migrations. Used so we don't return multiple conflicting migrations for the same
* shard.
*/
static std::vector<MigrateInfo> balance(const ShardStatisticsVector& shardStats,
const DistributionStatus& distribution,
std::set<ShardId>* usedShards);
/**
* Using the specified distribution information, returns a suggested better location for the
* specified chunk if one is available.
*/
static boost::optional<MigrateInfo> balanceSingleChunk(const ChunkType& chunk,
const ShardStatisticsVector& shardStats,
const DistributionStatus& distribution);
private:
/**
* Return the shard with the specified tag, which has the least number of chunks. If the tag is
* empty, considers all shards.
*/
static ShardId _getLeastLoadedReceiverShard(const ShardStatisticsVector& shardStats,
const DistributionStatus& distribution,
const std::string& tag,
const std::set<ShardId>& excludedShards);
/**
* Return the shard which has the least number of chunks with the specified tag. If the tag is
* empty, considers all chunks.
*/
static ShardId _getMostOverloadedShard(const ShardStatisticsVector& shardStats,
const DistributionStatus& distribution,
const std::string& chunkTag,
const std::set<ShardId>& excludedShards);
/**
* Selects one chunk for the specified zone (if appropriate) to be moved in order to bring the
* deviation of the shards chunk contents closer to even across all shards in the specified
* zone. Takes into account and updates the shards, which have already been used for migrations.
*
* The 'idealNumberOfChunksPerShardForTag' indicates what is the ideal number of chunks which
* each shard must have and is used to determine the imbalance and also to prevent chunks from
* moving when not necessary.
*
* Returns true if a migration was suggested, false otherwise. This method is intented to be
* called multiple times until all posible migrations for a zone have been selected.
*/
static bool _singleZoneBalance(const ShardStatisticsVector& shardStats,
const DistributionStatus& distribution,
const std::string& tag,
size_t idealNumberOfChunksPerShardForTag,
std::vector<MigrateInfo>* migrations,
std::set<ShardId>* usedShards);
};
} // namespace mongo
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