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/**
* Copyright (C) 2021-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 "mongo/db/namespace_string.h"
#include "mongo/db/operation_context.h"
namespace mongo {
/**
* Given a chunk, determines whether it satisfies the requisites to be auto-splitted and - if so -
* returns the split points (shard keys representing the lower bounds of the new chunks to create)
* along with a bool representing whether or not the max bson size for the vector of split points
* was reached. If the bool returns true, this function should be called on the same range again to
* retrieve the rest of the split points.
*
* The logic implemented can be summarized as follows: given a `maxChunkSize` of `x` MB, the
* algorithm aims to choose the split points so that the resulting chunks' size would be around
* `maxChunkSize`. As it is too expensive to precisely determine the dimension of a chunk, it is
* assumed a uniform distribution of document sizes, hence the aim is to balance the number of
* documents per chunk.
*
* ======= ALGORITHM DESCRIPTION =======
*
* The split points for a chunk `C` belonging to a collection `coll` are calculated as follows:
* - `averageDocumentSize` = `totalCollSizeOnShard / numberOfCollDocs`
* - `maxNumberOfDocsPerChunk` = `maxChunkSize / averageDocumentSize`
* - Scan forward the shard key index entries for `coll` that are belonging to chunk `C`:
* - (1) Choose a split point every `maxNumberOfDocsPerChunk` scanned keys.
* - (2) As it needs to be avoided the creation of small chunks, consider the number of documents
* `S` that the right-most chunk would contain given the calculated split points:
* --- (2.1) IF `S >= 80% maxNumberOfDocsPerChunk`, return the list of calculated split points.
* --- (2.2) ELSE IF `S` documents could be fairly redistributed in the last chunks so that their
* size would be at least `67% maxNumberOfDocsPerChunk`: recalculate the last split points (max 3).
* --- (2.3) ELSE simply remove the last split point and keep a bigger last chunk.
*
*
* ============== EXAMPLES =============
*
* ========= EXAMPLE (CASE 2.1) ========
* `maxChunkSize` = 100MB
* `averageDocumentSize` = 1MB
* `maxNumberOfDocsPerChunk` = 100
*
* Shard key type: integer
* Chunk `C` bounds: `[0, maxKey)` . Chunk `C` contains 190 documents with shard keys [0...189].
*
* (1) Initially calculated split points: [99].
* (2) The last chunk would contain the interval `[99-189]` so `S = 90`
* (2.1) `S >= 80% maxChunkSize`, so keep the current split points.
*
* Returned split points: [99].
* Returned continuation flag: false.
*
* ========= EXAMPLE (CASE 2.2) ========
* `maxChunkSize` = 100MB
* `averageDocumentSize` = 1MB
* `maxNumberOfDocsPerChunk` = 100
*
* Shard key type: integer
* Chunk `C` bounds: `[0, maxKey)` . Chunk `C` contains 140 documents with shard keys [0...139].
*
* (1) Initially calculated split points: [99].
* (2) The last chunk would contain the interval `[99-139]` so `S = 40`
* (2.2) `S` documents can be redistributed on the last split point by generating chunks of size >=
* 67% maxChunkSize. Recalculate.
*
* Returned split points: [69].
* Returned continuation flag: false.
*
* ========= EXAMPLE (CASE 2.3) ========
* `maxChunkSize` = 100MB
* `averageDocumentSize` = 1MB
* `maxNumberOfDocsPerChunk` = 100
*
* Shard key type: integer
* Chunk `C` bounds: `[0, maxKey)` . Chunk `C` contains 120 documents with shard keys [0...119].
*
* (1) Initially calculated split points: [99].
* (2) The last chunk would contain the interval `[99-119]` so `S = 20`
* (2.3) `S` documents can't be redistributed on the last split point by generating chunks of size
* >= 67% maxChunkSize. So remove the last split point.
*
* Returned split points: [].
* Returned continuation flag: false.
*/
std::pair<std::vector<BSONObj>, bool> autoSplitVector(OperationContext* opCtx,
const NamespaceString& nss,
const BSONObj& keyPattern,
const BSONObj& min,
const BSONObj& max,
long long maxChunkSizeBytes,
boost::optional<int> limit = boost::none,
bool forward = true);
/*
* Utility function for deserializing autoSplitVector/splitVector responses.
*/
static std::vector<BSONObj> parseSplitKeys(const BSONElement& splitKeysArray) {
uassert(ErrorCodes::TypeMismatch,
"The split keys vector must be represented as a BSON array",
!splitKeysArray.eoo() && splitKeysArray.type() == BSONType::Array);
std::vector<BSONObj> splitKeys;
for (const auto& elem : splitKeysArray.Obj()) {
uassert(ErrorCodes::TypeMismatch,
"Each element of the split keys array must be an object",
elem.type() == BSONType::Object);
splitKeys.push_back(elem.embeddedObject().getOwned());
}
return splitKeys;
}
} // namespace mongo
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