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rocksdb/include/leveldb/merge_operator.h

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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#ifndef STORAGE_LEVELDB_INCLUDE_MERGE_OPERATOR_H_
#define STORAGE_LEVELDB_INCLUDE_MERGE_OPERATOR_H_
#include <string>
#include <deque>
#include "leveldb/slice.h" // TODO: Remove this when migration is done;
namespace leveldb {
class Slice;
class Logger;
// The Merge Operator
//
// Essentially, a MergeOperator specifies the SEMANTICS of a merge, which only
// client knows. It could be numeric addition, list append, string
// concatenation, edit data structure, ... , anything.
// The library, on the other hand, is concerned with the exercise of this
// interface, at the right time (during get, iteration, compaction...)
//
// To use merge, the client needs to provide an object implementing one of
// the following interfaces:
// a) AssociativeMergeOperator - for most simple semantics (always take
// two values, and merge them into one value, which is then put back
// into rocksdb); numeric addition and string concatenation are examples;
//
// b) MergeOperator - the generic class for all the more abstract / complex
// operations; one method (FullMerge) to merge a Put/Delete value with a
// merge operand; and another method (PartialMerge) that merges two
// operands together. this is especially useful if your key values have a
// complex structure but you would still like to support client-specific
// incremental updates.
//
// AssociativeMergeOperator is simpler to implement. MergeOperator is simply
// more powerful.
//
// Refer to rocksdb-merge wiki for more details and example implementations.
//
class MergeOperator {
public:
virtual ~MergeOperator() {}
// Gives the client a way to express the read -> modify -> write semantics
// key: (IN) The key that's associated with this merge operation.
// Client could multiplex the merge operator based on it
// if the key space is partitioned and different subspaces
// refer to different types of data which have different
// merge operation semantics
// existing: (IN) null indicates that the key does not exist before this op
// operand_list:(IN) the sequence of merge operations to apply, front() first.
// new_value:(OUT) Client is responsible for filling the merge result here
// logger: (IN) Client could use this to log errors during merge.
//
// Return true on success.
// All values passed in will be client-specific values. So if this method
// returns false, it is because client specified bad data or there was
// internal corruption. This will be treated as an error by the library.
//
// Also make use of the *logger for error messages.
virtual bool FullMerge(const Slice& key,
const Slice* existing_value,
const std::deque<std::string>& operand_list,
std::string* new_value,
Logger* logger) const = 0;
// This function performs merge(left_op, right_op)
// when both the operands are themselves merge operation types
// that you would have passed to a DB::Merge() call in the same order
// (i.e.: DB::Merge(key,left_op), followed by DB::Merge(key,right_op)).
//
// PartialMerge should combine them into a single merge operation that is
// saved into *new_value, and then it should return true.
// *new_value should be constructed such that a call to
// DB::Merge(key, *new_value) would yield the same result as a call
// to DB::Merge(key, left_op) followed by DB::Merge(key, right_op).
//
// If it is impossible or infeasible to combine the two operations,
// leave new_value unchanged and return false. The library will
// internally keep track of the operations, and apply them in the
// correct order once a base-value (a Put/Delete/End-of-Database) is seen.
//
// TODO: Presently there is no way to differentiate between error/corruption
// and simply "return false". For now, the client should simply return
// false in any case it cannot perform partial-merge, regardless of reason.
// If there is corruption in the data, handle it in the FullMerge() function,
// and return false there.
virtual bool PartialMerge(const Slice& key,
const Slice& left_operand,
const Slice& right_operand,
std::string* new_value,
Logger* logger) const = 0;
// The name of the MergeOperator. Used to check for MergeOperator
// mismatches (i.e., a DB created with one MergeOperator is
// accessed using a different MergeOperator)
// TODO: the name is currently not stored persistently and thus
// no checking is enforced. Client is responsible for providing
// consistent MergeOperator between DB opens.
virtual const char* Name() const = 0;
};
// The simpler, associative merge operator.
class AssociativeMergeOperator : public MergeOperator {
public:
virtual ~AssociativeMergeOperator() {}
// Gives the client a way to express the read -> modify -> write semantics
// key: (IN) The key that's associated with this merge operation.
// existing_value:(IN) null indicates the key does not exist before this op
// value: (IN) the value to update/merge the existing_value with
// new_value: (OUT) Client is responsible for filling the merge result here
// logger: (IN) Client could use this to log errors during merge.
//
// Return true on success.
// All values passed in will be client-specific values. So if this method
// returns false, it is because client specified bad data or there was
// internal corruption. The client should assume that this will be treated
// as an error by the library.
virtual bool Merge(const Slice& key,
const Slice* existing_value,
const Slice& value,
std::string* new_value,
Logger* logger) const = 0;
private:
// Default implementations of the MergeOperator functions
virtual bool FullMerge(const Slice& key,
const Slice* existing_value,
const std::deque<std::string>& operand_list,
std::string* new_value,
Logger* logger) const override;
virtual bool PartialMerge(const Slice& key,
const Slice& left_operand,
const Slice& right_operand,
std::string* new_value,
Logger* logger) const override;
};
} // namespace leveldb
#endif // STORAGE_LEVELDB_INCLUDE_MERGE_OPERATOR_H_