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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>
namespace leveldb {
class Slice;
class Logger;
// The Merge Operator interface.
// Client needs to provide an object implementing this interface if Merge
// operation is accessed.
// Essentially, MergeOperator specifies the SEMANTICS of a merge, which only
// client knows. It could be numeric addition, list append, string
// concatenation, ... , anything.
// The library, on the other hand, is concerned with the exercise of this
// interface, at the right time (during get, iteration, compaction...)
// Note that, even though in principle we don't require any special property
// of the merge operator, the current rocksdb compaction order does imply that
// an associative operator could be exercised more naturally (and more
// efficiently).
//
// Refer to my_test.cc for an example of implementation
//
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
// value: (IN) The passed-in merge operand value (when Merge is issued)
// new_value:(OUT) Client is responsible for filling the merge result here
// logger: (IN) Client could use this to log errors during merge.
//
// Note: Merge does not return anything to indicate if a merge is successful
// or not.
// Rationale: If a merge failed due to, say de-serialization error, we still
// need to define a consistent merge result. Should we throw away
// the existing value? the merge operand? Or reset the merged value
// to sth? The rocksdb library is not in a position to make the
// right choice. On the other hand, client knows exactly what
// happened during Merge, thus is able to make the best decision.
// Just save the final decision in new_value. logger is passed in,
// in case client wants to leave a trace of what went wrong.
virtual void Merge(const Slice& key,
const Slice* existing_value,
const Slice& value,
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;
};
} // namespace leveldb
#endif // STORAGE_LEVELDB_INCLUDE_MERGE_OPERATOR_H_