75 lines
3.3 KiB
C
75 lines
3.3 KiB
C
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#ifndef STORAGE_LEVELDB_INCLUDE_MERGE_OPERATOR_H_
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#define STORAGE_LEVELDB_INCLUDE_MERGE_OPERATOR_H_
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#include <string>
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namespace leveldb {
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class Slice;
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class Logger;
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// The Merge Operator interface.
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// Client needs to provide an object implementing this interface if Merge
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// operation is accessed.
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// Essentially, MergeOperator specifies the SEMANTICS of a merge, which only
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// client knows. It could be numeric addition, list append, string
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// concatenation, ... , anything.
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// The library, on the other hand, is concerned with the exercise of this
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// interface, at the right time (during get, iteration, compaction...)
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// Note that, even though in principle we don't require any special property
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// of the merge operator, the current rocksdb compaction order does imply that
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// an associative operator could be exercised more naturally (and more
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// efficiently).
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//
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// Refer to my_test.cc for an example of implementation
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//
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class MergeOperator {
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public:
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virtual ~MergeOperator() {}
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// Gives the client a way to express the read -> modify -> write semantics
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// key: (IN) The key that's associated with this merge operation.
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// Client could multiplex the merge operator based on it
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// if the key space is partitioned and different subspaces
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// refer to different types of data which have different
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// merge operation semantics
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// existing: (IN) null indicates that the key does not exist before this op
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// value: (IN) The passed-in merge operand value (when Merge is issued)
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// new_value:(OUT) Client is responsible for filling the merge result here
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// logger: (IN) Client could use this to log errors during merge.
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//
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// Note: Merge does not return anything to indicate if a merge is successful
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// or not.
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// Rationale: If a merge failed due to, say de-serialization error, we still
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// need to define a consistent merge result. Should we throw away
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// the existing value? the merge operand? Or reset the merged value
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// to sth? The rocksdb library is not in a position to make the
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// right choice. On the other hand, client knows exactly what
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// happened during Merge, thus is able to make the best decision.
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// Just save the final decision in new_value. logger is passed in,
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// in case client wants to leave a trace of what went wrong.
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virtual void Merge(const Slice& key,
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const Slice* existing_value,
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const Slice& value,
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std::string* new_value,
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Logger* logger) const = 0;
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// The name of the MergeOperator. Used to check for MergeOperator
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// mismatches (i.e., a DB created with one MergeOperator is
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// accessed using a different MergeOperator)
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// TODO: the name is currently not stored persistently and thus
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// no checking is enforced. Client is responsible for providing
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// consistent MergeOperator between DB opens.
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virtual const char* Name() const = 0;
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};
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} // namespace leveldb
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#endif // STORAGE_LEVELDB_INCLUDE_MERGE_OPERATOR_H_
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