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103 Zeilen
3.9 KiB
103 Zeilen
3.9 KiB
// 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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//
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// A Cache is an interface that maps keys to values. It has internal
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// synchronization and may be safely accessed concurrently from
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// multiple threads. It may automatically evict entries to make room
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// for new entries. Values have a specified charge against the cache
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// capacity. For example, a cache where the values are variable
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// length strings, may use the length of the string as the charge for
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// the string.
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//
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// A builtin cache implementation with a least-recently-used eviction
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// policy is provided. Clients may use their own implementations if
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// they want something more sophisticated (like scan-resistance, a
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// custom eviction policy, variable cache sizing, etc.)
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#ifndef STORAGE_LEVELDB_INCLUDE_CACHE_H_
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#define STORAGE_LEVELDB_INCLUDE_CACHE_H_
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#include <cstdint>
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#include "leveldb/export.h"
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#include "leveldb/slice.h"
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namespace leveldb {
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class LEVELDB_EXPORT Cache;
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// Create a new cache with a fixed size capacity. This implementation
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// of Cache uses a least-recently-used eviction policy.
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LEVELDB_EXPORT Cache* NewLRUCache(size_t capacity);
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class LEVELDB_EXPORT Cache {
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public:
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Cache() = default;
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Cache(const Cache&) = delete;
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Cache& operator=(const Cache&) = delete;
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// Destroys all existing entries by calling the "deleter"
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// function that was passed to the constructor.
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virtual ~Cache();
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// Opaque handle to an entry stored in the cache.
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struct Handle {};
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// Insert a mapping from key->value into the cache and assign it
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// the specified charge against the total cache capacity.
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//
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// Returns a handle that corresponds to the mapping. The caller
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// must call this->Release(handle) when the returned mapping is no
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// longer needed.
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//
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// When the inserted entry is no longer needed, the key and
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// value will be passed to "deleter".
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virtual Handle* Insert(const Slice& key, void* value, size_t charge,
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void (*deleter)(const Slice& key, void* value)) = 0;
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// If the cache has no mapping for "key", returns nullptr.
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//
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// Else return a handle that corresponds to the mapping. The caller
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// must call this->Release(handle) when the returned mapping is no
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// longer needed.
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virtual Handle* Lookup(const Slice& key) = 0;
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// Release a mapping returned by a previous Lookup().
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// REQUIRES: handle must not have been released yet.
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// REQUIRES: handle must have been returned by a method on *this.
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virtual void Release(Handle* handle) = 0;
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// Return the value encapsulated in a handle returned by a
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// successful Lookup().
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// REQUIRES: handle must not have been released yet.
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// REQUIRES: handle must have been returned by a method on *this.
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virtual void* Value(Handle* handle) = 0;
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// If the cache contains entry for key, erase it. Note that the
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// underlying entry will be kept around until all existing handles
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// to it have been released.
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virtual void Erase(const Slice& key) = 0;
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// Return a new numeric id. May be used by multiple clients who are
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// sharing the same cache to partition the key space. Typically the
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// client will allocate a new id at startup and prepend the id to
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// its cache keys.
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virtual uint64_t NewId() = 0;
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// Remove all cache entries that are not actively in use. Memory-constrained
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// applications may wish to call this method to reduce memory usage.
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// Default implementation of Prune() does nothing. Subclasses are strongly
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// encouraged to override the default implementation. A future release of
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// leveldb may change Prune() to a pure abstract method.
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virtual void Prune() {}
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// Return an estimate of the combined charges of all elements stored in the
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// cache.
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virtual size_t TotalCharge() const = 0;
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};
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} // namespace leveldb
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#endif // STORAGE_LEVELDB_INCLUDE_CACHE_H_
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