1 // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file. See the AUTHORS file for names of contributors.
5 // A Cache is an interface that maps keys to values. It has internal
6 // synchronization and may be safely accessed concurrently from
7 // multiple threads. It may automatically evict entries to make room
8 // for new entries. Values have a specified charge against the cache
9 // capacity. For example, a cache where the values are variable
10 // length strings, may use the length of the string as the charge for
13 // A builtin cache implementation with a least-recently-used eviction
14 // policy is provided. Clients may use their own implementations if
15 // they want something more sophisticated (like scan-resistance, a
16 // custom eviction policy, variable cache sizing, etc.)
18 #ifndef STORAGE_LEVELDB_INCLUDE_CACHE_H_
19 #define STORAGE_LEVELDB_INCLUDE_CACHE_H_
22 #include "leveldb/slice.h"
28 // Create a new cache with a fixed size capacity. This implementation
29 // of Cache uses a least-recently-used eviction policy.
30 extern Cache* NewLRUCache(size_t capacity);
36 // Destroys all existing entries by calling the "deleter"
37 // function that was passed to the constructor.
40 // Opaque handle to an entry stored in the cache.
43 // Insert a mapping from key->value into the cache and assign it
44 // the specified charge against the total cache capacity.
46 // Returns a handle that corresponds to the mapping. The caller
47 // must call this->Release(handle) when the returned mapping is no
50 // When the inserted entry is no longer needed, the key and
51 // value will be passed to "deleter".
52 virtual Handle* Insert(const Slice& key, void* value, size_t charge,
53 void (*deleter)(const Slice& key, void* value)) = 0;
55 // If the cache has no mapping for "key", returns NULL.
57 // Else return a handle that corresponds to the mapping. The caller
58 // must call this->Release(handle) when the returned mapping is no
60 virtual Handle* Lookup(const Slice& key) = 0;
62 // Release a mapping returned by a previous Lookup().
63 // REQUIRES: handle must not have been released yet.
64 // REQUIRES: handle must have been returned by a method on *this.
65 virtual void Release(Handle* handle) = 0;
67 // Return the value encapsulated in a handle returned by a
68 // successful Lookup().
69 // REQUIRES: handle must not have been released yet.
70 // REQUIRES: handle must have been returned by a method on *this.
71 virtual void* Value(Handle* handle) = 0;
73 // If the cache contains entry for key, erase it. Note that the
74 // underlying entry will be kept around until all existing handles
75 // to it have been released.
76 virtual void Erase(const Slice& key) = 0;
78 // Return a new numeric id. May be used by multiple clients who are
79 // sharing the same cache to partition the key space. Typically the
80 // client will allocate a new id at startup and prepend the id to
82 virtual uint64_t NewId() = 0;
85 void LRU_Remove(Handle* e);
86 void LRU_Append(Handle* e);
87 void Unref(Handle* e);
94 void operator=(const Cache&);
97 } // namespace leveldb
99 #endif // STORAGE_LEVELDB_UTIL_CACHE_H_