rocksdb/include/rocksdb/cache.h

// Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree. An additional grant
// of patent rights can be found in the PATENTS file in the same directory.
//
// 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.
//
// A Cache is an interface that maps keys to values.  It has internal
// synchronization and may be safely accessed concurrently from
// multiple threads.  It may automatically evict entries to make room
// for new entries.  Values have a specified charge against the cache
// capacity.  For example, a cache where the values are variable
// length strings, may use the length of the string as the charge for
// the string.
//
// A builtin cache implementation with a least-recently-used eviction
// policy is provided.  Clients may use their own implementations if
// they want something more sophisticated (like scan-resistance, a
// custom eviction policy, variable cache sizing, etc.)

#pragma once

#include <stdint.h>
#include <memory>
#include "rocksdb/slice.h"
#include "rocksdb/statistics.h"
#include "rocksdb/status.h"

namespace rocksdb {

class Cache;

// Create a new cache with a fixed size capacity. The cache is sharded
// to 2^num_shard_bits shards, by hash of the key. The total capacity
// is divided and evenly assigned to each shard. If strict_capacity_limit
// is set, insert to the cache will fail when cache is full. User can also
// set percentage of the cache reserves for high priority entries via
// high_pri_pool_pct.
extern std::shared_ptr<Cache> NewLRUCache(size_t capacity,
                                          int num_shard_bits = 6,
                                          bool strict_capacity_limit = false,
                                          double high_pri_pool_ratio = 0.0);

// Similar to NewLRUCache, but create a cache based on CLOCK algorithm with
// better concurrent performance in some cases. See util/clock_cache.cc for
// more detail.
//
// Return nullptr if it is not supported.
extern std::shared_ptr<Cache> NewClockCache(size_t capacity,
                                            int num_shard_bits = 6,
                                            bool strict_capacity_limit = false);

class Cache {
 public:
  // Depending on implementation, cache entries with high priority could be less
  // likely to get evicted than low priority entries.
  enum class Priority { HIGH, LOW };

  Cache() {}

  // Destroys all existing entries by calling the "deleter"
  // function that was passed via the Insert() function.
  //
  // @See Insert
  virtual ~Cache() {}

  // Opaque handle to an entry stored in the cache.
  struct Handle {};

  // The type of the Cache
  virtual const char* Name() const = 0;

  // Insert a mapping from key->value into the cache and assign it
  // the specified charge against the total cache capacity.
  // If strict_capacity_limit is true and cache reaches its full capacity,
  // return Status::Incomplete.
  //
  // If handle is not nullptr, returns a handle that corresponds to the
  // mapping. The caller must call this->Release(handle) when the returned
  // mapping is no longer needed. In case of error caller is responsible to
  // cleanup the value (i.e. calling "deleter").
  //
  // If handle is nullptr, it is as if Release is called immediately after
  // insert. In case of error value will be cleanup.
  //
  // When the inserted entry is no longer needed, the key and
  // value will be passed to "deleter".
  virtual Status Insert(const Slice& key, void* value, size_t charge,
                        void (*deleter)(const Slice& key, void* value),
                        Handle** handle = nullptr,
                        Priority priority = Priority::LOW) = 0;

  // If the cache has no mapping for "key", returns nullptr.
  //
  // Else return a handle that corresponds to the mapping.  The caller
  // must call this->Release(handle) when the returned mapping is no
  // longer needed.
  // If stats is not nullptr, relative tickers could be used inside the
  // function.
  virtual Handle* Lookup(const Slice& key, Statistics* stats = nullptr) = 0;

  // Release a mapping returned by a previous Lookup().
  // REQUIRES: handle must not have been released yet.
  // REQUIRES: handle must have been returned by a method on *this.
  virtual void Release(Handle* handle) = 0;

  // Return the value encapsulated in a handle returned by a
  // successful Lookup().
  // REQUIRES: handle must not have been released yet.
  // REQUIRES: handle must have been returned by a method on *this.
  virtual void* Value(Handle* handle) = 0;

  // If the cache contains entry for key, erase it.  Note that the
  // underlying entry will be kept around until all existing handles
  // to it have been released.
  virtual void Erase(const Slice& key) = 0;
  // Return a new numeric id.  May be used by multiple clients who are
  // sharding the same cache to partition the key space.  Typically the
  // client will allocate a new id at startup and prepend the id to
  // its cache keys.
  virtual uint64_t NewId() = 0;

  // sets the maximum configured capacity of the cache. When the new
  // capacity is less than the old capacity and the existing usage is
  // greater than new capacity, the implementation will do its best job to
  // purge the released entries from the cache in order to lower the usage
  virtual void SetCapacity(size_t capacity) = 0;

  // Set whether to return error on insertion when cache reaches its full
  // capacity.
  virtual void SetStrictCapacityLimit(bool strict_capacity_limit) = 0;

  // Get the flag whether to return error on insertion when cache reaches its
  // full capacity.
  virtual bool HasStrictCapacityLimit() const = 0;

  // returns the maximum configured capacity of the cache
  virtual size_t GetCapacity() const = 0;

  // returns the memory size for the entries residing in the cache.
  virtual size_t GetUsage() const = 0;

  // returns the memory size for a specific entry in the cache.
  virtual size_t GetUsage(Handle* handle) const = 0;

  // returns the memory size for the entries in use by the system
  virtual size_t GetPinnedUsage() const = 0;

  // Call this on shutdown if you want to speed it up. Cache will disown
  // any underlying data and will not free it on delete. This call will leak
  // memory - call this only if you're shutting down the process.
  // Any attempts of using cache after this call will fail terribly.
  // Always delete the DB object before calling this method!
  virtual void DisownData(){
      // default implementation is noop
  };

  // Apply callback to all entries in the cache
  // If thread_safe is true, it will also lock the accesses. Otherwise, it will
  // access the cache without the lock held
  virtual void ApplyToAllCacheEntries(void (*callback)(void*, size_t),
                                      bool thread_safe) = 0;

  // Remove all entries.
  // Prerequisit: no entry is referenced.
  virtual void EraseUnRefEntries() = 0;

 private:
  // No copying allowed
  Cache(const Cache&);
  Cache& operator=(const Cache&);
};

}  // namespace rocksdb