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// Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#pragma once
#include <cstdint>
#include "rocksdb/rocksdb_namespace.h"
#include "rocksdb/slice.h"
#include "table/unique_id_impl.h"
namespace ROCKSDB_NAMESPACE {
class Cache;
// A standard holder for fixed-size block cache keys (and for related caches).
// They are created through one of these, each using its own range of values:
// * CacheKey::CreateUniqueForCacheLifetime
// * CacheKey::CreateUniqueForProcessLifetime
// * Default ctor ("empty" cache key)
// * OffsetableCacheKey->WithOffset
//
// The first two use atomic counters to guarantee uniqueness over the given
// lifetime and the last uses a form of universally unique identifier for
// uniqueness with very high probabilty (and guaranteed for files generated
// during a single process lifetime).
//
// CacheKeys are currently used by calling AsSlice() to pass as a key to
// Cache. For performance, the keys are endianness-dependent (though otherwise
// portable). (Persistable cache entries are not intended to cross platforms.)
class CacheKey {
public:
// For convenience, constructs an "empty" cache key that is never returned
// by other means.
inline CacheKey() : file_num_etc64_(), offset_etc64_() {}
inline bool IsEmpty() const {
return (file_num_etc64_ == 0) & (offset_etc64_ == 0);
}
// Use this cache key as a Slice (byte order is endianness-dependent)
inline Slice AsSlice() const {
static_assert(sizeof(*this) == 16, "Standardized on 16-byte cache key");
assert(!IsEmpty());
return Slice(reinterpret_cast<const char *>(this), sizeof(*this));
}
// Create a CacheKey that is unique among others associated with this Cache
// instance. Depends on Cache::NewId. This is useful for block cache
// "reservations".
static CacheKey CreateUniqueForCacheLifetime(Cache *cache);
// Create a CacheKey that is unique among others for the lifetime of this
// process. This is useful for saving in a static data member so that
// different DB instances can agree on a cache key for shared entities,
// such as for CacheEntryStatsCollector.
static CacheKey CreateUniqueForProcessLifetime();
protected:
friend class OffsetableCacheKey;
CacheKey(uint64_t file_num_etc64, uint64_t offset_etc64)
: file_num_etc64_(file_num_etc64), offset_etc64_(offset_etc64) {}
uint64_t file_num_etc64_;
uint64_t offset_etc64_;
};
constexpr uint8_t kCacheKeySize = static_cast<uint8_t>(sizeof(CacheKey));
// A file-specific generator of cache keys, sometimes referred to as the
// "base" cache key for a file because all the cache keys for various offsets
// within the file are computed using simple arithmetic. The basis for the
// general approach is dicussed here: https://github.com/pdillinger/unique_id
// Heavily related to GetUniqueIdFromTableProperties.
//
// If the db_id, db_session_id, and file_number come from the file's table
// properties, then the keys will be stable across DB::Open/Close, backup/
// restore, import/export, etc.
//
// This class "is a" CacheKey only privately so that it is not misused as
// a ready-to-use CacheKey.
class OffsetableCacheKey : private CacheKey {
public:
// For convenience, constructs an "empty" cache key that should not be used.
inline OffsetableCacheKey() : CacheKey() {}
// Constructs an OffsetableCacheKey with the given information about a file.
// This constructor never generates an "empty" base key.
OffsetableCacheKey(const std::string &db_id, const std::string &db_session_id,
uint64_t file_number);
// Creates an OffsetableCacheKey from an SST unique ID, so that cache keys
// can be derived from DB manifest data before reading the file from
// storage--so that every part of the file can potentially go in a persistent
// cache.
//
// Calling GetSstInternalUniqueId() on a db_id, db_session_id, and
// file_number and passing the result to this function produces the same
// base cache key as feeding those inputs directly to the constructor.
//
// This is a bijective transformation assuming either id is empty or
// lower 64 bits is non-zero:
// * Empty (all zeros) input -> empty (all zeros) output
// * Lower 64 input is non-zero -> lower 64 output (file_num_etc64_) is
// non-zero
static OffsetableCacheKey FromInternalUniqueId(UniqueIdPtr id);
// This is the inverse transformation to the above, assuming either empty
// or lower 64 bits (file_num_etc64_) is non-zero. Perhaps only useful for
// testing.
UniqueId64x2 ToInternalUniqueId();
inline bool IsEmpty() const {
bool result = file_num_etc64_ == 0;
assert(!(offset_etc64_ > 0 && result));
return result;
}
// Construct a CacheKey for an offset within a file. An offset is not
// necessarily a byte offset if a smaller unique identifier of keyable
// offsets is used.
//
// This class was designed to make this hot code extremely fast.
inline CacheKey WithOffset(uint64_t offset) const {
assert(!IsEmpty());
return CacheKey(file_num_etc64_, offset_etc64_ ^ offset);
}
// The "common prefix" is a shared prefix for all the returned CacheKeys.
// It is specific to the file but the same for all offsets within the file.
static constexpr size_t kCommonPrefixSize = 8;
inline Slice CommonPrefixSlice() const {
static_assert(sizeof(file_num_etc64_) == kCommonPrefixSize,
"8 byte common prefix expected");
assert(!IsEmpty());
assert(&this->file_num_etc64_ == static_cast<const void *>(this));
return Slice(reinterpret_cast<const char *>(this), kCommonPrefixSize);
}
};
} // namespace ROCKSDB_NAMESPACE
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