memtablerep.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.
//
// This file contains the interface that must be implemented by any collection
// to be used as the backing store for a MemTable. Such a collection must
// satisfy the following properties:
//  (1) It does not store duplicate items.
//  (2) It uses MemTableRep::KeyComparator to compare items for iteration and
//     equality.
//  (3) It can be accessed concurrently by multiple readers and can support
//     during reads. However, it needn't support multiple concurrent writes.
//  (4) Items are never deleted.
// The liberal use of assertions is encouraged to enforce (1).
//
// The factory will be passed an MemTableAllocator object when a new MemTableRep
// is requested.
//
// Users can implement their own memtable representations. We include three
// types built in:
//  - SkipListRep: This is the default; it is backed by a skip list.
//  - HashSkipListRep: The memtable rep that is best used for keys that are
//  structured like "prefix:suffix" where iteration within a prefix is
//  common and iteration across different prefixes is rare. It is backed by
//  a hash map where each bucket is a skip list.
//  - VectorRep: This is backed by an unordered std::vector. On iteration, the
// vector is sorted. It is intelligent about sorting; once the MarkReadOnly()
// has been called, the vector will only be sorted once. It is optimized for
// random-write-heavy workloads.
//
// The last four implementations are designed for situations in which
// iteration over the entire collection is rare since doing so requires all the
// keys to be copied into a sorted data structure.

#pragma once

#include <memory>
#include <stdexcept>
#include <stdint.h>
#include <stdlib.h>

namespace rocksdb {

class Arena;
class MemTableAllocator;
class LookupKey;
class Slice;
class SliceTransform;
class Logger;

typedef void* KeyHandle;

class MemTableRep {
 public:
  // KeyComparator provides a means to compare keys, which are internal keys
  // concatenated with values.
  class KeyComparator {
   public:
    // Compare a and b. Return a negative value if a is less than b, 0 if they
    // are equal, and a positive value if a is greater than b
    virtual int operator()(const char* prefix_len_key1,
                           const char* prefix_len_key2) const = 0;

    virtual int operator()(const char* prefix_len_key,
                           const Slice& key) const = 0;

    virtual ~KeyComparator() { }
  };

  explicit MemTableRep(MemTableAllocator* allocator) : allocator_(allocator) {}

  // Allocate a buf of len size for storing key. The idea is that a
  // specific memtable representation knows its underlying data structure
  // better. By allowing it to allocate memory, it can possibly put
  // correlated stuff in consecutive memory area to make processor
  // prefetching more efficient.
  virtual KeyHandle Allocate(const size_t len, char** buf);

  // Insert key into the collection. (The caller will pack key and value into a
  // single buffer and pass that in as the parameter to Insert).
  // REQUIRES: nothing that compares equal to key is currently in the
  // collection, and no concurrent modifications to the table in progress
  virtual void Insert(KeyHandle handle) = 0;

  // Like Insert(handle), but may be called concurrent with other calls
  // to InsertConcurrently for other handles
  virtual void InsertConcurrently(KeyHandle handle) {
#ifndef ROCKSDB_LITE
    throw std::runtime_error("concurrent insert not supported");
#else
    abort();
#endif
  }

  // Returns true iff an entry that compares equal to key is in the collection.
  virtual bool Contains(const char* key) const = 0;

  // Notify this table rep that it will no longer be added to. By default,
  // does nothing.  After MarkReadOnly() is called, this table rep will
  // not be written to (ie No more calls to Allocate(), Insert(),
  // or any writes done directly to entries accessed through the iterator.)
  virtual void MarkReadOnly() { }

  // Look up key from the mem table, since the first key in the mem table whose
  // user_key matches the one given k, call the function callback_func(), with
  // callback_args directly forwarded as the first parameter, and the mem table
  // key as the second parameter. If the return value is false, then terminates.
  // Otherwise, go through the next key.
  //
  // It's safe for Get() to terminate after having finished all the potential
  // key for the k.user_key(), or not.
  //
  // Default:
  // Get() function with a default value of dynamically construct an iterator,
  // seek and call the call back function.
  virtual void Get(const LookupKey& k, void* callback_args,
                   bool (*callback_func)(void* arg, const char* entry));

  virtual uint64_t ApproximateNumEntries(const Slice& start_ikey,
                                         const Slice& end_key) {
    return 0;
  }

  // Report an approximation of how much memory has been used other than memory
  // that was allocated through the allocator.  Safe to call from any thread.
  virtual size_t ApproximateMemoryUsage() = 0;

  virtual ~MemTableRep() { }

  // Iteration over the contents of a skip collection
  class Iterator {
   public:
    // Initialize an iterator over the specified collection.
    // The returned iterator is not valid.
    // explicit Iterator(const MemTableRep* collection);
    virtual ~Iterator() {}

    // Returns true iff the iterator is positioned at a valid node.
    virtual bool Valid() const = 0;

    // Returns the key at the current position.
    // REQUIRES: Valid()
    virtual const char* key() const = 0;

    // Advances to the next position.
    // REQUIRES: Valid()
    virtual void Next() = 0;

    // Advances to the previous position.
    // REQUIRES: Valid()
    virtual void Prev() = 0;

    // Advance to the first entry with a key >= target
    virtual void Seek(const Slice& internal_key, const char* memtable_key) = 0;

    // Position at the first entry in collection.
    // Final state of iterator is Valid() iff collection is not empty.
    virtual void SeekToFirst() = 0;

    // Position at the last entry in collection.
    // Final state of iterator is Valid() iff collection is not empty.
    virtual void SeekToLast() = 0;
  };

  // Return an iterator over the keys in this representation.
  // arena: If not null, the arena needs to be used to allocate the Iterator.
  //        When destroying the iterator, the caller will not call "delete"
  //        but Iterator::~Iterator() directly. The destructor needs to destroy
  //        all the states but those allocated in arena.
  virtual Iterator* GetIterator(Arena* arena = nullptr) = 0;

  // Return an iterator that has a special Seek semantics. The result of
  // a Seek might only include keys with the same prefix as the target key.
  // arena: If not null, the arena is used to allocate the Iterator.
  //        When destroying the iterator, the caller will not call "delete"
  //        but Iterator::~Iterator() directly. The destructor needs to destroy
  //        all the states but those allocated in arena.
  virtual Iterator* GetDynamicPrefixIterator(Arena* arena = nullptr) {
    return GetIterator(arena);
  }

  // Return true if the current MemTableRep supports merge operator.
  // Default: true
  virtual bool IsMergeOperatorSupported() const { return true; }

  // Return true if the current MemTableRep supports snapshot
  // Default: true
  virtual bool IsSnapshotSupported() const { return true; }

 protected:
  // When *key is an internal key concatenated with the value, returns the
  // user key.
  virtual Slice UserKey(const char* key) const;

  MemTableAllocator* allocator_;
};

// This is the base class for all factories that are used by RocksDB to create
// new MemTableRep objects
class MemTableRepFactory {
 public:
  virtual ~MemTableRepFactory() {}
  virtual MemTableRep* CreateMemTableRep(const MemTableRep::KeyComparator&,
                                         MemTableAllocator*,
                                         const SliceTransform*,
                                         Logger* logger) = 0;
  virtual const char* Name() const = 0;

  // Return true if the current MemTableRep supports concurrent inserts
  // Default: false
  virtual bool IsInsertConcurrentlySupported() const { return false; }
};

// This uses a skip list to store keys. It is the default.
//
// Parameters:
//   lookahead: If non-zero, each iterator's seek operation will start the
//     search from the previously visited record (doing at most 'lookahead'
//     steps). This is an optimization for the access pattern including many
//     seeks with consecutive keys.
class SkipListFactory : public MemTableRepFactory {
 public:
  explicit SkipListFactory(size_t lookahead = 0) : lookahead_(lookahead) {}

  virtual MemTableRep* CreateMemTableRep(const MemTableRep::KeyComparator&,
                                         MemTableAllocator*,
                                         const SliceTransform*,
                                         Logger* logger) override;
  virtual const char* Name() const override { return "SkipListFactory"; }

  bool IsInsertConcurrentlySupported() const override { return true; }

 private:
  const size_t lookahead_;
};

#ifndef ROCKSDB_LITE
// This creates MemTableReps that are backed by an std::vector. On iteration,
// the vector is sorted. This is useful for workloads where iteration is very
// rare and writes are generally not issued after reads begin.
//
// Parameters:
//   count: Passed to the constructor of the underlying std::vector of each
//     VectorRep. On initialization, the underlying array will be at least count
//     bytes reserved for usage.
class VectorRepFactory : public MemTableRepFactory {
  const size_t count_;

 public:
  explicit VectorRepFactory(size_t count = 0) : count_(count) { }
  virtual MemTableRep* CreateMemTableRep(const MemTableRep::KeyComparator&,
                                         MemTableAllocator*,
                                         const SliceTransform*,
                                         Logger* logger) override;
  virtual const char* Name() const override {
    return "VectorRepFactory";
  }
};

// This class contains a fixed array of buckets, each
// pointing to a skiplist (null if the bucket is empty).
// bucket_count: number of fixed array buckets
// skiplist_height: the max height of the skiplist
// skiplist_branching_factor: probabilistic size ratio between adjacent
//                            link lists in the skiplist
extern MemTableRepFactory* NewHashSkipListRepFactory(
    size_t bucket_count = 1000000, int32_t skiplist_height = 4,
    int32_t skiplist_branching_factor = 4
);

// The factory is to create memtables based on a hash table:
// it contains a fixed array of buckets, each pointing to either a linked list
// or a skip list if number of entries inside the bucket exceeds
// threshold_use_skiplist.
// @bucket_count: number of fixed array buckets
// @huge_page_tlb_size: if <=0, allocate the hash table bytes from malloc.
//                      Otherwise from huge page TLB. The user needs to reserve
//                      huge pages for it to be allocated, like:
//                          sysctl -w vm.nr_hugepages=20
//                      See linux doc Documentation/vm/hugetlbpage.txt
// @bucket_entries_logging_threshold: if number of entries in one bucket
//                                    exceeds this number, log about it.
// @if_log_bucket_dist_when_flash: if true, log distribution of number of
//                                 entries when flushing.
// @threshold_use_skiplist: a bucket switches to skip list if number of
//                          entries exceed this parameter.
extern MemTableRepFactory* NewHashLinkListRepFactory(
    size_t bucket_count = 50000, size_t huge_page_tlb_size = 0,
    int bucket_entries_logging_threshold = 4096,
    bool if_log_bucket_dist_when_flash = true,
    uint32_t threshold_use_skiplist = 256);

// This factory creates a cuckoo-hashing based mem-table representation.
// Cuckoo-hash is a closed-hash strategy, in which all key/value pairs
// are stored in the bucket array itself intead of in some data structures
// external to the bucket array.  In addition, each key in cuckoo hash
// has a constant number of possible buckets in the bucket array.  These
// two properties together makes cuckoo hash more memory efficient and
// a constant worst-case read time.  Cuckoo hash is best suitable for
// point-lookup workload.
//
// When inserting a key / value, it first checks whether one of its possible
// buckets is empty.  If so, the key / value will be inserted to that vacant
// bucket.  Otherwise, one of the keys originally stored in one of these
// possible buckets will be "kicked out" and move to one of its possible
// buckets (and possibly kicks out another victim.)  In the current
// implementation, such "kick-out" path is bounded.  If it cannot find a
// "kick-out" path for a specific key, this key will be stored in a backup
// structure, and the current memtable to be forced to immutable.
//
// Note that currently this mem-table representation does not support
// snapshot (i.e., it only queries latest state) and iterators.  In addition,
// MultiGet operation might also lose its atomicity due to the lack of
// snapshot support.
//
// Parameters:
//   write_buffer_size: the write buffer size in bytes.
//   average_data_size: the average size of key + value in bytes.  This value
//     together with write_buffer_size will be used to compute the number
//     of buckets.
//   hash_function_count: the number of hash functions that will be used by
//     the cuckoo-hash.  The number also equals to the number of possible
//     buckets each key will have.
extern MemTableRepFactory* NewHashCuckooRepFactory(
    size_t write_buffer_size, size_t average_data_size = 64,
    unsigned int hash_function_count = 4);
#endif  // ROCKSDB_LITE
}  // namespace rocksdb