leveldb 值memtable

参考 1，http://mingxinglai.com/cn/2013/01/leveldb-memtable

参考 2，http://www.aikaiyuan.com/4591.html

具体结构如下图所示：

其主要的结构就是一个SkipList，list中没有将key和value区分开而是全部当成一个key，在从其中解析出相应的key与value。上图中有这么三个key：Lookupkey，internal_key, user_key. user_key指示的是用户真实的key。

class MemTable {
 public:
  // MemTables are reference counted.  The initial reference count
  // is zero and the caller must call Ref() at least once.
  explicit MemTable(const InternalKeyComparator& comparator);

  // Increase reference count.
  void Ref() { ++refs_; }

  // Drop reference count.  Delete if no more references exist.
  void Unref() {
    --refs_;
    assert(refs_ >= 0);
    if (refs_ <= 0) {
      delete this;
    }
  }

  // Returns an estimate of the number of bytes of data in use by this
  // data structure.
  //
  // REQUIRES: external synchronization to prevent simultaneous
  // operations on the same MemTable.
  size_t ApproximateMemoryUsage();

  // Return an iterator that yields the contents of the memtable.
  //
  // The caller must ensure that the underlying MemTable remains live
  // while the returned iterator is live.  The keys returned by this
  // iterator are internal keys encoded by AppendInternalKey in the
  // db/format.{h,cc} module.
  Iterator* NewIterator();

  // Add an entry into memtable that maps key to value at the
  // specified sequence number and with the specified type.
  // Typically value will be empty if type==kTypeDeletion.
  void Add(SequenceNumber seq, ValueType type,
           const Slice& key,
           const Slice& value);

  // If memtable contains a value for key, store it in *value and return true.
  // If memtable contains a deletion for key, store a NotFound() error
  // in *status and return true.
  // Else, return false.
  bool Get(const LookupKey& key, std::string* value, Status* s);

 private:
  ~MemTable();  // Private since only Unref() should be used to delete it  私有析构函数表示该结构不能再栈上定义只能在堆上。且不能调用delete删除。最终的删除在<span style="font-family: Arial, Helvetica, sans-serif;">Unref中delete this。 </span>

  struct KeyComparator {
    const InternalKeyComparator comparator;
    explicit KeyComparator(const InternalKeyComparator& c) : comparator(c) { }
    int operator()(const char* a, const char* b) const;
  };
  friend class MemTableIterator;
  friend class MemTableBackwardIterator;

  typedef SkipList<const char*, KeyComparator> Table;    //主要的结构跳跃表

  KeyComparator comparator_; //比较函数
  int refs_;
  Arena arena_;             //内存池
  Table table_;

  // No copying allowed
  MemTable(const MemTable&);
  void operator=(const MemTable&);
};

Add函数：

void MemTable::Add(SequenceNumber s, ValueType type,
                   const Slice& key,
                   const Slice& value) {
  // Format of an entry is concatenation of:
  //  key_size     : varint32 of internal_key.size()
  //  key bytes    : char[internal_key.size()]
  //  value_size   : varint32 of value.size()
  //  value bytes  : char[value.size()]
  size_t key_size = key.size();     
  size_t val_size = value.size();
  size_t internal_key_size = key_size + 8;
  const size_t encoded_len =
      VarintLength(internal_key_size) + internal_key_size +
      VarintLength(val_size) + val_size;   //总共需要分配的内存大小
  char* buf = arena_.Allocate(encoded_len);
  char* p = EncodeVarint32(buf, internal_key_size);   //写入<span style="font-family: Arial, Helvetica, sans-serif;">internal_key_size，后面给据这个来获得internal_key 最终获得user_key</span>

  memcpy(p, key.data(), key_size);           //写入user_key
  p += key_size;
  EncodeFixed64(p, (s << 8) | type);          // 写入sequence nu 和type
  p += 8;  // <span style="font-family: Arial, Helvetica, sans-serif;">sequence nu 和type 总共8个字节</span>
 
  p = EncodeVarint32(p, val_size);             //写入value_size          
  memcpy(p, value.data(), val_size);           //写入value
  assert((p + val_size) - buf == encoded_len);  //断言写入数据的正确性
  table_.Insert(buf);                           //插入SkipList
}

bool MemTable::Get(const LookupKey& key, std::string* value, Status* s) {
  Slice memkey = key.memtable_key();     //生成lookupkey

  Table::Iterator iter(&table_);          // 产生SkipList
  iter.Seek(memkey.data());               //找出该小于等于该key的值
  if (iter.Valid()) {
    // entry format is:
    //    klength  varint32
    //    userkey  char[klength]
    //    tag      uint64
    //    vlength  varint32
    //    value    char[vlength]
    // Check that it belongs to same user key.  We do not check the
    // sequence number since the Seek() call above should have skipped
    // all entries with overly large sequence numbers.
    const char* entry = iter.key();     //拿到存储内容(包含value)
    uint32_t key_length;
    const char* key_ptr = GetVarint32Ptr(entry, entry+5, &key_length);   //32位变长存储，limit是5个字节所以加了一个 5
    if (comparator_.comparator.user_comparator()->Compare(
            Slice(key_ptr, key_length - 8),                  //获取用户的真实user_key
            key.user_key()) == 0) {
      // Correct user key
      const uint64_t tag = DecodeFixed64(key_ptr + key_length - 8);           //查看该值得类型，取最后一个字节
      switch (static_cast<ValueType>(tag & 0xff)) { 
        case kTypeValue: {
          Slice v = GetLengthPrefixedSlice(key_ptr + key_length);              //获取该key的值，该函数和上面一样都是先获取value的size在进行取值
          value->assign(v.data(), v.size());
          return true;
        }
        case kTypeDeletion:
          *s = Status::NotFound(Slice());
          return true;
      }
    }
  }
  return false;
}