LevelDB的memtable（内存中的数据）

LevelDB的memtable（内存中的数据）

存储在内存中的memtable的结构是跳表，有一个逻辑，当memtable的size达到了阈值后，就会将memtable转变为只读的imutable memtable，然后新生成一个memtable

代码文件：db/memtable.h，db/memtable.cc

namespace leveldb {

class InternalKeyComparator;
class MemTableIterator;

class MemTable {
 public:
  // MemTables are reference counted.  The initial reference count
  // is zero and the caller must call Ref() at least once.
  // 构造函数的参数就是比较器
  explicit MemTable(const InternalKeyComparator& comparator);

  MemTable(const MemTable&) = delete;
  MemTable& operator=(const MemTable&) = delete;

  // 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. It is safe to call when MemTable is being modified.
  // 这个函数是记录内存池的用量，其实就是memtable的大小
  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.
  // 在memtable封装了一个跳表的接口，通过这个迭代器，可以操作memtable的跳表
  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.
  // 将key和value转换成InternalKey后存储在跳表里面
  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.
  // 查询memtable，其实就是查询跳表
  bool Get(const LookupKey& key, std::string* value, Status* s);

 private:
  friend class MemTableIterator;
  friend class MemTableBackwardIterator;

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

  typedef SkipList<const char*, KeyComparator> Table;

  ~MemTable();  // Private since only Unref() should be used to delete it

  KeyComparator comparator_;
  int refs_;
  Arena arena_;
  Table table_;
};

}  // namespace leveldb

namespace leveldb {

// p+5就是limit，ptr>=p && ptr<p+5；所以一共就是读取前面4个字节
static Slice GetLengthPrefixedSlice(const char* data) {
  uint32_t len;
  const char* p = data;
  // 其实正常情况下，返回的p就是等于之前的p+5，也就是数据开始的地方
  p = GetVarint32Ptr(p, p + 5, &len);  // +5: we assume "p" is not corrupted
  // 由数据长度，读取数据，然后返回数据
  return Slice(p, len);
}

MemTable::MemTable(const InternalKeyComparator& comparator)
    // table_就是跳表，他的构造函数函数就是比较器和内存池
    : comparator_(comparator), refs_(0), table_(comparator_, &arena_) {}

MemTable::~MemTable() { assert(refs_ == 0); }
// 返回内存池的用量，其实就是memtable的大小
size_t MemTable::ApproximateMemoryUsage() { return arena_.MemoryUsage(); }

int MemTable::KeyComparator::operator()(const char* aptr,
                                        const char* bptr) const {
  // Internal keys are encoded as length-prefixed strings.
  Slice a = GetLengthPrefixedSlice(aptr);
  Slice b = GetLengthPrefixedSlice(bptr);
  return comparator.Compare(a, b);
}

// Encode a suitable internal key target for "target" and return it.
// Uses *scratch as scratch space, and the returned pointer will point
// into this scratch space.
static const char* EncodeKey(std::string* scratch, const Slice& target) {
  scratch->clear();
  PutVarint32(scratch, target.size());
  scratch->append(target.data(), target.size());
  return scratch->data();
}

// 重新封装了一个迭代器，就是一些操作跳表的接口
class MemTableIterator : public Iterator {
 public:
  explicit MemTableIterator(MemTable::Table* table) : iter_(table) {}

  MemTableIterator(const MemTableIterator&) = delete;
  MemTableIterator& operator=(const MemTableIterator&) = delete;

  ~MemTableIterator() override = default;

  bool Valid() const override { return iter_.Valid(); }
  void Seek(const Slice& k) override { iter_.Seek(EncodeKey(&tmp_, k)); }
  void SeekToFirst() override { iter_.SeekToFirst(); }
  void SeekToLast() override { iter_.SeekToLast(); }
  void Next() override { iter_.Next(); }
  void Prev() override { iter_.Prev(); }
  Slice key() const override { return GetLengthPrefixedSlice(iter_.key()); }
  Slice value() const override {
    Slice key_slice = GetLengthPrefixedSlice(iter_.key());
    return GetLengthPrefixedSlice(key_slice.data() + key_slice.size());
  }

  Status status() const override { return Status::OK(); }

 private:
  MemTable::Table::Iterator iter_;
  std::string tmp_;  // For passing to EncodeKey
};

Iterator* MemTable::NewIterator() { return new MemTableIterator(&table_); }

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()]
  // 由user_key到internalkey的大小，其实就是user_key.size()+8
  size_t key_size = key.size();
  size_t val_size = value.size();
  size_t internal_key_size = key_size + 8;
  // VarintLength的返回结果是int编码后的所用的长度，就是低7位存值，第8位为1说明数据没结束
  // encoded_len就是internal+value以及他们各自长度的所占的总字节数
  const size_t encoded_len = VarintLength(internal_key_size) +
                             internal_key_size + VarintLength(val_size) +
                             val_size;
  // 向内存池申请内存
  char* buf = arena_.Allocate(encoded_len);
  // 编码internalkey的长度
  char* p = EncodeVarint32(buf, internal_key_size);
  // 写入user_key
  std::memcpy(p, key.data(), key_size);
  p += key_size;
  // 写入序号和类型
  EncodeFixed64(p, (s << 8) | type);
  p += 8;
  // 编码value的长度
  p = EncodeVarint32(p, val_size);
  // 写入value
  std::memcpy(p, value.data(), val_size);
  assert(p + val_size == buf + encoded_len);
  // 将编码后的数据插入跳表
  table_.Insert(buf);
}

bool MemTable::Get(const LookupKey& key, std::string* value, Status* s) {
  // 获取memtable_key，就是长度加user_key+序号+类型
  Slice memkey = key.memtable_key();
  // 迭代器
  Table::Iterator iter(&table_);
  // 
  iter.Seek(memkey.data());
  if (iter.Valid()) {
    // 下面就是查出的所有结果
    // 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();
    uint32_t key_length;
    const char* key_ptr = GetVarint32Ptr(entry, entry + 5, &key_length);
    if (comparator_.comparator.user_comparator()->Compare(
            Slice(key_ptr, key_length - 8), key.user_key()) == 0) {
       // 比较得到user_key相同，说明在memtable中找到了正确的key
      // Correct user key
      // 获取序号加类型
      const uint64_t tag = DecodeFixed64(key_ptr + key_length - 8);
      switch (static_cast<ValueType>(tag & 0xff)) {
        case kTypeValue: {
          Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
          value->assign(v.data(), v.size());
          return true;
        }
        case kTypeDeletion:
          // 这是被删掉的key
          *s = Status::NotFound(Slice());
          return true;
      }
    }
  }
  return false;
}

}  // namespace leveldb