LevelDB的批处理（Batch）

主要是两个类，db/write_batch_internal.h，include/leveldb/write_batch.h，db/write_batch.cc

namespace leveldb {

class MemTable;

// WriteBatchInternal provides static methods for manipulating a
// WriteBatch that we don\'t want in the public WriteBatch interface.
// 一个batch的存储结构如下：
    // seq(8字节)+count(后面kv的个数，占4个字节)+kvs
    // 一个kv的存储结构
    // type(1个字节)+key_len(4字节)+key+value_len(4字节)+value
class WriteBatchInternal {
 public:
  // Return the number of entries in the batch.
  static int Count(const WriteBatch* batch);

  // Set the count for the number of entries in the batch.
  static void SetCount(WriteBatch* batch, int n);

  // Return the sequence number for the start of this batch.
  static SequenceNumber Sequence(const WriteBatch* batch);

  // Store the specified number as the sequence number for the start of
  // this batch.
  static void SetSequence(WriteBatch* batch, SequenceNumber seq);

  static Slice Contents(const WriteBatch* batch) { return Slice(batch->rep_); }

  static size_t ByteSize(const WriteBatch* batch) { return batch->rep_.size(); }
  // 用content中的内容替换batch中的内容
  static void SetContents(WriteBatch* batch, const Slice& contents);
  // 将batch中的所有条目插入memtable中，他们此时用的是同一个序号
  static Status InsertInto(const WriteBatch* batch, MemTable* memtable);
  // 向batch中增加条目
  static void Append(WriteBatch* dst, const WriteBatch* src);
};

}  // namespace leveldb

namespace leveldb {

class Slice;

class LEVELDB_EXPORT WriteBatch {
 public:
  class LEVELDB_EXPORT Handler {
   public:
    virtual ~Handler();
    virtual void Put(const Slice& key, const Slice& value) = 0;
    virtual void Delete(const Slice& key) = 0;
  };

  WriteBatch();

  // Intentionally copyable.
  WriteBatch(const WriteBatch&) = default;
  WriteBatch& operator=(const WriteBatch&) = default;

  ~WriteBatch();

  // Store the mapping "key->value" in the database.
  // 向batch中添加key-value
  void Put(const Slice& key, const Slice& value);

  // If the database contains a mapping for "key", erase it.  Else do nothing.
  // 向batch中添加一个删除的key，意思是类型为删除，只有key，没有value
  void Delete(const Slice& key);

  // Clear all updates buffered in this batch.
  void Clear();

  // The size of the database changes caused by this batch.
  //
  // This number is tied to implementation details, and may change across
  // releases. It is intended for LevelDB usage metrics.
  size_t ApproximateSize() const;

  // Copies the operations in "source" to this batch.
  //
  // This runs in O(source size) time. However, the constant factor is better
  // than calling Iterate() over the source batch with a Handler that replicates
  // the operations into this batch.
  // 将一个batch追加到末尾
  void Append(const WriteBatch& source);

  // Support for iterating over the contents of a batch.
  // 遍历batch中的所有的key-value，然后挨个处理他们
  Status Iterate(Handler* handler) const;

 private:
  friend class WriteBatchInternal;

  // 头部是8字节的序号seq+类型和4字节的batch里面条目的数量，所以初始头部长度为12字节
  std::string rep_;  // See comment in write_batch.cc for the format of rep_
};

}  // namespace leveldb

namespace leveldb {

// WriteBatch header has an 8-byte sequence number followed by a 4-byte count.
static const size_t kHeader = 12;

WriteBatch::WriteBatch() { Clear(); }

WriteBatch::~WriteBatch() = default;

WriteBatch::Handler::~Handler() = default;

void WriteBatch::Clear() {
  rep_.clear();
  rep_.resize(kHeader);
}

size_t WriteBatch::ApproximateSize() const { return rep_.size(); }

Status WriteBatch::Iterate(Handler* handler) const {
  Slice input(rep_);
  if (input.size() < kHeader) {
    return Status::Corruption("malformed WriteBatch (too small)");
  }

  input.remove_prefix(kHeader);
  Slice key, value;
  int found = 0;
  while (!input.empty()) {
    found++;
    // 获取一个字节的kv的类型
    char tag = input[0];
    input.remove_prefix(1);
    switch (tag) {
      case kTypeValue:
        // 内部是先获取前4个字节的长度，然后读取后面的数据，将数据写入key和value 
        if (GetLengthPrefixedSlice(&input, &key) &&
            GetLengthPrefixedSlice(&input, &value)) {
          handler->Put(key, value);
        } else {
          return Status::Corruption("bad WriteBatch Put");
        }
        break;
      case kTypeDeletion:
        // 如果是删除操作，那么就只有key，没有value
        if (GetLengthPrefixedSlice(&input, &key)) {
          handler->Delete(key);
        } else {
          return Status::Corruption("bad WriteBatch Delete");
        }
        break;
      default:
        return Status::Corruption("unknown WriteBatch tag");
    }
  }
  if (found != WriteBatchInternal::Count(this)) {
    return Status::Corruption("WriteBatch has wrong count");
  } else {
    return Status::OK();
  }
}

int WriteBatchInternal::Count(const WriteBatch* b) {
  return DecodeFixed32(b->rep_.data() + 8);
}

void WriteBatchInternal::SetCount(WriteBatch* b, int n) {
  EncodeFixed32(&b->rep_[8], n);
}

SequenceNumber WriteBatchInternal::Sequence(const WriteBatch* b) {
  return SequenceNumber(DecodeFixed64(b->rep_.data()));
}

void WriteBatchInternal::SetSequence(WriteBatch* b, SequenceNumber seq) {
  EncodeFixed64(&b->rep_[0], seq);
}

void WriteBatch::Put(const Slice& key, const Slice& value) {
  WriteBatchInternal::SetCount(this, WriteBatchInternal::Count(this) + 1);
  rep_.push_back(static_cast<char>(kTypeValue));
  PutLengthPrefixedSlice(&rep_, key);
  PutLengthPrefixedSlice(&rep_, value);
}

void WriteBatch::Delete(const Slice& key) {
  WriteBatchInternal::SetCount(this, WriteBatchInternal::Count(this) + 1);
  rep_.push_back(static_cast<char>(kTypeDeletion));
  PutLengthPrefixedSlice(&rep_, key);
}

void WriteBatch::Append(const WriteBatch& source) {
  WriteBatchInternal::Append(this, &source);
}

namespace {
class MemTableInserter : public WriteBatch::Handler {
 public:
  SequenceNumber sequence_;
  MemTable* mem_;

  void Put(const Slice& key, const Slice& value) override {
    mem_->Add(sequence_, kTypeValue, key, value);
    sequence_++;
  }
  void Delete(const Slice& key) override {
    mem_->Add(sequence_, kTypeDeletion, key, Slice());
    sequence_++;
  }
};
}  // namespace

Status WriteBatchInternal::InsertInto(const WriteBatch* b, MemTable* memtable) {
  MemTableInserter inserter;
  // 设置序号
  inserter.sequence_ = WriteBatchInternal::Sequence(b);
  inserter.mem_ = memtable;
  // 遍历batch中所有的kv，然后将他们插入memtable
  return b->Iterate(&inserter);
}

void WriteBatchInternal::SetContents(WriteBatch* b, const Slice& contents) {
  assert(contents.size() >= kHeader);
  b->rep_.assign(contents.data(), contents.size());
}

void WriteBatchInternal::Append(WriteBatch* dst, const WriteBatch* src) {
  SetCount(dst, Count(dst) + Count(src));
  assert(src->rep_.size() >= kHeader);
  dst->rep_.append(src->rep_.data() + kHeader, src->rep_.size() - kHeader);
}

}  // namespace leveldb