rocksdb/db/log_writer.h

//  Copyright (c) 2013, 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.
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#pragma once
#include <memory>
#include <stdint.h>
#include "db/log_format.h"
#include "rocksdb/slice.h"
#include "rocksdb/status.h"

namespace rocksdb {

class WritableFile;

using std::unique_ptr;

namespace log {

/**
 * Writer is a general purpose log stream writer. It provides an append-only
 * abstraction for writing data. The details of the how the data is written is
 * handled by the WriteableFile sub-class implementation.
 *
 * File format:
 *
 * File is broken down into variable sized records. The format of each record
 * is described below.
 *       +-----+-------------+--+----+----------+------+-- ... ----+
 * File  | r0  |        r1   |P | r2 |    r3    |  r4  |           |
 *       +-----+-------------+--+----+----------+------+-- ... ----+
 *       <--- kBlockSize ------>|<-- kBlockSize ------>|
 *  rn = variable size records
 *  P = Padding
 *
 * Data is written out in kBlockSize chunks. If next record does not fit
 * into the space left, the leftover space will be padded with \0.
 *
 * Record format:
 *
 * +---------+-----------+-----------+--- ... ---+
 * |CRC (4B) | Size (2B) | Type (1B) | Payload   |
 * +---------+-----------+-----------+--- ... ---+
 *
 * CRC = 32bit hash computed over the payload using CRC
 * Size = Length of the payload data
 * Type = Type of record
 *        (kZeroType, kFullType, kFirstType, kLastType, kMiddleType )
 *        The type is used to group a bunch of records together to represent
 *        blocks that are larger than kBlockSize
 * Payload = Byte stream as long as specified by the payload size
 *
 */
class Writer {
 public:
  // Create a writer that will append data to "*dest".
  // "*dest" must be initially empty.
  // "*dest" must remain live while this Writer is in use.
  explicit Writer(unique_ptr<WritableFile>&& dest);
  ~Writer();

  Status AddRecord(const Slice& slice);

  WritableFile* file() { return dest_.get(); }
  const WritableFile* file() const { return dest_.get(); }

 private:
  unique_ptr<WritableFile> dest_;
  int block_offset_;       // Current offset in block

  // crc32c values for all supported record types.  These are
  // pre-computed to reduce the overhead of computing the crc of the
  // record type stored in the header.
  uint32_t type_crc_[kMaxRecordType + 1];

  Status EmitPhysicalRecord(RecordType type, const char* ptr, size_t length);

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

}  // namespace log
}  // namespace rocksdb