rocksdb/db/log_writer.h
krad 58346b9e29 Log writer record format doc.
Summary: Added a ASCII doodle to represent the log writer format.

Test Plan: None

Reviewers: sdong

CC: leveldb

Task ID: 6179896

Blame Rev:
2015-04-07 16:25:56 -07:00

90 lines
2.9 KiB
C++

// 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