58346b9e29
Summary: Added a ASCII doodle to represent the log writer format. Test Plan: None Reviewers: sdong CC: leveldb Task ID: 6179896 Blame Rev:
90 lines
2.9 KiB
C++
90 lines
2.9 KiB
C++
// Copyright (c) 2013, Facebook, Inc. All rights reserved.
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// This source code is licensed under the BSD-style license found in the
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// LICENSE file in the root directory of this source tree. An additional grant
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// of patent rights can be found in the PATENTS file in the same directory.
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//
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#pragma once
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#include <memory>
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#include <stdint.h>
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#include "db/log_format.h"
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#include "rocksdb/slice.h"
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#include "rocksdb/status.h"
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namespace rocksdb {
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class WritableFile;
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using std::unique_ptr;
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namespace log {
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/**
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* Writer is a general purpose log stream writer. It provides an append-only
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* abstraction for writing data. The details of the how the data is written is
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* handled by the WriteableFile sub-class implementation.
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*
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* File format:
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*
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* File is broken down into variable sized records. The format of each record
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* is described below.
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* +-----+-------------+--+----+----------+------+-- ... ----+
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* File | r0 | r1 |P | r2 | r3 | r4 | |
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* +-----+-------------+--+----+----------+------+-- ... ----+
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* <--- kBlockSize ------>|<-- kBlockSize ------>|
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* rn = variable size records
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* P = Padding
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*
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* Data is written out in kBlockSize chunks. If next record does not fit
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* into the space left, the leftover space will be padded with \0.
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*
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* Record format:
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*
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* +---------+-----------+-----------+--- ... ---+
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* |CRC (4B) | Size (2B) | Type (1B) | Payload |
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* +---------+-----------+-----------+--- ... ---+
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*
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* CRC = 32bit hash computed over the payload using CRC
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* Size = Length of the payload data
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* Type = Type of record
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* (kZeroType, kFullType, kFirstType, kLastType, kMiddleType )
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* The type is used to group a bunch of records together to represent
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* blocks that are larger than kBlockSize
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* Payload = Byte stream as long as specified by the payload size
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*
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*/
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class Writer {
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public:
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// Create a writer that will append data to "*dest".
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// "*dest" must be initially empty.
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// "*dest" must remain live while this Writer is in use.
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explicit Writer(unique_ptr<WritableFile>&& dest);
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~Writer();
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Status AddRecord(const Slice& slice);
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WritableFile* file() { return dest_.get(); }
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const WritableFile* file() const { return dest_.get(); }
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private:
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unique_ptr<WritableFile> dest_;
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int block_offset_; // Current offset in block
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// crc32c values for all supported record types. These are
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// pre-computed to reduce the overhead of computing the crc of the
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// record type stored in the header.
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uint32_t type_crc_[kMaxRecordType + 1];
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Status EmitPhysicalRecord(RecordType type, const char* ptr, size_t length);
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// No copying allowed
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Writer(const Writer&);
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void operator=(const Writer&);
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};
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} // namespace log
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} // namespace rocksdb
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