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:
main
krad 10 years ago
parent db6569cd4a
commit 58346b9e29
  1. 6
      db/log_reader.h
  2. 34
      db/log_writer.h

@ -22,6 +22,12 @@ using std::unique_ptr;
namespace log {
/**
* Reader is a general purpose log stream reader implementation. The actual job
* of reading from the device is implemented by the SequentialFile interface.
*
* Please see Writer for details on the file and record layout.
*/
class Reader {
public:
// Interface for reporting errors.

@ -22,6 +22,40 @@ 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".

Loading…
Cancel
Save