wal

Write Ahead Log for LSM or bitcask storage.

Key Features

• Disk based, support large data volume
• Append only write, high performance
• Fast read, one disk seek to retrieve any value
• Support batch write, all data in a batch will be written in a single disk seek
• Iterate all data in wal with NewReader function
• Extremely fast read
• Support concurrent write and read, all functions are thread safe

Design Overview

Format

Format of a single segment file:

       +-----+-------------+--+----+----------+------+-- ... ----+
 File  | r0  |      r1     |P | r2 |    r3    |  r4  |           |
       +-----+-------------+--+----+----------+------+-- ... ----+
       |<---- BlockSize ----->|<---- BlockSize ----->|

  rn = variable size records
  P = Padding
  BlockSize = 32KB

Format of a single record:

+----------+-------------+-----------+--- ... ---+
| CRC (4B) | Length (2B) | Type (1B) |  Payload  |
+----------+-------------+-----------+--- ... ---+

CRC = 32-bit hash computed over the payload using CRC
Length = Length of the payload data
Type = Type of record
       (FullType, FirstType, MiddleType, LastType)
       The type is used to group a bunch of records together to represent
       blocks that are larger than BlockSize
Payload = Byte stream as long as specified by the payload size

Getting Started

func main() {
	wal, _ := wal.Open(wal.DefaultOptions)
	// write some data
	chunkPosition, _ := wal.Write([]byte("some data 1"))
	// read by the position
	val, _ := wal.Read(chunkPosition)
	fmt.Println(string(val))

	wal.Write([]byte("some data 2"))
	wal.Write([]byte("some data 3"))

	// iterate all data in wal
	reader := wal.NewReader()
	for {
		val, pos, err := reader.Next()
		if err == io.EOF {
			break
		}
		fmt.Println(string(val))
		fmt.Println(pos) // get position of the data for next read
	}
}