The Ultimate Guide to High Availability with MariaDB

Semi-Sync replication

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Contents

  1. Event Header Changes
    1. Semi Sync ACK Details
    2. Example of Heartbeat Event With Semi-Sync Protocol and CRC32
    3. Example of XID_EVENT, With CRC32
    4. Example of Semi-Sync ACK

Regular MariaDB replication is asynchronous.
MariaDB, since MariaDB 5.5, has included semisynchronous replication semi-synchronous Binlog Event.

Event Header Changes

If the user variable @rpl_semi_sync_slave is set, 2 exta bytes are added after the status byte of a binlog network stream and before the normal binlog event header.

  • uint<1> semi-sync indicator, always 0xef
  • uint<1> semi-sync flags, either 0x00 (no ACK) or 0x01 (ACK)

Note : The packet size, as in the network protocol header, is then: event_size + 1 byte status + 2 bytes semi-sync replication.

The MariaDB server sets the user variable whenever it is starting replication. For MariaDB Connector/C , the following query must be executed before the call to mariadb_rpl_open() is made to enable semi-sync replication.

SET @rpl_semi_sync_slave=1

If the semi-sync flag is set to 0x01, the master waits for a Semi Sync ACK packet from the slave and when the Semi Sync ACK is seen, the master acknowledges the client which has issued the transaction with a standard OK_Packet or a ERR_Packet.

The master can then write the transaction to the binary log and send the next events to the slave.

Note : The master only requests Semi Sync ACKs if rpl_semi_sync_master_enabled is enabled. If it is not enabled, the semi-sync flag will always be 0x00.

Semi Sync ACK Details

This event is sent by the slave only if the semi-sync flag is set to 0x01.

Note: this packet sent by the slave never includes the CRC32.

Sending an ACK when the semi-sync flag is set to 0x0 will cause an error and the connection is closed.

Example of Heartbeat Event With Semi-Sync Protocol and CRC32

We can clearly see:

  • 2a 00 00 [3 bytes] packet size:
  • 06 [1] sequence
  • 00 [1] status byte = 00 => OK
  • ef 00 [2] bytes => semi sync indicator (0xef) and semi-sync flag (00)

the ef 00 2 bytes after the OK byte 00

T 127.0.0.1:23240 -> 127.0.0.1:41054 [AP]
  2a 00 00 06 00 ef 00 00    00 00 00 1b d9 27 00 00    *............'..
  27 00 00 00 79 04 00 00    00 00 6d 79 73 71 6c 2d    '...y.....mysql-
  62 69 6e 2e 30 30 30 30    33 34 ed ef e1 f0          bin.000034....

Example of XID_EVENT, With CRC32

The master sets the Semi-Sync ACK request in the XID_EVENT event:

We see the 2 semi sync bytes: ef and 01.
The latter, being 1, means the slave server must send the Semi Sync ACK packet.

We also see in the binlog event header:

  • Event Type [1] = 10 XID_EVENT
  • Next Event pos [4] = 4a 05 00 00 => 1354 
22 00 00 0c 00 ef 01 17  d0 37 5a 17 d0 37 5a 10  "............7Z.
d9 27 00 00 1f 00 00 00  4a 05 00 00 00 00 6f 00  .?......J.....o.
00 00 00 00 00 00 44 30  aa fc                    ......D0..

Example of Semi-Sync ACK

This is sent by the slave server after the XID_EVENT receiving.

We see:

  • the semi sync indicator [1] = 0xef, sent before anything else
  • the Next Event position [8] = 4a 05 00 00 00 00 00 00 => 1354
    which is the next position of the XID_EVENT above
  • the binlog filename = mysql-bin.000034

Please note:

  • there is no terminating CRC32
  • the packet sequence now start starts from 0 
  19 00 00 00 ef 4a 05 00    00 00 00 00 00 6d 79 73    .....J.......mys
  71 6c 2d 62 69 6e 2e 30    30 30 30 33 34             ql-bin.000034

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