4-Semi-Sync Replication

Regular MariaDB replication is asynchronous. MariaDB also includes semisynchronous replication semi-synchronous Binlog Event.

Event Header Changes

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

• semi-sync indicator, always 0xef.

• 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.

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 replica.

Note : The master only requests Semi Sync ACKs if 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 replica only if the semi-sync flag is set to 0x01.

• semi-sync indicator, always 0xef;

• the next position of received event;

• binlog file name.

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

Example of Heartbeat Event With Semi-Sync Protocol and CRC32

We can 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).

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 replica 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.

Example of Semi-Sync ACK

This is sent by the replica 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.

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