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Introduction to semi-synchronous replication:

By default, the replication functions of MySQL 5.5 MariaDB 5.6 and MariaDB 10.1 are asynchronous, and asynchronous replication can provide the best performance. the master sends the Binlog logs to the slave library, which ends and does not verify whether the slave library has been received, but it also brings high risk, which means that when the master server or slave server fails It is possible that the slave computer does not receive the Binlog logs sent by the host, which will cause data inconsistency between the master server and slave server, and even cause data loss during recovery.

In order to solve the above problems, MySQL 5.5 introduces a semi-synchronous replication (Semi Replication) mode, which ensures that the Binlog log files sent by the master server are received from the server and written into its own relay log (Relay Log), and then it will give the master server a feedback that it has been received, and then the master database thread returns to the current session to inform the operation that the operation is complete, as shown in the following figure. When a timeout occurs, the source master server temporarily switches to asynchronous replication mode until at least one slave server that is set in semi-synchronous replication mode receives information in time (see figure 2-140).

In short, semi-synchronous replication ensures that committed transactions have been transferred to at least one standby database to a certain extent, so semi-synchronous replication further improves data integrity compared with asynchronous replication.

Note that semi-synchronous replication mode must be enabled on both the master and slave servers, otherwise the master server uses asynchronous replication mode by default.

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