MySQL master-slave synchronous asynchronous replication semi synchronous replication full synchronous replication

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To solve the problem of inconsistent master-slave synchronization

1、 Asynchronous replication

After the master database completes the commit transaction , Immediately execute asynchronously and return the result to the client , It doesn't matter whether it is received and processed from the library . If the processing is not received from the Library , There will still be inconsistency between master and slave data . period MySQL(5.5 before ) Only asynchronous replication is supported . And early support myisam engine .

2、 Semi-synchronous replication

The main library does not return to the client immediately after executing a transaction committed by the client , Instead, wait for at least one of the slave libraries to receive and write relay log Is returned to the client . As opposed to asynchronous replication , Semi-synchronous replication improves data security , At the same time, it also causes a certain degree of delay .
The emergence of semi synchronous replication , This is to ensure that the active and standby data are consistent at any time . As opposed to asynchronous replication , Semi synchronous replication requires the execution of every transaction , It is required that at least one standby database is successfully received , To return it to the user . The implementation principle is also very simple , After the local execution of the main database , Wait for the response message from the standby database （ It contains the latest data received by the standby database binlog（file,pos））, After receiving the standby database response message , And back to the user , Such a transaction is really completed . On the master database instance , There is a dedicated thread （ack_receiver） Receive the response message from the standby database , And notify the primary database and the standby database of the received logs through the notification mechanism , Can continue to execute .

3、 Full synchronous replication

When the master commits a transaction , All slave nodes must receive 、APPLY And commit these transactions , Then the main library thread can continue to do subsequent operations . Because you need to wait for all the transactions from the library to complete before returning , So the performance of full synchronous replication will be seriously affected .

Please correct me if there is any mistake