Mysql 45 study notes (twenty-four) MYSQL master-slave consistency

One. The basic principle of MySQL master and backup

Active-standby switching process:

• The client's read and write directly access node A, and node B is the standby database of A. It just synchronizes the updates of A and executes it locally, so that the data of nodes B and A are kept the same
• Switch to state 2, the client read and write access to node B, and node A is the standby database of B

Set Node B (standby database) to readonly mode

1. Some operational query statements will be placed in the backup database to check,Set read-only to prevent misuse;
2. Prevent switching logic from bugs.Double writing occurs during the switching process, resulting in inconsistency between active and standby;
3. readonly state, to determine the role of the node

Set standby to read-only, and follow The main library is kept up to date:

Because the readonly setting is invalid for super users, it is used for A thread that synchronizes updates has super permission.

Second, the UPDATE statement executes the steps of synchronizing to node B on node A:

• After the main library receives the follow-up request from the client, it executes the update logic of the internal transaction and writes the binlog at the same time
• A long connection is maintained between the standby database B and the primary database A.There is a special thread inside the main database A to serve the long connection of the standby database B

Three. The complete process of transaction log synchronization:

• Use the change master command on the standby database B to set the IP of the primary database A.Port, username, password and where to start requesting binlog, this location contains filename and log offset
• Execute the start slave command on the standby database B. At this time, the standby database will start two threads, namely io_thread and sql_thread in the figure.Among them, io_thread is responsible for establishing a connection with the main library
• After verifying the user name and password, the main database A starts to read the binlog from the local according to the location passed by the standby database B, and sends it to B
• After the standby database B gets the binlog, it writes it to the local file, which is called the relay log
• sql_thread reads the secondary school log, parses the commands in the log, and executes them.