How the MASTER_POS_WAIT() function works in Mariadb?

The MASTER_POS_WAIT() function is a useful tool for managing replication in Mariadb.

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The MASTER_POS_WAIT() function is a useful tool for managing replication in Mariadb. It allows you to wait until the current server has reached a specific binary log position or a specific GTID position from another server. This can help you ensure data consistency and synchronization across multiple servers in a replication topology.

Syntax

The syntax of the MASTER_POS_WAIT() function is as follows:

MASTER_POS_WAIT(log_name, log_pos[, timeout[, connection_name]])

The function takes four arguments:

  • log_name: A string that specifies the name of the binary log file to wait for. For example, mysql-bin.000001.
  • log_pos: An integer that specifies the position within the binary log file to wait for. For example, 1234.
  • timeout: An optional integer that specifies the maximum number of seconds to wait. If omitted or set to zero, the function will wait indefinitely until the position is reached or an error occurs.
  • connection_name: An optional string that specifies the name of the replication connection to use. If omitted, the function will use the default connection.

The function returns an integer value that indicates the result of the wait operation:

  • 0 or a positive value: The wait was successful and the current server has reached the specified position. The return value is the number of log events that the server had to wait for.
  • -1: The wait was interrupted by a user or a system event, such as a KILL statement or a server shutdown.
  • -2: The wait timed out and the current server has not reached the specified position.
  • NULL: The wait failed due to an invalid argument, such as an empty or nonexistent log name or position.

Examples

In this section, we will show some examples of how to use the MASTER_POS_WAIT() function in different scenarios.

Example 1: Waiting for a binary log position

Suppose you have a master server with the server ID 1 and a slave server with the server ID 2. You want to perform a backup on the slave server, but you want to make sure that it has applied the latest binary log event from the master server. You can use the MASTER_POS_WAIT() function to wait for the last binary log position from the master server before starting the backup. For example, if the last binary log position from the master server is mysql-bin.000001, 1234, you can execute the following statement on the slave server:

SELECT MASTER_POS_WAIT('mysql-bin.000001', 1234);

This will return 0 or a positive value if the slave server has applied the binary log position mysql-bin.000001, 1234, or a negative value or NULL if the wait was interrupted, timed out, or failed.

Example 2: Waiting for a GTID position

Suppose you have a master server with the server ID 1 and a slave server with the server ID 2. You are using GTID-based replication, and you want to perform a schema change on the master server, but you want to make sure that the slave server has applied all the transactions from the master server before the change. You can use the MASTER_POS_WAIT() function to wait for the last GTID position from the master server before executing the schema change. For example, if the last GTID position from the master server is 0-1-100, you can execute the following statement on the master server:

SELECT MASTER_POS_WAIT('', 0, 0, 'slave');

This will return 0 or a positive value if the master server has applied all the GTIDs from the slave server, or a negative value or NULL if the wait was interrupted, timed out, or failed.

Example 3: Waiting with a timeout

Suppose you have a master server with the server ID 1 and a slave server with the server ID 2. You want to perform a maintenance operation on the slave server, but you want to limit the downtime to 10 seconds. You can use the MASTER_POS_WAIT() function to wait for the last binary log position from the master server with a timeout of 10 seconds. For example, if the last binary log position from the master server is mysql-bin.000001, 1234, you can execute the following statement on the slave server:

SELECT MASTER_POS_WAIT('mysql-bin.000001', 1234, 10);

This will return 0 or a positive value if the slave server has applied the binary log position mysql-bin.000001, 1234 within 10 seconds, or -2 if the wait timed out. You can then decide whether to proceed with the maintenance operation or not based on the return value.

There are some other functions that are related to the MASTER_POS_WAIT() function and can be used to manage replication in Mariadb. Here are some of them:

  • SHOW MASTER STATUS: This statement shows the current binary log file name and position of the master server, as well as the GTID position if GTID-based replication is enabled. For example, SHOW MASTER STATUS will return something like:
+------------------+----------+--------------+------------------+----------------+
| File             | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+------------------+----------+--------------+------------------+----------------+
| mysql-bin.000001 |     1234 |              |                  | 0-1-100        |
+------------------+----------+--------------+------------------+----------------+
  • SHOW SLAVE STATUS: This statement shows the current replication status of the slave server, including the binary log file name and position of the master server that the slave is reading from, the relay log file name and position that the slave is executing from, and the GTID position if GTID-based replication is enabled.