One of the most common MySQL operations is replicating databases between master and slave servers. While most such connections are straightforward to establish and maintain, on occasion something goes amiss: some master data may not replicate on the slave, or read requests may be routed to the master rather than to the server, for example. Finding a solution to a replication failure sometimes requires a little extra detective work.
Replication is one of the most basic operations in MySQL — and any other database: it’s used to copy data from one database server (the master) to one or more others (the slaves). The process improves performance by allowing loads to be distributed among multiple slave servers for reads, and by limiting the master server to writes.
Additional benefits of replication are security via slave backups; analytics, which can be performed on the slaves without affecting the master’s performance; and widespread data distribution, which is accomplished without requiring access to the master. (See the MySQL Reference Manual for more on replication.)
As with any other aspect of database management, replication doesn’t always proceed as expected. The Troubleshooting Replication section of the MySQL Reference Manual instructs you to check for messages in your error log when something goes wrong with replication. If the error log doesn’t point you to the solution, ensure that binary logging is enabled in the master by issuing a SHOW MASTER STATUS statement. If it’s enabled, “Position” is nonzero; if it isn’t, make sure the master is running with the –log-bin option.
The manual offers several other replication-troubleshooting steps:
- The master and slave must both start with the –server-id option, and each server must have a unique ID value;
- Run SHOW SLAVE STATUS to ensure the Slave_IO_Running and Slave_SQL_Running values are both “yes”;
- Run SHOW_PROCESSLIST and look in the State column to verify that the slave is connecting to the master;
- If a statement succeeded on the master but failed on the slave, the nuclear option is to do a full database resynchronization, which entails deleting the slave’s database and copying a new snapshot from the master. (Several less-drastic alternatives are described in the MySQL manual.)
Solutions to real-world MySQL replication problems
What do you do when MySQL indicates the master-slave connection is in order, yet some data on the master isn’t being copied to the slave? That’s the situation described in a Stack Overflow post from March 2010.
The first step is to run “show master status” or “show master status\G” on the master database to get the correct values for the slave. The slave status above indicates the slave is connected to the master and awaiting log events. Synching the correct log file position should restore copying to the slave.
To ensure a good sync, stop the master, dump the database, record the master log file positions, restart the master, import the database to the slave, and start the slave in slave mode with the correct master log file position.
Another Stack Overflow post from March 2014 presents a master/slave setup using JDBC drivers in which transactions marked as read-only were still pinging the master. Since the MySQL JDBC driver was managing the connections to the physical servers — master and slave — the connection pool and Spring transaction manager weren’t aware that the database connection was linking to multiple servers.
The solution is to return control to Spring, after which the transaction on the connection will be committed. The transaction debug message will indicate that queries will be routed to the slave server so long as the connection is in read-only mode. By resetting the connection before it is returned to the pool, the read-only mode is cleared and the last log message will show that queries are now being routed to the master server.
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