# Introducing MySQL Slave Delay

MySQL Slave Delay implements an oft-requested replication feature: the ability to make a slave lag its master, a.k.a scheduling binlog events for some time in the future. This will probably be built into MySQL replication someday, but in the meantime, you can use this tool. A delayed slave is great if disaster strikes and propagates through replication, because it lets you stop the delayed slave before it gets corrupted, restart the slave until just before the offending statement, and recover quickly. This is a lot faster than restoring last night’s full backup and replaying a day’s worth of binary logs on the master.

This is the first tool largely contributed by someone else. Sergey Zhuravlev sent me the original program over email, and I tweaked it to watch the slave’s relay logs so it doesn’t need to connect to the master, added documentation, and released it.

### What it does

MySQL Slave Delay watches the slave’s relay log positions and makes the SQL thread lag the I/O thread by starting and stopping it as needed. It can also watch the master’s log positions, but this is usually not needed unless the slave’s I/O thread delay is large (typically it’s just milliseconds, but it depends on the network speed).

Since you can’t tell how far behind the slave is unless it’s running, MySQL Slave Delay samples and remembers the binlog positions. Later, when it needs to start the slave, it recalls the master’s position at the desired time ago, and makes the slave run until it hits that position.

Another way to think about it is that it schedules replication events for sometime in the future, instead of letting them be applied immediately.

All in all, a simple and easy way to accomplish delayed replication.

MySQL Toolkit is a set of essential tools for MySQL users, developers and administrators. The project’s goal is to make high-quality command-line tools that follow the UNIX philosophy of doing one thing and doing it well. They are designed for scriptability and ease of processing with standard command-line utilities such as awk and sed.