These are some sample pt-online-schema-change plugins based off of the ones we use at Yelp when running pt-online-schema-change. Also included is a sample pt-online-schema-change.cnf, which you can deploy in /etc/percona-toolkit alongside the plugin.
To learn more about how Yelp uses pt-online-schema-change, you can check out my talk at Percona Live 2016, a link to the slides is at the bottom of the page.
An example of using the local database connection to obtain the database name, and using it in a command to announce the start of the pt-online-schema-change run in the dba IRC channel.
Like init, this plugin announces the finish of a pt-online-schema-change run.
This plugin is more complicated because it returns a subroutine. You can do your setup before defining $lag. This sample plugin currently always returns a lag of 3.
Because pt-online-schema-change requires a user with the SUPER privilege, it's worth it to verify that it's running against a master withi read_only=0, and not, a replica with read_only=1.
We run pt-online-schema-change with --no-drop-old-table so that we can revert the change in case of an emergency. Having the master binary log coordinates is useful to examine changes from just before the point of swapping the new table in place.
To use just a plugin file like this one, add:
--plugin /your/path/here/pt-online-schema-change-plugin.pl
to your pt-online-schema-change command.
Or, use an pt-online-schema-change.cnf like the one provided, which includes a reference to the plugin file. We use the default location of /etc/percona-toolkit for both.