Running triggers on the slave for Row-based events
Contents
Running triggers on the slave
Starting from MariaDB 10.1.1, one can force the slave thread to run triggers for row based binlog events.
The setting is controlled by the slave_run_triggers_for_rbr global variable. It can be also specified as a command-line option or in my.cnf.
Possible values are:
| Value | Meaning |
|---|---|
| NO (Default) | Don't invoke triggers for Row-based events |
| YES | Invoke triggers for Row-based events, don't log their effect into the binary log |
| LOGGING | Invoke triggers for Row-based events, and log their effect into the binary log |
Note that if you just want to use triggers together with replication, you most likely don't need this option. Read below for details.
When to use slave_run_triggers_for_rbr
Background
Normally, MySQL's replication system can replicate trigger actions automatically.
- When one uses statement-based replication, the binary log contains SQL statements. Slave server(s) execute the SQL statements. Triggers are run on the master and on each slave, independently.
- When one uses row-based replication, the binary log contains row changes. It will have both the changes made by the statement itself, and the changes made by the triggers that were invoked by the statement. Slave server(s) do not need to run triggers for row changes they are applying.
Target usecase
One may want to have a setup where a slave has triggers that are not present on the master (Suppose the slave needs to update summary tables or perform some other ETL-like process).
If one uses statement-based replication, they can just create the required triggers on the slave. The slave will run the statements from the binary log, which will cause the triggers to be invoked.
However, there are cases where you have to use row-based replication. It could be because the master runs non-deterministic statements, or the master could be a node in a Galera cluster. In that case, you would want row-based events to invoke triggers on the slave. This is what the slave_run_triggers_for_rbr option is for. Setting the option to YES will cause the SQL slave thread to invoke triggers for row based events; setting it to LOGGING will also cause the changes made by the triggers to be written into the binary log.
The following triggers are invoked:
- Update_row_event runs an UPDATE trigger
- Delete_row_event runs a DELETE trigger
- Write_row_event action depends on whether the operation will require foreign key checks:
- when FK checks are not necessary, the operation will invoke a DELETE trigger if the record to be modified existed in the table. After that, an INSERT trigger will be invoked.
- when FK checks are necessary, either an UPDATE or or a combination of DELETE and INSERT triggers will be invoked.
Preventing multiple trigger invocations
There is a basic protection against triggers being invoked both on the master and slave. If the master modifies a table that has triggers, it will produce row-based binlog events with the "triggers were invoked for this event" flag. The slave will not invoke any triggers for flagged events.
See also
- Task in Jira, MDEV-5095.
