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Filtering operates by applying the filter within one, or more, of the stages configured within the replicator. Stages are the individual steps that occur within a pipeline, that take information from a source (such as MySQL binary log) and write that information to an internal queue, the transaction history log, or apply it to a database. Where the filters are applied ultimately affect how the information is stores, used, or represented to the next stage or pipeline in the system.
For example, a filter that removed out all the tables from a specific database would have different effects depending on the stage it was applied. If the filter was applied on the master before writing the information into the THL, then no slave could ever access the table data, because the information would never be stored into the THL to be transferred to the slaves. However, if the filter was applied on the slave, then some slaves could replicate the table and database information, while other slaves could choose to ignore them. The filtering process also has an impact on other elements of the system. For example, filtering on the master may reduce network overhead, albeit at a reduction in the flexibility of the data transferred.
In a standard replicator configuration with MySQL, the following stages are configured in the master, as shown in Figure 11.1, “Filters: Pipeline Stages on Masters”.
Within the slave, the stages configured by default are shown in Figure 11.2, “Filters: Pipeline Stages on Slaves”.
Filters can be applied during any configured stage, and where the filter is applied alters the content and availability of the information. The staging and filtering mechanism can also be used to apply multiple filters to the data, altering content when it is read and when it is applied.
Where more than one filter is configured for a pipeline, each filter is executed in the order it appears in the configuration. For example, within the following fragment:
... replicator.stage.binlog-to-q.filters=settostring,enumtostring,pkey,colnames ...
For certain filter combinations this order can be significant. Some filters rely on the information provided by earlier filters.