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Using NDB API Events to mask/hide colum data when replicating

If you  have asynchronous replication where the slave database is using MySQL Cluster then you can use the NDB API events functionality to mask/overwrite data. You might do this for example if the replica is to be used for generating reports where some of the data is sensitive and not relevant to those reports. Unlike stored procedures, NDB API events will be triggered on the slave.

The first step is to set up replication (master->slave rather than multi-master) as described in Setting up MySQL Asynchronous Replication for High Availability).

In this example, the following table definition is used:

The following code should be compiled and then executed on a node within the slave Cluster:

From the master Cluster, insert some values (note that the example can easily be extended to cover updates too):

and then check that on the slave the value has been set to 0 for each of the entries:

How this works…. The table data is replicated as normal and the real values are stored in the slave. The “slave_filter” process has registered against insert operations on this table and when it’s triggered it sets the VALUE field to 0. The event is processes asynchronously from the replication and so there will be some very narrow window during which the true values would be stored in the slave.