Developing custom arbiters for multi-master replication

Change collisions might occur if the same records can be changed simultaneously in two places. In a multi-master replication topology, catalog service domains detect collisions automatically. When a catalog service domain detects a collision, it invokes an arbiter. Typically, collisions are resolved with the default collision arbiter. However, an application can provide a custom collision arbiter.

Before you begin

See Planning multiple data center topologies for more information about planning and designing the multi-master replication topology.
See Configuring multiple data center topologies for more information about setting up links between catalog service domains.

About this task

If a catalog service domain receives a replicated entry that collides with a collision record, the default arbiter uses the changes from the lexically lowest named catalog service domain. For example, if domain A and B generate a conflict for a record, then the change from domain B is ignored. Domain A keeps its version and the record in domain B is changed to match the record from domain A. Domain names are converted to uppercase for comparison.

An alternative option is for the multi-master replication topology to call on a custom collision plug-in to decide the outcome. These instructions outline how to develop a custom collision arbiter and configure a multi-master replication topology to use it.

Procedure

Develop a custom collision arbiter and integrate it into your application.
The class must implement the interface:
```
com.ibm.websphere.objectgrid.revision.CollisionArbiter
```
A collision plug-in has three choices for deciding the outcome of a collision. It can choose the local copy or the remote copy or it can provide a revised version of the entry. A catalog service domain provides the following information to a custom collision arbiter:
- The existing version of the record
- The collision version of the record
- A Session object that must be used to create the revised version of the collided entry
The plug-in method returns an object that indicates its decision. The method invoked by the domain to call the plug-in must return true or false, where false means to ignore the collision. When the collision is ignored, the local version remains unchanged and the arbiter forgets that it ever saw the existing version. The method returns a true value if the method used the provided session to create a new, merged version of the record, reconciling the change.

In the objectgrid.xml file, specify the custom arbiter plug-in.

The ID must be CollisionArbiter.

<dgc:objectGrid name="revisionGrid" txTimeout="10">
    	<dgc:bean className="com.you.your_application.
						CustomArbiter" id="CollisionArbiter">
    		<dgc:property name="property" type="java.lang.String"
						value="propertyValue"/>
    	</dgc:bean>
</dgc:objectGrid>