[1]We sometimes get questions from users on how to ensure Master-Worker pattern within peer-to-peer (P2P) architecture in GridGain [2]. When designing our API and our deployment model, we purposely went with P2P architecture because we wanted to have ultimate freedom on how a grid node is used. As a result, in GridGain a node can act as master or worker or both, depending on your configuration. Moreover, you don't even have to change a single line of code to get this to work.

The following example shows on how it can be done. In GridGain every node has a notion of attributes which it gets at startup. Here is an example that shows how a node can get a "worker" attribute from Spring XML configuration file at startup:

<bean 
    id="grid.cfg" 
    class="org.gridgain.grid.GridConfigurationAdapter" 
    scope="singleton">
    ...
    <property name="userAttributes">
        <map>
            <!-- Make this node a worker node. -->
            <entry key="segment.worker" value="true"/>
        </map>
    </property>
    ...
</bean>

Now, we need to make sure that only worker nodes are passed into GridTask.map(...) method on the master nodes. To do this, on the master nodes we need to configure GridAttributesTopologySpi [3], the purpose of which is to filter nodes based on their attributes. Here is how the configuration will look like:

<bean 
    id="grid.cfg" 
    class="org.gridgain.grid.GridConfigurationAdapter" 
    singleton="true">
        ...
        <property name="topologySpi">
            <bean class="org.gridgain.grid.spi.topology.attributes.GridAttributesTopologySpi">
               <property name="attributes">
                    <map>
                        <!-- Include only worker nodes. -->
                        <entry key="segment.worker" value="true"/>
                    </map>
                </property>
            </bean>
        </property>
        ...
</bean>

That's it! To verify that it works, we can add assertion into our GridTask [4] implementation to make sure that all included nodes are indeed "worker" nodes as follows:

public class FooBarGridTask 
    extends GridTaskAdapter<String, String> {
    ...
    public Map<GridJob, GridNode> map(
        List<GridNode> topology, String arg) {

        Map<GridJob, GridNode> jobs = 
            new HashMap<GridJob, GridNode>(topology.size());

        for (GridNode node : topology) {
            String workerAttr = 
                node.getAttribute("segment.worker");

            // Assert that worker attribute is present and
            // is assigned value "true".
            assert workerAttr != null;
            assert Boolean.getBoolean(workerAttr) == true;

            jobs.put(new FooBarWorkerJob(arg), node);
        }

        return jobs;
    }
    ...
}

Note, that although we only segmented grid into 2 segments, masters and workers, you can configure as many segments as you like by providing additional node attributes. For example, you can have several worker groups each responsible for processing only a certain subset of jobs.

For addtional examples take a look at Segmenting Grid Nodes [5] article on our Wiki [6].