Page History

...

You can think aggregators as a tree, that the leafs leaves (which are the graph vertices) are sending messages to the root (the master aggregator), and then the root is combining all these messages to a value. In the end, this combined values is are distributed back to the leafsleaves. Aggregators are useful for statistics (think of an histogram of vertex degrees) or for global controllingmanagement.

Registering aggregators

To start using aggregators, you must register declare them in your GraphJob.

e.g.

HamaConfiguration conf = new HamaConfiguration(new Configuration());
GraphJob graphJob = new GraphJob(conf, MyClass.class);

// To add an average aggregator
graphJob.setAggregatorClass(AverageAggregator.class);

// To add a sum aggregator
graphJob.setAggregatorClass(SumAggregator.class);

There are lots of multiple different aggregators and you can also make your own. Look You can look for already implemented aggregators in org.apache.hama.graph package.

Start working with aggregators

To start aggregating values from your vertices, is very easy and strait forward. From your In order to aggregate values, from your vertex, use:

this.aggregate(index,value);

The index parameter of this method is a number that is equivalent to the order of the registered aggregator. (The first registered aggregator is number has index 0, second is number has index 1 etc.)

Get results

Inside your vertex, you can get the results of each aggregator by using this the method:

this.getAggregatedValue(index);

...

To write your own aggregator, you have o to extend AbstractAggregator class and implement the methods of #aggregate(M value) and #getValue(). For more, please see the default implemented implementation of aggregators in org.apache.hama.graph package.