BSPModel

Overview

The Hama is based on the Bulk Synchronous Parallel Model (BSP), in which a computation involves a number of supersteps, each having many distributed computational peers that synchronize at the end of the superstep. Basically, a BSP program consists of a sequence of supersteps. Each superstep consistsof the following three phases:

• Local computation
• Process communication
• Barrier synchronization
  

BSP Function

Hama provides a user-defined function “bsp()” that can be used to write your own BSP program. The bsp() function handles the whole parallel part of the program. In the 0.2 version, it only takes one argument, which is a communication protocol interface. Later, more arguments such as input or reporter could also be included.

Communication Model

Within the bsp() function, you can use the powerful communication functions for many purposes using BSPPeer. We tried to follow the standard library of BSP world as much as possible. The following table describes all the functions you can use:

The send() and all the other functions are very flexible. Here is an example that sends a message to all peers:

    @Override
    public void bsp(
        BSPPeer<NullWritable, NullWritable, Text, DoubleWritable> peer)
        throws IOException, SyncException, InterruptedException {

      for (String peerName : peer.getAllPeerNames()) {
        peer.send(peerName, 
          new LongMessage("Hello from " + peer.getPeerName(), System.currentTimeMillis()));
      }

      peer.sync();
    }

Synchronization

When all the processes have entered the barrier via the sync() function, the Hama proceeds to the next superstep. In the previous example, the BSP job will be finished by one synchronization after sending a message "Hello from ..." to all peers.

But, keep in mind that the sync() function is not the end of the BSP job. As was previously mentioned, all the communication functions are very flexible. For example, the sync() function also can be called in a for loop so that you can use to program the iterative methods sequentially:

    @Override
    public void bsp(
        BSPPeer<NullWritable, NullWritable, Text, DoubleWritable> peer)
        throws IOException, SyncException, InterruptedException {

      for (int i = 0; i < 100; i++) {
        // send some messages
        peer.sync();
      }

    }

The BSP job will be finished only when all processes have no more local and outgoing queues entries and all processes done or is killed by the user.

Inputs and Outputs

Counters

Combiners are used for performing message aggregation to reduce communication overhead in cases when messages can be summarized arithmetically e.g., min, max, sum, and average at the sender side. Suppose that you want to send the integer messages to a specific processor from 0 to 1000 and sum all received the integer messages from all processors.

    public void bsp(BSPPeer<NullWritable, NullWritable, NullWritable, NullWritable> peer) throws IOException,
        SyncException, InterruptedException {

      for (int i = 0; i < 1000; i++) {
    	peer.send(masterTask, new IntegerMessage(peer.getPeerName(), i));
      }
      peer.sync();

      if (peer.getPeerName().equals(masterTask)) {
        IntegerMessage received;
        while ((received = (IntegerMessage) peer.getCurrentMessage()) != null) {
          sum += received.getData();
        }
      }
    }

If you follow the previous example, Each bsp processor will send a bundle of thousand Integer messages to a masterTask. Instead, you could use a Combiners in your BSP program to perform a sum Integer messages and to write more concise and maintainable as below, that is why you use Combiners.

  public static class SumCombiner extends Combiner {

    @Override
    public BSPMessageBundle combine(Iterable<BSPMessage> messages) {
      BSPMessageBundle bundle = new BSPMessageBundle();
      int sum = 0;

      Iterator<BSPMessage> it = messages.iterator();
      while (it.hasNext()) {
        sum += ((IntegerMessage) it.next()).getData();
      }

      bundle.addMessage(new IntegerMessage("Sum", sum));
      return bundle;
    }

  }

Setup and Cleanup

— General Information —

In Apache Hama, you can implement your own BSP method by extending from org.apache.hama.bsp.BSP class. Apache Hama provides in this class a user-defined function bsp() that can be used to write your own BSP program.

The bsp() function handles whole parallel part of the program. (So it just gets called once, not all over again)

There are also setup() and cleanup() which will be called at the beginning of your computation, respectively at the end of the computation.

cleanup() is guranteed to run after the computation or in case of failure.

You can simply override the functions you need from BSP class.

Basically, a BSP program consists of a sequence of supersteps. Each superstep consists of the three phases:

• Local computation
• Process communication
• Barrier synchronization
  

NOTE that these phases should be always sequential order.

In Apache Hama, the communication between tasks (or peers) is done within the barrier synchronization.