Overview

TODO:

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.

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The "bsp()" function is a user-defined function that handles the whole parallel part of the program. It only takes one argument "BSPPeer", which contains an communication, counters, and IO interfaces.

Inputs and Outputs

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Input and Output

General Information

Since Hama 0.4.0 we provide a input and output system for BSP Jobs.

TODO: Some blahblah about key value and stuff What's in case when no input is configured? and stuff like that should be documented here..

Input

Configuring Input

When setting up a BSPJob, you can provide a InputFormat and a Path where to find the input.

    BSPJob job = new BSPJob();
    // detail stuff omitted
    job.setInputPath(new Path("/tmp/test.seq");
    job.setInputFormat(org.apache.hama.bsp.SequenceFileInputFormat.class);

Another way to add input paths is following:

 
   SequenceFileInputFormat.addInputPath(job, new Path("/tmp/test.seq"));

You can also add multiple paths by using this method:

 
   SequenceFileInputFormat.addInputPaths(job, "/tmp/test.seq,/tmp/test2.seq,/tmp/test3.seq");

Note that these paths must be separated by a comma.

In case of a SequenceFileInputFormat the key and value pair are parsed from the header.

When you use want to read a basic textfile with TextInputFormat the key is always LongWritable which contains how much bytes have been read and Text which contains a line of your input.

Using Input

You can now read the input from each of the functions in BSP class which has BSPPeer as parameter. (e.G. setup / bsp / cleanup)

In this case we read a normal text file:

 @Override
  public final void bsp(
      BSPPeer<LongWritable, Text, KEYOUT, VALUEOUT> peer)
      throws IOException, InterruptedException, SyncException {
      
      // this method reads the next key value record from file
      KeyValuePair<LongWritable, Text> pair = peer.readNext();

      // the following lines do the same:
      LongWritable key = new LongWritable();
      Text value = new Text();
      peer.readNext(key, value);
  }

Consult the docs for more detail on events like end of file.

There is also a function which allows you to re-read the input from the beginning.

This snippet reads the input five times:

  for(int i = 0; i < 5; i++){
    LongWritable key = new LongWritable();
    Text value = new Text();
    while (peer.readNext(key, value)) {
       // read everything
    }
    // reopens the input
    peer.reopenInput()
  }

Custom Inputformat

You can implement your own inputformat blabla

Output

Configuring Output

Using Input

Custom Outputformat

Implementation notes

Internal implementation details

BSPJobClient

1. Create the splits for the job 2. writeNewSplits() 3. job.set("bsp.job.split.file", submitSplitFile.toString()); 4. Sets the number of peers to split.lenth

JobInProgress

1. Receives splitFile 2. Add split argument to TaskInProgress constructor
   

Task

1. Gets his split from Groom 2. Initializes everything in BSPPeerImpl
   

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:

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    @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.

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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.

Counters

Setup and Cleanup

Combiners

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