Hama学习笔记（6）-获取各个peer(task)的信息、确定master task

有时候在bsp job中需要确定一个master task，这就需要获取各个peer(task)的信息，如host name、端口号等等。

hama中获取peer的主机名和端口号很方便：

  @SuppressWarnings("unchecked")  private final static <KEYIN, VALUEIN, KEYOUT, VALUEOUT, M extends Writable> void runBSP(      final BSPJob job,      BSPPeerImpl<KEYIN, VALUEIN, KEYOUT, VALUEOUT, M> bspPeer,      final BytesWritable rawSplit, final BSPPeerProtocol umbilical)      throws Exception {    BSP<KEYIN, VALUEIN, KEYOUT, VALUEOUT, M> bsp = (BSP<KEYIN, VALUEIN, KEYOUT, VALUEOUT, M>) ReflectionUtils        .newInstance(job.getConfiguration().getClass("bsp.work.class", BSP.class),            job.getConfiguration());    // The policy is to throw the first exception and log the remaining.    Exception firstException = null;    try {      bsp.setup(bspPeer);      bsp.bsp(bspPeer);    } catch (Exception e) {      LOG.error("Error running bsp setup and bsp function.", e);      firstException = e;    } finally {      try {        bsp.cleanup(bspPeer);      } catch (Exception e) {        LOG.error("Error cleaning up after bsp executed.", e);        if (firstException == null)          firstException = e;      } finally {        try {          bspPeer.close();        } catch (Exception e) {          LOG.error("Error closing BSP Peer.", e);          if (firstException == null)            firstException = e;        }        if (firstException != null)          throw firstException;      }    }  }

可见，调用setup之后紧接着就是调用bsp方法，setup中不加sync的话，可能master task正在发送消息，其他的peer已经进入bsp函数执行第一个超步了，这时他们试图读取master task发来的消息是读不到的。当然啦，也可以发第一轮消息发送放在第一个超步中。