Samza的ApplicationMaster

当Samza ApplicationMaster启动时，它做以下的事情：

通过STREAMING_CONFIG环境变量从YARN获取配置信息(configuration)
在随机端口上启动一个JMX server
实例化一个metrics registry和reporter来追踪计量信息
将AM向YARN的RM注册
使用每个stream的PartitionManager来获取总共的partition数量
从Samza的job configuration里获取总的container数量
将partition分给container(在Samza AM的dashboard里，称为Task Group)
为每个container向YARN发送一个ResourceRequest
每秒向YARN RM poll一次，检查allocated and released containers

SamzaAppMaster的实现

并不是提交AppMaster，只是向RM注册这个AppMaster。因为此时,AppMaster已经启动了。

1.在SamzaAppMasterLifecycle对象的onInit()方法中，使用amCient.registerApplicationMaster

2 val response = amClient.registerApplicationMaster (host , state.rpcPort, "%s:%d" format (host, state. trackingPort))

amClient对象的类：

org.apache.hadoop.yarn.client.api.async.AMRMClientAsync<T extends org.apache.hadoop.yarn.client.api.AMRMClient.ContainerRequest>

AMRMClientAsync handles communication with the ResourceManager and provides asynchronous updates on events such as container allocations and completions. It contains a thread that sends periodic heartbeats to the ResourceManager. It should be used by implementing a CallbackHandler:

 class MyCallbackHandler implements AMRMClientAsync.CallbackHandler {
   public void onContainersAllocated(List<Container> containers) {
     [run tasks on the containers]
   }
   
   public void onContainersCompleted(List<ContainerStatus> statuses) {
     [update progress, check whether app is done]
   }
   
   public void onNodesUpdated(List<NodeReport> updated) {}
   
   public void onReboot() {}
 }

The client's lifecycle should be managed similarly to the following:

 AMRMClientAsync asyncClient = 
     createAMRMClientAsync(appAttId, 1000, new MyCallbackhandler());
 asyncClient.init(conf);
 asyncClient.start();
 RegisterApplicationMasterResponse response = asyncClient
    .registerApplicationMaster(appMasterHostname, appMasterRpcPort,
       appMasterTrackingUrl);
 asyncClient.addContainerRequest(containerRequest);
 [... wait for application to complete]
 asyncClient.unregisterApplicationMaster(status, appMsg, trackingUrl);
 asyncClient.stop();

这个类是用来做为一个Client和RM进行通信，并且注册一个用于回调的对象来处理container 的allocation和completion事件。它启动一个线程，周期性地发送hearbeat至ResourceManager