1. 写在前面
正如前几篇文章所述，YARN/MRv2是一个资源统一管理系统，它上面可以运行各种计算框架，而所有计算框架的client端编写方法类似，本文拟以MapReduce计算框架的client端代码为例进行说明。
2. 两个相关协议
需要通过两个协议提交作业：
ClientProtocol：Hadoop中的JobClient通过该协议向JobTracker提交作业
ClientRMProtocol：Yarn中的client通过该协议向ResourceManager提交作业。
3. Client设计方法
为了使Hadoop MapReduce无缝迁移到Yarn中，需要在client端同时使用这两个协议，采用的方法是：
【继承+组合的设计模式】
设计新类YARNRunner，实现ClientProtocol接口，并将ClientRMProtocol对象作为内部成员。当用户提交作业 时，会直接调用YARNRunner中的submitJob函数，在该函数内部，会接调用ClientRMProtocol的 submitApplication函数，将作业提交到ResourceManager中。此处的submitApplication函数实际上是一个 RPC函数，由ResourceManager实现。
我们看一下ClientRMProtocol接口中的所有方法：
public SubmitApplicationResponse submitApplication( SubmitApplicationRequest request) throws YarnRemoteException;
向ResourceManager提交新的application，client调用该函数时，需要在参数request中指定application所在队列，ApplicationMaster相关jar包及启动方法等信息。
public KillApplicationResponse forceKillApplication( KillApplicationRequest request) throws YarnRemoteException;
client要求ResourceManager杀死某个application。
public GetApplicationReportResponse getApplicationReport( GetApplicationReportRequest request) throws YarnRemoteException;
client通过该函数向ResourceManager查询某个application的信息，如id，user，time等信息。
4. 整个流程分析
Client首先通过ClientRMProtocal#getNewApplication获取一个新的“ApplicationId”，然后使 用ClientRMProtocal#submitApplication提交一个application，当调用 ClientRMProtocal#submitApplication时 ，需要向Resource Manager提供足够的信息以便启动第一个container（实际上就是Application Master）。Client需要提供足够的细节信息，如运行application需要的文件和jar包，执行这些jar包需要的命令，一些unix环 境设置等。
这之后，Resource Manager会首先申请一个container，并在它里面启动ApplicationMaster，之后ApplicationMaster会通过 AMRMProtocal和ContainerManager分别与Resource Manager和Node Manager通信进行资源申请和container启动。
具体细节：
（1） Client向Resource Manager发动一个连接，更具体 一些，实际上是向ResourceManager的ApplicationsManager发动一个连接。
YarnRPC rpc = YarnRPC.create(this.conf);
InetSocketAddress rmAddress =
NetUtils.createSocketAddr(this.conf.get(
YarnConfiguration.RM_ADDRESS,
YarnConfiguration.DEFAULT_RM_ADDRESS),
YarnConfiguration.DEFAULT_RM_PORT,
YarnConfiguration.RM_ADDRESS);
LOG.info("Connecting to ResourceManager at " + rmAddress);
applicationsManager =
(ClientRMProtocol) rpc.getProxy(ClientRMProtocol.class,
rmAddress, this.conf);
（2） 一旦获取一个连接到ASM的handler，client要求ResourceManager分配一个新的ApplicationId。
SubmitApplicationRequest request = recordFactory.newRecordInstance(SubmitApplicationRequest.class);
request.setApplicationSubmissionContext(appContext);
applicationsManager.submitApplication(request);
LOG.info("Submitted application " + applicationId + " to ResourceManager");
（3） ASM返回的response中也包含cluster的信息，如该cluster中最少/最大可用资源量，这可以帮助我们合理的设置Application Master需要的资源量，关于更多细节，可查看GetNewApplicationResponse类。
Client最重要的任务是设置对象ApplicationSubmissionContext，它定义了ResourceManager启动ApplicationMaster所需的全部信息。Client需要在该context中设置一下信息：
[1] 队列，优先级信息：该application将要提交到哪个队列，以及它的优先级是多少。
[2] 用户：哪个用户提交的application，这主要用于权限管理。
[3] ContainerLaunchContext：启动并运行ApplicationMaster的那个container的相关信息，包括：本地资源 （binaries，jars，files等），安全令牌（security tokens），环境变量设置（CLASSPATH等）和运行命令。
// Create a new ApplicationSubmissionContext
ApplicationSubmissionContext appContext =
Records.newRecord ( ApplicationSubmissionContext . class ) ;
// set the ApplicationId
appContext.setApplicationId ( appId ) ;
// set the application name
appContext.setApplicationName ( appName ) ;
// Create a new container launch context for the AM'scontainer
ContainerLaunchContext amContainer =
Records.newRecord ( ContainerLaunchContext . class ) ;
// Define the local resources required
Map < String , LocalResource > localResources =
new HashMap < String , LocalResource > ( ) ;
// Lets assume the jar we need for our ApplicationMaster is available in
// HDFS at a certain known path to us and we want to make it available to
// the ApplicationMaster in the launched container
Path jarPath ; // <- known path to jar file
FileStatus jarStatus = fs.getFileStatus ( jarPath ) ;
LocalResource amJarRsrc = Records.newRecord ( LocalResource . class ) ;
// Set the type of resource - file or archive
// archives are untarred at the destination by the framework
amJarRsrc.setType ( LocalResourceType.FILE ) ;
// Set visibility of the resource
// Setting to most private option i.e. this file will only
// be visible to this instance of the running application
amJarRsrc.setVisibility ( LocalResourceVisibility . APPLICATION ) ;
// Set the location of resource to be copied over into the
// working directory
amJarRsrc.setResource ( ConverterUtils . getYarnUrlFromPath ( jarPath ) ) ;
// Set timestamp and length of file so that the framework
// can do basic sanity checks for the local resource
// after it has been copied over to ensure it is the same
// resource the client intended to use with the application
amJarRsrc.setTimestamp ( jarStatus . getModificationTime ( ) ) ;
amJarRsrc.setSize ( jarStatus . getLen ( ) ) ;
// The framework will create a symlink called AppMaster.jar in the
// working directory that will be linked back to the actual file.
// The ApplicationMaster, if needs to reference the jar file, would
// need to use the symlink filename.
localResources.put ( "AppMaster.jar" , amJarRsrc ) ;
// Set the local resources into the launch context
amContainer.setLocalResources ( localResources ) ;
// Set up the environment needed for the launch context
Map < String , String > env = new HashMap < String , String > ( ) ;
// For example, we could setup the classpath needed.
// Assuming our classes or jars are available as local resources in the
// working directory from which the command will be run, we need toappend
// "." to the path.
// By default, all the hadoop specific classpaths will already be available
// in $CLASSPATH, so we should be careful not to overwrite it.
String classPathEnv = "$CLASSPATH:./*:" ;
env . put ( "CLASSPATH" , classPathEnv ) ;
amContainer . setEnvironment ( env ) ;
// Construct the command to be executed on the launched container
String command =
"${JAVA_HOME}" + / bin / java " +
" MyAppMaster" +
" arg1 arg2 arg3" +
" 1>" + ApplicationConstants . LOG_DIR_EXPANSION_VAR + "/stdout" +
" 2>" + ApplicationConstants . LOG_DIR_EXPANSION_VAR + "/stderr" ;
List < String > commands = new ArrayList < String > ( ) ;
commands.add ( command ) ;
// add additional commands if needed
// Set the command array into the container spec
amContainer.setCommands ( commands ) ;
// Define the resource requirements for the container
// For now, YARN only supports memory so we set the memory
// requirements.
//If the process takes more than its allocated memory, it will
// be killed by the framework.
// Memory being requested for should be less than max capability
// of the cluster and all asks should be a multiple of the min capability.
Resource capability = Records . newRecord ( Resource . class ) ;
capability.setMemory ( amMemory ) ;
amContainer.setResource ( capability ) ;
// Set the container launch content into the ApplicationSubmissionContext
appContext.setAMContainerSpec ( amContainer ) ;
(4) 这之后client可以向ASM提交application：
// Create the request to send to the ApplicationsManager SubmitApplicationRequest appRequest = Records.newRecord ( SubmitApplicationRequest . class ) ; appRequest.setApplicationSubmissionContext ( appContext ) ; // Submit the application to the ApplicationsManager // Ignore the response as either a valid response object is returned on //success or an exception thrown to denote the failure applicationsManager. submitApplication ( appRequest ) ;
（4） 到此为止，ResourceManager应该已经接受该application，并根据资源需求分配一个container，最终在分配的 container中启动ApplicationMaster。Client有多种方法跟踪实际任务的进度：可以使用 ClientRMProtocal#getApplicationReport与ResourceManager通信以获取application执行当 前情况报告。
GetApplicationReportRequest request = recordFactory .newRecordInstance(GetApplicationReportRequest.class); request.setApplicationId(appId); GetApplicationReportResponse response = applicationsManager .getApplicationReport(request); ApplicationReport applicationReport = response.getApplicationReport();
从ResourceManager中获取的ApplicationReport包含以下内容：
[1] 一般的application信息，如：ApplicationId，application所在队列，application对应用户等
[2] ApplicationMaster信息：ApplicationMaster所在的host，接收用户请求的rpc port以及client与ApplicationMaster通信需要的token等。
[3] 追踪Application的相关信息：如果application支持进度追踪，可以设置一个tracking url，通过该url，client可以直接获取进度。
[4] ApplicationStatus：client通过ApplicationReport#getYarnApplicationState可从 ResourceManager那获取application的当前状态，如果ApplicationState为FINISHED，client需要调 用ApplicationReport#getFinalApplicationStatus检查application运行成功或者失败，如果运行失 败，可调用ApplicationReport#getDiagnostics获取application失败的详细信息。
[5] 如果ApplicationMaster支持，client可直接通过host：rpcport向ApplicationMaster查询其执行进度。当然，也可以使用上面提到的tracking url。