[Hadoop Wiki] Update of "WritingYarnApps" by Arun C Murthy

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The "WritingYarnApps" page has been changed by Arun C Murthy:
http://wiki.apache.org/hadoop/WritingYarnApps

New page:
We describe, at a high-level, the way to implement new Applications for YARN.

=== Concepts & Flow ===
The general concept is: the 'application submission client' submits an 'application' to the ResourceManager via ClientRMProtocol#getNewApplicationId/ClientRMProtocol#submitApplication. During the call to ClientRMProtocol#submitApplication you need to provide sufficient information to the ResourceManager to 'launch' the application's first container i.e. the ApplicationMaster - see ApplicationSubmissionContext for details. You need to provide app-specific command line, unix env (optional) etc. - effectively you need to describe the unix process which needs to be launched for your ApplicationMaster.

The RM then launches the ApplicationMaster (as specified) which then communicates to the RM via AMRMProtocol. 

First it needs to register (AMRMProtocol#registerApplicationMaster. Then uses AMRMProtocol#allocate to request/receive for containers for it's task.

To launch a container for its task the AM uses ContainerManager#startContainer (i.e. talks to NodeManager which implements this x'face) and passes a ContainerLaunchContext which, similar to ApplicationSubmissionContext, has command-line, env etc.

=== Interfaces ===
The interfaces you'd most like be concerned with are:
ContainerManager -> Protocol for AM to talk to NodeManager to start/stop containers
ClientRMProtocol -> Protocol for job-client (launching program from gateway) to talk to RM to launch a new application i.e. application master itself
AMRMProtocol -> Protocol for AM to use to register/stop itself and get resources from the Scheduler