The Application Master is the process that coordinates the execution of an application in the cluster. Each application has its own unique Application Master that is tasked with negotiating resources (Containers) from the Resource Manager and working with the Node Managers to execute and monitor the tasks. In the YARN design, MapReduce is just one application framework; the design permits building and deploying distributed applications using other frameworks. For example, YARN ships with a Distributed Shell application that permits running a shell script on multiple nodes in a YARN cluster.
Once the Application Master is started (as a Container), it will periodically send heartbeats to the Resource Manager to affirm its health, and to update the record of its resource demands. After building a model of its requirements, the Application Master encodes its preferences and constraints in a heartbeat message to the Resource Manager. In response to subsequent heartbeats, the Application Master will receive a lease on Containers bound to an allocation of resources at a particular node in the cluster. Depending on the Containers it receives from the Resource Manager, the Application Master may update its execution plan to accommodate the excess or lack of resources. Container allocation/de-allocation can take place in a dynamic fashion as the application progresses.
