How Yarn works

YARN / Map Reduce 2 (Yet Another Resource Negotiator)

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Resource Manager

The ResourceManager is the ultimate authority that arbitrates resources among all the applications in the system.

The ResourceManager maintains the list of applications running on the cluster and the list of available resources on each live NodeManager.

Scheduler (Capacity & Fair scheduler):The Scheduler is responsible for allocating resources to the various running applications subject to familiar constraints of capacities, queues etc.

The Scheduler is pure scheduler in the sense that it performs no monitoring or tracking of status for the application.

It offers no guarantees about restarting failed tasks either due to application failure or hardware failures.

It manages who gets cluster resources (in the form of containers) and when. When the ResourceManager accepts a new application submission, one of the first decisions the Scheduler makes is selecting a container in which ApplicationMaster will run.

Application Manager: The ApplicationsManager is responsible for accepting job-submissions, negotiating the first container for executing the application specific ApplicationMaster and provides the service for restarting the ApplicationMaster container on failure.

It manages running Application Masters in the cluster, i.e., it is responsible for starting application masters and for monitoring and restarting them on different nodes in case of failures.

Application Master

The per-application ApplicationMaster is, in effect, a framework specific library and is tasked with negotiating resources from the ResourceManager and working with the NodeManager(s) to execute and monitor the tasks.

After the ApplicationMaster is started, it will be responsible for a whole life cycle of this application. First and foremost, it will be sending resource requests to the ResourceManager to ask for containers needed to run an application’s tasks. A resource request is simply a request for a number of containers that satisfies some resource requirements.

If and when it is possible, the ResourceManager grants a container (expressed as container ID and hostname) that satisfies the requirements requested by the ApplicationMaster in the resource request.

After a container is granted, the ApplicationMaster will ask the NodeManager (that manages the host on which the container was allocated) to use these resources to launch an application-specific task. This task can be any process written in any framework (such as a MapReduce task or a Giraph task). The NodeManager does not monitor tasks; it only monitors the resource usage in the containers and, for example, it kills a container if it consumes more memory than initially allocated.

The ApplicationMaster spends its whole life negotiating containers to launch all of the tasks needed to complete its application. It also monitors the progress of an application and its tasks, restarts failed tasks in newly requested containers, and reports progress back to the client that submitted the application. After the application is complete, the ApplicationMaster shuts itself down and releases its own container.

Though the ResourceManager does not perform any monitoring of the tasks within an application, it checks the health of the ApplicationMasters. If the ApplicationMaster fails, it can be restarted by the ResourceManager in a new container. You can say that the ResourceManager takes care of the ApplicationMasters, while the ApplicationMasters takes care of tasks.

Node Manager

The NodeManager is the per-machine framework agent who is responsible for containers, monitoring their resource usage (cpu, memory, disk, network) and reporting the same to the ResourceManager/Scheduler.

The NodeManager runs services to determine the health of the node it is executing on. The services perform checks on the disk as well as any user specified tests. If any health check fails, the NodeManager marks the node as unhealthy and communicates this to the ResourceManager, which then stops assigning containers to the node.

Container

Containers execute tasks as specified by the Application Master.

Resource manager restart

An application recovery after the restart of ResourceManager (YARN-128). The ResourceManager stores information about running applications and completed tasks in HDFS. If the ResourceManager is restarted, it recreates the state of applications and re-runs only incomplete tasks.

NNK

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