The JobTracker is a crucial component in the Hadoop MapReduce framework that manages resources and coordinates the processing of data across the Hadoop cluster. Its primary role is to schedule, monitor, and manage the execution of MapReduce jobs, thereby ensuring the efficient utilization of cluster resources.
Here are some key responsibilities of JobTracker:
1. Job submission: When a client submits a MapReduce job, JobTracker accepts the job and stores it in the shared storage.
2. Job initialization: JobTracker initializes the job by splitting the input data into multiple chunks, called input splits. Each input split is processed by a single map task.
3. Task assignment: JobTracker assigns map and reduce tasks to TaskTrackers, which are the worker nodes that execute tasks. JobTracker considers the availability of TaskTrackers, their computing capacity, and data locality while assigning tasks.
4. Fault tolerance: JobTracker monitors the status of running tasks on the TaskTrackers. In case of task failures, it reschedules the failed tasks on different TaskTrackers, thereby ensuring fault tolerance. JobTracker can also blacklist a TaskTracker if it detects frequent task failures on that node.
5. Progress monitoring: JobTracker keeps track of the job progress and provides this information to the client. It keeps track of completed tasks, pending tasks, and failed tasks for every job.
6. Resource management: JobTracker controls the allocation of resources within the cluster and ensures the efficient usage of computing power and memory.
7. Task coordination: JobTracker takes care of the synchronization and coordination between map and reduce tasks. It ensures that reduce tasks start only after all the map tasks have completed.
In summary, the JobTracker is the central controller in the Hadoop MapReduce framework, overseeing job execution, resource management, fault tolerance, and coordination among map and reduce tasks.
It is important to note that JobTracker is used in Hadoop version 1.x. In Hadoop version 2.x, the processing framework switched to YARN (Yet Another Resource Negotiator), where the JobTracker’s roles are divided into two separate components: ResourceManager and ApplicationMaster. ResourceManager takes care of resource allocation, while ApplicationMaster manages the life cycle of a job, including task assignment and monitoring.