Task Execution

Task Execution :

• After the task tracker assigns a task, the next step is for it to run the task.
• First, it localizes the job JAR by copying it from the shared filesystem to the tasktracker’s filesystem.
• It also copies any files needed from the distributed cache by the application to the local disk.
• Second, it creates a local working directory for the task and un-jars the contents of the JAR into this directory.
• Third, it creates an instance of TaskRunner to run the task.
• TaskRunner launches a new Java Virtual Machine to run each task so that any bugs in the user-defined map and reduce functions don’t affect the task tracker (by causing it to crash or hang, for example).
• It is, however, possible to reuse the JVM between tasks.
• The child process communicates with its parent through the umbilical interface.
• This way it informs the parent of the task’s progress every few seconds until the task is complete.