In the real world, user code is buggy, processes crash, and machines fail. One of the major benefits of using Hadoop is its ability to handle such failures and allow your job to complete successfully. We need to consider the failure of any of the following entities the task, the application master, the node manager, and the resource manager.
Consider first the case of the task failing. The most common occurrence of this failure is when user code in the map or reduce task throws a runtime exception. If this happens, the task JVM reports the error back to its parent application master before it exits. The error ultimately makes it into the user logs. The application master marks the task attempt as failed, and frees up the container so its resources are available for another task.
Another failure mode is the sudden exit of the task JVM perhaps there is a JVM bug that causes the JVM to exit for a particular set of circumstances exposed by the MapReduce user code. In this case, the node manager notices that the process has exited and informs the application master so it can mark the attempt as failed.
Hanging tasks are dealt with differently. The application master notices that it hasn’t received a progress update for a while and proceeds to mark the task as failed. The task JVM process will be killed automatically after this period. The timeout period after which tasks are considered failed is normally 10 minutes and can be configured on a per-job basis (or a cluster basis) by setting the mapreduce.task.timeout property to a value in milliseconds.
Setting the timeout to a value of zero disables the timeout, so long-running tasks are never marked as failed. In this case, a hanging task will never free up its container, and over time there may be cluster slowdown as a result. This approach should therefore be avoided, and making sure that a task is reporting progress periodically should suffice.
When the application master is notified of a task attempt that has failed, it will reschedule execution of the task. The application master will try to avoid rescheduling the task on a node manager where it has previously failed. Furthermore, if a task fails four times, it will not be retried again. This value is configurable. The maximum number of attempts to run a task is controlled by the mapreduce.map.maxattempts property for map tasks and mapreduce.reduce.maxattempts for reduce tasks. By default, if any task fails four times (or whatever the maximum number of attempts is configured to), the whole job fails.
For some applications, it is undesirable to abort the job if a few tasks fail, as it may be possible to use the results of the job despite some failures. In this case, the maximum percentage of tasks that are allowed to fail without triggering job failure can be set for the job. Map tasks and reduce tasks are controlled independently, using the mapreduce.map.failures.maxpercent and mapreduce.reduce.failures.maxpercent properties.
A task attempt may also be killed, which is different from it failing. A task attempt may be killed because it is a speculative duplicate or because the node manager it was running on failed and the application master marked all the task attempts running on it as killed.
Killed task attempts do not count against the number of attempts to run the task (as set by mapreduce.map.maxattempts and mapreduce.reduce.maxattempts), because it wasn’t the task’s fault that an attempt was killed.
Application Master Failure
Just like MapReduce tasks are given several attempts to succeed (in the face of hardware or network failures), applications in YARN are retried in the event of failure. The maximum number of attempts to run a MapReduce application master is controlled by the mapreduce.am.max-attempts property. The default value is 2, so if a MapReduce application master fails twice it will not be tried again and the job will fail.
YARN imposes a limit for the maximum number of attempts for any YARN application master running on the cluster, and individual applications may not exceed this limit. The limit is set by yarn.resourcemanager.am.max-attempts and defaults to 2, so if you want to increase the number of MapReduce application master attempts, you will have to increase the YARN setting on the cluster, too.
The way recovery works is as follows. An application master sends periodic heartbeats to the resource manager, and in the event of application master failure, the resource manager will detect the failure and start a new instance of the master running in a new container which is managed by a node manager.
In the case of the MapReduce application master, it will use the job history to recover the state of the tasks that were already run by the application so they don’t have to be rerun. Recovery is enabled by default, but can be disabled by setting yarn.app.mapreduce.am.job.recovery.enable to false.
Node Manager Failure
If a node manager fails by crashing or running very slowly, it will stop sending heartbeats to the resource manager (or send them very infrequently). The resource manager will notice a node manager that has stopped sending heartbeats if it hasn’t received one for 10 minutes (this is configured, in milliseconds, via the yarn.resourcemanager.nm.liveness-monitor.expiry-interval-ms property) and remove it from its pool of nodes to schedule containers on.
Any task or application master running on the failed node manager will be recovered using the mechanisms described in the previous two sections. In addition, the application master arranges for map tasks that were run and completed successfully on the failed node manager to be rerun if they belong to incomplete jobs, since their intermediate output residing on the failed node manager’s local filesystem may not be accessible to the reduce task.
Node managers may be blacklisted if the number of failures for the application is high, even if the node manager itself has not failed. Blacklisting is done by the application master, and for MapReduce the application master will try to reschedule tasks on different
nodes if more than three tasks fail on a node manager. The user may set the threshold with the mapreduce.job.maxtaskfailures.per.tracker job property.
Note : Note that the resource manager does not do blacklisting across applications (at the time of writing), so tasks from new jobs may be scheduled on bad nodes even if they have been blacklisted by an application master running an earlier job.
Resource Manager Failure
Failure of the resource manager is serious, because without it, neither jobs nor task containers can be launched. In the default configuration, the resource manager is a single point of failure, since in the (unlikely) event of machine failure, all running jobs fail—and can’t be recovered.
To achieve high availability (HA), it is necessary to run a pair of resource managers in an active-standby configuration. If the active resource manager fails, then the standby can take over without a significant interruption to the client.
Information about all the running applications is stored in a highly available state store (backed by ZooKeeper or HDFS), so that the standby can recover the core state of the failed active resource manager.
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