Major changes when migrating to MapReduce 2

Reviewing the differences between MapReduce version 1 (MRv1) and MapReduce version 2 (MRv2) helps you to understand the changes to the capabilities and services that have replaced the deprecated ones.

Architectural changes

MapReduce in Hadoop 2 is split into two components:
  • YARN (Yet Another Resource Negotiator): cluster resource management capabilities
  • MapReduce: MapReduce-specific capabilities

In the MapReduce version 1 (MRv1) architecture, the cluster is managed by a service called the JobTracker. TaskTracker services lived on each host and would launch tsks on behalf of jobs. The JobTracker would serve information about completed jobs.

In MapReduce version 2 (MRv2), the functions of the JobTracker are split between four services:

  • ResourceManager
  • ApplicationMaster
  • JobHistory Server
  • NodeManager

Configuration options with new name

Many configuration options have new names to reflect the shift. As JobTrackers and TaskTrackers no longer exist in MRv2, all configuration options pertaining to them no longer exist, although many of them have corresponding options for the ResourceManager, NodeManager, and JobHistoryServer.

The vast majority of job configuration options that were available in MRv1 work in MRv2 as well. For consistency and clarity, many options have been given new names. The older names are deprecated, but will still work for the time being. The exceptions to this are mapred.child.ulimit and all options relating to JVM reuse, as these are no longer supported.

Managing resources

One of the larger changes in MRv2 is the way that resources are managed. In MRv1, each host was configured with a fixed number of map slots and a fixed number of reduces slots. Under YARN, there is no distinction between resources available for maps and resources available for reduces - all resources are available for both.

The notion of slots has been discarded, and resources are now configured in terms of amounts of memory (in megabytes) and CPU (in “virtual cores”, which are described below).

In MRv1, the and mapred.tasktracker.reduce.tasks.maximum properties dictated how many map and reduce slots each TaskTracker had. These properties no longer exist in YARN. Instead, YARN uses yarn.nodemanager.resource.memory-mb and yarn.nodemanager.resource.cpu-vcores, which control the amount of memory and CPU on each host, both available to both maps and reduces.

Administration commands

The jobtracker and tasktracker commands, which start the JobTracker and TaskTracker, are no longer supported because these services no longer exist. They are replaced with yarn resourcemanager and yarn nodemanager, which start the ResourceManager and NodeManager respectively. hadoop mradmin is no longer supported. Instead, yarn rmadmin can be used.

Secure cluster setup

As in MRv1, a configuration must be set to have the user that submits a job own its task processes. The equivalent of the MRv1 LinuxTaskController is the LinuxContainerExecutor. In a secure setup, NodeManager configurations should set yarn.nodemanager.container-executor.class to org.apache.hadoop.yarn.server.nodemanager.LinuxContainerExecutor. Properties set in the taskcontroller.cfg configuration file should be transitioned to their analogous properties in the container-executor.cfg file.

Queue access control lists (ACLs) are now placed in the Capacity Scheduler configuration file instead of the JobTracker configuration.

High Availability

The underlying architecture of active-standby pair is similar to JobTracker HA in MRv1. A major improvement over MRv1 is: in YARN, the completed tasks of in-flight MapReduce jobs are not re-run on recovery after the ResourceManager is restarted or failed over. Further, the configuration and setup has also been simplified. The main differences are:
  • Failover controller is moved from a separate ZKFC daemon to be a part of the ResourceManaget itself, meaning that there is no need to run an additional daemon.
  • Clients, applications, and NodeManagers do not requre configuring a proxy-provider to talk to the active ResourceManager.