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FAQ for the REEF


29 April 2015

What are the things that YARN doesn’t provide but REEF would have?

REEF is used to build on resource managers like YARN. REEF is not, in fact, an alternative to YARN. Resource managers manage the resources of a cluster in containers, which for the sake of simplicity can be thought of as slices of a machine: Some amount of CPU, memory, network and other resources on a physical computer. Different resource managers differ on the exact set of resources managed and how to acquire them, though. For example: Apache YARN uses a request based model, while Apache Mesos uses resource offers.

Given a resource manager of the above kind, REEF provides a control flow framework: It manages the containers as Evaluators and provides a centralized, event driven, control flow abstraction. To do so, REEF provides its own runtime for the coordination container (application master in YARN) and the slave containers. Together these two, called Job Driver and Evaluator, provide the following functionality beyond the resource manager:

  • Portability: REEF is designed to be portable between resource managers. Hence, applications built on REEF are portable. The current code base does, in fact, offer two such ports: One running on YARN and one running on a single machine that uses processes for containers.
  • Container reuse:REEF Evaluators outlive Tasks executed on them. When a Task finishes, a REEF application has the choice to re-use the Evaluator for another Task or to return it. The Evaluator process also offers facilities to hold on to state to be presented to subsequent Tasks, which forms the basis for recursive processing and cross-application handover.
  • Event coordination: REEF collects all events happening in the cluster and presents them to (application supplied) event handlers running in the Job Driver. Hence, the very hard problem of building a control flow for a distributed application is reduced to the (still hard, but) easier problem of building a centralized control flow for a distributed application when using REEF. Example events are: Evaluator allocation, Task start and stop and messages from a Task. Additionally, REEF also centralizes error handling in the form of events fired at the Job Driver. Where possible, those events even include the exceptions thrown by user code executing in the Evaluator.
  • Pre-built libraries:Because REEF standardizes the control flow, it can provide a common platform for additional libraries to be used by applications. One such library is the name service: It keeps track of which Evaluator executes which Task at all times. Crucially, this service is not hard-coded in REEF but instead is built as a plug-in library using the REEF API. We expect to build additional such libraries over time.
  • Communication:REEF provides mechanisms for the Job Driver to communicate with Tasks and vice versa.
  • Configuration:In conjunction with REEF, we developed Tang: A configuration framework for distributed systems based on dependency injection. Tang is built out of the experience with Hadoop’s configuration mechanism and provides a strong emphasis on early checking and configuration consistency.

What are similar products to REEF? How are they different?

The closest that comes to mind is Apache Twill. Like REEF, it provides a higher level API on top of Apache YARN. Unlike REEF, Twill exposes a thread-based API and is accessible to Java only. Essentially, it presents a YARN cluster as a system that can execute Java threads in a distributed fashion. As such, it can logically be viewed as on a higher layer than REEF: In addition to mechanisms, Twill contains policies. Namely the ones for a thread-based programming abstraction. In fact, we have a Twill-like abstraction built on REEF in a development branch. REEF provides considerably more flexibility than Twill: REEF is designed from the ground up to support mixed language environments and is also not limited to a thread abstraction.

REEF is also somewhat related to the various big data systems out there: Dryad, Apache Tez, Apache Spark,Apache Giraph, Hyracksetc. All of these have to provide similar functionality to REEF (failure detection, checkpointing ,…). Crucially, none of them expose these basic mechanisms as a layer in its own right. Instead, user code enters these systems at the level of the programming abstraction: Dataflows or graphs, usually. REEF aims to factor out common functionality of those systems into a layer of its own to avoid the pitfalls associated with repetitive functionality.

Does it run on any platform?

REEF is regularly tested on Windows Server 2012R2 as well as Ubuntu Linux 12.04, both running Java 7. REEF is developed on Workstations and Laptops running Mac OS X 10.8 and 10.9, various versions of Linux and Windows Pro in version 7 and 8.1. We’d expect it to run on any platform supported by Apache Hadoop with the potential exception of the .NET integration: That is currently tested only on Windows Server 2012R2 and Windows 8.1.

How do I try things in my laptop?

The easiest way is to run REEF applications using the REEF local runtime. That is a REEF implementation that uses local processes in lieu of a resource manager installation. The next level of complexity would be to setup a single-node YARN installation. Instructions on how to do so can be found in the documentation of your favorite Hadoop distribution.

Where is the best place to get a Hadoop distribution?

REEF is coded against the public API of Apache Hadoop version 2.2 or newer. Hence, we expect REEF applications to run on any distribution that ships with this version of Hadoop.

That said, we regularly test REEF using the Hortonworks Data Platform (HDP) as well as our own builds of development versions of Apache Hadoop. We do not test on any other distributions, but would welcome your success (or failure) stories.

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