Distributed Computing Working Group Progress Report: 2016
Data sizes continue to increase, while single core performance has stagnated. We scale computations by leveraging multiple cores and machines. Large datasets are expensive to replicate, so we minimize data movement by moving the computation to the data. Many systems, such as Hadoop, Spark, and massively parallel processing (MPP) databases, have emerged to support these strategies, and each exposes its own unique interface, with little standardization.
Developing and executing an algorithm in the distributed context is a complex task that requires specific knowledge of and dependency on the system storing the data. It is also a task orthogonal to the primary role of a data scientist or statistician: extracting knowledge from data. The task thus falls to the data analysis environment, which should mask the complexity behind a familiar interface, maintaining user productivity. However, it is not always feasible to automatically determine the optimal strategy for a given problem, so user input is often beneficial. The environment should only abstract the details to the extent deemed appropriate by the user.
R needs a standardized, layered and idiomatic abstraction for computing on distributed data structures. R has many packages that provide parallelism constructs as well as bridges to distributed systems such as Hadoop. Unfortunately, each interface has its own syntax, parallelism techniques, and supported platform(s). As a consequence, contributors are forced to learn multiple idiosyncratic interfaces, and to restrict each implementation to a particular interface, thus limiting the applicability and adoption of their software and hampering interoperability.
The idea of a unified interface stemmed from a cross-industry workshop organized at HP Labs in early 2015. The workshop was attended by different companies, universities, and R-core members. Immediately after the workshop, Indrajit Roy, Edward Ma, and Michael Lawrence began designing an abstraction that later became known as the CRAN package ddR (Distributed Data in R). It declares a unified API for distributed computing in R and ensures that R programs written using the API are portable across different systems, such as Distributed R, Spark, etc.
The ddR package has completed its initial phase of development; the first release is now on CRAN. Three ddR machine-learning algorithms are also on CRAN, randomForest.ddR, glm.ddR, and kmeans.ddR. Two reference backends for ddR have been completed, one for R’s parallel package, and one for HP Distributed R. Example code and scripts to run algorithms and code on both of these backends are available in our public repository at https://github.com/vertica/ddR.
The overarching goal of the ddR project was for it to be a starting point in a collaborative effort, ultimately leading to a standard API for working with distributed data in R. We decided that it was natural for the R Consortium to sponsor the collaboration, as it should involve both industry and R-core members. To this end, we established the R Consortium Working Group on Distributed Computing, with a planned duration of a single year and the following aims:
- Agree on the goal of the group, i.e., we should have a unifying framework for distributed computing. Define success metric.
- Brainstorm on what primitives should be included in the API. We can use ddR’s API of distributed data-structures and dmapply as the starting proposal. Understand relationship with existing packages such as parallel, foreach, etc.
- Explore how ddR like interface will interact with databases. Are there connections or redundancies with dplyr and multiplyr?
- Decide on a reference implementation for the API.
- Decide on whether we should also implement a few ecosystem packages, e.g., distributed algorithms written using the API.
We declared the following milestones:
- Mid-year milestone: Finalize API. Decide who all will help with developing the top-level implementation and backends.
- End-year milestone: Summary report and reference implementation. Socialize the final package.
This report outlines the progress we have made on the above goals and milestones, and how we plan to continue progress in the second half of the working group term.
Results and Current Status
The working group has achieved the first goal by agreeing that we should aim for a unifying distributed computing abstraction, and we have treated ddR as an informal API proposal.
We have discussed many of the issues related to the second goal, deciding which primitives should be part of the API. We aim for the API to support three shapes of data --- lists, arrays and data frames --- and to enable the loading and basic manipulation of distributed data, including multiple modes of functional iteration (e.g., apply() operations). We aim to preserve consistency with base R data structures and functions, so as to provide a simple path for users to port computations to distributed systems.
The ddR constructs permit a user to express a wide variety of applications, including machine-learning algorithms, that will run on different backends. We have successfully implemented distributed versions of algorithms such as K-means, Regression, Random Forest, and PageRank using the ddR API. Some of these ddR algorithms are now available on CRAN. In addition, the package provides several generic definitions of common operators (such as colSums) that can be invoked on distributed objects residing in the supporting backends.
Each custom ddR backend is encapsulated in its own driver package. In the conventional style of functional OOP, the driver registers methods for generics declared by the backend API, such that ddR can dispatch the backend-specific instructions by only calling the generics.
The working group explored potential new backends with the aim of broadening the applicability of the ddR interface. We hosted presentations from external speakers on Spark and TensorFlow, and also considered a generic SQL backend. The discussion focused on Spark integration, and the R Consortium-funded intern Clark Fitzgerald took on the task of developing a prototype Spark backend. The development of the Spark backend encountered some obstacles, including the immaturity of Spark and its R interfaces. Development is currently paused, as we await additional funding.
During the monthly meetings, the working group deliberated on different design improvements for ddR itself. We list two key topics that were discussed. First, Michael Kane and Bryan Lewis argued for a lower level API that directly operates on chunks of data. While ddR supports chunk-wise data processing, via a combination of dmapply() and parts(), its focus on distributed data structures means that the chunk-based processing is exposed as the manipulation of these data structures. Second, Clark Fitzgerald proposed restructuring the ddR code into two layers that includes chunk-wise processing while retaining the emphasis on distributed data structures. The lower level API, which will interface with backends, will use a Map() like primitive to evaluate functions on chunks of data, while the higher level ddR API will expose distributed data structures, dmapply, and other convenience functions. This refactoring would facilitate the implementation of additional backends.
Discussion and Future Plans
The R Consortium-funded working group and internship has helped us start a conversation on distributed computing APIs for R. The ddR CRAN package is a concrete outcome of this working group, and serves as a platform for exploring APIs and their integration with different backends. While ddR is still maturing, we have arrived at a consensus for how we should improve and finalize the ddR API.
As part of our goal for a reference implementation, we aim to develop one or more prototype backends that will make the ddR interface useful in practice. A good candidate backend is any open-source system that is effective at R use cases and has strong community support. Spark remains a viable candidate, and we also aim to further explore TensorFlow.
We plan for a second intern to perform three tasks: (1) refactor the ddR API to a more final form, (2) compare Spark and TensorFlow in detail, with an eye towards the feasibility of implementing a useful backend, and (3) implement a prototype backend based on Spark or TensorFlow, depending on the recommendation of the working group.
By the conclusion of the working group, it will have produced:
- A stable version of the ddR package and at least one practical backend, released on CRAN,
- A list of requirements that are relevant and of interest to the community but have not yet been met by ddR, including alternative implementations that remain independent,
- A list of topics that the group believes worthy of further investigation.
 Clark Fitzgerald. https://github.com/vertica/ddR/wiki/Design