Sunday, August 28, 2016

Taking a Detour with Apache Spark

Almost two years ago, while preparing for a talk I was giving at the now defunct Seattle Eastside Scala Meetup, I started a public GitHub project collecting and organizing Apache Spark code examples in Scala. I had stumbled on a way to run the examples on all supported platforms without setting up or deploying to a cluster, so the overheads of experimenting with the Spark APIs and programming idioms were remarkably low. It seemed like this approach was not well known at the time, so I shared it via the GItHub project and by posting here. Other than avoiding the overheads of a Spark cluster, the main feature of the project has been a "baby steps" approach to the examples. I've tried to demonstrate each API feature with multiple, slightly varying examples and (with notable, unfortunate exceptions) comments, to build intuitions before leaving the readers to take their chances with the Scaladoc.

Two years and about sixty sample programs later, I'm still not sure of the project's role and future, except that it has been tremendously helpful to my learning about Spark and Scala. The Apache Spark project's documentation and examples have improved, as has test coverage -- the latter always being a good way to learn about a new feature, except when there isn't any. The Databricks blog has also made a difference. And yet, the project continues to be useful to me, and I occasionally hear from others who find it helpful, including one local company that uses it in their training program. I like the "baby steps" approach to learning an API, and apparently I'm not the only one.

But lately I've had to ask myself some hard questions about the project. As I hope to post separately about soon, the evolution of Spark SQL's object model (remember SchemaRDD?) has made the task of keeping the project organized rather challenging lately -- I don't like to move examples around so I don't break links from the blog, StackOverflow and elsewhere. Another problem that's been nagging at me lately is my choice of Scala for the examples. I enjoy using Scala, have enjoyed learning it, and the Apache Spark project continues to keep the Scala APIs as a first class citizen. Indeed, Spark is written in Scala, but as I'll discuss later, that's no guarantee of strong support for Scala APIs. I've never been interested in the Python or R APIs, even though I believe they're of tremendous importance to the industry: I'm not part of the target audience (broadly, the data scientist) and I don't enjoy programming in either language. That leaves Java.

Time to explore the Java APIs

Many of you have seen the various Typesafe/Lightbend surveys showing Scala to be more popular than Java for Spark development -- the latest one has it at 76% Scala, 58% Java 8 and 34% Java 7 or lower. Clearly, there is overlap, so it's not clear whether Java or Scala are more popular overall. I see several reasons to explore Spark from the Java perspective:

  • Java is clearly an important part of the Spark ecosystem, as the surveys show.
  • The Java APIs are not merely an afterthought in Spark: real effort seems to have been invested in making Java programming practical and a reasonable approach.
  • While even a quick examination of the Spark project's Java examples (which date back to Java 7) shows them to be verbose and awkward compared with the Scala examples, the introduction of functional programming features in Java 8 raises the possibility of Java catching up.
  • I see a certain hesitation about Scala in the "big data" ecosystem. Lightbend has taken the "we don't have to choose" approach, and seems to be pretty sincere about it -- and of course they should be if they believe their own survey results. Confluent's decision about Apache Kafka is a bit more interesting: Kafka is also written in Scala, but only supports a Java API, with others provided by the community. While Cake Solutions actively develops the scala-kafka-client project, the Scala APIs are definitely not a first class citizen.
  • I've been a Java programmer, on and off, for 18 years. Before Scala, it was my recreational language of choice, and I still like it. I'm curious about Java 8, which I've only used a little, for another recent project.

Together, these certainly don't motivate me to abandon Scala, but they do motivate me to understand the tradeoffs better than I do now. The bottom line is that I've started adding some Java examples to the project, and started marking my commit messages with "[Scala]" or "[Java]" as appropriate.

Important Questions

I'm definitely making this up as I go, so let me expose some of the decisions I'm trying to make.

Which Examples?

I started with Dataset and DataFrame, since I had recently worked on those in Scala. But I'd at least like to get a cross section of the different areas: core RDDs, SQL, streaming and perhaps GraphX. Then I'll probably focus more on the areas that bring out interesting differences, whichever they turn out to be. There's no point exploring Spark SQL as a query language comprehensively in both Java and Scala, so I won't do it in Java.

Which Version(s) of Spark?

This is easy: much of the reason I invest in the project is to keep up with Spark evolution, and it takes a lot of effort. I'll continue adopting each new Spark release as soon as I can, and use its new features.

Java 8 or Earlier?

Java 8 seems to be getting a lot of adoption, and the new features definitely make it better suited to Spark. But the APIs have a number of features that were intended to work around the deficiencies of earlier versions of Java (such as all of, so it seems interesting to explore them for a while. Yet I'll probably change to Java 8 soon to keep the project from becoming a museum.

One or Two Projects on GitHub?

So far I've used the parallel Scala/Java source tree structure of sbt projects to locate the Java code in the same project as the Scala code, but I'm already feeling like this was a bad idea. I think it hinders my ability to serve the community, since Java programmers are much more likely to be familiar with Maven than sbt, and the one Java class I had written to support the Scala code (hiveql.SumLargeSalesUDAF) is now tangled up with the Java examples. I think you can expect to see a separate project soon. (Splitting the projects also allows me to use different Java versions.)

Parallel Organization?

As I mentioned earlier, the evolution of the object model around Spark SQL has made it hard to keep the project organized, and the Scala examples are getting out of hand. I'm not going to inflict this entropy on Java developers, and will try to organize the Java examples according to my current understanding of how Spark fits together. In due course this may help me sort out the organization of the Scala examples too -- in any case I'm hoping to write a separate post on this topic.

How Much Effort?

I don't know how much I'll balance my effort on Scala and Java examples, or even whether I'll keep working on the Java ones for much longer. It depends on feedback, how much insight I get, where the community ends up going, and how Java and Scala (and Spark) evolve.

Abandoning Scala?

I've already made this decision: definitely not. It's now my recreational language of choice, and I think it has a future. At the very least, I plan to keep up my attempts at covering major Spark features in Scala as they evolve.

Feedback please!

While my "baby steps" approach to Spark examples seems to have had some impact on the community, I get very little direct feedback. Occasional questions have inspired some of the examples, which I hope were helpful to those people, and one local professional who reached out and told me how he has found the project valuable has dramatically increased my motivation. I'd be delighted to hear about your experiences, either about the examples themselves, or about Spark in general.