Haskell meets large scale distributed analytics
Alp Mestanogullari, Mathieu Boespflug   |   25 February 2016

Large scale distributed applications are complex: there are effects at scale that matter far more than when your application is basked in the warmth of a single machine. Messages between any two processes may or may not make it to their final destination. If reading from a memory bank yields corrupted bytes about once a year, with 10,000 nodes this is likely to happen within the hour. In a million components system some hardware somewhere will be in a failed state, continuously. And it takes cunning to maximize throughput when network latencies are vastly superior to processing units’ crunch power. The key idea behind distributed computing middlewares such as Hadoop is to capture common application patterns, and solve coping with these effects once and for all for each such pattern, so that applications writers don’t have to do so themselves from scratch every time. Today we’re introducing a tech preview of sparkle. The motto: implement a robust and scalable distributed computing middleware for Haskell, by reusing Apache Spark (itself built on top of parts of Hadoop).

Why Spark? We could as well have built a complete platform starting from the likes of Cloud Haskell, which we maintain. And distributed computing engines is increasingly becoming a crowded space. But we started by asking a simple question: if I’m a data scientist seeking to train a model with state-of-the-art machine learning techniques, what is my best option to get the job done? How can I do that without giving up Haskell’s strong static guarantees and concise syntax?

Spark is a popular piece of the puzzle that leverages the huge Hadoop ecosystem for storage and cluster resource management to make it easy to write robust and scalable distributed applications as the composition of basic but familiar combinators to us FP aficionados: (distributed!) map, filter, zip, reduce, concat and many of their friends. These patterns generalize the suprisingly effective MapReduce framework of old. And on top of those, Sparks builds an impressively large set of general machine learning techniques as a library (we’ll see an example of using these in this post).

Today, Spark is already available to write scalable Scala, Java, R or Python applications. Haskell is a great language for writing clearly the kind of intricately complex algorithms common in analytics, so we’re throwing Haskell into the mix. With Haskell, you get the benefit of a language and ecosystem ruthlessly focused on refactorability, backed by a state of the art optimizing native code compiler supporting SIMD intrinsics when you need them.

sparkle basics

So what is it like in practice? sparkle’s “Hello World” on a hosted Amazon EMR cluster:

# Build it
$ stack build hello
# Package it
$ mvn package -f sparkle -Dsparkle.app=sparkle-example-hello
# Run it
$ spark-submit --master 'spark://IP:PORT' sparkle/target/sparkle-0.1.jar

The code looks something like this:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE StaticPointers #-}

import Control.Distributed.Spark as RDD
import Data.Text (isInfixOf)

main :: IO ()
main = do
    conf <- newSparkConf "Hello sparkle!"
    sc <- newSparkContext conf
    rdd <- textFile sc "s3://some-bucket/some-file.txt"
    as <- RDD.filter (static (\line -> "a" `isInfixOf` line)) rdd
    numAs <- RDD.count as
    putStrLn $ show numAs ++ " lines with the letter 'a'."

Nothing too fancy here: assuming we have a file somewhere (on S3 in this case), we’d like to count the number of lines that contain at least one “a”. If the file is very large, Spark, the underlying middleware that we’re using to write this app, could choose to do this piecewise on multiple nodes in parallel.

Spark will break down this high-level description into a number of low-level tasks, then schedule these tasks to be executed on whatever nodes in your cluster are available. If any of those tasks fail (e.g if a node gets disconnected from the cluster), Spark will automatically reschedule this task on another node, which means distributed applications written with Spark are fault-tolerant out of the box.

Spark goes far beyond your basic map, filter or fold: it also provides modules for large-scale graph processing, stream processing, machine learning and dataframes, all based on a simple abstraction for distributed collections called “Resilient Distributed Dataset” (RDD).

What sparkle does is provide bindings for the API of this middleware, along with a framework to conveniently write Spark based apps in Haskell and package them for distribution cluster-wide. There’s a lot more that we can do with this than counting lines - in just a moment we’ll have a look at a full scale machine learning example. But first let’s take a quick peek under the hood.

Shipping functions across language boundaries and across machines

The core computational content of our app above is captured in the lambda passed to `filter. Note that once again, Spark may choose to perform the filtering over the dataset given a predicate on one or more remote nodes in the cluster, in parallel. But if it is an arbitrary closure, how does this one get shipped around the cluster? Does sparkle somehow know how to serialize closures into a sequence of bytes, hand it to Spark, and tell Spark how to deserialize this closure on the other end of the wire?

That’s where the -XStaticPointers extension comes in. If you haven’t heard of this before, you might want to follow the link because what we do in sparkle is very similar to what’s done at the end of that post, in the Static Closures section. The gist is that we don’t know how to serialize arbitrary closures, but we do know, thanks to the distributed-closure package, how to serialize and ship so-called static closures, i.e. closures composed of only top-level bindings and serializable arguments. In Haskell, we need to ask the compiler to check that the closure really is static by using the static keyword. Spark in Scala has similar limitations, but the static closure check is entirely implicit (no keyword required, at the cost of some significant extra magic in the compiler).

So operationally, Spark handles the input data, notices that our application is one big filter over the whole input data set, figures out a smart allocation of available nodes to perform parts of the map, ships a symbolic representation of the function to map (a “static pointer”) to those nodes as necessary, and combines the results transparently so that we don’t have to.

It’s a nice enough toy example, but we’ll explore next how to use sparkle for a real machine learning use case.

Online Latent Dirichlet Allocation, from Haskell

We’ll try this Scala application in Haskell. The goal: classify articles according to the overall topic they’re likely to be covering (zoology? television? distributed computing?). The method we’ll use for this is called Latent Dirichlet Allocation (LDA), as described here using Scala, but here’s a one-sentence summary: given a collection of text documents, the algorithm tries to infer the different topics discussed in the said documents by looking at the distribution of words used in those documents. The “online” variation consists in learning an LDA model incrementally, instead of having to go through the entire collection of documents before it can give us a model. For the curious, this variation is described in this paper.

We will go through this step by step. The first thing to do is to initialize a few (internal) Spark datastructures, collectively called a context (as in the first example). We’ll be using Spark’s recently introduced Data Frames, so we need the context to include that too:

main :: IO ()
main = do
  conf <- newSparkConf "Spark Online LDA"
  sc <- newSparkContext conf
  sqlc <- newSQLContext sc

Next, we load a collection of “stop words” (from S3) as an RDD and make a good old Haskell list of values out of that, which we can pass around conveniently. These are the words we want the algorithm to ignore when analyzing the documents, usually a list of the most common words that do not inform the topic discussed in a document (such as “and”, “the” and so on).

  stopwords <- textFile sc "s3://some-bucket/stopwords.txt"
           >>= collect

We’re now ready to load our collection of documents, using Spark’s wholeTextFiles function, mapping a directory (local or distant) to an RDD of key-value pairs of the form (filename, content). For our purposes we’ll drop the file names, keep the values and assign each one a numeric index.

  docs <- wholeTextFiles sc "s3://some-bucket/documents/"
      >>= justValues
      >>= zipWithIndex

Before we hand the documents to the LDA algorithm, we need to tokenize their contents, remove the stopwords and do some other preprocessing. This is all provided by Spark but at the level of Data Frames, not on plain old RDDs. The conversion from RDD to data frame is not particularly interesting but here it is:

  docsRows <- toRows docs
  docsDF <- toDF sqlc docsRows "docId" "text"

We end up with a dataframe where each row has 2 columns, the first named “docId” corresponding to the identifier, the second named “text” corresponding to the document’s content.

To process this big data frame of all documents, we’ll want to build a pipeline consisting of a tokenizer, a stop words remover and a “count vectorizer”, which despite its fancy name basically just counts the number of times each word is used in a given document. Each step of the pipeline takes as argument the name of the column it should consider as its input and the name of the column it will place the output in, in the resulting data frame.

  tok <- newTokenizer "text" "words"
  tokenizedDF <- tokenize tok docsDF
  swr <- newStopWordsRemover stopwords "words" "filtered"
  filteredDF <- removeStopWords swr tokenizedDF
  cv <- newCountVectorizer vocabSize "filtered" "features"
  cvModel <- fitCV cv filteredDF
  countVectors <- toTokenCounts cvModel filteredDF "docId" "features"

At this stage, we have a distribution of “word usage” for each document. We can now initialize the algorithm and run it against the aforementionned distributions, with parameters that depend on the dataset you are running LDA against. Finally, let’s print a summary of the topic model inferred by LDA:

  lda <- newLDA batchFraction numTopics maxIterations
  ldamodel <- runLDA lda countVectors
  describeResults ldamodel cvModel maxTermsPerTopic

    numTopics = 10
    batchFraction = 1
    vocabSize = 600
    maxTermsPerTopic = 10
    maxIterations = 50

By way of example, we applied the above on a dataset consisting of articles from the New York Times. The algorithm had to identify 10 topics, the first two summarized as follows:

>>> Topic #0
atlanta -> 0.05909878836489215
journal -> 0.03736792191830798
constitution -> 0.03532890625936911
moved -> 0.03254662858728971
service -> 0.02101874084187926
beach -> 0.01934327641726217
washington -> 0.015744658734434487
column -> 0.011533062032191725
editor -> 0.01027877462336505
coxnews -> 0.010027441771679654
>>> Topic #1
rubin -> 0.040416783706726876
citigroup -> 0.015552806663132827
enron -> 0.014951083930487013
first -> 0.014679582063525697
company -> 0.01296990190006682
clinton -> 0.012818751484368918
former -> 0.012721129987043
policy -> 0.010760148602112128
business -> 0.010315259378148107
world -> 0.009556332963292368
.... (a bunch more topics)

Complete code and examples

All the code for sparkle, including for the two demos from this blog post, is available here. The apps/ folder contains the code for the demos while the sparkle/ folder contains all the code that makes the demos work.

Where to from here?

We set out on a pragmatic mission: marry two different ecosystems that rarely cross over for a quick, painless but very robust solution to support common analytics workflows in Haskell. To do so we had to teach JVM to Haskell, and Haskell to the JVM. It turns out that doing so was nowhere near painless (more on that in the next post!), but quicker than scattering both communities’ resources on reimplementing their own platforms for medium to large scale analytics. Do note that sparkle is still just a tech preview, but we can already realize these benefits:

Deep within sparkle lies a proto inline-java waiting to break free, much in the style of inline-r and inline-c. That is to say, in pursuit of running Haskell on Spark, we ended up with most of the basic elements to enable writing mixed syntax source files including both Java and Haskell for seamless interop. So from here what we’d like to do is:

In short, plenty of new directions to contribute towards. :)

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