thurber

Apache Beam and Google Cloud Dataflow on ~~steroids~~ Clojure.

This is alpha software. Bleeding-edge and all that. Buyer beware.

• Principles
• Quickstart
• Guide
  • Transforms
  • Coders
• Demos
  • Word Count
  • Mobile Gaming Example
• Make It Fast

Principles

• Enable Clojure
  • Bring Clojure's powerful, expressive tookit (destructuring, immutability, REPL, async tools, etc etc) to Apache Beam.
• REPL Friendly
  • Build and test your pipelines incrementally in the REPL.
  • Learn Beam semantics (windowing, triggering) interactively.
• No Macros
• No AOT
• Bypassable API
  • No API lock-in. Pipelines can be composed of Clojure/thurber and Java transforms. Don't like thurber? Incrementally refactor your pipeline back to Java.
• Not Afraid of Java Interop
  • Wherever Clojure's Java Interop is performant and works cleanly with Beam, embrace it.

Quickstart

1. Clone & cd into this repository.
2. lein repl
3. Copy & paste:

(ns try-thurber
   (:require [thurber :as th]
             [clojure.string :as str])
   (:import (org.apache.beam.sdk.io TextIO)))

(defn- extract-words [sentence]
  (remove empty? (str/split sentence #"[^\p{L}]+")))

(defn- format-as-text [[k v]]
  (format "%s: %d" k v))

(.run
    (doto (th/create-pipeline)
      (th/apply!
        (-> (TextIO/read)
          (.from "demo/word_count/lorem.txt"))
        #'extract-words
        #'th/->kv
        (th/count-per-key)
        #'format-as-text
        #'th/log-elem*)))

Guide

Beam's Java API is oriented around constructing a Pipeline and issuing a series of .apply invocations to mutate the pipeline and build up a directed, acyclic graph of stages.

thurber/apply! applies a series of transforms to a Pipeline or PCollection. (When pipeline graphs fork, multiple apply! invocations may be needed to create the different paths.)

thurber/comp* is how composite transforms are made. The result of comp* is a transform that can be used again within a subsequent call to apply! or comp*.

Transforms

Any PTransform instance can be provided to apply! and comp* but thurber supports other transform representations:

• A Clojure var can be provided and will convert to a ParDo transform. The var must ref a function:
  • e.g., #'extract-words
  • The will be invoked with the processing element as its arg.
  • The return value of the function will automatically emit downstream via ProcessContext.output().
  • If the function returns a seq then all items in the seq are emitted. Often these seqs are lazy and produce many elements.
  • May return nil in which case no automatic output occurs.
  • Some complex ParDo implementations will need first-class access to the current ProcessContext instance.
    • thurber/*process-context* is always bound to this current context.
    • When emitting elements explicitly via this bound var, functions will typically return nil so that no automatic emissions occur.
• thurber/partial*, filter*, combine-per-key, and other thurber functions produce transform representations that can be used in apply! and comp*

Coders

Every Beam PCollection must have a Coder.

The thurber/nippy coder de/serializes nippy supported data types (this includes all Clojure core data types).

thurber/nippy-kv codes Beam KV values that contain nippy-supported data types for the key and value.

Coders can be specified as Clojure metadata on function vars, or directly within a thurber transformation representation.

The default coder used is thurber/nippy which is appropriate for var-referenced functions (ParDo) that output Clojure data types.

Demos

Each namespace in the demo/ source directory is a pipeline written in Clojure using thurber. Comments in the source highlight salient aspects of thurber usage.

These are the best way to learn thurber's API and serve as recipes for various scenarios (use of tags, side inputs, windowing, combining, Beam's State API, etc etc.)

To execute a demo, start a REPL and evaluate (demo!) from within the respective namespace.

Word Count

The word_count package contains ports of Beam's Word Count Examples to Clojure/thurber.

Mobile Gaming Example

Beam's Mobile Gaming Examples have been ported to Clojure using thurber.

These are fully functional ports but require deployment to GCP Dataflow. (How-to notes coming soon.)

Make It Fast

First make your pipeline work. Then make it fast.

Streaming/big data implies hot code paths.

Use Clojure type hints liberally.

References

• https://write.as/aaron-d/clojure-data-streaming-and-dodging-static-types

License

Copyright © 2020 Aaron Dixon

Like Clojure distributed under the Eclipse Public License.