Jackdaw Streams API

Rationale

The Jackdaw Streams API is a thin wrapper around the underlying Kafka Streams DSL. It allows the definition of streaming applications using idiomatic Clojure functions rather than the corresponding Java interop.

Kafka Streams may be a good choice if you'd like to apply complex transformations or aggregations to one or more data streams that can be made available as Kafka topics, and you'd like to make the resulting output highly available.

If it's just a simple transformation you're after, you might consider SMT Transforms in combination with Kafka Connect.

Usage

If you've used the Java API, you'll be aware that the core operators are defined as methods on the KStream and KTable classes. In Jackdaw, we expose these methods as functions in the jackdaw.streams namespace with names that are hyphenated versions of the corresponding Java method.

The API docs should be consulted for full details but the essential elements of a typical streams app are described below

Topic Definition

(def topic-metadata

  {:input
   {:topic-name "input"
    :partition-count 1
    :replication-factor 1
    :key-serde (jackdaw.serdes.edn/serde)
    :value-serde (jackdaw.serdes.edn/serde)}

   :output
   {:topic-name "output"
    :partition-count 1
    :replication-factor 1
    :key-serde (jackdaw.serdes.edn/serde)
    :value-serde (jackdaw.serdes.edn/serde)}})

App Definition

(ns my.example.word-count
  (:require
    [clojure.string :as str]
    [jackdaw.streams :as j]))

(defn split-lines
  [input-string]
  (str/split (str/lower-case input-string) #"\W+"))

(defn topology-builder
  [topic-metadata]
  (fn [builder]
    (let [text-input (j/kstream builder (:input topic-metadata))

          counts (-> text-input
                     (j/flat-map-values split-lines)
                     (j/group-by (fn [[_ v]] v))
                     (j/count))]

      (-> counts
          (j/to-kstream)
          (j/to (:output topic-metadata)))

      builder)))

Start the App

(defn -main
  [& args]
  (let [app-config (parse-args args)
        builder (j/streams-builder)
        topology ((topology-builder topic-metadata) builder)
        app (j/kafka-streams topology app-config)]
    (j/start app)
    app))