Ketu

A Clojure Apache Kafka client with core.async api

[com.appsflyer/ketu "1.1.0"]

Features

• Channels API: Take kafka data from a channel and send data to kafka through a channel.
• Consumer Source: Polls records from kafka and puts them on a channel.
• Producer Sink: Takes records from a channel and sends them to kafka.
• Shapes: Transform the original objects of the java client to clojure data and back.
• Simple Configuration: Friendly, validated configuration.

Minimal Example

Consume a name string from kafka and produce a greeting string for that name back into kafka, all through channels:

(ns example
  (:require [clojure.core.async :refer [chan close! <!! >!!]]
            [ketu.async.source :as source]
            [ketu.async.sink :as sink]))

(let [<names      (chan 10)
      source-opts {:name       "greeter-consumer"
                   :brokers    "broker1:9092"
                   :topic      "names"
                   :group-id   "greeter"
                   :value-type :string
                   :shape      :value}
      source      (source/source <names source-opts)

      >greets     (chan 10)
      sink-opts   {:name       "greeter-producer"
                   :brokers    "broker2:9091"
                   :topic      "greetings"
                   :value-type :string
                   :shape      :value}
      sink        (sink/sink >greets sink-opts)]

  ;; Consume a name and produce a greeting. You could also do this with e.g. clojure.core.async/pipeline.
  (->> (<!! <names)
       (str "Hi, ")
       (>!! >greets))

  ;; Close the source. It automatically closes the source channel `<names`.
  (source/stop! source)
  ;; Close the sink channel `>greets`. It causes the sink to close itself as a consequence.
  (close! >greets))

Configuration reference

Anything that is not documented is not supported and might change.

Read more about the default values used by the underlying Kafka clients v3.3.1 here

Note: int is used for brevity but can also mean long. Don't worry about it.

Common options (both source and sink accept these)

Key	Type	Req?	Notes
:brokers	string	required	Comma separated host:port values e.g "broker1:9092,broker2:9092"
:topic	string	required
:name	string	required	Simple human-readable identifier, used in logs and thread names
:key-type	:string,:byte-array	optional	Default :byte-array, used in configuring key serializer/deserializer
:value-type	:string,:byte-array	optional	Default :byte-array, used in configuring value serializer/deserializer
:internal-config	map	optional	A map of the underlying java client properties, for any extra lower level config

Consumer-source options

Key	Type	Req?	Notes
:group-id	string	required
:shape	:value:, [:vector <fields>],[:map <fields>], or an arity-1 function of ConsumerRecord	optional	If unspecified, channel will contain ConsumerRecord objects. Examples
:ketu.source/consumer-decorator	fn [consumer-context poll-fn] -> Iterable<ConsumerRecord>	optional	Decorates the internal poll function. when provided the decorator will be called with the following params: consumer-context: {:ketu.source/consumer consumer} pool-fn: fn [] -> Iterable Returns an iterable collection of consumerRecord. The decorator should call the poll-fn on behalf of the consumer source.

Producer-sink options

Key	Type	Req?	Notes
:shape	:value, [:vector <fields>],[:map <fields>], or an arity-1 function of the input returning ProducerRecord	optional	If unspecified, you must put ProducerRecord objects on the channel. Examples
:compression-type	"none" "gzip" "snappy" "lz4" "zstd"	optional	Default "none", values are same as "compression.type" of the java producer
:workers	int	optional	Default 1, number of threads that take from the channel and invoke the internal producer

Data shapes

You don't have to deal with ConsumerRecord or ProducerRecord objects.
To get a clojure data structure with any of the ConsumerRecord fields, configure the consumer shape:

; Value only:
{:topic      "names"
 :key-type   :string
 :value-type :string
 :shape      :value}
(<!! consumer-chan)
;=> "v"

; Vector:
{:shape [:vector :key :value :topic]}
(<!! consumer-chan)
;=> ["k" "v" "names"]

; Map
{:shape [:map :key :value :topic]}
(<!! consumer-chan)
;=> {:key "k", :value "v", :topic "names"}

Similarly, to put a clojure data structure on the producer channel:

; Value only:
{:key-type   :string
 :value-type :string
 :shape      :value}
(>!! producer-chan "v")

; Vector:
{:shape [:vector :key :value]}
(>!! producer-chan ["k" "v"])

; Vector with topic in each message:
{:shape [:vector :key :value :topic]}
(>!! producer-chan ["k1" "v1" "names"])
(>!! producer-chan ["k2" "v2" "events"])

Consumer Decorator

The consumer decorator allows running custom logic on the consumer polling thread.
This allows custom control on the consumer behavior including manual offset management.
Custom decorator logic may require different consumer configurations.
for example when managing the offset manually, auto-commit should usually set to false.

In this example we use the decorator to run commands in the polling thread context.
The consumer is paused/resumed based on commands sent from the application.
The decorator processes all immediately available commands in the commands-chan, and only then calls (poll-fn).

(ns consumer-decorator-example
  (:require [clojure.core.async :as async]
            [ketu.async.source :as source]
            [ketu.async.sink :as sink]))

(let [commands-chan (async/chan 10)
      consumer-chan (async/chan 10)
      consumer-opts {:name                           "consumer-example"
                     :brokers                        "broker1:9092"
                     :topic                          "example"
                     :group-id                       "example"
                     :value-type                     :string
                     :shape                          :value
                     :ketu.source/consumer-decorator (fn [consumer-ctx poll-fn]
                                                       (loop []
                                                         (when-let [command (async/poll! commands-chan)]
                                                           (command consumer-ctx)
                                                           (recur)))
                                                       (poll-fn))}
      source        (source/source consumer-chan consumer-opts)

      producer-chan (async/chan 10)
      sink-opts     {:name       "producer-example"
                     :brokers    "broker1:9092"
                     :topic      "example"
                     :value-type :string
                     :shape      :value}
      sink          (sink/sink producer-chan sink-opts)

      ; periodically produce data to the topic
      producing     (future
                      (dotimes [i 20]
                        (async/>!! producer-chan (str i))
                        (Thread/sleep 300))
                      (async/>!! producer-chan "done")
                      (async/close! producer-chan))

      ; read from the consumer channel and print to the screen
      processing    (future
                      (loop []
                        (let [message (async/<!! consumer-chan)]
                          (println message)
                          (when (not= message "done")
                            (recur)))))]
  (try
    (Thread/sleep 2000)                                     ; consumer is consuming normally
    (let [paused  (promise)
          resumed (promise)]

      ; Send the commands channel a function that will pause the consumer
      (async/>!! commands-chan (fn [{consumer :ketu.source/consumer}]
                                 (.pause consumer (.assignment consumer))
                                 (deliver paused true)))

      @paused
      (println "consumer is paused")
      (Thread/sleep 2000)

      ; Send the commands channel a function that will resume the consumer
      (async/>!! commands-chan (fn [{consumer :ketu.source/consumer}]
                                 (.resume consumer (.paused consumer))
                                 (deliver resumed true)))

      @resumed
      (println "consumer is resumed")

      ; Wait for all futures to finish
      @producing
      @processing)
    (finally
      (source/stop! source))))

Development & Contribution

We welcome feedback and would love to hear about use-cases other than ours. You can open issues, send pull requests, or contact us at clojurians slack.