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A library for building applications using Yahoo S4 distributed stream platform in the Clojure programming language.


A sample application project is included in the sampleapp directory of this project.

Defining events

You can define a event object that can be received by processing elements using the def-s4-message macro. The macro works in a similar way to the deftype macro, generating a new class with the specified name and fields, but it will also generate standard getter and setter methods so the message object can be easilly wired using Spring. def-s4-message also deals with some serialization issues involving the use of Clojure data types and the Kryo serialization library used in S4 by default. Messages can be built from Clojure maps and transformed back into maps using the msg-to-map function:

 ;; Generates a cljs4.tests.Msg class with fields 'foo' with type int
 ;; and 'bar' of type Object
 (def-s4-message cljs4.tests.Msg [[int :foo] :bar])
 (def *msg* (cljs4.tests.Msg. {:foo 1 :bar "test"}))
 (.getFoo *msg*)
 ; returns 1
 (msg-to-map *msg*)
 ; returns {:foo 1 :bar "test"}

Defining Processing Elements

S4 processing elements (PEs) can be defined using the def-s4-pe macro. The macro receives as arguments the name of the class for the PE, the fields containing the state of the PE, and implementations for the processEvent and output functions. The state will be stored in a Clojure reference that can be accessed using the state java method on the this reference:

(def-s4-pe cljs4.NumAggregatorPE [:acum]

  :process-event [[Object] (fn [this event]
                             (let [event (msg-to-map event)]
                               (dosync (alter (.state this)
                                              (fn [old-state]
                                                (assoc old-state :acum
                                                       (+ (or (:acum old-state) 0) (:num event))))))))]

  :output (fn [this] (println (str "NumAgreggatorPE: Acum output: " (.getAcum this)))))

S4 PEs can emit new events invoking the dispatch-event function from the processEvent method:

; Dispatches an event using the message class defined earlier
(dispatch-event this stream-name (cljs4.tests.Msg. {:foo 1 :bar "test"}))

State manipulation methods

Instead of using the .state getter to manipulate the state of a PE, the functions read-state, write-state and alter-state can be used. These functions offer a simpler interface for the manipulation of the PE's state:

(def-s4-pe cljs4.PosPersister [:filePath :acum :textId]
  :init (fn [this]
          (let [acum {}]
            (write-state this :acum acum)))
  :process-event [[Object] (fn [this token]
                             (let [token-map (msg-to-map token)]
                               (write-state this :textId (:textId token-map))
                               (alter-state this :acum
                                            (fn [old] (let [old-count (get old (:pos token-map))]
                                                       (if (nil? old-count) (assoc old (:pos token-map) 1)
                                                           (assoc old (:pos token-map) (inc old-count))))))))]
  :output (fn [this]
            (let [ac (read-state this :acum)]
              (spit (str (read-state this :filePath) "/" (read-state this :textId) ".txt")
                    (reduce str "" (map (fn [[pos count]] (str "POS: " pos "COUNT: " count " \r\n"))
                                        (read-state this :acum)))))))

Defining adapters

S4 dispatchers can be defined using the def-s4-adapter macro. This macro expects a few arguments: the class, the state fields and a couple of function implementations. The functions to implement are the init function to init the state of the adapter and the run function with the code for the main loop of the adapter:

(def-s4-adapter cljs4.RandomNumberAdapter [:stream]

  :init (fn [this args]
     (.start (Thread. this)))

  :run (fn [this]
  (loop [num (int (Math/floor (* (rand) 100)))]
    (generate-event this (.getStream this) (cljs4.Number. {:num num}) [(mod num 10)])
      (Thread/sleep 3000)
        (recur (int (Math/floor (* (rand) 100)))))))

Adapters can emit new events using the generate-event function.


clj-s4 includes some functions to describe the wiring of S4 PEs, adapters, etc using Lisp s-expressions instead of XML files.

PEs can be wired using the wire-pe function:

(wire-pe {:id "OddNumsAggregatorPE"
   :class "cljs4.NumAggregatorPE"
   :keys ["OddNumbers *"]
     [{:name "id" :value "OddNumbersAggregatorPE"}]})

Other java beans can be wired using the wire-bean function:

    {:id "randomNumbersGenerator"
     :class "cljs4.RandomNumberAdapter"
     :properties [{:name "stream" :value "RandomNumbers"}]})

Collections of beans (PEs and other components) can be collected in a S4 application wiring using the wire-app function:

(wire-app "RandomNumbers"

 (wire-pe {:id "bifurcationPE"
  :class "cljs4.BifurcationPE"
  :keys ["RandomNumbers *"]
    [{:name "id" :value "bifurcationPe"}
     {:name "dispatcher" :ref "numbersDispatcher"}]})

      ; other beans


In the same way collections of beans can be collected in a S4 adapter wiring using the wire-adapters function:

(wire-adapters "RandomNumbersAdapter"

  {:id "randomNumbersGenerator"
   :class "cljs4.RandomNumberAdapter"
   :properties [{:name "stream" :value "RandomNumbers"}]}))

Beans wiring XML, applications and adapters generated wiring XML can be generated using the functions gen-xml and gen-xml-app passing as an argument the ID of the bean or the name passed as the first parameter to the wire-app and wire-adapters functions.


clj-s4 includes a Leiningen task to bundle a S4 application. This task uses the information for the application that must be defined in the project.clj file for the leiningen project:

 (defproject sampleapp "1.0.0-SNAPSHOT"
   :description "a demo S4 app built using clj-s4"
     :dependencies [[org.clojure/clojure "1.2.0"]
     		 [org.clojure/clojure-contrib "1.2.0"]
                     [clj-s4/clj-s4 "0.2.1"]]
   :s4-app {:name "RandomNumbers"
            :namespace ""
            :configuration ["sampleapp.wiring" "RandomNumbers"]
            :adapters ["sampleapp.wiring" "RandomNumbersAdapter"]})

The :s4-app key must include a map defining the name of the application, the namespace including the implementation of the PEs, adapters, etc. the namespace of the configuration including the wiring for the application and the name of the configuration file and, optionally, the namespace and file name for the wiring for a S4 adapter.

The S4 application bundle can be generated using the s4-deploy leiningen task.

$lein bundle $DEPLOY_PATH

Running the sample application

Before building and deploying the sample application, S4 must be installed in redbutton mode following the instructions found at the S4 wiki

Then we can cd into the sampleapp directory:

$ cd CLJ_S4_DIR/sampleapps/random-numbers

We must retrieve the dependencies for the application:

$ lein deps

Then we can build and deploy the application using Leiningen:

$ lein s4-deploy PATH_TO/s4image/s4_app

If we check the directory for S4 apps we should be able to see a new application directory named RandomNumbers:

$ ls PATH_TO/s4image/s4_apps/RandomNumbers/ RandomNumbers_conf.xml adapter_conf.xml lib

We can now start S4:

$ cd PATH_TO/s4image/bin $ ./

We should be able to see in the output of the shell script how the RandomNumbers PE are registered:

[/Users/antoniogarrote/Development/Projects/s4/s4image/bin/./../s4_apps/RandomNumbers/RandomNumbers_conf.xml] Adding processing element with bean name bifurcationPE, id bifurcationPe adding pe: cljs4.BifurcationPE@6a69ed4a Using ConMapPersister .. Adding processing element with bean name OddNumsAggregatorPE, id OddNumbersAggregatorPE adding pe: cljs4.NumAggregatorPE@15db4492 Using ConMapPersister .. Adding processing element with bean name EvenNumsAggregatorPE, id EvenNumbersAggregatorPE adding pe: cljs4.NumAggregatorPE@35e09eab Using ConMapPersister ..

Now we can start the adapter included into the RandomNumbers application:

$ cd PATH_TO/s4image/bin $ ./ -x -u ../s4_apps/RandomNumbers/lib/sampleapp*.jar -d ../s4_apps/RandomNumbers/adapter_conf.xml

After starting the adapter, we should we able to start noticing events, and creation of PEs related to the application.

After a while, in the shell where the S4 node was started, we should be able to see the output from the accumulator PE:

... dispatchin to dispatcher: io.s4.dispatcher.Dispatcher@7e8905bd stream EvenNumbers event cljs4.Number@63ff5c98 dispatchin to dispatcher: io.s4.dispatcher.Dispatcher@7e8905bd stream EvenNumbers event cljs4.Number@5979b7b0 NumAgreggatorPE: Acum output: 54 NumAgreggatorPE: Acum output: 77 NumAgreggatorPE: Acum output: 82 NumAgreggatorPE: Acum output: 94 NumAgreggatorPE: Acum output: 84 NumAgreggatorPE: Acum output: 135 NumAgreggatorPE: Acum output: 162 NumAgreggatorPE: Acum output: 234 NumAgreggatorPE: Acum output: 260 dispatchin to dispatcher: io.s4.dispatcher.Dispatcher@7e8905bd stream OddNumbers event cljs4.Number@4ab83be0 NumAgreggatorPE: Acum output: 206 dispatchin to dispatcher: io.s4.dispatcher.Dispatcher@7e8905bd stream OddNumbers event cljs4.Number@3a87d472 NumAgreggatorPE: Acum output: 273 ...

A different sample application dealing with the distributed POS-tagging of texts can be found in the wordcount subdirectory of the sampleapps directory.


Copyright (C) 2010 Antonio Garrote

Distributed under the Eclipse Public License, the same as Clojure.


Clojure library for the Yahoo S4 distributed streams framework



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