At its core, an S4 application consists of a number of Processing Elements (PEs) that are wired together with the help of a spring configuration file that defines the PEs and the flow of events in the system. Also, events are produced by event producers that listen that sends these events to the client adapter for S4, from where, the S4 platform takes over and dispatch it to appropriate processing elements. After processing these events, PEs can choose to dispatch them to other PEs for further processing or they can choose to produce output events. Thus, arbitrarily complex behavior can be derived together by wiring a simple set of PEs.

S4 comes with a few example applications, but here is a much simpler S4WordCount application that shows how to:

1. Keep state in a PE.
2. Dispatch events from a PE.
3. Process multiple events from a single PE.
4. Write a simple java client for sending events to S4.

In S4WordCount, we will build a simple WordReceiverPE, that will receive events in the form of word and will simply print these words on stdout. It will also identify sentences in the word stream and then forward these sentences for further processing to SenteceReceiverPE. WordReceiverPE will also produce receive sentence events and print them to stdout.

First lets have a look at Word and Sentence, the event object used in our example:

package test.s4;

public class Word {
	private String string;

	public String getString() {
		return string;
	}

	public void setString(String message) {
		this.string = message;
	}

	@Override
	public String toString() {
		return "Word [string=" + string + "]";
	}
}

package test.s4;

public class Sentence {
	private String string;

	public Sentence(){
		// default constructor
	}

	public Sentence(String string) {
		this.string = string;
	}

	public String getString() {
		return string;
	}

	public void setString(String message) {
		this.string = message;
	}

	public String getSentenceId(){
		// all sentences have the same key
		return "1";
	}
	public void setSentenceId(String id){
		// do nothing
	}

	@Override
	public String toString() {
		return "Sentence [string=" + string + "]";
	}

}

S4 uses keys, which is a set of some properties of the event object, for routing/dispatching events. In this case since Word is the key-less entry point into the system, and we don’t have any key for it, but for Sentence which will be processed further, we have a Sentence.sentenceId as the key. (For simplicity, all Sentence have the same sentenceId, 1)

Now let’s have a look at our first PE, i.e. WordReceiverPE:

package test.s4;

import io.s4.dispatcher.Dispatcher;
import io.s4.processor.AbstractPE;

/**
 * This class receive word and sentence events and print them to stdout
 */
public class WordReceiverPE extends AbstractPE {
	private StringBuilder builder = new StringBuilder();

	/**
	 * Dispatcher that will dispatch events on <code>Sentence *</code> stream.
	 */
	private Dispatcher dispatcher;

	public Dispatcher getDispatcher() {
		return dispatcher;
	}

	public void setDispatcher(Dispatcher dispatcher) {
		this.dispatcher = dispatcher;
	}

	/**
	 * Process word events. This prints out the received word and also builds
	 * the sentence that will be dispatched once we found the end of the
	 * sentence identified by <code>.</code>
	 * 
	 * @param word
	 *            Word received on <code>RawWords *</code> stream.
	 */
	public void processEvent(Word word) {
		System.out.println("Received: " + word);

		// keep building the sentence
		builder.append(' ').append(word.getString().trim());

		if (builder.toString().endsWith(".")) {
			System.err.print("End of sentence found");

			// dispatch a Sentence event
			dispatcher.dispatchEvent("Sentence", new Sentence(builder.toString()));

			// reset buffer.
			builder.setLength(0);
		}
	}

	/**
	 * Process sentence events. This method demonstrated that one class can
	 * recieve multiple type of events on different Streams.
	 * 
	 * @param sentence
	 *            Sentence recieved on <code>Sentence *<code> stream.
	 */
	public void processEvent(Sentence sentence) {
		System.out.println("Received Sentence(WordReceiverPE) : " + sentence);
	}

	@Override
	public void output() {
		// TODO Auto-generated method stub
	}

	@Override
	public String getId() {
		return this.getClass().getName();
	}
}

We define a StringBuilder that will be used to accumulate words to form a Sentence. The processEvent(Word) method simply prints the received word on stdout and appends the word to the builder. It then checks if the sentence is complete by checking for . at the end of the builder and if so, it creates a Sentence event(object) and dispatches it to the Sentence stream. Once dispatched the processEvent(Sentence) will receive that event and will again print the sentence to stdout.

Now let’s have a look at SenteceReceiverPE, our second PE which does nothing but print the received Sentence on stdout.

package test.s4;

import io.s4.processor.AbstractPE;

/**
 * This class receives Sentence events and print them on stdout.
 */
public class SentenceReceiverPE extends AbstractPE {

	/**
	 * Print received sentence event.
	 * @param sentence
	 */
	public void processEvent(Sentence sentence){
		System.out.println("Received Sentence: " + sentence);
	}

	@Override
	public void output() {
		// TODO Auto-generated method stub
	}

	@Override
	public String getId() {
		return this.getClass().getName();
	}

}

Finally, lets see the content of the application config file that wires all this together and forms a valid S4 application. The name of the config file should follow the naming convention of <AppName>-config.xml. In this case we will call the config file S4WordCount-conf.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans-2.0.xsd">

	<bean id="wordCatcher" class="test.s4.WordReceiverPE">
	   <property name="dispatcher" ref="dispatcher"/>

		<property name="keys">
		  <!--  Listen for both words and sentences -->
			<list>
				<value>RawWords *</value>
				<value>Sentence *</value>
			</list>
		</property>
	</bean>

	   <bean id="sentenceCatcher" class="test.s4.SentenceReceiverPE">
        <property name="keys">
            <list>
                <value>Sentence *</value>
            </list>
        </property>
    </bean>

	<bean id="dispatcher" class="io.s4.dispatcher.Dispatcher"
		init-method="init">
		<property name="partitioners">
			<list>
			<!-- Partition on senteceId which is "1" for all sentences. -->
				<ref bean="sentenceIdPartitioner" />
			</list>
		</property>
		<property name="eventEmitter" ref="commLayerEmitter" />
		<property name="loggerName" value="s4" />
	</bean>

	<bean id="sentenceIdPartitioner" class="io.s4.dispatcher.partitioner.DefaultPartitioner">
		<property name="streamNames">
			<list>
				<value>Sentence</value>
			</list>
		</property>
		<property name="hashKey">
			<list>
				<value>sentenceId</value>
			</list>
		</property>
		<property name="hasher" ref="hasher" />
		<property name="debug" value="true" />
	</bean>

</beans>

This is a spring bean definition file. The first bean, wordCatcher is an object of class test.s4.WordReceiverPE and it injects a dispatcher called dispatcher into WordRecieverPE. We will look at properties of dispatcher later. keys define the stream on which our PE will be listening for event. RawWords * means that it will be receiving all events on the stream named RawWords irrespective of their keys. Similar is the intention for Sentence *.

Our second bean is sentenceCatcher which is an object of class test.s4.SentenceReceiverPE and will accept all events on stream called Sentence.

Third is the definition for the dispatcher which we injected into wordCatcher. The dispatcher needs a partitioner that partitions the events based on some key and then dispatches them to appropriate PEs. In this case we are using the DefaultPartitioner whose properties are defined later by sentenceIdPartitoner bean which says that partition the event objects on Sentence stream by sentenceId property. dispatcher uses the S4 provided commLayerEmitter to emit the events.

Running the application

To run this application on the S4:

1. Setup S4 as documented here.
2. Create a S4WordCount.jar from above classes.
3. Deploy the application on S4, by creating the following directory structure:

   	/$S4_IMAGE
   		/s4-apps
   			/S4WordCount
   				S4WordCount-conf.xml
   				/lib
   					S4Wordcount.jar

4. Start S4: $S4_IMAGE/scripts/start-s4.sh -r client-adapter &
5. Start client adapter: $S4_IMAGE/scripts/run-client-adapter.sh -s client-adapter -g s4 -d $S4_IMAGE/s4-core/conf/default/client-stub-conf.xml &

Now S4 is ready to receive events. Following is an event sender that uses the java client library to send events to S4. It reads one word at a time from stdin and sends it to S4.

package test.s4.generator;

import io.s4.client.Driver;
import io.s4.client.Message;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.Reader;

public class TestMessageSender {
	public static void main(String[] args) {
		if (args.length < 1) {
            System.err.println("No host name specified");
            System.exit(1);
        }
        String hostName = args[0];
        
        if (args.length < 2) {
            System.err.println("No port specified");
            System.exit(1);
        }
        
        int port = -1;
        try {
            port = Integer.parseInt(args[1]);
        } catch (NumberFormatException nfe) {
            System.err.println("Bad port number specified: " + args[1]);
            System.exit(1);
        }
        
        if (args.length < 3) {
            System.err.println("No stream name specified");
            System.exit(1);
        }
        String streamName = args[2];
        
        if (args.length < 4) {
            System.err.println("No class name specified");
            System.exit(1);
        }
        String clazz = args[3];       
        
        Driver d = new Driver(hostName, port);
        Reader inputReader = null;
        BufferedReader br = null;
        try {
            if (!d.init()) {
                System.err.println("Driver initialization failed");
                System.exit(1);
            }
            
            if (!d.connect()) {
                System.err.println("Driver initialization failed");
                System.exit(1);           
            }
            
            inputReader = new InputStreamReader(System.in);
            br = new BufferedReader(inputReader);

            for  (String inputLine = null; (inputLine = br.readLine()) != null;) {
                String string = "{\"string\":\""+inputLine+"\"}";
                System.out.println("sending " + string);
				Message m = new Message(streamName, clazz, string);
                d.send(m);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        finally {
            try { d.disconnect(); } catch (Exception e) {}
            try { br.close(); } catch (Exception e) {}
            try { inputReader.close(); } catch (Exception e) {}
        }
	}
}

Run client: java TestMessageSender localhost 2334 RawWords test.s4.Word

Here is the sample output on the S4 console:

Received: Word [string=this]
Received: Word [string=is]
Received: Word [string=a]
Received: Word [string=sentence.]

Using fast path!
Value 1, partition id 0
wrapper is stream:Sentence keys:[{sentenceId = 1}:0] keyNames:null event:Sentence [string= this is a sentence.] => 0
Received Sentence(WordReceiverPE) : Sentence [string= this is a sentence.]
Received Sentence: Sentence [string= this is a sentence.]

A lot of java frameworks and IDEs tries to hide this complexity by generating code – from simple getters and setters to entire DAO layers and what not – that might give small productivity gains in the beginning but eventually every additional line of code in your project, either hand-written or generated by a state of art code generator, someone will need to maintain and evolve it, which according to some is almost 90% of the total software costs.

Thats why framework like Play feels like fresh air and it seems to be making inroads in the bloated enterprise java world.

1. Create a user glassfish (you can call it anything you want) under which Glassfish will run.

   #useradd glassfish

2. Install glassfish in /home/glassfish.
3. Create the startup script /etc/init.d/glassfish for glassfish.
   

   #!/bin/bash
   #
   # glassfish:          Startup script for Glassfish Application Server.
   #
   # chkconfig: 3 80 05
   # description:      Startup script for domain1 of Glassfish Application Server.
   
   GLASSFISH_HOME=/home/glassfish/glassfish;
   export GLASSFISH_HOME
   
   GLASSFISH_OWNER=glassfish;
   export GLASSFISH_OWNER
   
   start() {
           echo -n "Starting Glassfish: "
           echo "Starting Glassfish at `date`" >> $GLASSFISH_HOME/domains/domain1/logs/startup.log
           su $GLASSFISH_OWNER -c "$GLASSFISH_HOME/bin/asadmin start-domain domain1" >> $GLASSFISH_HOME/domains/domain1/logs/startup.log
           sleep 2
           echo "done"
   }
   
   stop() {
           echo -n "Stopping Glassfish: "
           echo "Stopping Glassfish at `date`" >> $GLASSFISH_HOME/domains/domain1/logs/startup.log
           su $GLASSFISH_OWNER -c "$GLASSFISH_HOME/bin/asadmin stop-domain domain1" >> $GLASSFISH_HOME/domains/domain1/logs/startup.log
           echo "done"
   }
   
   # See how we were called.
   case "$1" in
           start)
                   start
                   ;;
           stop)
                   stop
                   ;;
           restart)
                   stop
                   start
                   ;;
           *)
                   echo $"Usage: glassfish {start|stop|restart}"
                   exit
   esac

   view raw glassfish This Gist brought to you by GitHub.
4. Install the service

   #chmod +x /etc/init.d/glassfish
   #chkconfig --add glassfish
   #chkconfig --level 3 glassfish on

5. Start glassfish.

   #/etc/init.d/glassfish start