To configure a cluster of Jboss it’s very easy, but for having a right JMS Messaging configuration could be too difficult.

For example, in my case I configured a jboss cluster, with two node (A & B) but just one node at time read the messages from the queue. It was an active-passive cluster. I would a real active-active cluster with load balacing policy.

First of all you need to configure at least 2 nodes starting from “all” configuration (e.g. nodeA & nodeB).

In this example I am going to explain how to configure the JMS messaging with a Oracle persistence.

Before to configure the messaging “plugin”, you have to check that the following component have been defined, under deploy folder:

1. Oracle datasource (e.g. jdbc/myapp/ds as name) (Jboss doc)
2. JMS Queues, with attribute “clustered” set to true

<mbean code="org.jboss.jms.server.destination.QueueService" 
    name="jboss.messaging.destination:service=Queue,
    name=FileProcess" xmbean-dd="xmdesc/Queue-xmbean.xml">
    <depends optional-attribute-name="ServerPeer">jboss.messaging:service=ServerPeer</depends>
    <depends>jboss.messaging:service=PostOffice</depends>
    <attribute name="Clustered">false</attribute>
</mbean>

Now we can go inside deploy/messaging folder (for each nodes) to perform the following steps:

1. Remove file hsqldb-persistence-service.xml
2. Copy the file oracle-persistence-service.xml from folder JBOSS_HOME\docs\examples\jms
• Change any occurrence of DefaultDS with your datasource name (e.g. jdbc/myapp/ds)
• Change any occurrence of java:/DefaultDS with your datasource name (e.g. jdbc/myapp/ds)
• Change the value of <attribute name=”Clustered”>false</attribute> to true
3. Open file messaging-jboss-beans.xml to change the name of datasource of application-policy definition
• Change the value of <module-option name=”dsJndiName”>
4. Open file messaging-service.xml
• Remove (or comment) ClusterPullConnectionFactoryName

<attribute name="ClusterPullConnectionFactoryName">
jboss.messaging.connectionfactory:service=ClusterPullConnectionFactory
</attribute>

It’s all.. now you can start the nodes, for example with the following commands:

Node A
run -c node-A -g MyCluster -b 0.0.0.0 -Djboss.messaging.ServerPeerID=1

Node B
run -c node-B -g MyCluster -b 0.0.0.0 -Djboss.messaging.ServerPeerID=2 -Djboss.service.binding.set=ports-01

Last week I bought a new very nice and cheap VPS hosting (wooservers.com), I chosen Linux Debian 6 as operating system,  and immediately after I moved on it some web-site/application hosted in different placed.

To do it I configured an Apache Web server for managing HTML and PHP sites (domain1.com / domain2.com) and a Tomcat Server for hosting Java application (domain3.com / domain4.com).

Every website has a really domain associated. Hereafter I collected all steps followed in order to configure several virtual-hosts on Apache Webserver for all domains, and a sub-set of them virtual-hosts on Tomcat for Java application.

Sure, first of all you need to install the following software:

• Java (in my case 1.6.0_26)
• Apache 2 (in my case the version coming from apt-get install apache2)
• Tomcat 6 or above (in my case the version coming from apt-get install tomcat6)
• MOD JK plugin (apt-get install libapache2-mod-jk)

Normally, on debian O.S., the apache installation is based in /etc/apache2

You can configure and enable MOD_JK plugin changing jk.load configuration file
vim /etc/apache2/mods-available/jk.load

#jk.load configuration file
LoadModule jk_module /usr/lib/apache2/modules/mod_jk.so
JkWorkersFile   /etc/apache2/worker.properties
JkLogFile       /var/log/apache2/mod_jk.log

Here you specified a worker.properties file to configure the tomcat server connector (ip, port, name etc etc).
And example of worker.properties could be:

# Define 1 real worker using ajp13
worker.list=worker1
# Set properties for worker1 (ajp13)
worker.worker1.type=ajp13
worker.worker1.host=127.0.0.1
# Specify the connector-port, not HTTP Port. I mean 8009 and not 8080
worker.worker1.port=8009

Then, you can continue with virtual-host configuration for each domains which you need.
On Debian there is very easy way to do it; you can create a configuration file for each virtual host under sites-available folder.

Please create a text file with the name of your domain (e.g. domain.com).
I am posting in this post two differents configuratione examples:

• The first for a virtual-host hosted by Apache Web-Server (Php/Html), to specify the options and document root of application files.
• The second for a virtual-host to route to Tomcat server by mod_jk plugin

Virtual-Host for domain hosted by Apache Web-Server

<VirtualHost *:80 >
        ServerAdmin info@domain1.com
        ServerName www.domain1.com
        ServerAlias domain1.com
        DocumentRoot /var/www/domain1/public

                Order Deny,Allow
                Allow from all
                Options -Indexes

                AllowOverride None
                Options ExecCGI -MultiViews +SymLinksIfOwnerMatch -Indexes
                Order allow,deny
                Allow from all

        ErrorLog /var/log/apache2/error_domain1.log
        LogLevel warn
        CustomLog /var/log/apache2/access_domain1.log combined
        ServerSignature On
</VirtualHost>

Virtual-Host for domain hosted by Tomcat Server

<VirtualHost *:80 >
        ServerAdmin info@domain3.com
        ServerName www.domain3.com
        ServerAlias domain3.com
        JkMount /* worker1
        ErrorLog /var/log/apache2/error_domain3.log
        LogLevel warn
        CustomLog /var/log/apache2/access_domain3.log combined
        ServerSignature On
</VirtualHost>

Now you can add and enable the virtual hosts with a simply command

a2ensite /etc//etc/apache2/sites-available/domain1.com
a2ensite /etc//etc/apache2/sites-available/domain2.com
/etc/init.d/apache2 reload

Woow, now you Php/HTML websites are available for all users on internet.
It’s missing the Tomcat configuration for Java application, but we can do it in 2 easy steps.

First of all it’s important to be sure that Tomcat Connector is enabled. (On my VPS Tomcat configuration files are stored into /var/lib/tomcat6/conf)

Please ensure that the following lines are uncommented in your server.xml

<Connector port="8080" protocol="HTTP/1.1" connectionTimeout="20000"
           URIEncoding="UTF-8" redirectPort="8443" />

Now you have to add the virtual-hosts, for domains routed to Tomcat, to specify the main webapps folder.
You can do it always in server.xml

<Host name="domain3.com" debug="0" appBase="domain3" unpackWARs="true">
      <Alias>www.domain3.com</Alias>
      <Logger className="org.apache.catalina.logger.FileLogger"
              directory="logs" prefix="domain3_log1." suffix=".log" timestamp="true"/>
</Host>

please keep in mind that appBase attribute indicates the name of folder in your tomcat directory (usually followed by ROOT directory) where there are the files of your application.

Enjoy your domains…

“Hi Davide… our company website seems down since yesterday afternoon. Could you check? Yesterday there was a power problem in our datacenter”.

The error reported it was:

DB function failed with error number 145
'TABLE\ jos_session' is marked as crashed and should be repaired
SQL=SELECT session_id FROM jos_session WHERE session_id ....

Woow, It’s very simple to solve the problem.
Go into your Mysql administrator website (via phpmyadmin, sqlyoog or others similar tools) for executing the following commands:

REPAIR TABLE jos_session

DELETE FROM jos_session

Thanks to my RSS feed I found an interesting article to discuss three different approach to read – process- update massive data.

The problem is:

1. Load data based on Database based on groups
2. Process the data
3. Update the records loaded in step 1 with modifications

We have to handle 100s of groups and each group will have around 40K records. You can imagine the amount of time it would take if we were to perform this exercise in a synchronous fashion. Image here explains this problem in an effective way.

So, The first, stupid, scenario is with a single worker that reads and processes single group at a time.

Immediately, you can image to “break” the synchronous mechanism to introduce async.

Sure with this approach the “external” actors can be happy because they work can be very fast and they don’t have to wait the process done by Worker 2. There is still one worker who has to do all the work.

The main problem can be related to memory issue,  The producers will pile up their data in a queue and it needs resources to hold the same. Just as in this example the cabinet can fill up, the same can happen with the JVM resources too. We need to be careful how much data we are going to place in memory and in some cases it may not be much.

In this case, the best solution suggested is to introduce a lot of consumers queues and a pool of consumer to process the data; as in the real-life such as Cinema ticket-office.

Java provides a concurrency package to solve this issue and the java developer of source-blog font helped me in understanding the use of BlockingQueue in a very effective manner.

There are trhee 3 main classes to implement it (you can find the source hosted on SVN code here)

• Producer.java
• Consumer.java
• Broker.java

The Producer

public class Producer implements Runnable{
    private Broker broker;
    public Producer(Broker broker){
        this.broker = broker;
    }

   @Override
    public void run(){
        try{
           for (Integer i = 1; i < 5 + 1; ++i){
                System.out.println("Producer produced: " + i);
                broker.put(i);
            }
            this.broker.continueProducing = Boolean.FALSE;
            System.out.println("Producer finished its job; terminating.");
        }
        //...
    }
}

The Consumer

public class Consumer implements Runnable{
    private String name;
    private Broker broker;

    public Consumer(String name, Broker broker)   {
        this.name = name;
        this.broker = broker;
    }

    @Override
    public void run(){
        try{
            Integer data = broker.get();
            while (broker.continueProducing || data != null){
                System.out.println("Consumer " + this.name + " processed data from broker: " + data);
                data = broker.get();
            }
            System.out.println("Comsumer " + this.name + " finished its job; terminating.");
        }
        //...
    }
}

The loop to process data is an endless loop, that runs on two conditions – until the producer is consuming and there is some data with the broker

The Broker

public class Broker{
    public ArrayBlockingQueue queue = new ArrayBlockingQueue(100);
    public Boolean continueProducing = Boolean.TRUE;
    public void put(Integer data) throws InterruptedException{
        this.queue.put(data);
    }
    public Integer get() throws InterruptedException{
        return this.queue.poll(1, TimeUnit.SECONDS);
    }
}

The queue uses a FIFO logic.
If the producers are slow the consumers wait for data to process and if the broker is full, the producers wait to fill up it.

How to test it

Broker broker = new Broker();
ExecutorService threadPool = Executors.newFixedThreadPool(3);
//The consumers
threadPool.execute(new Consumer("1", broker));
threadPool.execute(new Consumer("2", broker));

Future producerStatus = threadPool.submit(new Producer(broker));
// this will wait for the producer to finish its execution.
producerStatus.get();

The next step is to implement this approach in JEE scenario on application server and different JVM…I hope I can post soon on this blog some pieces of codes.

[via Java code geeks]

At the moment there are just 2 certification:

• Oracle Certified Associate, JAVA SE 7 Programmer
• Oracle Certified Professional, JAVA SE 7 Programmer

As usual, Associate certification is a pre-requisit of Professional certification.

Oracle Certified Associate (OCAJP)

1. 90 Questions
2. 150 minutes
3. 75% To be passed
4. 238€

Oracle Certified Associate (OCPJP)

1. 120-150 Questions
2. 2.5 – 3 Hours
3. TBD To be passed
4. 39€

The OCPJP is the equivalent of SCJP but for Java 7.

More info on Oracle certification website.

I found two interesting posts on regulargeek.com with programming languages trends:

1. Tradional Programming languanges trends.
2. “New” programming languanges trends.. or better to say unusual languages for enterprise sectors.

Tradination Programming language trends

As you can see, thanks below chart, the main language, since 2006, seems to be “Java”.

Very interesting is also the next graph which makes a zoom for last year (2011).

Sure, the grown of Java language seems paused, however we can say that Java is the most commonly used.

Alternative programming languange  trends

Here, we can see jobs “posting” on indeed website for “alternative” programming language skill as:

• Lua
• Scheme
• Action scripts
• Erlang
• Groovy
• Scala
• Clojure

Very interesting as in last months a big growth has been done by Lua and Clojure.

Here, some very easy tips:

TO DO

• Use mute “function” on your phone where you are not speaking.
• Stay away from other people / colleagues.
• Don’t speak too close to your microphone.
• Don’t use “hold” function on your phone… so don’t answer to another call that you can receive in the meantime.

• Prefer an indoor room, don’t stay outdoor.
• If you have a internet connection, use IM chat to speak directly to another partecipant.
• Be on time at the begin of call conference… new people that enter in the call cause much trouble.
• Prefer a system call conference system instead of multi-calls among phone.
• Stay focused and avoid multitasking with your pc or “items” close to you.
• Have a conference call leader to lead the goals and actors conversation

at the end… but not less important:

ENCOURAGE PARTICIPATION

I found a nice post about “encourage participation”; normally one or two people can dominate the conversation.

You will get better results if you encourage everyone to participate, asking quiet people what they think if they don’t… Sure this a task of “conference call leader”.

Here you find a little description and the link to download it.

Oracle Database development virtual machine [Download]

• Oracle Linux 5
• Oracle Database 11g Release 2
• Oracle XML DB
• Oracle SQL Developer
• Oracle JDeveloper

Oracle Java Enterprise development virtual machine [Download]

• Oracle Linux 5 + JDK 1.6
• Oracle WebLogic Server 12.1.1
• Oracle Coherence 3.7
• Hudson 2.2
• Subversion 1.4.2
• Maven 3.0.1
• NetBeans IDE 7.1 RC2
• Oracle Enterprise Pack for Eclipse 12.1.1 / Eclipse 3.7

Oracle Java Enterprise development virtual machine [Download]

• Oracle WebLogic Server 10.3
• Oracle Database 11g Express Edition

SOA & BPM Development virtual machine [Download]

• Oracle Linux 5
• Oracle Database XE 10.2
• Oracle WebLogic Server 10.3.3.0
• Oracle SOA Suite 11g R1 PS2
• Oracle BAM 11g R1 PS2
• Oracle B2B 11g R1 PS2
• Oracle JDeveloper 11.1.1.3

These virtual machine can help you to speed up your development in a project with Oracle technologies.

Very good thing if you want add others questions or doubts as comments… thanks!

First of all I think it’s better to ask a self-evaluation about skills / main frameworks then with some quickly questions you can confirm it.

Java overview questions (Junior developer):

• Differences between override / overload.
• Can you create an instance of abstract class?
• What’s an annotation? Please give me some example of annotations that you know or used.
• What’s Singleton Pattern? Please try to write a simple class as singleton.
• What’s Factory Pattern? Please give a good situation to use a Factory.
• Do you know the differences among Exception, Throwable and Error?
• Which is the difference between operator == and method .equals()? and compareTo?
• What’s an Interface?
• Can you extends more than one class?

Java overview questions (developer):

• What are the differences between linkedList and arrayList? When you prefere one over the other one?
• How and When make an object as immutable?
• What is the scope of a variable?
• What is an hash algorithm? Could you give me some famous hash algorithm and some libraries to implement their?
• How can I make an object as serializable? When do you need a serializable object?

Java overview questions (senior developer):

• How Does an hashmap work? Please explain the difference between HashMap and ConcurrentHashMap.
• What is a Reentrant Lock?
• Please try to give me an example of Java deadlock.
• Please try to “design” very simple cache if you need to implement, without any framework, to store data to avoid a lot of database queries on configuration tables… then, which are the weaknesses of this solution?

Java EE questions:

• Can you give some examples of improvements in the Java EE5/6 specification in comparison to the J2EE specification?
• Considering two enterprise systems developed in different platforms, which good options do you propose to exchange data between them?
• What do you suggest to implement asynchronous code in Java EE?
• What is the difference between queue and topic in a message queuing system?
• Which strategies do you consider to import and export XML content?
• Which are the differences between SOAP and REST interface? When is it better to use REST and when SOAP?
• How would you configure an environment to maximize productivity of a development team?
• How can you implement a cache in a cluster Context?

at the end… but very important, Methodologies and general skills:

• What do you suggest to versioning your code? Please explain the reason.
• Did you implement or take part in a open source project?
• Which is the last technology that you studied for your interest? Why?
• Do you know some IT software Methodologies?
• What do you suggest to test your software parts?
• Which kinds of solutions can you adopt to limit logs performance impacts on your application?
• What is the two-face-commit? How can you implement it in a J2EE context?
• What do you suggest to schedule some jobs / batch activities?
• Try to imagine that you are in a new project and you don’t have a skill about a framework. What is the approach that will you use? (e.g. You ask a training course,  You apply a self-study to improve your skills…)
• Try to imagine that you are responsible to write a document about software guide lines in a project, could you give me a draft of document index?

Please keep in mind, as said by java geek code blog:

Try to formulate the questions in a way that you don’t get a single answer, but a short analysis from the candidate.
People can easily find answers on the Internet, but good analysis can be provided only with accumulated experience!