Friday, January 18, 2013

What is Maven repository ?

Maven Repository :

A repository is a place where all the project jars, library jar, plugins or any other project specific artifacts are stored and can be used by Maven easily.

There are three types of repository :
  •  local
  • central
  • remote 

Local Repository :

The maven local repository is a local folder that is used to store all your project’s dependencies (plugin jars and other files which are downloaded by Maven). In simple, when you build a Maven project, all dependency files will be stored in your Maven local repository.

The default name of the Maven's local repository is .m2.

Central Repository

Maven central repository is repository provided by Maven community. It contains a large number of commonly used libraries.

When you build a Maven’s project, Maven will check your pom.xml file, to identify which dependency to download. First, Maven will get the dependency from your local repository, if not found, then get it from the default Maven central repository

This repository is managed by Maven community, required to be configured and it requires internet access to be searched.

Remote Repository

Sometime, Maven does not find a mentioned dependency in central repository as well then it stopped build process and output error message to console. To prevent such situation, Maven provides concept of Remote Repository which is developer's own custom repository containing required libraries or other project jars.

For example, using below mentioned POM.xml,Maven will download dependency (not available in central repository) from Remote Repositories mentioned in the same pom.xml.
<id>JBoss repository</id>

Maven Dependency Search Sequence :

When we execute Maven build commands, Maven starts looking for dependency libraries in the following sequence:

Step 1 - Search dependency in local repository, if not found, move to step 2 else exit.
Step 2 - Search dependency in central repository, if not found, move to step 3 else exit.
Step 3 - Search dependency in remote repository or repositories, if not found then it is prompt error message else exit.

Wednesday, January 9, 2013

how to install MAVEN on linux

What is MAVEN ?

Read here :

Here are the steps to download and install Maven on linux :

Step 1 : Download the latest binary from the
Step 2 : Untar it using tar command.
              tar -zxvf /usr/local/apache-maven-3.0.4-bin.tar.gz
Step 3: Add Maven binary Path to the System Path i,e add in .bash_profile path
          $ cd $HOME
          $ vi ~/.bash_profile
Set PATH and M2_HOME as follows
save the file by pressing esc : wq button

Note: Don't delete the previous PATH, Just append the M2_HOME path after :

Now save and close the file then logout and login back to see the effects, To confirm where the installation has done properly or not, check :
mvn --version
you will see following ouput :
Apache Maven 3.0.4 (r1232337; 2012-01-17 00:44:56-0800)
Maven home: /usr/local/apache-maven-3.0.4
Java version: 1.7.0_10, vendor: Oracle Corporation
Java home: /usr/local/jdk1.7.0_10/jre
Default locale: en_US, platform encoding: UTF-8
OS name: "linux", version: "2.6.18-92.el5", arch: "amd64", family: "unix"

Congratulation... you are done here  :)

Wednesday, January 2, 2013

How ExceptionHandler return JSON in spring MVC

I am working on one project where client/server response is in JSON format. It is easy to send object in JSON format but what if some exception occured and you want to send the Exception also in JSON format ?
After n number of trial. I finally able to do the above task

Step 1 : Add following annotation "AnnotationMethodHandlerExceptionResolver" in your <CONTROLLER>-servlet.xml file.

<!-- JSON format support for Exception -->
    <bean id="methodHandlerExceptionResolver"
        <property name="messageConverters">
                <ref bean="jacksonMessageConverter" />

    <bean id="jacksonMessageConverter"

Step 2: Make sure you have added jackson jars into your classpath.

Step 3: In controller class :

    @RequestMapping(value="/test", method = RequestMethod.GET)
    public String toTest() throws MxlServiceException {

   try {
      int i = 10/0; // it will throw exception which will be caught by handleException(...)
  }catch (Exception e) {
    throw e;
             return "hello";

then catch them both by writing an exception handler that looks like this:

@ExceptionHandler({ Exception.class })
    public ErrorResponse handleException(Exception ex,
            HttpServletRequest request, HttpServletResponse response) {
        ErrorResponse errorResponse = new ErrorResponse();
        return errorResponse;


{"message": "your_message"}

Feel free to comment :)

Wednesday, November 28, 2012

What is Hibernate Caching?

In a typical application, you perform lot of operations like instantiate objects, load object from the database and so on. Sometime in multiuser application you may face a situation in handling multiple call of databases.

Hibernate offers caching functionality which is designed to reduces the amount of necessary database access. This is a very powerful feature if used correctly. It increases your application performance and works between your application and the database as it avoids the number of database hit as many as possible.

Hibernate Cache Types :

Hibernate uses different types of caches. Each type of cache is used for different purposes. Let us first have a look at this cache types.
  • First level cache
  • Second level cache
  • Query level cache

1. First level cache :

First-level cache is the session cache and is always Associates with the Session object. Hibernate uses this cache by default. The Session object keeps an object under its own cache before committing to the database. Here, it processes one transaction after another one, means wont process one transaction many times. Mainly it reduces the number of SQL queries it needs to generate within a given transaction. That is instead of updating after every modification done in the transaction, it updates the transaction only at the end of the transaction.

2. Second level cache :

Second-level cache is an optional cache and is always associates with the Session Factory object. The second-level cache can be configured on a per-class and per-collection basis and mainly responsible for caching objects across sessions. While running the transactions, in between it loads the objects at the Session Factory level, so that those objects will available to the entire application, don’t bounds to single user. Since the objects are already loaded in the cache, whenever an object is returned by the query, at that time no need to go for a database transaction. In this way the second level cache works.

Hibernate supports four open-source cache implementations named EHCache (Easy Hibernate Cache), OSCache (Open Symphony Cache), Swarm Cache, and JBoss Tree Cache.

Each cache has different performance, memory use, and configuration possibilities.
S.N.Cache NameDescription 
1EHCacheIt can cache in memory or on disk and clustered caching and it supports the optional Hibernate query result cache. 
2OSCacheSupports caching to memory and disk in a single JVM, with a rich set of expiration policies and query cache support. 
3warmCacheA cluster cache based on JGroups. It uses clustered invalidation but doesn't support the Hibernate query cache 
4JBoss CacheA fully transactional replicated clustered cache also based on the JGroups multicast library. It supports replication or invalidation, synchronous or asynchronous communication, and optimistic and pessimistic locking. The Hibernate query cache is supported

3. Query level cache :

Hibernate also implements a cache for query resultsets that integrates closely with the second-level cache. This is an optional feature and requires two additional physical cache regions that hold the cached query results and the timestamps when a table was last updated. This is only useful for queries that are run frequently with the same parameters.

Useful java Keytool Command

Generate a Java keystore and key pair :

keytool -genkey -alias mycert -keyalg RSA -keystore keystore.jks -keysize 1024
Generate a keystore and self-signed certificate :

 keytool -genkey -keyalg RSA -alias selfsigned -keystore keystore.jks -storepass password -validity 360 -keysize 2048
keytool command to view certificate details from keyStore :

keytool -list -v -keystore keystore.jks

Check a particular keystore entry using an alias:
keytool -list -v -keystore keystore.jks -alias mydomain

keytool command option is -printcert which prints details of a certificate stored in .cer file :
keytool -printcert -file test.cer

Export a certificate from a keystore:
keytool -export -alias mydomain -file mydomain.crt -keystore keystore.jks
 keytool -export -alias mydomain -keypass keypass -keystore keystore.jks -storepass jkspass -rfc -file keytool_crt.pem

Note: "keytool -export" command uses DER format by default. The "-rfc" option is to change it to PEM (RFC 1421) format.

Friday, November 2, 2012

How to install on Cent OS

The Basics - What is mod_jk?

The mod_jk connector is an Apache HTTPD module that allows HTTPD to communicate with Apache Tomcat instances over the AJP protocol.


1. Download the latest apache connector from
2. Untar the download by
         tar zxvf <filename>
3. Goto native directory of connector
         cd <connector dir>/native/
4. Run the scripts
Note: If you get any issue like "autocong" not installed then install following things:
       yum install autoconf
       yum install libtool

5. You need "httpd-devel" tools to build it. So make sure you have already installed it by
         yum list installed | grep httpd-devel    else install it "yum install httpd-devel"
6. From the native directory Run
      ./configure --with-apxs=/usr/sbin/apxs


Congrats!! Now you can see the module under your native directory.

Tuesday, October 30, 2012

Load Balancing on Web Application server clusters


A cluster is a group of servers running a Web application simultaneously, appearing to the world as if it were a single server. To balance server load, the system distributes requests to different nodes within the server cluster, with the goal of optimizing system performance. This results in higher availability and scalability -- necessities in an enterprise, Web-based application.
High availability can be defined as redundancy. If a single Web server fails, then another server takes over, as transparently as possible, to process the request.
Scalability is an application's ability to support a growing number of users. If it takes an application 10 milliseconds(ms) to respond to one request, then it should take 10 ms to respond to 10,000 concurrent requests.
Of the many methods available to balance a server load, the main two are:
  • DNS round robin and
  • Hardware load balancers.

DNS Round Robin

To balance server loads using DNS, the DNS server maintains several different IP addresses for a site name. The multiple IP addresses represent the machines in the cluster, all of which map to the same single logical site name. Using our example, could be hosted on three machines in a cluster with the following IP addresses:
In this case, the DNS server contains the following mappings:
When the first request arrives at the DNS server, it returns the IP address, the first machine. On the second request, it returns the second IP address: And so on. On the fourth request, the first IP address is returned again.

Advantages of DNS Round Robin

The main advantages of DNS round robin are that it's cheap and easy:

Inexpensive and easy to set up

  • The system administrator only needs to make a few changes in the DNS server to support round robin, and many of the newer DNS servers already include support.
  • It doesn't require any code change to the Web application.


  • It does not require any networking experts to set up or debug the system in case a problem arises.

Disadvantages of DNS Round Robin

Two main disadvantages of this software-based method of load balancing are :

No support for server affinity

  • Server affinity is a load-balancing system's ability to manage a user's requests, either to a specific server or any server, depending on whether session information is maintained on the server or at an underlying, database level.
    Without server affinity, DNS round robin relies on one of three methods devised to maintain session control or user identity to requests coming in over HTTP, which is a stateless protocol.
    • cookies
    • hidden fields
    • URL rewriting
    When a user makes a first request, the Web server returns a text-based token uniquely identifying that user. Subsequent requests include this token using either cookies, URL rewriting, or hidden fields, allowing the server to appear to maintain a session between client and server. When a user establishes a session with one server, all subsequent requests usually go to the same server.
    The problem is that the browser caches that server's IP address. Once the cache expires, the browser makes another request to the DNS server for the IP address associated with the domain name. If the DNS server returns a differnt IP address, that of another server in the cluster, the session information is lost.

No support for high Availability

  • Consider a cluster of n nodes. If a node goes down, then every nth request to the DNS server directs you to the dead node.
  • Changes to the cluster take time to propagate through the rest of the Internet. One reason is that many large organizations -- ISPs, corporations, agencies -- cache their DNS requests to reduce network traffic and request time. When a user within these organizations makes a DNS request, it's checked against the cache's list of DNS names mapped to IP addresses. If it finds an entry, it returns the IP address to the user. If an entry is not found in its local cache, the ISP sends this DNS request to the DNS server and caches response.
    When a cached entry expires, the ISP updates its local database by contacting other DNS servers. When your list of servers changes, it can take a while for the cached entries on other organizations' networks to expire and look for the updated list of servers. During that period, a client can still attempt to hit the downed server node, if that client's ISP still has an entry pointing to it. In such a case, some users of that ISP couldn't access your site on their first attempt, even if your cluster has redundant servers up and running.
  • This is a bigger problem when removing a node than when adding one. When you drop a node, a user may be trying to hit a non-existing server. When you add one, that server may just be under-utilized until its IP address propogates to all the DNS servers. Although this method tries to balance the number of users on each server, it doesn't necessarily balance the server load. Some users could demand a higher load of activity during their session than users on another server, and this methodology cannot guard against that inequity.

Hardware Load Balancers

The above problem can be solved through virtual IP addresses. The load balancer shows a single (virtual) IP address to the outside world, which maps to the addresses of each machine in the cluster. So, in a way, the load balancer exposes the IP address of the entire cluster to the world.


When a request comes to the load balancer, it rewrites the request's header to point to other machines in the cluster. If a machine is removed from the cluster, the request doesn't run the risk of hitting a dead server, since all of the machines in the cluster appear to have the same IP address. This address remains the same even if a node in the cluster is down. Moreover, cached DNS entries around the Internet aren't a problem. When a response is returned, the client sees it coming from the hardware load balancer machine. In other words, the client is dealing with a single machine, the hardware load balancer.

Advantages of Hardware Load Balancers

Support Server affinity

  • The hardware load balancer reads the cookies or URL readings on each request made by the client. Based on this information, it can rewrite the header information and send the request to the appropriate node in the cluster, where its session is maintained.
  • Hardware load balancers can provide server affinity in HTTP communication, but not through a secure channel, such as HTTPS. In a secure channel, the messages are SSL-encrypted, and this prevents the load balancer from reading the session information.

High Availability Through Failover

    Failover happens when one node in a cluster cannot process a request and redirects it to another. There are two types of failover:
  • Request Level Failover. When one node in a cluster cannot process a request (often because it's down), it passes it along to another node.
  • Transparent Session Failover. When an invocation fails, it's transparently routed to another node in the cluster to complete the execution.
    Hardware load balancers provide request-level failover; when the load balancer detects that a particular node has gone down, it redirects all subsequent requests to that dead node to another active node in the cluster. However, any session information on the dead node will be lost when requests are redirected to a new node.
    Transparent session failover requires execution knowledge for a single process in a node, since the hardware load balancer can only detect network-level problems, not errors. In the execution process of a single node, hardware load balancers do not provide transparent session failover.
  • To achieve transparent session failover, the nodes in the cluster must collaborate among each other and have something like a shared memory area or a common database where all the session data is stored. Therefore, if a node in the cluster has a problem, a session can continue in another node.
  • Metrics. Since all requests to a Web application must pass through the load-balancing system, the system can determine the number of active sessions, the number of active sessions connected in any instance, response times, peak load times, the number of sessions during peak load, the number of sessions during minimum load, and more. All this audit information is used to fine tune the entire system for optimal performance.

Disadvantages of Hardware Load Balancers

The drawbacks to the hardware route are the costs, the complexity of setting up, and the vulnerability to a single point of failure. Since all requests pass through a single hardware load balancer, the failure of that piece of hardware sinks the entire site.

Load Balancing HTTPS Requests

As mentioned above, it's difficult to load balance and maintain session information of requests that come in over HTTPS, as they're encrypted. The hardware load balancer cannot redirect requests based on the information in the header, cookies, or URL readings. There are two options to solve this problem:
  • Web server proxies
  • Hardware SSL decoders.

Implementing Web Server Proxies

A Web server proxy that sits in front of a cluster of Web servers takes all requests and decrypts them. Then it redirects them to the appropriate node, based on header information in the header, cookies, and URL readings.
The advantages of Web server proxies are that they offer a way to get server affinity for SSL-encrypted messages, without any extra hardware. But extensive SSL processing puts an extra load on the proxy.
Apache and Tomcat. In many serving systems, Apache and Tomcat servers work together to handle all HTTP requests. Apache handles the request for static pages (including HTML, JPEG, and GIF files), while Tomcat handles requests for dynamic pages (JSPs or servlets). Tomcat servers can also handle static pages, but in combined systems, they're usually set up to handle dynamic requests.
You can also configure Apache and Tomcat to handle HTTPS requests and to balance loads. To achieve this, you run multiple instances of Tomcat servers on one or more machines. If all of the Tomcat servers are running on one machine, they should be configured to listen on different ports. To implement load balancing, you create a special type of Tomcat instance, called a Tomcat Worker.
As shown in the illustration, the Apache Web server receives HTTP and HTTPS requests from clients. If the request is HTTPS, the Apache Web server decrypts the request and sends it to a Web server adapter, which in turn sends the request to the Tomcat Worker, which contains a load-balancing algorithm. Similar to the Web server proxy, this algorithm balances the load among Tomcat instances.

Hardware SSL Decoder

There are hardware devices capable of decoding SSL requests.that sit in front of the hardware load balancer, allowing it to decrypt information in cookies, headers and URLs.
These hardware SSL decoders are faster than Web server proxies and are highly scalable. But as with most hardware solutions, they cost more and are complicated to set up and configure.


As per the above information, I guess Hardware load balancing would be more better to go for web Server load balancing. It supports server affinity and High Availability and also the audit information(Metrics) can be used to fine tune the entire system for optimal performance.

How TOPT Works: Generating OTPs Without Internet Connection

Introduction Have you ever wondered how authentication apps like RSA Authenticator generate One-Time Passwords (OTPs) without requiring an i...