Ruminating on Hibernate Entities and DTO

Today we had a lengthy debate on the hot topic of Hibernate entities vs DTO to be used across layers of a n-tiered application. Summarizing the key concepts that were clarified during our brainstorming :)

Q. Is is possible to use Hibernate entities in SOA style web services?

Ans. Yes. It is possible, but with a few important caveats and serious disadvantages.

If you are using Hibernate annotations to decorate your entities, then it would mandate that your service clients also have the necessary Hibernate/JPA jar files. This can be a big issue if you have non-Java clients (e.g. a .NET webservice client). If you use a mapping file (*.xml), then you are good to go as then there are no dependencies on Hibernate jars. But any change in your data model will affect your entities and this will result in changes to all your webservice consumers. So a lot of tight coupling :(

Also you have to understand that Hibernate has used Javaassist to create new dynamic proxies of your Hibernate entities. So the entities that you are referencing are actually that of the generated dynamic proxy - that contains a reference to the actual entity object. So U would need to deproxy the entity and then use it - either using dozer or a similar mapping library.

Person cleanPerson = mapper.map(person, Hibernate.getClass(person));

Note: In the above code, Hibernate.getClass() returns the actual entity class of the proxy object. 

If you are using lazy loading (which most applications do), then you might encounter the famous "LazyInitializationException". This occurs when you detact the Hibernate entity and the serialization process would trigger the lazy loading for a property. To understand this exception, you need to understand the concept of sessions and transactions as given here.
If you do not have lazy loading and do a eager load of all your entities, then it would work - but this is only a short term solution that may work only for small applications.


Q. Ok. I understand the pitfalls. So if I go with DTO, will my problems be solved?

Ans. Yes, DTO is the best option, but you may encounter a few issues again.

Many developers encounter the "LazyInitializationException" when they try to use libraries such as a Dozer to copy properties. This happens because Dozer uses reflection behind the scenes to copy properties and again attempts to access uninitialized lazy collections. There are 2 ways to resolve this problem. Use a custom field mapper as shown here or use the latest version of Dozer that has a HibernateProxyResolver class to get the real entity class behind the Javaassist proxy. This is explained in the proxy-handling section of Dozer site.

Annotations are used at compile-time or runtime?

There is a lot of confusion among folks on the scope of annotations - whether annotations are used only at compile-time or also at runtime?

The answer is that it depends on the RETENTION POLICY of the annotation. An annotation can have one of the three retention policies.

RetentionPolicy.SOURCE: Available only during the compile phase and not at runtime. So they aren't written to the bytecode. Example: @Override, @SuppressWarnings

RetentionPolicy.CLASS: Available in the *.class file, but discarded by the JVM during class loading. Useful when doing bytecode-level post-processing. But not available at runtime.

RetentionPolicy.RUNTIME: Available at runtime in the JVM loaded class. Can be assessed using reflection at runtime. Example: Hibernate / Spring annotations.

Serializing JPA or Hibernate entities over the wire

In my previous post, we discussed about the need to map properties between Hibernate entities and DTOs. This is required, because Hibernate instuments the byte-code of the Java entity classes using tools such as JavaAssist.

Many developers often encounter the LazyInitializationException of Hibernate when you try to use the Hibernate/JPA entities in your webservices stack. To enable you to serialize the same Hibernate entites across the wire as XML, we have 2 options -

1. Use the entity pruner library - This library essentially removes all the hibernate dependencies from your entity classes by pruning them. Though this library is quite stable, developers should be careful about when to prune and unprune.
2. Use AutoMapper libraries - Using libraries such as Dozer makes it very easy to copy properties from the domain hibernate entity to a DTO class. For e.g.

PersonDTO cleanPerson = mapper.map(person, PersonDTO.class);

Out of the above 2 approaches, I would recommed to use the second one as it is clean and easy to use. It also enforces clean separation of concerns. A good discussion on StackOverFlow is available here.

How RSA Protected Configuration Provider works behind the scenes?

We were using the "RSA Protected Configuration provider" to encrypt sensitive information in our config files. I was suprised to see that the generated config file also had a triple-DES encrypted key.

So that means the config section is actually encrypted/decrypted using this symmetric key. But where is the key that has encrypted this key. It is here that the RSA public/private key pair come into picture. The public key in the RSA container is used to encrypt the DES key and the private key is used to decrypt the DES key. A good forum tread discussing this is available here.

There is also a W3C standard for XML encryption available here.

How does DPAPI work behind the scenes?

In my previous post, we saw how DPAPI can be used on Windows platforms to encrypt sensitive information without having to worry about key management. The advantage of using DPAPI is that data protection API is a core part of the OS and no additional libraries are required.

DPAPI is essentially a password-based data protection service and hence requires a password to provide protection. By default, the logged-on users password (hash) would be used for the same.  A good explanation of the internal working of DPAPI is given here:  http://msdn.microsoft.com/en-us/library/ms995355.aspx

Its interesting to see how MS has used concepts such as MasterKey and PRNG to generate keys that would be actually used for encryption.
I was intrigued to understand how DPAPI works when the user/administrator changes the password. Snippet from the article:
"DPAPI hooks into the password-changing module and when a user's password is changed, all MasterKeys are re-encrypted under the new password."

Can we use DPAPI from Java?

In my previous post, we discussed on how DPAPI makes it simple to encrypt sensitive information without worrying about key generation and management.

I wondered if there was a Java API though which we can use DPAPI on windows machines. Found out that there is an open source JNI wrapper available for the same.
http://jdpapi.sourceforge.net/index.html

Also worth reading is this excellent post on encryption key management. 

Ecrypting sensitive information in database

Recently for PCI complaince, we needed to encrypt credit card information (PAN) before storing it in the database. We decided to use AES 256 bit encryption for the same and developed a .NET component in the middle tier for the encryption/decryption.

After this, we faced the chicken-n-egg problem of storing the encryption key :)
In the past, we had used techniques such as storing the key in the windows registry and using RBAC to control access. Other option was to split the key into multiple files and use file-based OS permissions to control access.

But in the lastest version of .NET, you have another good option - i.e. DPAPI (Data Protection API). Using DPAPI, we can delegate key management to the operating system.
We do not provide a key with which to encrypt the data. Rather, the data is encrypted with a key derived from the logged-in user or system credentials. Thus we can pass any "sensitive" information to DPAPI and it would encrypt it using the "password" of the logged-in user or machine level authentication credentials.

The following MSDN links give detailed information on how to achieve this for connection strings - a common requirement scenario.

http://msdn.microsoft.com/en-us/library/ms998280.aspx


In our case, we placed our encryption key in a configuration section and encrypted that section using the methods described in the above link.
Because we were using a web farm scenario with two servers, we had 2 options- either use DPAPI on each server to encrypt the data using the machine specific key or use a separate key store/container and use RSA for encryption. In the first option, we would end up with different 'cipher-value' for the same input data. In the second option, the 'cipher-value' would be the same, but we would need to import the RSA keys on each server in the farm.

More information at this link:   http://msdn.microsoft.com/en-us/library/ms998283.aspx 

Why do we need Base64 encoding?

We often use Base64 encoding when we need to embed an image (or any binary data) within a HTML or XML file. But why do we need to encode it as Base64? What would happen if we don't encode it as Base64?  A good discussion on this topic is available here. 

Base64 was originally devised as a way to allow binary data to be attached to emails as MIME. We need to use Base64 encoding whenever we want to send binary data over a old ASCII-text based protocol. If we don't do this, then there is a risk that certain characters may be improperly interpreted. For e.g.

• Newline chars such as 0x0A and 0x0D
• Control characters like ^C, ^D, and ^Z that are interpreted as end-of-file on some platforms
• NULL byte as the end of a text string
• Bytes above 0x7F (non-ASCII)

We use Base64 encoding in HTML/XML docs to avoid characters like '<' and '>' being interpreted as tags.

Beware of using Hibernate 'increment' id generator

While reviewing one of the development projects using Hibernate as the ORM tool, I noticed that they were using the "increment" ID generation feature.

The way the implementation of this feature works is that it checks the highest value of the ID in the database and then increments it by one. But this approach fails miserable in a clusterned environment, because many threads might be writing to database concurrently and there is no locking.

It is best to stick to database specific features for auto-id generations, e.g. sequences in SQLServer 2012, Oracle 11g and identity columns in old versions of SQL Server.

Delegated Authentication through a Login Provider

Nowadays it is common to see many internet sites delegating the authentication process to some third-party providers such as Google, Facebook, Twitter, etc.

So essentially you do not have to register and create a new login/password for the site. You can just use your existing login credentials of google, facebook, twitter, OpenID, LiveID, etc. Behind the scenes, the application and the login provider use standards such as OAuth and OpenID to do the magic.

The advantages of delegating authentication to a popular third-party provider are:

1. Users don't have to register and create another set of username/password for your site. Thus the user has fewer passwords to remember.
2. The application does not have to worry about creating a Login form and SSL enabling it.

Found 2 good libraries on google-code for enabling OAuth delegated authentication on your application:
http://code.google.com/p/socialauth/

http://code.google.com/p/socialauth-net/

How does Oracle RAC manage locks?

Typically databases store locks in-memory and hence it is challenging to load-balance or cluster 'write' operations to a shared database across multiple database instances.
For e.g. In my previous blog, I mentioned how SQL Server does not have any Oracle RAC equivalent.

So how does Oracle RAC managed to maintain locks & data consistency across multiple nodes of the RAC cluster?

The secret sauce is the high speed private network called the "interconnect". RAC has something called as "Cache Fusion Mechanism" that allows for inter-instance block transfers and cache consistency. The global cache services (GCS) is a set of processes that ensures that only one instance modifies the block at any given time. GCS also flags in-memory blocks as "invalid" whenever the blocks are changed in other nodes.

The following links have a good explanation on the RAC consistency mechanism:
http://www.rampant-books.com/art_rac_global_block_management.htm

http://www.rampant-books.com/art_burleson_rac_cache_coherency.htm

Ruminating on SQLServer HA and DR strategy

Recently for one of our customers, we were evaluating the options for configuring SQLServer 2008 for High Availability and Disaster Recovery.

Having successfully used Oracle RAC technology in the past many a times, I was suprised to realize that SQLServer does not have any equivalent to Oracle RAC. There is essentially no concept of load-balancing "read-write" requests between server instances working on a shared database storage system (e.g. SAN, RAID-10 array).

The only near equivalent to the RAC concept is using the SQLServer 2008 peer-to-peer transaction replication with updatable subscriptions. This is horribly complex to configure and maintain with data being replicated to peer servers. Also MS has finally decided not to support this feature anymore.

The clustering techniques of SQLServer 2008 use confusing words such as "active/active" and "active/passive". But in reality, there is no load-balancing of the database requests. The secondary server is typically in stand-by mode (as in active/passive). The 'active/active' concept essentially means that the SQL Servers are accessing two separate databases or a partitioned database. If one server fails, the processing is offloaded to the second machine. So now the second machine is running 2 server instances and this bogs down the resources of the server.

In SQLServer 2012, MS has tried to move closed to the RAC feature. We have a new feature of "AlwaysOn" that enables us to create HA groups and define replication strategies between the nodes in a HA group. The 'secondary' server would always be 'read-only' and can be used for reporting purposes or for 'read' operations. This is far better, as now we can atleast offload our 'read' operations to a different server, but we need to funnel all 'write' operations through the 'primary' server. A good discussion on this topic is available here.

After HA, we moved over to define our DR strategy. We first compared log-shipping vs mirroring. In Log Shipping, the secondary database is marked as 'read-only' and can be used for reporting, but the time taken for replication can be upto 30 mins. Hence it cannot be used if you desire instantaneous failover.
In Mirroring, there is almost instantaneous update to the secondary failover database, but since the database is always in recovery mode, it cannot be used for any purpose.

Again in Mirroring, we have 'synchronous' and 'asynchronous' replication. IMHO, synch replication is a disaster for any OLTP application and should actually never be used. Found a good case study on MS site that details out the strategy for "identity columns" when there is a potential for data-loss using async mirroring. Another option that can be considered for DR is SQL Replication.

Spring Security SecurityContextHolder

We have been happily using the static methods of "SecurityContextHolder" in our application. But as architects, it is important to understand what happends behind the scenes..

Where does Spring store the Principle object? My rough guess was that it would either be in Session or the ThreadLocal containers.
Found this great write-up that explains how the "Strategy" design pattern is used to store the UserDetails (Principle) object in the HTTPSession and then through a filter assigning it to the ThreadLocale container.

http://stackoverflow.com/questions/6408007/spring-securitys-securitycontextholder-session-or-request-bound

The advantage of this approach is that one can obtain the current 'Principle' anywhere in the code using simple static methods rather than being dependant on the request object or session object.

Connection Strings Reference.

Found this cool site containing a consolidated listing of "connection-strings" that can be applied to most of the databases out there. Quite valuable for quick reference :)

http://www.connectionstrings.com/

Estimating the size of SQL Server Database

MSDN has a very good link that explains in simple language on how we can estimate the size of the database. The size of the database would depend on the size of the tables and the indexes. The below link gives us easy formulas that can be used to calculate the database size.
http://msdn.microsoft.com/en-us/library/ms187445

A good samaritan also created excel templates that use these formulas for database sizing.

On Oracle, you can use PL/SQL SPROCs to calculate the table and index sizes. For e.g. you have the CREATE_TABLE_COST procedure of the DBMS_SPACE package that can be used to find out the bytes required for a table. You just have to input the potential number of rows expected.

Ruminating on RACI matrix

The RACI matrix (Responsible, Accountable, Consulted, Informed) is an excellent tool for mapping between processes/functions and roles.
It brings in immediate clarity on Who’s Responsible for something, Who's Accountable for the final result, Who needs to be Consulted and Who all are kept Informed.

There are several templates available on the net that can be used for the RACI matrix. One good template is available here. Once data is collated on such a matrix, then we can analyse questions such as -

a) Are there too many A's? Usually its best to have only 1 person/role accountable for a process/function.
b) Are there too many I's? Too much information that is not needed? etc.

We can use RACI matrices anywhere. You could "RACI" any deliverable in your project. Also in EA governance, we can use RACI to clearly separate the responsibilities across the centralized EA team and project architects.Given below is an example of EA Governance RACI Matrix.

Centralized Clustered ESB vs Distributed ESB

Often folks ask me the difference between traditional EAI (Enterprise Application Integration) products and ESB (Enterprise Service Bus). Traditionally EAI products followed the hub-spoke model and when we look at a lot of topology options of ESB's then one would see that the hub-spoke model is still followed!

Given the fact that almost all EAI product vendors have metamorphized their old products to 'ESB' adds to the confusion.  For e.g. The popular IBM Message Broker product line is being branded as 'Advanced ESB'. Microsoft has released a ESB Guidance Package that depends on the BizTalk platform, etc.

In theory, there is a difference between EAI (hub-n-spoke) architecture and ESB (distributed bus) architecture. In a hub/spoke model, the hub 'broker' becomes the single point of failure as all traffic is routed through it. This drawback can be addressed by clustering brokers for high availability.

In a distributed ESB, there is a network of brokers that colloborate to form a messaging fabric. So in essence, there is a small lightweight ESB engine that runs on every node where a SOA component is deployed. This lightweight ESB engine would do the message transformation and routing.

For e.g. in Apache ServiceMix, you can create a network of message brokers (ActiveMQ). Multiple instances of ServiceMix can be networked together using ActiveMQ brokers. The client application sees it as one logical normalized message router (NMR). But here again, the configuration info (e.g. routing information, service names, etc.) are centralized somewhere.


So then what is the fundamental difference between hub-n-spoke and ESB? IBM did a good job in clearing this confusion, by brining in 2 concepts/terms - "centralization of control config" and "distribution of infrastructure".  A good blog explaining these concepts from IBM is here. Loved the diagram flow on this post. Point made :)

Snippet from IBM site:
"Both ESB and hub-and-spoke solutions centralize control of configuration, such as the routing of service interactions, the naming of services, etc. 
Similarly, both solutions might deploy in a simple centralized infrastructure, or in a more sophisticated, distributed manner. In fact, if the Hub-and-spoke model has these features it is essentially an ESB."

As explained earlier, some opensource ESBs such as Apache Service Mix and Petals ESB are lightweight and have the core esb engine (aka service engine) deployed on each node. These folks call themselves "distributed ESB". Other vendors such as IBM, use the concept of "Federated ESBs" for distributed topologies across ESB domains.

Translate Chinese unicode code points to English

One of our legacy applications had the UI in chinese and it was required to convert it to English.
Instead of hiring a translator, we decided to use Google Translation Services.

But the application was picking up chinese lables/messages from a properties file. The properties file had the chinese characters expressed as unicode code points. The Google Translate webpage expected chinese characters to be typed or copy pasted onto the form. We searched for a similar translation service that would accept unicode code points, but in vain.

Finally, we decided to write a simple program that would write the chinese unicode codepoints to a file and then open the file using a program such as notepad++ or MS word. These programs support chinese characters and would allow you to copy paste them onto the Google Translation page.

Given below is the simple Java code snippet to write to a file. Please open this file using MS Word (or any other program that supports UTF-8 font rendering).
-------------------------------------------------

import java.io.File;
import com.google.common.io.Files;

public class Chinese_Chars {
    public static void main (String arg[])throws Exception{

        String str = "\u6587\u4EF6";
        byte[] array = str.getBytes("UTF-8");
        
        File file = new File("d:/temp.txt");
        Files.write(array, file);
    }
}

---------------------------------------------------

Show below are some screen shots of Google translate page and MS Word opening the file.

Understanding REST

In one of my old posts, I had elaborated on the differences between REST and simple POX/HTTP.

Recently came across an interesting post by Ryan Tomayko; where-in he trys to explain REST in a simple narrative style. A must read :)    http://tomayko.com/writings/rest-to-my-wife

Another interesting discussion thread on REST is available at StackOverFlow - regarding verbs and error codes.

File Upload Security - Restrict file types

In web applications, we often have to restrict the file types that can be uploaded to the server. One way to restrict it is by checking the file extensions. But what if someone changes the file extension and tries to upload a file.

For common file types such as GIF, PDF, JPEG we can check the contents of the file for a "signature" or "magic number". More information given in this blog post - http://hrycan.com/2010/06/01/magic-numbers/

The Apache Tika project can be used to quickly extract meta-data information from a file stream. 

List of names...

Very often, we quickly need to populate a sample database with a list of names. In the past, we often did this by using random numbers appended to some common names.

But found this cool site on the web that gives us thousands of sample names that can be used to populate our databases for demo purposes.

http://www.listofnames.info/

Ruminating on JSF

In the past, I always hated JSF, the same way I hated EJB 2.x. But of-late, I am seeing a renewed interest in JSF, especially since a lot of pain areas were resolved in the JSF 2.0 specification.

Over the last couple of days, I have been evaluating PrimeFaces - an opensource implementation of JSF 2.0 and I would say that I am pretty impressed. Going through the sample demo pages, I was mighty pleased with the neat and clean code on both the XHTML file and the POJO beans Java code.

Also PrimeFaces has a plethora of components that should suffice for 90-95% of all basic web application requirements.

In general, if you do not require heavy UI customization and can hence sacrifice absolute control over the generated HTML, CSS and JS, then I would say that using PrimeFaces would greatly increase the productivity of an average development team. IMHO, the productivity gain could be as high as 50% over doing the conventional plumbing using MVC frameworks and JQuery.

But if there is a special requirement that cannot be sufficed by the standard UI components provided by PrimeFaces, then you are in trouble. You would then need deeper expertise to write your own JSF component or customize existing ones.

Based on my study and the common challenges we faced, I am jotting down some FAQ's that should be useful for folks embarking on using PrimeFaces.

Q) How to manipulate JSF components on the client side using plain JS or JQuery? How to use JQuery JS API or any other JS library on the client side with JSF?

http://stackoverflow.com/questions/7927716/how-to-select-primefaces-ui-or-jsf-components-using-jquery

http://stackoverflow.com/questions/5457292/jquery-conflicts-with-primefaces

Q) How to include JS file or inline JS code in a JSF XHTML page?
A) There are 3 ways to handle this.

• Escape all special chars like 'greater than' or 'lesser than'
• Use <![CDATA[ ... ]]> to hold your JavaScript code
• Put the JavaScript code in a separate .js file, and use in the JSF page  

http://www.mkyong.com/jsf2/how-to-include-javascript-file-in-jsf/

Q) How do I output HTML text in JSF? Do I need to use the 'verbatim' tag?
A) <h:outputtext escape="false" value="#{daBean.markedUpString}"></h:outputtext>

Q) Can I mix HTML tags with JSF tags?
A) You can. It is not as much as a pain as in JSF 1.x, but you need to be aware of issues.
http://stackoverflow.com/questions/5474178/jsf-facelets-why-is-it-not-a-good-idea-to-mix-jsf-facelets-with-html-tags

SOA interoperability - .NET WCF from Java

Recently, one of our teams was struggling to integrate a Java application with a .NET WCF service. The exception that was thrown on the Java side was a SOAPFault as shown below:

SOAPFaultException: The message with Action 'https://tempService:48493/VirtualTechnician/AcceptVehicleTermsOfService' cannot be processed at the receiver, due to a ContractFilter mismatch at the EndpointDispatcher. This may be because of either a contract mismatch (mismatched Actions between sender and receiver) or a binding/security mismatch between the sender and the receiver. Check that sender and receiver have the same contract and the same binding (including security requirements, e.g. Message, Transport, None). 

After a lot of debugging using SOAPUI and WireShark, we found out that the problem was not in the SOAP message, but in the HTTP header. The SOAP Action HTTP header needs to be set in the HTTP Post Request.

On JAX-WS, it can be done with the following code snippet:

BindingProvider bp = (BindingProvider) smDispatch;
            bp.getRequestContext().put(BindingProvider.SESSION_MAINTAIN_PROPERTY, Boolean.TRUE);
            bp.getRequestContext().put(BindingProvider.SOAPACTION_USE_PROPERTY, Boolean.TRUE);
            bp.getRequestContext().put(BindingProvider.SOAPACTION_URI_PROPERTY, "http://tempuri.org/IVtOwl/AcceptVehicleTermsOfService");// Correct SOAP Action

Custom PMD rules using XPath

Writing custom rules in PMD using XPath is an exciting concept, but unfortunately there are not many good tutorials or reference guides available on the internet for this.

Recently, we wanted to write custom PMD rules to extract Spring JDBC calls from the code base. We utilized the PMD desiger that is provided OOTB in Eclipse to easily write the rules.

Just open Eclipse -> Preferences -> PMD -> Rule Designer.
In Rule Designer, copy-paste your source code and check the AST (Abstract Syntax Tree) that is formed. You can also copy the AST XML from the menu bar and paste it on to a text editor. Writing the XPath expression then becomes very very simple !!!

For e.g. for finding our the Spring JDBC query calls the XPath was:
//PrimaryPrefix[Name[starts-with(@Image,'jdbcTemplate.query')]]

Ruminating on "Trickle Batch Load".

Traditionally most of the batch jobs were run at end-of-day; where the entire day transaction log was pulled out and processed in some way. During the good old days, when the volume of data was low, these batch processes could easily meet their business SLAs. Even if there was a failure, there was sufficient time to correct the data and rerun the process and still meet the SLA's.

But today, the volume of data has grown exponentially. Across many of our customers, we have seen challenges around meeting SLA's due to large data volumes. Also business stakeholders have become more demanding and want to see sales reports and spot trends early - many times during a day. To tackle these challenges, we have to resort to the 'trickle batch' load design pattern.

In trickle batch, small delta changes from source systems are sent for processing mutiple times a day. There are various advantages of such a design -

• The business gets real-time access to critical information. For e.g. sales per hour.
• Business SLA's can be easily met, as EOD processes are no longer a bottleneck. 
• Operational benefits include low network latency and reduced CPU/resource utilization.
• Early detection of issues - network issues, file corruption, etc. 

The typically design strategies used to implement "trickle batch" are using the CDC (Change Data Capture) capabilities of Databases and ETL tools.
Today almost all ETL tools such as IBM InfoSphere DataStage, Informatica, Oracle Data Integrator, etc. have in-built CDC capabilities. 

Trickle batches typically feed a ODS (Operational Data Store) that can run transactional reports. Data from the ODS is then fed to a DW appliance for MIS reporting.

Ruminating on Session Replication

In any web application cluster, we need to configure Session Replication for failover and high availability. While configuring session replication, the following common queries often come across our minds. I have attempted to answer these questions in a product agnostic way.

Q) Is the session replicated sychronously or asynchronously?
A) Session replication can occur either synchronously or asynchronously. In sychronous replication, the request does not return until the session has been replicated across all members of the cluster. This obviously has performance implications. In asynchronous replication, the response is returned and the session data is queued to be replicated across the cluster nodes. Typically asynchronous replication is the default mode on most app servers and this is configured with "Session Affinity".

Q) Is the entire session replicated or only the delta of what has changed?
A) It could be quite difficult to keep track of all session data modifications accurately. For e.g. someone would just refer a object in session and change its properties, without calling setAttribute() again. Hence typically app servers would replicate the entire session object each time for every request.

Q) What are the different topology options for configuring the memory-to-memory session replication?
A) For small clusters, we can set up all-to-all peer replication - i.e. the session is replicated across all the nodes of the cluster. For large clusters, we can set up "replication groups" or "buddy groups" that are essentially a group of nodes that would replicate session data between themselves. In some environments such as WebSphere Extreme Scale, one can configure dedicated JVMs in a grid for storing sessions.

Exploring Google Guava

I am pretty impressed with the simplicity of Google Guava API. We have been using Apache Commons for many years, but Guava has good support for generics and hence is a better choice for new Java development.

For e.g. to read a text file as a list of strings, you need just 2 lines of code.

//read a file from the classpath, as we want to avoid absolute paths..
File file = new File(getClass().getResource("/com/company/project/test.txt").getFile());
//pass a proper charset to ensure proper encoding
List<String>lines = Files.readLines(file, Charsets.UTF_8); 

Another alternate method to read the entire content of the file as a string -

public static String toString(File file,Charset charset)

To read a binary file into a byte array, use the following utility method -

public static byte[] toByteArray(File file)

Similar simple methods exist for writing files too !..

Some interesting code..to tickle your brain cells

Found this code over the internet :) ...Took me some time to debug. Please copy-paste the below code in eclipse and start analysing - why this is happening?

public class TimePass {
    public static void main(String[] args) {
        if ( false == true ) { //these characters are ignored?: \u000a\u007d\u007b
            System.out.println("false is true!");
        }
    }
}

Hint: linefeed and curly braces :)

Using Netty - Lessons learned

We were building a ISO-8583 equivalent card transaction simulator using the powerful Netty framework.The simplicity of the netty design is a great wow factor. Also most of the complexities of NIO are abstracted by the framework by concepts such as Channel, ChannelBuffer, ChannelEvent, Decoders/Encoders, etc.

While we were using the LengthFieldBasedFrameDecoder class, we faced an intriguing problem. The socket client was sending the length of the record in the first 2 bytes. For e.g. "11abcdefghijk"
But strangely, the length derived by the  BigEndianHeapChannelBuffer was something different.
To get to the bottom of this, we enabled debugging and saw the raw buffer byte array that reached the server.

The buffer array for "11abcdefghijk" was [49, 49, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107]
The first 2 bytes contained the unicode charset number of the character '1'. Hence the lenght was also encoded from a character into bytes using the default charset, which resulted in a very large number.. Obviously the LengthFieldBasedFrameDecoder failed to decode the message.

To get around this problem, we had to send the first 2 bytes without any characted encoding. We achieved this using unicode escape sequences for the lenght field - i.e. \u0000 and \u000b; essentially 0 and 11.
The buffer array for "\u0000\u000babcdefghijk" was [0, 11, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107]

Now the  netty decoder would work. But this approach has a serious flaw...for 2 chars - \u000A and \u000D. These chars correspond to \n (linefeed) and \r (carriage return) and hence would not compile itself as the Unicode escapes are pre-processed before the compiler is run.

Hence the best approach is to write the lenght field to the socket stream without using any charset encoding (write raw bytes) and then write the record as a string.

An alternate workaround is to write your own decoder, which too is very simple. 

Given below is a sample decoder that extracts the first 2 chars of a string for the length and returns the buffer upstream.

-----------------------------------------------------------------

import java.util.logging.Logger;

import org.jboss.netty.buffer.ChannelBuffer;
import org.jboss.netty.channel.Channel;
import org.jboss.netty.channel.ChannelHandlerContext;
import org.jboss.netty.handler.codec.frame.FrameDecoder;
import org.jboss.netty.util.CharsetUtil;

public class NarenLengthFieldDecoder extends FrameDecoder {

    private static final Logger logger = Logger.getLogger(NarenLengthFieldDecoder.class.getName());

    public int lengthFieldLength = 2; // some default value

    public NarenLengthFieldDecoder(int length) {
        this.lengthFieldLength = length;
    }

    @Override
    protected Object decode(ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception {
        // wait until the length prefix is available.
        if (buffer.readableBytes() < lengthFieldLength) {
            // return null to inform frame decoder that frame is not yet
            // complete and to continue reading data
            return null;
        }

        // read length field...create a byteArray for the length field and copy
        // the first bytes into it..
        byte[] array = new byte[lengthFieldLength];
        buffer.readBytes(array); // Imp: readBytes also forwards the readerIndex

        // Mark the current buffer position before reading the length field
        // because the whole frame might not be in the buffer yet.
        // We will reset the buffer position to the marked position if
        // there's not enough bytes in the buffer.
        buffer.markReaderIndex();

        int dataLength = getLength(array);// length of the record

        // wait until the whole data is available.
        if (buffer.readableBytes() < dataLength) {
            // The whole bytes were not received yet - return null.
            // This method will be invoked again when more packets are received
            // and appended to the buffer.

            // Reset to the marked position to read the length field again next
            // time.
            buffer.resetReaderIndex();
            return null;
        }

        // forward remaining buffer to higher up handlers
         // There's enough bytes in the buffer. Read it.
         ChannelBuffer frame = buffer.readBytes(dataLength);
         // Successfully decoded a frame.  Return the decoded frame.
         return frame;
    }

    /**
     * Returns the first 2 or 3 sequence of bytes...that specify the length of
     * the record
     * 
     * @param array
     *            The byte array containing the length
     * @return An integer representing the length of the record
     */
    public int getLength(byte[] array) {
        String temp = new String(array, CharsetUtil.ISO_8859_1);

        int length = 0;
        try {
            length = Integer.parseInt(temp);
        } catch (Exception ex) {
            logger.info("Could not parse the length field of the record >>>" + temp);
        }
        return length;
    }
}

UIaaS - UI as a Service

Some time back, Salesforce popularized the term - UIaaS (UI as a Service), when they launched VisualForce.

UIaaS essentially means the ability to create new user interfaces using pre-built components. UI components could be pages, controls, static resources, etc. So the concept is to not start from scratch, but use off-the-shelf UI widgets. Typically UI designers are web-based and allow for in-browser UI design.

The underlying technologies for creating UIaaS are –
Server Side: JSF, Portlets, .NET WebParts
Client Side: Dojo controls, JQuery controls, Ext-JS controls, etc.

IMHO, the term is just old wine in new bottle. We used to have concepts of UI Widget Factory that encompasses creating reusable widgets and storing them (with meta-data) in a repository. Application developers would then pick and choose their widgets and design new pages. The reusable widgets could be technical widgets such as a "tab-bar","menu-bar","calendar", etc. or business widgets such as "Healthcare Provider Search", "Google Maps Overlay", etc.