Software Twist – Adrian Tosca Blog

Handling Enterprise Web Application Sessions

Posted in Patterns and Practices, Software Architecture by Adrian Tosca on 2011, January 30

The basics of web session handling

Web server session is a well-known mechanism used in web applications to handle state in connections for an environment that is by very nature stateless. At its base, the HTTP protocol has no notion of state, is just a request-response protocol. Unfortunately the stateless protocol has some issues to cope with certain type of applications that need persistent information over a number of request-response cycles.  A shopping cart is an example of the necessity to save some temporary information over a number of request-response cycles until the check-out is finalized.

Over the years, there were many attempts to improve the communication environment with state information: cookies, information saved in the delivered page and posted-back with the form (a mechanism perfected by Microsoft ASP .Net) and of course the web session.

Web session has many mode of implementations but all have in common that some data is saved on the server and there is a mean of  identifying the data between requests. By using this identifier the data can be retrieved with the next requests. Of course the identifier still needs to be persisted between the requests as is described later.

Mechanisms for saving data on the server

  • Data saved in files

This is a simple way to persists data on the server, the data is serialized in simple files and the name is used to identify  it. This mechanism is often used with the PHP framework. The mechanism is very simple to implement and use but is not safe enough for some uses.

  • Data saved in the same process as the server component

The server component that is processing requests has usually a long living time on the server and can hold the data between requests in memory. Of course a crash of the component will cause the loose of all saved data.

  • Data saved in a dedicated process on the server

This is a variation of the previous method where data is saved in a dedicated process, specially build to handle session data and thus more robust and more unlikely to crash. This does not come for free as it incur some performance penalty because of the need to send data to another process. The data is still lost if the server crashes or needs a restart.

  • Data saved on a separate server

The data is still saved in memory but this time on a separate server or farm of servers. The cost of transferring data over network can be pretty high but the reliability might be more important.

  • Data saved in a database

This method avoid loosing the data but again some performance must be traded in. The performance cost of saving the data to the database might be high but the permanent storage and transactionality might deserve the trade off.

Most current web frameworks including J2EE and .NET Framework have implementations of all these mechanisms.

Mechanisms for identifying the data between requests

  • Using a cookie

A short identifier that uniquely identify the data on the server or more usually the entire session is saved as a session cookie. The cookie  is persisted on client side and sent with each requests to the server usually until the client application that handles the series of requests is closed.

  • Using the URL

The identifier is directly written in the URL of the page. The URL is parsed on the server and the extracted identifier is extracted to identify the data or the entire session.

  • Using a hidden field

The identifier is written in a hidden HTML form field and sent back to the server when a new request is made.

Most current frameworks support all these mechanisms. The cookie is usually the default mechanism but in some cases the other mechanisms are used as a fall back method.

Handling the web session in a more powerful way

All the mechanism covered above are used to simply save some data over a series of requests. The data can be anything and the frameworks do not care of its meaning. The framework that implements the web session mechanism just saves a bunch of data and return it when requested. A obvious limitation is that the session is, well, just a session, limited in time by the time the client is up and running.

Have you used Amazon to buy something? It is interesting how they handle the shopping card data. Instead of saving this data for the duration of the session (as long as your web browser is opened for example) they persist it indefinitely. This is a shift from the classical way of doing things but a very powerful and significant one. Instead of giving the impression of something temporary they basically tell you: “this is your place, you can do anything you want and we will keep your data exactly as you left it until you come back!”.

But this approach also need a shift from the usual session handling. Because to implement a permanent “session” the application needs to understand the data. Imagine for example what would happen if the shopping cart saved an item with 100$ price tag and after a month and several price reductions the user  is coming back and buys the product with the original price: he will not be very happy if he finds out!

The way to handle the permanent “session” is to not treat it like session but as normal application model data. If you are using a relational database to persists the application data you just need to have a couple of more tables to handle the “temporary” data. In the case of the shopping cart example you can have a table with reference to product, the original price and date of the last access. The price can be directly retrieved from the current product price when the shopping cart page is displayed.

But this mechanism can be used not only for fancy web sites but also for normal enterprise class applications. Complex applications usually need a number of steps to enter or modify data. Filling up this data can take quite some time, it can be very frustrating to have to do it at once especially when it happens that the network is temporary unavailable or the damn browser just needed a rest. Imagine you can have a web enterprise “session” mechanism in place and any data that is saved, presumably using several complex forms, is treated as temporary data until the user commit it. The final validation can take place at the last stage when all the data is available and the user has the chance to correct some of first information entered before the last save is performed.

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What is software quality? Depends who is asking

Posted in Patterns and Practices, Software Architecture by Adrian Tosca on 2011, January 3

Sometimes the software quality is an elusive subject. Defining quality is complex because a different perspective will, most of the time, give a whole new definition.

Take for example what the user perceives as quality attributes of a system. When thinking about the user perspective, usability is the first to come to mind. Most users just muddle through an application functions without taking time to learn it. If the user doesn’t figure out what to do he will just leave. If the user really needs to do a task, such as at work using a company system, he will be very angry if he cannot find its way through the system functions.

Another one of the first thing a user notices about an application is the performance. The performance is a show stopper if is not enough. On the other hand, trying to have extreme performance is not needed most of the time and can have a negative impact on other quality attributes. The availability of the system is also an important aspect from the user perspective. A system must be there when needed or is useless.

Security is also perceived as fundamental from the user point of view, especially in the recent years more and more emphasis is put on securing personal data.

On the other hand if looking from the developer perspective quality of a system looks a bit different. The developer will (or should ) think about maintainability as the system will more likely change in one form or the other as a result of new functions or corrections of the ones already implemented. Some components of the system might be reused if the proper thinking is applied and reusability can have a big impact on future implementations.

One of the latest trends in software development has been to use tests from the inception phases of the project. Techniques such as test driven development are applied with great success for a large class of problems. But to be effective the testability of the system must be build into the product from the begging not as an after thought.

From the business perspective the cost of building the system or the time to marked can be very important as these can decide if it is build at all. The projected life time of a system can be also important because a system needs to be operated and maintained, maybe for several years, and this will incur some king of operating costs. A new system will most likely not exists in isolation and certainly will not change over night what a company does, so integration with legacy systems might be an important aspect. Other important aspects for the business can also be thinks like roll-out time or robustness.

So, in conclusion, what is software quality? Well, it seems it depends who is asking!

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Benchmarking – a marketing tool

Posted in Patterns and Practices by Adrian Tosca on 2009, September 1

There has been a recently heated discussion when http://ormbattle.net/ published benchmarks that demonstrates that NHibernate performance is poor compared with other ORM frameworks. At first sight it looks like an valuable tool for assessing different frameworks speeds, but at a deeper analysis and in the light of the comments on Ayanide post «Benchmarks are useless, yes, again» it isn’t as clear anymore:

Trying to compare different products without taking into account their differences is a flawed approach. Moreover, this benchmark is intentionally trying to measure something that NHibernate was never meant to perform. We don’t need to try to optimize those things, because they are meaningless, we have much better ways to resolve things.

Making a benchmark is simple, make a loop and measure things. But this kind of benchmark only shows as much as the speed of runing some meaningless instructions in a non realistic loop.  Making a benchmark that handles real world scenarios is a much harder undertaking, one that could possibly show real insight into using a framework or another. But the frameworks are usualy very different and one way to do things in the first framework has no direct correspondence  in the other. It will just be a different thing to measure. In the end is all about marketing stuff.

If you want to show something is faster you can certainly make a benchmark to show it as another interesing battle at a whole another level clearly demonstrates. Microsoft published on its site a benchmark report entitled «Benchmarking IBM WebSphere 7 on IBM Power6 and AIX vs. Microsoft .NET on HP BladeSystem and Windows Server 2008» that demonstrates .NET on Windows is much better in terms of speed than WebShepre on AIX. IBM response was to make a benchmark of its own showing the opposite.

The bottom line is that benchmarks should be seen with reserves as most often are just marketing tools.

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Working hard versus working smart

Posted in Patterns and Practices by Adrian Tosca on 2009, July 11

Working hard on a project doesn’t always give the best productivity, there are actually more times when it doesn’t. Here are some of the approaches I’ve seen on projects:

Working hard

Work by starting hard

The code like hell approach may work for a small project, but on any project larger than a couple of month it will go wrong. After the initial time the fatigue will prevail and the end will not make anyone on the team happy even if the project eventually delivers it’s product.

Fuzzy start

Fuzzy start

There are times when the start of a project is not so clear, nobody knows what to do and there is a lot of fuzziness in the work that it is done.  When starting a big project it seems that there is so much time that  a lot of activities that are not very productive are allowed to take place. But after some time the schedule start to ring a bell and everyone is starting to work hard. In the end everybody is tired and the overall productivity is not very high.

Working smart

Work smart\

But there is a way, working smart means to work as hard as you can to keep a steady peace forever. The productivity may not be the highest when measured on small periods but overall is higher than with any other approach.

So try to work smart and you will do more than trying to work hard!

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Understanding Liskov Substitution Principle

Posted in Patterns and Practices, Software Development by Adrian Tosca on 2009, June 14

The Liskov Substitution Principle, in its simplified, object oriented language way, says that:

Derived classes must be substitutable for their base classes.

The principle looks simple and obvious; at first look we are tempted to question that the opposite is true: can one make a class that is not substitutable for their base class? Technically speaking no. If one makes a derived class and the compiler does not complain everything should be fine. Or is it not?

Let’s see a simple example:

public class MusicPlayer {
    public virtual void Play(string fileName) {
        // reference implementation for playing
    }
}

// Optimized mp4 player
public class Mp4MusicPlayer : MusicPlayer {
    public override void Play(string fileName) {
        if (Path.GetExtension(fileName) != "mp4")
            throw new ArgumentException("can only play mp4");
        // optimized implementation for playing
    }
}

In the above example we have a reference implementation for a music player. The reference implementation would play all types of files but maybe not with the best performance or quality. An optimized player for a certain type of file, for example for a MP4 file type, is as far as the c# interface is concerned a specialized type of music player.

So what’s the problem? The above Mp4MusicPlayer class just violated the Liskov substitution principle. One cannot substitute MusicPlayer with Mp4MusicPlayer bacause the later works only with certain specific music files. Where MusicPlayer would work with a MP3 file, the Mp4MusicPlayer would throw an exception and the program will fail.

Where did this go wrong? As it seems, there is more to an interface that meets the eye. In our case the preconditions of the derived class are stronger than those of the base class and even if in c# the interface does not contain in it’s definition the preconditions you will have to keep them in mind when designing the inheritance tree.

But there is more than that. The above is only one way in which the Liskov substitution principle can be violated. In a more general terms is a problem of abstractions. When one defines a base class or an interface it actually defines an abstraction. And when one makes a derived class implicitly agrees to satisfy the same abstraction.

In the example above the MusicPlayer abstraction is “something that can play any music file”
But Mp4MusicPlayer abstraction is “something that can play MP4 music files”

The abstractions are different, and this is the root cause of the problem. When the abstractions are not appropriate the situation can get nasty.

Liskov substitution principle is all about abstractions

Liskov substitution principle is all about abstractions

There are multiple ways in which the abstraction can be broken. A wrong abstraction can be hacked in simple cases but eventually it will come back to byte you. We might be able to install an electronic device on the wooden horse in the right of the above image if we need the “horse” abstraction to neigh but we will never persuade it to eat a hand of hay.

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