Windows DNA is a platform for building distributed applications using the Microsoft Windows operating system and related software products. That statement leaves a considerable amount of room for confusion. What is a "platform"? What are "related products"? Even the term distributed computing has been given various definitions.

It's worth the effort to sort out the confusion because multi-tier architectures like DNA are widely regarded as the future of large-scale development. As we will see in this chapter, such architectures offer significant benefits in the construction of mission-critical and enterprise-scale systems. This book will introduce and overview, just about everything you want to know about building modern, forward- looking applications for Windows. The Windows DNA architecture is a blueprint and set of guidelines for doing this.

Before we can begin, however, we need to examine a few concepts. When we talk about modern applications, we are almost always talking about network applications. It's rare to develop an application today that does not either make use of distributed resources or incorporate technologies developed for the Web. Even standalone desktop applications make use of techniques and technologies influenced by distributed programming, such as data source abstraction and component development.

In this chapter, we'll look at:

• The nature of modern, networked applications, and a brief review of the history that brought us to the current state of software development
• What we want to get out of an application, and the inherent problems we need to overcome when designing distributed applications
• The goals that Windows DNA is designed to tackle over and above other architectures

Before we embark on our voyage through Windows DNA, let's examine the nature and needs of a modern application. Building such applications is what Windows DNA is about.

Internet Application Design
In case you haven't noticed, most of the new software development taking place today centers around delivering functionality via the Internet. What does the typical Internet-based application look like today?

There really is no single answer to that question. There are so many types of business solutions being built using Internet technology that there are probably thousands of different types of Internet-based applications. However, there is one question we can probably answer: what are the basic characteristics of these applications?

Internet Application Requirements
In general, an Internet-based application will:

• Present a unified view of data from multiple data sources
• Allow a user to update data
• Provide full e-commerce capabilities
• Be fast
• Be scalable to thousands of concurrent users

Now let's take a look at each of these characteristics in a little more detail.

Presenting a Unified View of Data From Multiple Data Sources
Presenting a unified view of data coming from multiple sources creates a number of problems. A lot of businesses have multiple types of data stores, ranging from mainframe-based VSAM applications, to SQL Server databases, to Oracle databases, to e-mail stores and directory services. There needs to be a way to "tie" all of this data together. So obviously we are going to need a robust data access mechanism, which allows us to access multiple types of data sources that might even reside on different platforms. In addition, we might need host, or mainframe, integration capability. Just getting data from a mainframe application to a web page is a huge technical feat.

Allowing a User to Update Data
If our application allows a user to purchase something, or initiate financial transactions, or update personal data, we're going to need transactional capability. By having transactional capability, we need to somehow make certain that either all parts of a piece of work, or transaction, complete successfully or none of the transaction is allowed to occur.

To make matters more complicated, we already know that the likelihood of having data in multiple sources is fairly high. The consequence is we will need the ability to define transactions that span multiple data sources, while still having full two-phase commit and rollback.

Full e-Commerce Capabilities
Providing full e-commerce capability is a must these days. If you're selling products over the Internet, you will need a framework that provides a shopping cart, and management tools to manage product catalogs, run promotions and sales, and present cross-selling opportunities to your users. Additionally, you will need this framework to be extensible so you can incorporate your own business logic, such as calculating taxes in foreign countries. It would also be really nice if this e-commerce framework used the same transactional capability described above when users commit their purchases.

Fast
You might get users to your web site the first time, but if your site is so slow that they have a bad user experience, they might never come back. So you're going to need to be able to architect a solution that solves your business problem, but is fast at the same time.

Being able to distribute requests among many machines is one way to help achieve a speedy solution. Other design characteristics, such as performing work asynchronously, can also help speed up things.

An example, albeit a crude one, might be an online purchasing application. When a user actually places an order, they probably don't need any kind of response other than "we received your order, here is your confirmation number". You could then place the order on a queue and process it later. The user doesn't know their order won't be processed until later, but they've got the result they wanted quickly.

The implication here is that you'll need a queuing or messaging mechanism at your disposal, so you can incorporate it into your application design.

Scalable to Thousands of Concurrent Users
Not only does your site need to be fast, it probably needs to support thousands of concurrent users. Again, load balancing across multiple machines will help solve this problem. Not only will load balancing help you handle more users, it will also improve your "uptime", and help ensure you are running 24x7x365 like all users expect these days.

Platform Capability Requirements
So now you have a better picture of some characteristics of applications being built today. There's obviously a lot of infrastructure needed to build these applications. What we can do now is take these characteristics, and from them create a list of the capabilities we're going to need from our platform.

These include:

• A web server
• Transaction processing and object brokering
• Message queuing
• A data access mechanism
• Security
• Load balancing capabilities

We'll see later in the book that Windows 2000 provides these, but that's jumping the gun a little. We should probably turn our attention first to identifying the things that drive us when designing applications.

Application Design Goals
For most Internet-based applications you can usually identify two sets of goals: