Physical Model

   

The physical model defines where the physical resources of the application are deployed to support the requirements of the other enterprise sub-models, and the process through which the competing interests of the sub-models are resolved. The completed physical model of an enterprise application is the application’s architecture. Some of the questions this model answers include:

• How will physical resources of computers, network bandwidth and protocols, databases, components, operating system and back-office services, and third party features be used to meet the overall needs of the business (such as scalability and robustness)?
• How will existing deployments migrate to desired deployments?
• How can resources be accessed most efficiently (for example, local machine versus the Internet)?
• How can usability and throughput performance requirements be met in execution environments that include slow LAN/WAN access times, intermittently disconnected network servers, and unreliable Internet connections?

How the physical model interacts with other models

As the diagram in the Enterprise Application Model shows, the physical model directly interacts with the user model, the logical model, and the technology model. The following table characterizes these interactions and gives brief examples of each.

Sub-model	How the physical model relates to it	Example
User model	Provides the means by which the application is made available to the user.	The physical architecture must be able to support the user’s desktop configuration and scale efficiently to the number of users and their processing requirements.
Logical model	Constrains the extent to which logical constructs can be mapped to the physical infrastructure.	An application that provides online transaction processing must define components that can interface with a transaction management infrastructure.
Technology model	Limits the technology that can be supported on the physical infrastructure.	An application deployed to the public on the Internet must use "least common denominator" technology for user interface elements that must run on a variety of platforms and browsers.

Since the development model permeates all of the Enterprise sub-models, there are no "typical" interactions because you must account for every design and implementation decision in the development model.

How the Internet affects the physical model

Examples of the way the Internet has impacted the physical model include:

• An enormous increase in the importance of using highly scalable architectures to support unknown or unlimited numbers of Internet users.
• A requirement to access application functionality (business logic) from many different client platforms.
• A great increase in the use of distributed, asynchronous resources.
• The enabling of new application types, including:

  • Simple informational (simple HTML)

  • Active informational (interactive HTML/ActiveX™/Java)

  • Traditional desktop-oriented application embedded in the Web

• The availability of installation-free user interface (browser-based HTML).
• The need for corporate firewalls between internal LANs and the public Internet.
• On-demand, server-based, application installation (ActiveX components and Java applets).
• Packaging (.cab and .zip files) and digital signing of components.

For more information   The physical model is explored in greater depth in "Designing the Physical Architecture," in Chapter 3, “Creating the Application Architecture.”