Logical and Physical Independence

The architectural dimension illustrated in Figure 2–4 emphasizes the logical and physical independence of components, represented by four separate tiers. These tiers signify the partitioning of application logic across various computers in a networked environment:

• Logical independence. The four tiers in the architectural model represent logical independence: You can modify application logic in one tier (for example, in the business service tier) independently of the logic in other tiers. You can change your implementation of business logic without having to change or upgrade logic in the presentation or client tier. This independence means, for example, that you can introduce new types of client components without having to modify business service components.

• Physical independence. The four tiers also represent physical independence: You can deploy the logic in different tiers on different hardware platforms (that is, different processor configurations, chip sets, and operating systems). This independence allows you to run distributed application components on the computers best suited to their individual computing requirements and best suited to maximizing network bandwidth.

  How you map application components or infrastructure components to a hardware environment (that is, your deployment architecture) depends on many factors, depending on the scale and complexity of your software solution. For very small deployments, a deployment architecture might involve only a few computers. For large-scale deployments, the mapping of components to a hardware environment might take into account factors such as the speed and power of different computers, the speed and bandwidth of network links, security and firewall considerations, and component replication strategies for high availability and scalability.