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The N-Tier Application Model and Workflow Rules

Many sites view their deployments as an n-tier application architecture -- a model in which the application is divided into several service tiers or layers that can be modified independently.  Adherence to this organization results in a highly flexible and highly reusable application. It also results in an application that can utilize a variety of Web servers, application servers, and database servers because the interfaces for each layer follow industry-standard protocols.  The essential tiers are:

• Presentation tier. This tier holds the views, forms and reports, often running on their own Web server (e.g. Internet Information Server, Apache) for static content.  Often even individuals can create their own views and reports since the presentation tier does not affect any of the inter-departmental agreements in place concerning managing the data and workflows.
• Business tier. This tier holds the intensive logic that forms the brains of your application (the workflow rules).  The business tier advances work to the next state or responsible party, sends notifications, and runs calculations.  It runs on the application server (such as, WebSphere or WebLogic).  Typically the business logic is the result of deliberate design or explicit agreement between stakeholders; for instance, your business logic may define how lease costs are to be charged internally to departments.  As such, the business tier is encapsulated and is not open to ad-hoc changes.
• Data tier.  This tier holds the data saved in the database management system (such as Oracle, SQL Server, or Sybase).  

The workflow rules embody the business logic for your applications and provide individual steps or services that your organization can tie together to use in their own  or expression of business policy.