Project Description
A working version of the model can be found here
Overview
Multi-tier architectures are traditionally used for database applications. The middle tier separates presentation and business functions and its services allow communication between programs based on different technologies and programming languages.
Different technologies for realization of the middle tier exist (e.g. transaction processing, message-oriented, object-oriented, and Web-based). They differ in communication protocols and service allocation.Multi-tier architecture provides many benefits over traditional client/server architecture:
Installing and deploying the user interface is virtually instantaneous - only the Web interface in the middle tier needs to be updated. Without a "thick" client interface, it is easier to deploy, maintain, and modify applications - no matter where the client is located. Because the application itself is server-based, users always access the most up-to-date version. These benefits explain the growing popularity of the multi-tier architecture, and why almost every client/server application provider has retooled or is retooling to support Web-based clients.
Web-based Multi-tier model
The presented model generally consists of four tiers. The tiered architecture is chosen for flexibility and separation of presentation and business roles within the real automation. The additive benefit is the security . every tier can connect only to its direct neighbors. So the data and the business rules cannot be directly accessed from the Internet, and thus cannot be harmed (see figure below).
On the top tier are the clients . Client tier. They request services from the system using standard internet browser. Different HTTP pages are constructed for the different kinds of devices (e.g. PCs, Laptop, PDA, cellphone).
The next tier is the Presentation tier. This tier is responsible for handling the clients. requests and forming the view of the responses. After receiving a request it is analyzed, transformed into XML encoded queries, and dispatched to the appropriate server from the next tier.
The Services tier works bellow the presentation tier. On this tier all the functionality of the model is placed. The different services work on different servers, so a hardware failure of a single server affects only the corresponding service. This modular approach increases the flexibility and reliability of the whole model. Example functions of the servers in the services tier are: data logging, plant control and monitoring, commerce services, customers support services, training services, accounting services, etc.
The lowest tier of the model is the Data tier. Its role is inherited from the three-tier database model. It depends on the upper tier servers. In the case of logging server the data tier is a database. In the case of transaction server the data tier is presented of controller network (figure 1). Other forms of data tier occur in other cases. Generally the role of the data tier is to produce or store data.
