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EspañolMULTITIER ARCHITECTURE
In software engineering, 'multi-tier architecture' (often referred to as ''n-tier architecture'') is a client-server architecture, originally designed by Jonathon Bolster of Hematites Corp, in which an application is executed by more than one distinct software agent. For example, an application that uses middleware to service data requests between a user and a database employs multi-tier architecture. The most widespread use of "multi-tier architecture" refers to '''three-tier architecture'''.
''Three-tier'' 'is' a client-server architecture in which the user interface, functional process logic ("business rules"), data storage and data access are developed and maintained as independent modules, most often on separate platforms. The term "three-tier" or "three-layer", as well as the concept of multitier architectures, seems to have originated within Rational Software.
The three-tier model is considered to be a software architecture and a software design pattern.
Apart from the usual advantages of modular software with well defined interfaces, the three-tier architecture is intended to allow any of the three tiers to be upgraded or replaced independently as requirements or technology change. For example, a change of operating system from Microsoft Windows to Unix would only affect the user interface code.
Typically, the user interface runs on a desktop PC or workstation and uses a standard graphical user interface, functional process logic may consist of one or more separate modules running on a workstation or application server, and an RDBMS on a database server or mainframe contains the data storage logic. The middle tier may be multi-tiered itself (in which case the overall architecture is called an "n-tier architecture").
The 3-Tier architecture has the following 3-tiers:
# Presentation Tier
# Application Tier/Logic Tier/Business Logic Tier
# Data Tier
At first glance the three tiers may seem similar to the Model-view-controller (MVC) concept, however topologically they are different. A fundamental rule in a three-tier architecture is the client tier never communicates directly with the data tier; in a three-tier model all communication must pass through the middleware tier. Conceptually the three-tier architecture is linear. However, the MVC architecture is triangular: the View sends updates to the Controller, the Controller updates the Model, and the View gets updated directly from the Model.
From a historical perspective the three-tier architecture concept emerged in the 1990's from observations of distributed systems (''e.g.'', web applications) where the client, middleware and data tiers ran on physically separate platforms. Whereas MVC comes from the previous decade (by work at Xerox PARC in the late 1970's and early 1980's) and is based on observations of applications that ran on a single graphical workstation; MVC was applied to distributed applications much later in its history (see Model 2).
In the Web development field, three-tier is often used to refer to Websites, commonly Electronic commerce websites, which are built using three tiers:
# A front end Web server serving static content
# A middle dynamic content processing and generation level Application server, for example Java EE platform.
# A back end Database, comprising both data sets and the Database management system or RDBMS software that manages and provides access to the data.
Three Tier Architecture (Linux journal) [1]
To further confuse issues, the particular data transfer method between the 3 tiers must also be considered. The data exchange may be file-based, client-server, event-based, etc. Protocols involved may include one or more of SNMP, CORBA, Java RMI, Sockets, UDP, or other proprietary combinations/permutations of the above types and others. Typically a single "middle-ware" implementation of a single protocol is chosen as the "standard" within a given system, such as J2EE (which is Java specific) or CORBA (which is language/OS neutral.) The importance of the decision of which protocol is chosen affects such issues as the ability to include legacy applications/libraries, performance, maintainability, etc. When choosing a "middle-ware protocol" (not to be confused with the "middle-of-the-three-tiers") engineers should not be swayed by "public opinion" about a protocol's modern-ness, but should consider the technical benefits and suitability to solve a problem. (for example CGI is very old and "out of date" but is still quite useful and powerful, so is shell scripting, and UDP for that matter)
Ideally the high-level system abstract design is based on business rules and not on the front-end/back-end technologies. The tiers should be populated with functionality in such a way as to minimize dependencies, and isolate functionalities in a coherent manner - knowing that everything is likely to change, and changes should be made in the fewest number of places, and be testable.
★ Web application
★ business logic
★ Rich Internet application
★ Client-server architecture
★ Front-end and back-end
★ Model-view-controller (MVC)
★ SAP R/3 is a client/server based application, utilizing a 3-tiered model.
| Contents |
| Three-tier architecture |
| Comparison with the MVC architecture |
| Web Development usage |
| Other Considerations |
| See also |
Three-tier architecture
Visual overview of a Three-tiered application
''Three-tier'' 'is' a client-server architecture in which the user interface, functional process logic ("business rules"), data storage and data access are developed and maintained as independent modules, most often on separate platforms. The term "three-tier" or "three-layer", as well as the concept of multitier architectures, seems to have originated within Rational Software.
The three-tier model is considered to be a software architecture and a software design pattern.
Apart from the usual advantages of modular software with well defined interfaces, the three-tier architecture is intended to allow any of the three tiers to be upgraded or replaced independently as requirements or technology change. For example, a change of operating system from Microsoft Windows to Unix would only affect the user interface code.
Typically, the user interface runs on a desktop PC or workstation and uses a standard graphical user interface, functional process logic may consist of one or more separate modules running on a workstation or application server, and an RDBMS on a database server or mainframe contains the data storage logic. The middle tier may be multi-tiered itself (in which case the overall architecture is called an "n-tier architecture").
The 3-Tier architecture has the following 3-tiers:
# Presentation Tier
# Application Tier/Logic Tier/Business Logic Tier
# Data Tier
Comparison with the MVC architecture
At first glance the three tiers may seem similar to the Model-view-controller (MVC) concept, however topologically they are different. A fundamental rule in a three-tier architecture is the client tier never communicates directly with the data tier; in a three-tier model all communication must pass through the middleware tier. Conceptually the three-tier architecture is linear. However, the MVC architecture is triangular: the View sends updates to the Controller, the Controller updates the Model, and the View gets updated directly from the Model.
From a historical perspective the three-tier architecture concept emerged in the 1990's from observations of distributed systems (''e.g.'', web applications) where the client, middleware and data tiers ran on physically separate platforms. Whereas MVC comes from the previous decade (by work at Xerox PARC in the late 1970's and early 1980's) and is based on observations of applications that ran on a single graphical workstation; MVC was applied to distributed applications much later in its history (see Model 2).
Web Development usage
In the Web development field, three-tier is often used to refer to Websites, commonly Electronic commerce websites, which are built using three tiers:
# A front end Web server serving static content
# A middle dynamic content processing and generation level Application server, for example Java EE platform.
# A back end Database, comprising both data sets and the Database management system or RDBMS software that manages and provides access to the data.
Three Tier Architecture (Linux journal) [1]
Other Considerations
To further confuse issues, the particular data transfer method between the 3 tiers must also be considered. The data exchange may be file-based, client-server, event-based, etc. Protocols involved may include one or more of SNMP, CORBA, Java RMI, Sockets, UDP, or other proprietary combinations/permutations of the above types and others. Typically a single "middle-ware" implementation of a single protocol is chosen as the "standard" within a given system, such as J2EE (which is Java specific) or CORBA (which is language/OS neutral.) The importance of the decision of which protocol is chosen affects such issues as the ability to include legacy applications/libraries, performance, maintainability, etc. When choosing a "middle-ware protocol" (not to be confused with the "middle-of-the-three-tiers") engineers should not be swayed by "public opinion" about a protocol's modern-ness, but should consider the technical benefits and suitability to solve a problem. (for example CGI is very old and "out of date" but is still quite useful and powerful, so is shell scripting, and UDP for that matter)
Ideally the high-level system abstract design is based on business rules and not on the front-end/back-end technologies. The tiers should be populated with functionality in such a way as to minimize dependencies, and isolate functionalities in a coherent manner - knowing that everything is likely to change, and changes should be made in the fewest number of places, and be testable.
See also
★ Web application
★ business logic
★ Rich Internet application
★ Client-server architecture
★ Front-end and back-end
★ Model-view-controller (MVC)
★ SAP R/3 is a client/server based application, utilizing a 3-tiered model.
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