5-Layer Web Architecture

Synopsis

As any other IT system, web systems have to concentrate on architecture considerations. This is all the more true that web applications put together many different techonologies, on both client and server side, with sometimes important overlapping.

In deed, layer definition in IT system is one of the easiest way to achieve sustainable long term development. The well known Internet Protocol is the best example of such a layered system that has proved consistency.

The intention of the article is to define Mediboard architecture the most simple and comprehensive way, based on a multi-layer architecture, more precisely on 5-layer architecture concepts.

Each layer is described with a general purpose definition and when needed, with an explanation/example applied to Mediboard project.

The various layers are also categorized according their (vertical) scope:

• Global: layer is module independant
• Local: layer is module specific

At last, each layer is cut into layer item, themselves fed with a name, a specific technology/language and responsibilities.

The big picture

Layer 1: Data persistence

• Scope: global

The data persistence layer is the low-level layer handling any kind of data in the project. Here you find databases and files systems. You also make decisions on file encoding and native — if necessary — database systems.

For instance you might need transactions or foreign keys and decide to prefer PostgreSQL to mySQL. You could also want to store files as database BLOBs or use file system for instance.

What	Technologies	Responsibilities
Relational object mapping	MySQL	CRUD (Create-Read-Update-Delete) operations
File storage	MySQL, File System	version control, file management

Layer 2: Server-side Framework

• Scope: global

The — server-side — framework layer is probably the most important and time-consuming layer. Being generic enough will allow stability and homogeneousness of the application, and being feature rich will make concrete (module) development much faster and reliable.

It also concerns decisions on internationalization, modularization, HTTP calls, etc., but also coding standards and library use.

What	Technologies	Responsibilities
Framework language	PHP	Server side scripting
Framework standards	PEAR	coding standards, QA, GP helper classes
Framework fondation classes	proprietary	modularization, internationalization, form validation, HTTP requests, HTTP parameters
Data access	ADOdb	DB Abstraction, DB indepandance, DB query forgery (SQL or other)

Layer 3: Business logics

• Scope: local

Business logics layer is the applicative layer that will provide the users with business-related functionality. Provided a good layer 2 framework, you'll be able to prevent creeping featuritis. That layer defines concrete data metamodel, business rules and constraints.

In our case, everything that cares about patients, hospitalisation, medecins, etc., and relations between them and there properties has to be defined here.

In a few words, it handles the module-specific concrete classes implementation, in terms of data computation, form production, etc.

What	Technologies	Responsibilities
Business logics	PHP, Framework fondation classes	concrete class specialisation, form production, data computation, ...
Content production	PHP arrays, PHP variables	prepares to templating

Layer 4: Presentation structure

• Scope: local

The general purpose of Layer 4 is to generate content structure from server-side script, here PHP, in a user agent independant manner, without any positioning or skinning considerations. Just structure the ouput.

The purest way to do so is often to produce any XML compatible information. The classical way is to choose (X)HTML for browser navigation but you could also have RSS feeds for webservice-like information sharing among different websites.

This layer could also be used to produce alternative content structure with the same Layer 3 logics.

What	Technologies	Responsibilities
Presentation production	Smarty	Structures logical information, logics to presentation one-way mapping
Presentation expression	XHTML 1.0 strict	Document logics, skin abstraction

Layer 5: GUI client-side customization

• Scope: mostly global, could be local (the more global/generic the better)

Layer 5 is dedicated to media dependant front-end and skinning cutomization. It's also used to add a bunch of interactivity with client-side scripts.

Most of modern web applications make an extensive use of CSS. Former technologies focused on old and deprecated HTML 4.0 faicilities, with a strong depandance on web browsers implementation.

The good point is that CSS allows clean, reliable, factorized customization at very low cost/effort.

An important remark though, some poor browsers are not yet fully compatible with very advanced CSS2 techniques, so it's the stylesheet developer's responsibility to have at least correct output in all browsers, even if not absolutly identical.

What	Technologies	Responsibilities
GUI skinning	CSS1 CSS2	positioning, coloring, font
GUI interaction	DOM, JavaScript	form validation, interactivity, event handling

Related links

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