Introduction
System architecture consist of the abstract concepts that constitute an application, environment, or proposed framework. Sakai is no different. In order to provide a better understanding of Sakai, this document defines an Abstract Sakai Architecture. Technology is specifically absent from this description, as that is what a design consists of. The Sakai Java Framework is an example of an implementation strategy that can be drawn from this.
Principles
The architecture presented here is intended to provide design support for the following guiding principles:
- Create a system in which different kinds of applications, some potentially outside of the Sakai environment can be aggregated to create a single user experience.
- Provide for separation of application and presentation logic.
- Provide an environment which allows tools and services to be migrated and re-used between other Sakai environments, and potentially other (non-Sakai) environments.
- Capture educational, application, common, and system capabilities into re-usable services which can be migrated in Sakai and non-Sakai environments.
- Create an environment which allows tools and services to be adapted to local system system requirements including enterprise and back office services.
Architecture Diagram
The Sakai architecture consist of the following elements:
Client
Sakai is intended to be run as a client / server application pair. While the majority of the clients will be standard, off the shelf web browsers, customized browsers and other network aware applications may be used in some situations. The majority of Sakai applications will have their output aggregated and presented to the client using a markup language, such as HTML Specialized clients may communicate directly with Sakai services provided they are enabled for such transactions (content authoring, for example).
Aggregator
The output of one or more Sakai application (and potentially non-Sakai applications as well) can be combined using an aggregator server application. This aggregator allocates and manages screen real estate and certain user interface transactions (to be defined). Support for accessibility is provided using a combination of standard UI elements in the presentation layer and the aggregator.
Presentation
The presentation layer combines data from a Sakai tool and a user interface description to create a mark up fragment which is aggregated before delivery to the user. The user interface description is ideally contained in a resource external to the software and makes use of standard user interface elements designed to deliver a consistent Sakai user experience.
Tools
A Sakai tool is an application which marries presentation logic with application logic contained in services. Tools provide code to respond to user interface requests and events, which may (or not) modify data managed by services. The tool may draw on services to provide data to the presentation layer.
Services
A service is a collection of classes which manage data via a defined set of behaviors. This data may or may not be persistent across user sessions. Where possible data should be modeled and represented using accepted industry standards. Behavior is represented using a published Application Programming Interface (API). Services may call other services, creating dependencies on them. Services are intended to be modular, reusable and portable across Sakai environments, and potentially to non-Sakai environments also.
System
The system is the server environment which the Sakai environment resides, plus any remote capabilities available to it. This environment may include web servers,database servers, operating systems, file and resource repositories, enterprise and back office systems, etc.