Learning Management Systems (LMS) are software systems to implement different e-learning scenarios in practice. Developing LMS that satisfy the needs and desires of teachers is crucial in e-learning. In a simple case, teachers need to provide a set of files to be downloaded by students (library) and participate in an asynchronous discussion (discussion forum). In more complex cases, students are provided with synchronous communication and collaboration tools, implement practical assignments, use virtual reality landscapes, etc. Teachers see a particular LMS from the perspective whether it is capable of implementing an e-learning scenario needed by the teacher, or not. Thus, a modern LMS should demonstrate the ability to implementa variety of different e-learning scenarios, and should support an evolutionary development paradigm to adopt the system functionality to permanent changesof user requirements.

This paper proposes a novel object-oriented architecture for implementing learning management systems suitable for evolutionary development of variety of e-learning scenarios on a single software platform.

In accordance with recent trends in developing modern internet applications we distinguish between client-side (front-end) and server-side (back-end) software components. Communication between such components is carried out by means of HTTP requests generated on the client-side, and HTTP responses from the server-site.

Server-side components of the system may be seen as a cloud service consisting of a number of abstract data classes (ADC) roughly corresponding to main components of an e-learning course such as: course announcements, course curriculum, course library, course forum, course chat, etc. Instances of abstract data classes can be created by means of messages sent to particular ADCs. Such instances of ADCs are called abstract data objects (ADO). Each instance (ADO) has a unique identifier and, hence, is addressable on the cloud. Each ADO may be seen as a combination of persistent private memory and methods that can be applied to elements of the private memory. The private memory is a set of other ADOs. Therefore, a particular e-learning course can be seen as an ADO encapsulating all the necessary course components as other ADOs. For example, a simple e-learning course may encapsulate an announcements object, library object and forum object. The methods applicable to ADOs, provide all the functionality needed from teacher and student perspectives. For example, announcement objects can be edited, created, deleted and accessed.

There are three aspects of the architecture that should be especially pointed out:
- Flexible adaptation of course functionality: a particular course may encapsulate an arbitrary number of different ADOs providing a wide range of functionality. At any time, a certain ADO may be added or removed to adjust the course functionality to particular needs.
- Evolutionary development of the whole system: the system may be further developed by adding new Abstract Data Classes implementing desired e-learning concepts.
- Reuse of course components: the same ADO may be reused in context of different courses allowing sharing functionality of some common components. As a consequence, different courses may use a single shared library, shared forum, chat, etc.

The approach is implemented as an LMS called TeachCenter. TeachCenter is used at TU Graz, Austria for a number of years.

Keywords: LMS, Architecture.