Simile's submodel concept provides a flexible but powerful approach to modularity — building a model up from a number of modules or components. At one extreme, Simile supports plug-and-play modularity: you can simply load a submodel, select an interface file, and all the links between the submodel and the main model are automatically made. This enables a community of researchers to agree on an interface standard, then engineer submodels to that standard.

Simile simulates the behaviour of models by generating a computer program and then running this program. The standard language used is Tcl/Tk, which is an interpreted language and hence does not run particularly fast, roughly of the same order as other visual modelling packages. However, if you have a recommended C++ compiler installed, then you can simply choose to run the models in C++ : the whole run-time environment is identical. Models then run several hundred times faster : in many cases, roughly at the same speed as hand-written C programs.

Simile is capable of handling models with several hundred equations. It is also capable of handling models with large number of instances (tens of thousands) of the same type of object. Both aspects mean that you do not hit a bottleneck as you move from prototype models to serious research models.

Many models are developed within a particular modelling paradigm (using the term paradigm in the dictionary sense of “a conceptual framework within which scientific theories are developed”). Thus, a particular model might be described as differential equation model, a System Dynamics model, an age-class model, a spatial model, a cellular automaton model, an object-oriented model, or an agent-based model. Some large-scale ecosystem models do combine two or more paradigms, but these tend to be implemented as large, unwieldy programs.

Modelling is not just about producing results through running a simulation model. It is a social activity, involving many people (other researchers, managers, local people) who are able to contribute to the formulation of a model, but who have no wish to model themselves. Because of Simile's intuitive interface, including familiar, every-day concepts such as stock, flow, influence and object, many people are able to contribute to the model-design process; while the modeller knows that the result of the conceptual-modelling phase can be gradually fleshed out to produce a fully-defined model.

Simile's visual modelling environment is based on the widely-used System Dynamics notation, involving the drawing of stock-and-flow and influence diagrams. Both concepts are intuitive, and thus readily learnt.

This chapter sets out the features of Simile.

This section describes the basic concepts involved in modelling with Simile.