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All systems are composed of interconnected components. The boundaries of a system may be physical, or, most often, conceptual – i.e. what comprises a system is defined by the human observer. For example, if one talks about the system for a department of some company, it might (or might not) include the clients, the technical staff, the company-wide computer network. If one talks about a heating system, it might consist of just the hot water tank and thermostat, or it may include the pipes around the house and to the cold water storage tank, or it might also include the tap fittings and bath, sinks etc. and so on. Of course, at the end of the day, a system analyst will make decisions as to the boundaries of the system for a particular project. Resulting from the idea of the boundaries of a system is the associated idea of the external system that exists outside the boundaries. It is up to the system analyst to identify the particular system appropriate for a project, and also the interactions that this system has with the external system. For example, the boundaries for a sales department system might be set in terms of the tasks performed by members of the department, so members of the external system would include customers, suppliers, other departments and so on. The members of the external system interact with the system, so customers place orders to the department, supplied supply goods and present invoices for payment, and the sales department sends information and goods to other departments to arrange payments and delivery. The figure below illustrates how a software system is made up of communicating software component objects. Each oval represents a software component object, and the lines between the objects illustrate how some objects that make up the system communicate to each other. As can be seen, in one case a component object is itself a system, i.e. a component object can itself be a sub-system of a larger system.
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