An architecture is a set of primary component types that characterize a system. All systems have an architecture. An architecture is similar to the list of components of a system but is more about the types of components rather than the instances. There are many types of architectures.

The one most people are aware of is building architecture. In fact it is called architectural engineering.

This is a historical consequence of early designing being all about buildings where the Greek transliteration of arkytektone is the master builder. I knew those Greek classes would come in handy some day. Electrical, mechanical, civil and software systems each have architectures.

In addition to architectural domains there are abstract architectures called logical architectures. In fact, systems engineers prefer to work with logical architectures and not worry about the actual type of physical architecture that will be employed.

Logical Architecture

A logical architecture is not to suggest that there is an illogical architecture but an alternative or derived physical architecture. Logical architecture is the decomposition of a subject system into a set of logical subsystems. These are largely treated as black boxes that will later be assigned to physical systems.

The systems engineer prefers to work at the logical architecture level because it allows him to make clear distinctions about who is responsible for what parts of the system precluding many turf wars.

Logical architecture diagrams are not that interesting to look at. They contain a few boxes that have bland sounding names. But the lines, the lines tell the story. They say what can be communicated among the boxes. They name the interfaces and which boxes use them. This stuff is system gold. When you can get people to agree on the lines then you have saved yourself from a world of hurt. This leaves many domain specific decisions to actual hardware or software designers but precludes them from stepping on each other's territory. If a function is assigned to a software logical block then it is clear to everyone what group is responsible for the implementation.

Physical Architecture

Physical architecture is the type we are most familiar with. A physical architecture shows the specific types of equipment that will be used in the development of a system. Technically, physical architecture design is no longer a requirements related activity but a high level design process. This is because, the boundaries and interfaces are the domain of the requirements. Once established, these define what is inside and hence design. Sometimes the parts are buy outs while at other times the parts are made in-house.

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