March 08, 2011

"Designing Software, Drawing Pictures

Not a huge amount of new stuff in this session, but a couple of useful things:

The goal of architecture: in particular up-front architecture, is first and foremost to communicate the vision for the system, and secondly to reduce the risk of poor design decisions having expensive consequences.

The Context->Container->Component diagram hierarchy:

The Context diagram shows a system, and the other systems with which it interacts (i.e. the context in which the system operates). It makes no attempt to detail the internal structure of any of the systems, and does not specify any particular technologies. It may contain high-level information about the interfaces or contracts between systems, if
appropriate.

The Container diagram introduces a new abstraction, the Container, which is a logical unit that might correspond to an application server, (J)VM, databases, or other well-isolated element of a system. The container diagram shows the containers within the system, as well as those immediately outside it (from the context diagram), and details the
communication paths, data flows, and dependencies between them.

The Component diagram looks within each container at the individual components, and outlines the responsibilties of each. At the component
level, techniques such as state/activity diagrams start to become useful
in exploring the dynamic behaviour of the system.

(There’s a fourth level of decomposition, the class diagram, at which we start to look at the high-level classes that make up a component, but I’m not sure I really regard this as an architectural concern)

The rule-of-thumb for what is and what isn’t architecture:

All architecture is design, but design is only architecture if it’s costly to change, poorly understood, or high risk. Of course, this means that “the architecture” is a moving target; if we can reduce the cost of change, develop a better understanding, and reduce the risk of an element then it can cease to be architecture any more and simply become part of the design.


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