Context

One of the hardest things about software development is being asked to come up with a design when all you're given is a set of requirements and a blank sheet of paper. One of the first things I do is step back to see the big picture. What exactly is the context here? What is this all about? What is it we're building and how does it fit in with the existing environment?

A really useful starting point can be to draw a simple block diagram showing your system and all of the other systems that it interfaces with. For example, if you're building an Internet Banking system you might have an existing core banking system that contains all of the information about customers and their accounts, along with your new systems that allow customers and administrative staff to access this information. Detail isn't important here. It's your wide angle view showing a big picture of the system landscape.

Using a whiteboard or a sheet of flip chart paper and a set of index cards is an excellent way to get started. In the diagram above, each index card represents a separate, distinct software system that has been annotated with a very high-level statement of its responsibilities. This is very similar to Class-Responsibility-Collaboration (CRC) cards although we're working at a very coarse level of granularity and showing collaborations as explicit connections between the cards.

Such diagrams help you determine the context that your system operates in, which is useful to identify who you potentially need to go and talk to as far as inter-system interfaces are concerned. There are a number of ways to draw these types of diagrams, such as showing your system in the centre of the diagram with inbound/outbound connections surrounding it or you could also choose to show the key actors that have interactions with your system. It's also common to annotate the inter-system connections with some information about their purpose although, as with any diagram, a short supporting commentary is essential.

Abstraction is the key here and detail isn't important as this is a starting point for other diagrams.