Relationships between Functional Units - IJOY94/flow-design-cheatsheet GitHub Wiki

Functional units (FU) are the data processors in flow designs. The operate on input data and produce output data - sometimes while using in-memory state or a hardware resource.

In order to work together towards a common goal - some overall software behavior - FUs need to be connected, thus forming data flows and abstraction hierarchies.

The two fundamental relationships between FUs are:

	Explanation
	Data enclosed in () is flowing from the producer A to the consumer B.
	A depends on B for some service.

Note that in contrast to UML diagrams a dependency relationship is not denoted by an arrow. The arrow symbol is reserved for data flows.

In flow designs the most common relationship of course are data flows. This serves two purposes:

• Decoupling: Functional units in data flows are fundamentally decoupled, i.e. the do know not of each other and therefore do not rely on each other. They are independent and follow the Principle of Mutual Oblivion (PoMO).
• Explicit data: Data in common software structure depictions like class diagram or flow charts is strangely absent. That leads to a neglect of designing the relationship of functional units to it. In Flow-Design the countermeasure is to focus on data with data flows and to always be specific about which data is flowing along a connection. For that purpose data should always be written next to the arrow enclosed in bracket.

Although Flow-Design is about visualization of software structures on paper or whiteboards sometimes the need arises to describe a data flow using just text.

Although this is cumbersome it can be done:

• Use letters, words, phrases for functional units, e.g. Split, Determine next player.
• Use > or arrows -> for data flow relationships. Include data using brackets, e.g. -(sum, average)->.
• Use -* to denote dependencies or use brackets attached to FUs, e.g. Determine next player(game board).

In general feel free to invent your own textual language. Just stay true to the principles of Flow-Design and be clear and consistent.