Span of Control - The Mosque

One of the ways to evaluate a design is to look at the span of control of each component.  If the span is too wide, the component may have absorbed too many details from lower components.  If is too narrow, the partitioning may not be being effective at reducing complexity.

The too narrow case has a sharp bound.  If there is only one thing below a component, that component should be looked at.

The too wide case is a little fuzzier.  It's probably something like 5 plus or minus 2. 

There are exceptions.  If the too narrow case is being called from many places and it makes a semantic transformation to what it is calling, that is probably a good thing.

If the too wide case is something like a command parser where the core is a giant switch statement where each case is one of the commands, then it is probably not usefully reduceable. In that case there should be no other control flow statements in the procedure and each switch case should be a single line.  Hoist surrounding control flow into an outer function and create functions for any multiline cases.

One of the consequences of this is that good designs tend to show a mosque shape.  The get wider until a maximum level and then narrow slightly.  This arises from the delegation structure at the top calling unique routines but the final work being done by fewer common routines (possibly from a library).