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2.3 Causes of complexity
Now that you know the high-level symptoms of complexity and why
complexity makes software development difficult, the next step is to
understand what causes complexity, so that we can design systems to avoid
the problems. Complexity is caused by two things: dependencies and
obscurity. This section discusses these factors at a high level; subsequent
chapters will discuss how they relate to lower-level design decisions.
For the purposes of this book, a dependency exists when a given piece of
code cannot be understood and modified in isolation; the code relates in some
way to other code, and the other code must be considered and/or modified if
the given code is changed. In the Web site example of Figure 2.1(a), the
background color creates dependencies between all of the pages. All of the
pages need to have the same background, so if the background is changed for
one page, then it must be changed for all of them. Another example of
dependencies occurs in network protocols. Typically there is separate code
for the sender and receiver for the protocol, but they must each conform to
the protocol; changing the code for the sender almost always requires
corresponding changes at the receiver, and vice versa. The signature of a
method creates a dependency between the implementation of that method and
the code that invokes it: if a new parameter is added to a method, all of the
invocations of that method must be modified to specify that parameter.
Dependencies are a fundamental part of software and can’t be completely
eliminated. In fact, we intentionally introduce dependencies as part of the
software design process. Every time you write a new class you create
dependencies around the API for that class. However, one of the goals of
software design is to reduce the number of dependencies and to make the
dependencies that remain as simple and obvious as possible.
Consider the Web site example. In the old Web site with the background
specified separately on each page, all of the Web pages were dependent on
each other. The new Web site fixed this problem by specifying the
background color in a central place and providing an API that individual
pages use to retrieve that color when they are rendered. The new Web site
eliminated the dependency between the pages, but it created a new
dependency around the API for retrieving the background color. Fortunately,
the new dependency is more obvious: it is clear that each individual Web
page depends on the bannerBg color, and a developer can easily find all the
places where the variable is used by searching for its name. Furthermore,
compilers help to manage API dependencies: if the name of the shared
variable changes, compilation errors will occur in any code that still uses the
old name. The new Web site replaced a nonobvious and difficult-to-manage
dependency with a simpler and more obvious one.
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