What is a pattern?

What is common between the words blueprint, scaffolding, and maintenance? These software development terms have been borrowed from the world of building construction and architecture. However, one of the most influential terms comes from a treatise on architecture and urban planning written in 1977 by the leading Austrian architect Christopher Alexander and his team consisting of Murray Silverstein, Sara Ishikawa, and several others.

The term pattern came in vogue after their seminal work, A Pattern Language: Towns, Buildings, Construction (volume 2 in a five-book series), based on the astonishing insight that users know about their buildings more than any architect ever could. A pattern refers to an everyday problem and its proposed but time-tested solution.

In the book, Christopher Alexander states the following:

"Each pattern describes a problem, which occurs over and over again in our environment, and then describes the core of the solution to that problem in such a way that you can use this solution a million times over, without ever doing it the same way twice."

For example, his wings of light pattern describe how people prefer buildings with more natural lighting and suggests arranging the building so that it is composed of wings. These wings should be long and narrow, never more than 25 feet wide. Next time you enjoy a stroll through the long well-lit corridors of an old university, be grateful to this pattern.

Their book contained 253 such practical patterns, from the design of a room to the design of an entire city. Most importantly, each of these patterns gave a name to an abstract problem and together formed a pattern language.

Remember when you first came across the word déjà vu? You probably thought: "wow, I never knew that there was a word for that experience." Similarly, architects were not only able to identify patterns in their environment but could also, finally, name them in a way that their peers could understand.

In the world of software, the term design pattern refers to a general repeatable solution to a commonly occurring problem in software design. It is a formalization of best practices that a developer can use. Like in the world of architecture, the pattern language has proven to be extremely helpful to communicate a certain way of solving a design problem to other programmers.

There are several collections of design patterns, but some have been considerably more influential than the others.