In software engineering we often try to draw parallels with our profession and other engineering or creation industries. These analogies often make sense initially but breakdown as the analogy goes deeper into the process. It also encourages misinterpretation and sets an unrealistic expectation of how software is created.

The typical analogy that is put forward is that building software is like building a house or a bridge.

Initially this makes sense as both processes have the following phases:

  1. Engineers consult with the customer to understand their needs
  2. A design of the entity is created
  3. Engineers work together to construct the entity
  4. The entity is completed and handed over the customer to begin using

When we look at this high level process. The two disciplines have a lot in common. Unfortunately it is only at this higher level that things fall into place. Where software engineering begins to drift away from other engineering disciplines is:

  1. A house or bridge has known environment variables - the weather, climate and seasons are typically well understood before design and will not change significantly for the lifetime of the entity. This provides some predictability to the engineers so their design can be built with enough resilience to meet extreme changes in environment variables. In contrast, the environment around a software system is constantly changing at a rapid pace, the users skill levels, technologies, devices are constantly changing, attempting to future proof a design to be resilient to these changes is impossible as no one can predict with certainty the direction the environment around software will take.
  2. The needs of the users of a house or bridge will not significantly change over time and the solutions to those needs is a solved well understood problem. A house or bridge consists of several well known components, for example a bathroom, kitchen, bedroom and lounge room. A customer of a house will usually be able to predict with high certainty how many of each component they will require and the capacity that each component needs to support. That is a house will contain 3 to 4 residents, as such 3 bathrooms are required, 4 bedrooms and a single kitchen and lounge room. Software systems however may start with predictable capacities but grow at exponential speeds, as such those initial components might not scale with the growth in user numbers. It is hard to imagine an engineer designing a house to sleep 4 that could in a few years be turned into a hotel serving millions or that even though they asked for a kitchen, they have since learned that everyone eats out, no one uses the kitchen , so they'd like it replaced with another bathroom. This however is a common story with software systems.
  3. Software is easier to change than physical infrastructure which de-emphasizes the need to "get everything right at the start". If you design a software solution with the expectation that it will not be easy to change (like a house or bridge) you build in unneeded, excess complexity. Which is not required as software is more malleable and can be changed to adapt.

The common theme between the above points is that when building a software system, variables external and internal to the system are constantly and sometimes rapidly changing. These constantly changing variables require a different process and different methodology to engineering.

When we draw parallels between other engineering practices and software, we underplay these variables and as such create the expectation that software can be predictably and uniformly executed like other disciplines.


This article was first noted down on the 10th of February, 2016.