Overlapping magisteria

I've written that unit tests and acceptance tests represent two non-overlapping magisteria. I've realised I need to revise this.

Unit tests and acceptance tests specify internal and external behaviour respectively. There is no inherent reason why external behaviour should be trivially translatable to internal behaviour, because the structure of the internals need not bear any resemblance to the model the system presents to the world.

However, that is only true of systems in general. Systems produced using a domain-driven design methodology deliberately enforce structural similarity between the system's implementation and the domain it represents.

Because of this, it's possible to make inferences between external and internal properties of the system. For example, if a

LoginFailure

object returns

true

to the method

shouldLockAccount()

when its

int

field called

previousAttempts

is

2

or greater, there's a good chance that if a user thrice enters the wrong password that their account will be disabled.

If we know something about a well-designed system's external behaviour, we can generally extrapolate about its internal behaviour (and vice-versa).

Continuous receipt - sacrificing versions

Last week I received an email that convinced me that the way we think about versioning has irrevocably
changed. Here is the meat of the mail:

IntelliJ's Perforce plugin now has job support.

While this is a valuable feature for me in my current project, it's hardly a major event for software in general.

What struck me was the author's implicit assumption that IntelliJ is a single continuous entity that changes state. IntelliJ "now" behaves differently to what it did a month ago.

To a user, "IntelliJ" is becoming less a description of an immutable executable than the name of an evolving organism of features. Whereas once we spoke of the 2010 incarnation of a product verses the 2011 product, soon we will speak of how a piece of software behaved in 2012 compared to 2013.

Web applications like Facebook, Twitter and Gmail have been doing this for some time. It's becoming the norm for desktop apps too.

The frantic pace of continuously delivered versions is obliterating the end-user's concept of a version altogether. Where previously we expected to pull new features by explicitly choosing to upgrade, we now accept that our software will frequently be pushed changes - almost in real time.

This large number of minutely differing versions is similar to how animation generates the illusion of motion. The more frequent the changes, the smoother the effect. As video games have increased their frame-rates they have made older games look clunky and awkward by comparison. Publishing games with 1995's industry standard frame-rate would put a studio at a serious competitive disadvantage today.

Developers who are unable to embrace the concepts of continuous delivery will find users perceive their software in much the same way.