blog/content/books/things-i-learnt/integration-tests/index.md

+++
title = "Things I Learnt The Hard Way - Unit Tests Are Good, Integration Tests Are Gooder"
date = 2019-06-19

[taxonomies]
tags = ["en-au", "book", "things i learnt", "unit tests", "integration tests"]
+++

The view of the whole is greater than the sum of its parts. And that includes
tests for the whole compared to tests of single things.

<!-- more -->

First, I just don't want to into a discussion about what's the "unit in a unit
test"[^1], so let's take the point that a unit test is a test that tests a
class/function, not the whole system, which would require data flowing through
several classes/functions.

There are several libraries/frameworks that actually split this in a way that
you can't test the whole.
[Spring](https://spring.io/)+[Mockito](https://site.mockito.org/) is one of
those combinations -- and one that I worked with. Due the bean container of
Java, the extensive use of Beans by Spring and the way Mockito interacts with
the container, it's pretty easy to write tests that involve only one class:
You can ask Mockito to mock every dependency injection in one class and mock
every injected class, simply using annotations.

And this is cool and all. But the fact that we are making sure each class does
what it does, it doesn't give a proper view of the whole; you can't see if
that collection of perfectly tested classes actually solve the problem the
system is responsible for solving.

Once, in C++, I wrote an alarm system
[daemon](https://en.wikipedia.org/wiki/Daemon_(computing)) for switches. There
were three different levels of things the alarm system should do: It could
only log the message of the incoming error, it could log the error and send a
SNMP message, or it could log the error, send a SNMP message and turn a LED in
the front panel on.  Because each piece had a well defined functionality, we
broke the system in three different parts: One for the log, one for the SNMP
and one for the LED.  All tested, all pretty. But I still had a nagging
feeling that something was missing. That's when I wrote a test that would
bring the daemon up, send some alarms and see the results.

And, although each module was well tested, we still got one things we were
doing it wrong. If we never wrote an integration test, we would never catch
that.

Not only that, but because we wrote a test that interacted with the daemon, we
could get a better picture of its functionality and the test actually _made
sense_ -- as in, if you read the unit tests, they seemed disconnected from
what the daemon was expected to do, but the integration tests actually read
like "Here, let me show that we actually did what you asked". And yes, this
was akin to [Gherkin](/books/things-i-learnt/gherkin) tests, although I didn't
know Gherkin at the time.

Personally, I think over time integration tests are more important that unit
tests. The reason is that I still have the feeling that unit tests check if
the classes/functions have _adherence_ to the underlying _design_ -- Does your
view can actually work without the controller? Is the controller using
something from the model or using things that should be in the view? -- but
adherence to the design gets better over time -- developers start using the
layout from previous examples, so they capture the design by osmosis, while
the big picture starts to get more and more complex, with lots of moving
parts.

[^1]: There is no "unit" in "unit tests". "Unit test" means the test _is_ a
  unit, indivisible and dependent only on itself.

{{ chapters(prev_chapter_link="/books/things-i-learnt/gherkin", prev_chapter_title="Gherkin Is Your Friend to Understand Expectations") }}