Automated testing is a process by which we rely on special software to continuously run pre-defined tests that verify the integrity of our application. To this end, automated tests are a collection of steps that cover the functionality of an application and compare triggered outcomes to expected ones.
Manual testing is a great way to ensure that a piece of written functionality works as expected. The main problem encountered by most adopters of this strategy, especially those who use it exclusively, is regression. Once a piece of functionality is tested, the only way they can guarantee regressions (or bugs) were not introduced by another piece of functionality is by retesting it. And as the application grows, this becomes impossible to handle. This is where automated tests come in.
Automated testing uses special software that has an API that allows us to automate the steps involved in testing the functionality. This means that we can rely on machines to run these tests as many times as we want, and the only thing stopping us from having a fully-working application is the lack of proper test coverage with well-defined tests.
There's a lot of different software available for performing such tests and it's usually geared toward specific types of automated testing. For example, Behat is a powerful PHP-based open source behavior testing framework that allows the scripting of tests that mirror quite closely what a manual tester would do—interact with the application through the browser and test its behavior. There are other testing frameworks that go much lower in the level of their testing target. For example, the PHP industry standard tool, PHPUnit, is widely used for performing unit tests. This type of testing focuses on the actual code at the lowest possible level; it tests that class methods work properly by verifying their output after providing them with different input. A strong argument in favor of this kind of testing is that it encourages better code architecture, which can be (partly) measured by the ease with which unit testing can be written for it.
We also have functional or integration tests which fall somewhere in between the two examples. These go higher than code level and enlist application subsystems in order to test more comprehensive sets of functionality, without necessarily considering browser behavior and user interaction.
It is not difficult to agree that a well-tested application features a combination of the different testing methodologies. For example, testing the individual architectural units of an application does not guarantee that the entire subsystem works, just as testing only the subsystem does not guarantee that its individual components will work properly under all circumstances. Also, the same is true for certain subsystems that depend on user interaction—these require test coverage as well.
In this chapter, we will see how automated testing works in Drupal 8. More specifically, we will go through and explain all testing methodologies available for us as module developers and exemplify them with two tests each. By the end of this chapter, you'll be ready to write your own tests and be familiar enough with the code to further explore the available testing capabilities.