June 20, 2018

...Learn TDD with Codemanship

Design Principles Are The Key To A Testing Pyramid

On the 3-day Codemanship TDD workshop, we discuss the testing pyramid and why optimising your test suites for fast execution is critical to achieving continuous delivery.



The goal with the pyramid is to be able to test as much of our software as possible as quickly as possible, so we can re-test and reassure ourselves that our code is shippable very frequently (i.e., continuously).

If our tests take hours to run, then we can only run them every few hours. Those are hours during which we don't know if the software's shippable.

So the bulk of our automated tests - the base of testing pyramid - should be fast-running "unit" tests. This typically means tests that have no external dependencies. (That's my working definition of "unit" test, for the purposes of making the argument for excluding file systems, databases, web services and the like from the majority of our tests.)

The purpose of our automated tests is to detect when code is broken. Every time we change a line of code, it can break the software. Therefore we need a test to catch every potential broken LOC.

The key to a good testing pyramid is to minimise the tests that have external dependencies, and the key to that is minimising the amount of code that has external dependencies.

I explain in the workshop how our design principles help us achieve this - and three in particular:

* Single Responsibility
* Don't Repeat Yourself
* Dependency Inversion

Take the example of a module that has a method which:

1. Formats a SQL string using data from a business object
2. Connects to a database to execute that query
3. Unpacks the response (recordset or array) into a business object

To test any part of this logic, we must include a trip to the database. If we break it up into 3 methods, each with a distinct responsibility, then it becomes possible to test 1. and 3. without including 2. That's a third as many "integration" tests.

In a similar vein, imagine we have data access objects, each like our module above. Each can format a SQL string using an object's data - e.g., CustomerDAO, InvoiceDAO, OrderDAO. Each connects to the database for fetch and save that object type's data. Each knows how to unpack the database response into the corresponding object type.

There's repetition in this design: connecting to the database. If we consolidate that code into a single module, we again reduce the number of integration tests we need.

Finally, we have to consider the call stack in which database connections are being made. Consider this poor design for a video rental system:



When we examine the code, we see that the methods that have direct external dependencies are not swappable within the overall call stack.



We cannot test pricing a video rental without paying a visit to the external video ratins service. We cannot test rentals without trips to the database, either.

To exclude these external dependencies from a set of tests for Rental, we have to turn those dependencies upside-down (make them swappable by dependency injection, basically).



This is often what people mean when they talk about "testable" code. In effect, it means there's enough "swappability" in our design to allow us to test the bulk of the logic by mocking or stubbing external dependencies. The win-win here is that we not only get a better-proportioned testing pyramid, we also get a more flexible design that can more readily accommodate change. e.g., getting video ratings from Rotten Tomatoes instead.)



Posted 5 months, 4 days ago on June 20, 2018