I love testing. I’ve always been a big fan of testing, but in the last 5 years, I’ve seen a lot of really terrible tests written. Today I’m going to explain to you the levels of testing I believe are most useful.

Where testing has gone off the rails

Testing started out great. Test Driven Development (TDD) slowly became the king especially with “Red, Green, Refactor” as its slogan. TDD has fallen off the rails. It’s far too close to the code to be of any help.

Far too often the test cases are too narrow and too specific to how the code was written. The concern with testing for far too long has been “how many lines of code are covered” or “the test suite is not consistent on the testing servers.”

There are levels of testing. While TDD is great for unit tests, I think unit tests have extremely low value in application design and should be used minimally I know this goes against common thought in our industry, but since our time is limited and valuable we want to use the best tools for the job.

There are 5 levels of testing worth doing


Penetration testing

Yes, this is the most important test you could conduct on your system. Developers don’t write automated tests for this nearly enough. This needs to change.

Performance testing

This helps us know how our application will handle under load and spot issues like N+1 query issues.

Unit level tests for complex logic

I think unit tests are great when dealing with input that has to be regexed, transformed, or computed. The question I ask is: “In reading this code, how complex or complicated is the logic?” If I can understand it in about a minute or two, it’s not worth testing.

UI tests

Did the page draw? Do the buttons work? Does it look okay in IE? Chrome? Firefox? iPhone? Android? Cool. When the UI consumes an API, I tend to mock those out and simulate use cases, errors etc… I try not to use a real API -just mocks.

Contract testing

This is what we’re talking about today. Basically, we test in a similar fashion as “black box” testing with some BDD similarities.

Rules for Contract testing


Rule 1: Tests are loosely coupled with implementation

Coupling in software leads to inflexibility which leads to unhappy builds and test suites. When writing and building software, I think of what are the things this software I’m writing is going to be doing. What is the point?

That should leave you with a few possible outcomes:

  1. I’m building a system script for some purpose
  2. I’m building an API that others will consume
  3. I’m building a UI that someone will use

Contract testing is best applied to 2 and 3 - when there is some finite outcome.

Rule 2: Tests are purely functional

Contract testing is functional testing. It’s purely black box. For example, this could be helpful in implementing a Tax Rate API. You supply $6.17 in sales for groceries and in Denver, Colorado the city sales tax is 0% for food. Therefore the total price is $6.17. In Boulder, though, the sales tax is 6% so you have to account for each situation:

  • The input price
  • The location
  • The expected outcome

Unless tax rates change for a given location, the expected outcome should be consistent regardless of the technology. There are no side effects.

More generally, contract testing is focusing on outcome versus implementation.

Rule 3: Testing that is platform or technology agnostic

This is derived from rule 2. The technology of the testing mechanism is platform independent. Your tests aren’t necessarily written in the application’s language. They’re implemented separately. The advantage of this when it comes to API testing is that when your tests are separate, the code base can change without breaking the tests.

When the code base changes, the tests don’t have to unless there is a rule that the tests didn’t account for. This style of testing can be likened to black box testing.

This approach is really helpful when building Microservices. The idea is that you are testing “I send in x,y,z data” and I should get the same response every time. The code can change. In fact, as long as you implement the correct format, structure, and return a consistent response based on input, you can change everything. The database, the tech can go from Ruby to Elixir or Node. It can be deployed to Google Compute instead of Amazon. Everything can change.

Why contract testing?

I think it can be summarized nicely.

Testing the contracts:

  • Allows true flexibility in design of the software
  • Creates consistency
  • Ensures that your code can change while your tests are fixed
  • Prevents bugs from creeping into your software through refactoring or other redesign efforts
  • Saves time from having to write large, complex test suites that break every other build

How do I do contract testing?

This is where it gets harder. This is not as common in the industry as TDD, so there are fewer tools available. The way I approach this is a bit interesting:

Every API has a schema

It can be XML or Json. I prefer Json schema as most developers in the last 5 years can read it far easier to XML. It defines the keys, the acceptable parameters, and the associated validations as well as if the field is required. This can be helpful for generating mock test data. There is a really good tool for generating these types of files: [RAML] (https://raml.org/)

Using the schema, I can generate mocks. Also, I publish a dynamically created schema from the API. From this I can write a test that says:

Does my stored schema match the one the API is returning?

If it doesn’t match, that could lead to interesting and unexpected bugs for the consumers of our APIs.

Every API endpoint has at least one related test

Each API needs a bunch of different tests for each endpoint. You need testing for things like authorization keys, headers, client-side errors, server-side errors and the list go on and on in terms of the error checking you need to be doing.

More importantly, you need to test each possible scenario. If you’re writing an API that takes zip codes and returns the sales tax for that area, you need to account for oddball zip codes (such as those that end with 9 or start with 0) Fun fact: 00501 is the zip code for Holtsville, New York. Test the bounds, the ranges, and the edge cases. By doing this at the API level, you’re not tied to how the code was written to get there but instead to the outcomes.

Almost any testing suite can do API calls. In Javascript, I’ve been liking Jest quite a lot. There are also things like RSpec with Ruby (You don’t need Rails).

In a future post, I’ll give you a demo of how this all works so you can see and poke at living examples.

Am I off the rails or do you agree with this article? I post these things to Reddit and Hacker News, so if you have something to say please say it over there. I’d love to hear from you.