At Zillow, I've done a lot of work on the design and development of
the test infrastructure we use for full-stack tests. It's always fun
to watch your tool become popular, but even more interesting is the
discussions around test suite design that come with it.

Many discussions later, I have a good idea of what I want in a test suite.
Here's what I think about:

Tests are a question of cost

At the end of the day, tests have a cost. Each and every test has a
value / cost ratio. Things that increase the value of a test include:

  • consistency: given the same inputs, give the same results, every time.
  • speed: the faster the test is, the faster the feedback. The faster
    the feedback, the faster one can take action, and the more often we
    can execute the tests to get feedback.

In contrast, the things that increase the cost of a test include:

  • maintenance time: maintenance takes time, and development time is expensive.
    probably the biggest cost to consider.
  • cpu / memory to execute the test: although arguably cheap in this world
    of cloud providers, cpu and memory are real concerns, and tests that use
    a lot of these resources are expensive.
  • the time to execute the test: time is a huge cost, especially as the
    technology world we live in demands for more changes, more
    quickly. Depending on how fast you ship, tests that take too long will
    be prohibitively expensive, and thus not used.

When I look at the value of a test, I look at these factors. In
practice, I've found that the most important metric of them all is
maintenance time: test that have little to no maintenance survive
refactors, rewrites, and pretty much anything that could happen to
code besides deprecation.

On the other hand, the more the test requires maintenance, the more likely
it'll suffer one of two outcomes:

  • the test is thrown out because it takes too much time to maintain,
    despite the value.
  • the test is not given the time it needs, and continues to fall into
    disarray until it is ignored.

Basically: low maintenance tests last forever, high maintenance tests probably won't.

Designing cheap tests

So how do we make tests that require little to no maintenance? From what I've observed, there are two types of maintenance:

  • functional maintenance, which modifies the test to reflect changes in the code itself
    • e.g. for a web page, the login form fields are modified
  • operational maintenance, which requires keeping a service dependency in a good state to test.
    • e.g. for an office application with cloud sync, keeping the cloud syncing service up.

Functional maintenance is unavoidable: as code changes, one must
ensure that any tests that validate that code are kept up to date. In
addition, for most tests, functional maintenance is relatively cheap
in time: except in the cases of extreme redesigns or refactorings, the
changes tend to be small in nature.

Operational maintenance costs can vary wildly, and it can become very
expensive. Tests that have multiple dependencies can become a game of
juggling an environment where all of those are functional. It becomes
even harder if there's a small team maintaining this environment:
executing the tests consistently requires a production-quality
environment, and that's more difficult the more services there are to

However, unlike functional maintenance, operational maintenance, for
the most part, is avoidable. Taking advantage of heavy mocking, it's
possible to remove dependencies like databases and APIs. Google
Testing Blog has a good article about

Summary: tests with fewer operational dependencies are cheaper to maintain.

What kind of test distribution: the testing pyramid

When testing software, there are multiple levels at which one could author tests:

  • at the "unit" level, typically written in the same language and validating a single function or behaviour
  • at the integration level, typically written in the same language, and validating the communication between your code and an external application
  • at the end-to-end level, not necessarily written in the same language, and validating a complete workflow that a user would be performing.

Although all are important and should be included in a test suite,
each test is not created equally. Going back to the idea that tests
with the least maintenance will last the longest, we should be trying
to have as many of those as possible.

Unit tests are the cheapest. They:

  • have no dependencies (or else they would at least be considered an integration test),
  • run quickly (no waiting for network, or other delay from communication)

If we could capture all behaviour of our application with just unit
tests, that would be perfect. Unfortunately, many things can go wrong
when composing multiple pieces of these units together, so some level
of integration and end-to-end tests will be needed. But the larger
tests should be fewer in number, since they are harder to maintain.

A good model to visualize a good distribution is the "testing pyramid", as explained
by Martin Fowler and Google:

The more expensive tests are fewer in number, while the cheaper tests
are much more common.

How many tests should be in a suite

Adequate test coverage varies wildly between applications: medical
software than monitors heart rate should probably have a lot more
coverage than a non-critical social media website. The only common
rule of thumb I've found is: add the absolute minimum number of tests
to achieve your desired confidence in quality.

Testing is important, but at the end of the day, it's not a
user-facing feature. On the other hand, quality is. Adding additional
tests does increase quality, but it comes at the cost of development
and maintenance time toward other features that help your application
provide value. A properly sized testing suite comes right at the line
of too little testing, and hover around that. This gives developers
as much time as possible on features, while ensuring that an
important feature (quality) is not neglected.


  • the best tests are the cheapest tests: low maintenance and executes quickly and low CPU/RAM resources
  • the cheapest tests have the fewest number of dependencies on other applications, like DBs or APIs
  • try to keep test coverage as low level as possible, and cheap tests are worth 10x expensive ones.
  • expensive tests validate the whole infrastructune, so they're almost
    always necessary: refer to the test pyramid for a rough sketch of a good distribution.
  • never add more or less coverage than you need: more coverage results
    in more maintenance that detracts from development time, and less coverage means an application
    whose quality is not up to the desired standards.
  • how much coverage do I need? Depends on how critical the application
    is, and how critical it continues to work. A payment path needs high
    quality, so should have high coverage. The alignment of a button on
    a dialog three pages deep probably needs less quality assurance.

How do you design your test suite?