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Hyperpragmatic pure FP testing
with
distage-testkit
Functional Scala 2019
Septimal Mind Ltd
team@7mind.io
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Tests and Test Taxonomies 2/31
Not all the tests are equal
Tests are important. We know that.
Tests are the simplest way to define and verify important contracts.
And contracts are crucial for project maintainability and viability.
We write tests. A lot of.
But
. . .
some tests prove themselves useful and some do not.
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Not all the tests are equal
Which tests are good and which are bad?
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Bad test criterias
Bad tests are:
◮ Slow,
◮ Unstable: they fail randomly,
◮ Nonviable: they don’t survive refactorings,
◮ Demanding: they require complex preconditions to be met: external services up and
running, fixtures loaded, etc, etc,
◮ Incomprehensible: they signal about a problem but don’t help us to localize the
cause.
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We spend more resources to write and
maintain bad tests
than the value they bring us.
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How may we make our tests better?
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Tests and Test Taxonomies 7/31
Let’s ditch Unit/Functional/Integration trinity. . .
. . . and introduce some new terminology
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Test Taxonomy: Encapsulation Axis
Let’s say that every test falls under one of the following categories:
1. Blackbox tests check just interfaces not knowing anything about implementations
behind them,
2. Whitebox tests may know about implementations and check them directly, sometimes
even breaking into some internal state to verify if it conforms to test expectations.
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Test Taxonomy: Isolation Axis
Let’s say that every test falls under one of the following categories:
◮ Atomic tests check just one “unsplittable” software component,
◮ Group tests check multiple software components,
◮ Communication tests communicate with outer world (databases, API providers, etc,
etc).
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Tests and Test Taxonomies 10/31
You may extend and modify this Test Taxonomy as it would be convenient for you.
More about test taxonomies:
https://blog.7mind.io/constructive-test-taxonomy.html 10 / 31

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Test Taxonomy: Why So?
According to our experience the best tests are
Blackbox tests with Atomic or Group Isolation Level.
This makes sense: such tests are fast and refactoring-resilient.
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Communication tests
But in real projects most of the tests fall under Communication category (“Integration”
tests).
Why?
1. Engineers want to test The Whole Thing,
2. It’s hard to separate components,
3. etc, etc. . .
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Communication tests can be more useful
We may replace integration components with Dummies1.
This way we may turn
Communication tests
into
Group or Atomic2
1
people also call them “Fakes”, “in-memory implementations” or “Mocks”
2
this statement is valid for Blackbox tests only
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Dual Test Tactic 14/31
Dual Test Tactic
The same test scenario executed with both Production and Dummy implementations of
integration components is beneficial:
1. We can test business logic quickly, without any interference,
2. We still able to verify business logic behaviour with “real” integrations,
3. Dual Test Tactic enforces us to follow LSP and design better.
Refactorings are crucial for long-term health of the project.
Blackbox tests enable refactorings.
Dual Test Tactic drastically reduces the price of refactorings.
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Dual Test Tactic: ideas
1. Ignore Communication tests in case their dependencies are unsatisfied (service
unavailable), don’t fail,
2. Avoid automatic “Mocks”, they prevent encapsulation,
3. Never run heavy dependencies in-process
◮ don’t bring whole Kafka or Cassandra into the classpath. PLEASE
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Dual Test Tactic: The Main Issue
It’s easy to setup test dependencies while working with Dummies
But you have to do things differently for Production dependencies.
We have to take care of:
1. Resource acquisition and Cleanups,
2. Speed,
3. Memoization and resource reuse,
4. Interference,
5. . . .
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Dual Test Tactic: dummy repositories testcase
make[UserRepo]
make[UserService]
make[AccountsRepo]
make[AccountsService]
run[MyTestCase] run[AnotherTestCase]
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Dual Test Tactic: production repositories testcase
Setup
Execute
Cleanup
probe database port
make[DbDriver]
setup users table setup accounts table
make[UserRepo] make[AccountsRepo]
remove users table
close driver
remove accounts table
make[UserService] make[AccountsService]
run[MyTestCase]run[AnotherTestCase]
1. Order
2. How to avoid unneccessary job?
2.1 Memoize resources?..
2.2 In a singleton?..
2.3 When we close resources?..
2.4 On JVM shutdown?.. Oops, SBT. . .
2.5 Disambiguation (same class, different
parameters)?..
3. Resource deallocation
3.1 . . . even after a failure
3.2 Order!
4. Other integrations, e.g. run Dockers
4.1 . . . and await until they open ports
4.2 . . . and stop them after the tests
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Dual Test Tactic: Real testcase steps
◮ Integration points stack together
◮ . . . and make the problem notably hard
◮ Manual wiring is hard to maintain
◮ . . . and suffers from combinatoric explosion of possible code paths
◮ Cake pattern doesn’t make much difference
◮ Conventional DI frameworks fail
It’s hard to setup variable contexts.
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Dual Test Tactic
may be very pricy under usual circumstances.
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Can we make it cheap?
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Yes.
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The Goal
We want to write code like this and never care about setting things up:
1 class JustATest[F[+_, +_]] {
2 "service" must {
3 "do something" in {
4 (users: UserService[F], accounts: AccountingService[F]) =>
5 for {
6 user <- users.create()
7 balance <- accounts.getBalance(user)
8 } yield {
9 assert(balance == 0)
10 }
11 }
12 }
13 }
14
15 object JustATestZioProd extends JustATest[zio.IO] with Prod
16 object JustATestZioDummy extends JustATest[zio.IO] with Dummy 23 / 31

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dist✪ge
is a Dependency Injection mechanism for Scala
But it lacks negative traits of a typical DI thingy. . .
. . . and has many unique properties.
. . . and it’s blazing fast.
You may call it a module system for Scala. . .
. . . with automatic garbage-collecting solver
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distage
1. Supports FP, wires Resources, Effects, ZIO Environments. . .
2. transparent and predictable
◮ first plan then do
◮ test the outcome without effect execution!
◮ compile-time checks and tests
3. no scala-reflect nor Java reflection1,
◮ ScalaJS support is coming
4. is non-invasive.
◮ You can add it to your project keeping business logic intact. . .
◮ . . . and remove it.
5. is not an ad-hoc thing, it has strong theory behind.
1
Since version 0.10.0, see https://blog.7mind.io/lightweight-reflection.html
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Docker support in distage
While we develop and test we may need our integrations up and running. . .
. . . We have docker-compose for that. . .
But manual rituals are annoying and Compose is imperfect
(wait until service opens port? Hah)
Container orchestration is not easy.
But. . .
We made a distage extension allowing to get rid of Compose in most of the cases.
It can run test containers, reuse and shutdown them.
And it can await until the service is ready.
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Let’s look at a real example. . .
Source code: https://github.com/7mind/distage-example
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distage performs optimal ahead of time planning.
It doesn’t do any unnecessary job.
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One More Thing: Roles
distage allow you to fuse microservices into “flexible monoliths”.
We may:
1. Develop services (Roles) separately, even in a multirepo,
2. Build a single Docker image with multiple Roles in it,
3. Run several Roles within one process
◮ We define roles we want to start as commandline parameters
4. ⇒ higher computation density, savings on infrastructure,
5. ⇒ substantial development simplification
◮ full environment can be started on a low-end machine
◮ with just one command
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distage-framework is the most productive way to write
maintainable pure functional applications with ZIO.
(And any other monad)
distage-testkit is the best way to make your tests performant and reliable.
distage is adopted by several different companies and
tested by two years of production usage.
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Thank you for your attention
distage website: https://izumi.7mind.io
Septimal Mind is an Irish software consultancy and R&D Company
We’re looking for clients, contributors, adopters and colleagues ;)
Contact: team@7mind.io
Github: https://github.com/7mind
Slides: https://github.com/7mind/slides
Follow us on Twitter
https://twitter.com/shirshovp
https://twitter.com/kai_nyasha
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