Using oracles to test the service and data layer

Getting rid of mocks in your service layer tests and test your database for real


A common architectural style is the 3-layer model (data, service, and API/view layer) for writing web services. With this style, the data layer is tested with unit tests, H2 database or not. The service layer is tested with mocks, where the calls to the database are emulated.

The approach with the data layer has some downsides. Inserting, updating, and reading things can be seen as encoding/decoding data from a medium. As you might like encoding and decoding JSON, decoding might go wrong. When working with databases, schema changes might cause decode errors when you do not update your domain model. Or when you introduce a new enum member it might that the database cannot store this yet. To assert that we have symmetric encoding and decoding effects, we can use property-based tests. Also if you use Postgres and H2 for testing, there might be discrepancies like H2 doesn’t have PostGIS or other unsupported features.

The approach with the service layer has some downsides, What if the behavior of the repository method changes over time or the emulated repository method is invalid? You’ll test with the wrong assumptions and you might introduce bugs.

A solution

To tackle both pitfalls we can use oracles and model-based testing which are according to F# for fun and profit:

The way it works is that, in parallel with your (complex) system under test, you create a simplified model. Then, when you do something to the system under test, you do the same (but simplified) thing to your model. In the end, you compare your model’s state with the state of the system under test. If they are the same, you’re done.

In our case, we use a mirror implementation of the interface. When working with the data layer, we have a real database implementation and an in-memory implementation. The in-memory implementation can be used as an oracle. The oracle can also be used in both testing the data and service layer. In the data layer tests, the oracle is used to verify that the in-memory variant mirrors the behavior of the database and in the service layer tests we use the oracle to mock the database.

In this case, we work with a functional scala tech stack: Doobie and ZIO. We use ZIO mainly in the upper layers like the service layer and API layer to handle side effects.

The data layer

Coding a repository interface

When coding a repository in Scala you can choose to commit to an effect type like cats.effect.IO, scala.concurrent.Future or zio.Task from the start. However, this also has downsides.

  • What if you would like to compose several methods?
  • In testing evaluating the effect of an effect type like zio.Task has an immediate effect leaving a dirty database that can interfere with other tests

When you use doobie you could use ConnectionIO or if you use slick DBIO to implement these repositories in terms of the effect type which is transactional and can be rolled back. This means you can compose multiple repository methods like inserting and reading an entity while rolling back the whole operation, and leaving the database clean while you have tested the behavior.

A repository interface could look like this:

final case class Person(id: UUID, name: String, age: Int)

trait PersonRepository[F[_]] {
  def insertMany(persons: List[Person]): F[Long]
  def deleteWhenOlderThen(age: Long): F[Long]
  def listAll(): F[List[Person]]

object PersonRepository {
  implicit val functorK: FunctorK[PersonRepository] = Derive.functorK
  implicit val semigroupalK: SemigroupalK[PersonRepository] = Derive.semigroupalK

We use FunctorK and SemigroupalK here, which are explained on the cats-tagless documentation. In essence, they give you functions that allow you to transform the interface in interesting ways. Like with FunctorK we can transform the effect type and with SemigroupalK we can execute two interpreters at the same time which is needed for our data layer testing.

Coding a repository database implementation

When it comes to coding a doobie implementation it is pretty straightforward. We implement PersonRepository in terms of ConnectionIO.

One thing to note is that we separate out the queries from the interface. By doing this we can test queries later with the doobie-specs2 package which allows you to test the syntax of queries.

object DoobiePersonRepository extends PersonRepository[ConnectionIO] {

  object queries {
    def deleteWhenOlderThen(age: Long): Update0 =
      fr"delete from persons where age > $age".update

    def listAll: Query0[Person] =
      fr"select id, name, age from persons".query[Person]

  def insertMany(persons: List[Person]): ConnectionIO[Long] =
    Update[Person]("insert into persons (id, name, age) values (?, ?, ?)").updateMany(persons).map(_.toLong)

  def deleteWhenOlderThen(age: Long): ConnectionIO[Long] =

  def listAll(): ConnectionIO[List[Person]] =[List]

Coding a repository in-memory implementation

To code an in-memory implementation we would like to emulate a database and its operations. How you could do that?

  • A database can be emulated by using a case class that has fields and where each field is a table in the database. Each field should be a List[A]. If every field is a List[A] you could potentially derive a Monoid for free.
  • We need a common set of combinators that allow you to query and mutate.

The first part is simple, we could for example create a case class that will hold our state of the database like this.

case class Universe(persons: List[Person])

Now to query or mutate we need to emulate the behavior of transaction as well, but also when we want to query the database we need to have access to the whole Universe.

In functional programming luckily we have the State monad which is perfect for this. If the repository is implemented in terms of State[Universe, *] we can chain together multiple mutations to form a transaction as well if a query is weaved inside the transaction it will work, because the internal state is updated.

To write universal combinators we need setters to mutate the Universe structure and getters to query the Universe structure. In functional programming, we also have an abstraction for this: lenses. In this case, I’ll use the excellent library Monocle. When you annotate your Universe case class with the @Lenses annotation, Monocle will automatically generate lenses on the companion object of Universe. In this case, we will have a lens defined Universe.persons which is of type Lens[Universe, List[Person]].

Now with all the ingredients we can start writing our first combinators:

final case class Septic[D, A] private (db: State[D, A]) {
  def run(state: D): A = db.runA(state).value

object Septic {
  def all[D, A](at: Lens[D, List[A]]): Septic[D, List[A]] =

  def insertMany[D, A](at: Lens[D, List[A]])(elements: List[A]): Septic[D, Long] =

  def insertMany_[D, A](at: Lens[D, List[A]])(elements: List[A]): Septic[D, List[A]] =
    Septic(State.modify[D](s => at.modify(_ ++ elements)(s)) *> State.pure(elements))

  def insert[D, A](at: Lens[D, List[A]])(element: A): Septic[D, Long] =

  def delete[D, A](at: Lens[D, List[A]])(filter: A => Boolean): Septic[D, Long] =

  def delete_[D, A](at: Lens[D, List[A]])(filter: A => Boolean): Septic[D, List[A]] =
    Septic {
      for {
        elements <- State.get[D]
        (toDelete, toKeep) = at.get(elements).partition(filter)
        _ <- State.modify[D](s => at.modify(_ => toKeep)(s))
      } yield toDelete

The first thing to note is that we create a new type (in Scala 3 we could use opaque types) called Septic wrapping a State monad which has constrained combinators. The nice thing about these general combinators is:

  • They infer the Septic type when you supply it the Lens[D, List[A]]
  • When used with an atomic reference, you could even use the implementation to a bootup server and use it locally for testing for example

With these combinators, we can code our PersonRepository:

final case class Universe(
  persons: List[Person]

object Universe {
  def zero: Universe = Universe(Nil)

object SepticPersonRepository extends PersonRepository[Septic[Universe, *]] {
  def insertMany(persons: List[Person]): Septic[Universe, Long] =

  def deleteWhenOlderThen(age: Long): Septic[Universe, Long] =
    Septic.delete(Universe.persons)(_.age > age)

  def listAll(): Septic[Universe, List[Person]] =

Testing our in-memory and data layer implementation

As stated before, if we want to assert that our data layer is right we need to run for example a database program (like an insert and read) in parallel. This is where SemigroupalK comes into play.

In my proof of concept library I’ve created a Harnass:

class Harnass[Alg[_[_]], F[_], Tx[_], D](initState: D, db: Alg[Tx], model: Alg[Septic[D, *]], tx: Tx ~> F) {

  // create a effect type which is higher kinded tuple which has the doobie version and Septic version
  type Eff[A] = Tuple2K[Tx, Septic[D, *], A]
  // set the effect type of the repository interface
  type Paired = Alg[Eff]

  // a eval function which uses the Paired and returns a `F[(A,A)]`
  trait Evaluator {
    def eval[A](f: Paired => Eff[A]): F[(A, A)]

  def model(implicit S: SemigroupalK[Alg], F: Functor[F]): Evaluator = {
    val paired: Paired = S.productK(db, model)
    new Evaluator {
      override def eval[A](f: Paired => Eff[A]): F[(A, A)] = {
        //here we get the `Tuple2K` from `f`
        val effectTuple: Eff[A] = f(paired)
        //we run the connection against a rollback transactor, and get the result
        val dbValue: F[A] = tx(effectTuple.first)
        //we run the state monad and get the value
        val stateValue: A = -> stateValue)

Don’t be daunted by the generic parameters. I’ll go quickly over them:

  • Alg is the repository type PersonRepository in our case
  • F is the effect type like cats.effect.IO
  • Tx is the transaction type, this is ConnectionIO from Doobie
  • D is the state type used for Septic, in our case, this is Universe

Like stated before, it creates out of SemigroupalK[Alg] a higher-kinded paired version. So we combine two interpreters of PersonRepository, like: PersonRepository[ConnnectionIO] and PersonRepository[Septic[Universe, *]] into a PersonRepository[Tuple2K[ConnectionIO, Septic[Universe, *], *]].

A few tests in my proof of concept look like this:

  def harnass: Harnass[PersonRepository, IO, ConnectionIO, Universe] =
    new Harnass(, DoobiePersonRepository, SepticPersonRepository, xa.trans)

  "PersonRepository" should {
    "should insert and read" in {
      prop { persons: List[Person] =>
        assertMirroring {
          harnass.model.eval { x =>
              x.insertMany(persons) *>

    "should delete people older then" in {
      prop { (persons: List[Person], age: Int) =>
        assertMirroring {
          harnass.model.eval { x =>
              x.insertMany(persons) *>
              x.deleteWhenOlderThen(age) *>

In this case, we use specs2 with scalacheck to do property-based testing. We ask scalacheck to generate arbitrary lists of Person instances and run our database program by using harnass.model.eval. This is wrapped by assertMirroring is a little helper method that asserts that the values in the returned tuple are equal.

The *> can be read as followed. Alternatively you could also write a for comprehension if that is easier for you. Another nice thing to note is that we can configure the Transactor[IO] to be a rollback transactor by setting always on the strategy to connection.rollback *> connection.close

Using the Oracle in service layer tests

As stated before we use ZIO for our service layer. When writing flows you can just put them on objects for example like this:

object PersonService {
  def deletePersonsOlderThen(age: Int): RIO[Pg, Unit] =
    for {
      _ <- ZIO.when(age < 0)(
      _ <- Pg.query(_.persons.deleteWhenOlderThen(age))
    } yield ()

The nice thing about doing this is that you don’t have problems with circular dependencies as the dependencies are residing in the RIO effect type offered by ZIO as you can read here.

I don’t like to assert invariants in the API layer, as it’s harder to test and the API layer its concern is to decode incoming requests and encode responses. In this case, the invariant is that the age should be greater than zero. When it’s smaller we use to stop the program and exit with the AppError.InvalidAge. After that, we use the data layer by using the Pg service.

In this case, we require the Pg service which I like to define like this:

trait PostgresRepos[F[_]] {
  def persons: PersonRepository[F]

object PostgresRepos {
  implicit val functorK: FunctorK[PostgresRepos] = Derive.functorK

object Pg {

  trait Service {
    // This is `ConnectionIO` in production.
    type ConnIO[A]

    // Collection of all the `ConnectionIO` based repositories
    protected val postgresReposConnIO: PostgresRepos[ConnIO]

    // The natural transformation which transforms `ConnectionIO` to a `Task`
    protected val transTask: ConnIO ~> Task

    lazy val postgresReposTask: PostgresRepos[Task] =

    def query[A](f: PostgresRepos[Task] => Task[A]): Task[A] =

  def query[A](f: PostgresRepos[Task] => Task[A]): Eff[Pg, A] =
    ZIO.accessM(_.get.query(f).mapError(err => AppError.Unexpected(err)))

The query accessor method has access to PostgresRepos which is a trait that is a collection of all the repositories.

Now comes the trick. When you use it in production, you’ll use the DoobieXXX version and when you unit test your service methods, you use the SepticXXX versions which are asserted to be equal to the DoobieXXX versions.


I hope this article gave you insight into how to test your data layer and have better service layer tests as well.

By using the oracle we solve a few problems

  • In the data layer tests, we test with the real database without making the database dirty by using rollback on each ConnectionIO
  • We assert encoding/decoding symmetry from our domain model. You might miss out on decoding existing entries in the database though.
  • In the service layer tests, we don’t use mocks, but in-memory variants which mirror the behavior of the real implementation asserted in the data layer tests

I’ve actually coded the Septic thing into a repository and you can find it up here. It’s a proof of concept, but I’ve used this methodology at DHL Netherlands. It needs some work on testing all the combinators at Septic. If someone wants to continue the work release this to Maven Central go ahead. It would be great to mention my work if you do.

Created by

Mark de Jong

Mark de Jong

Software Creator