Testing with MockServer

I've just found out MockServer and it looks awesome 🤩 so I wanted to check it out repeating the steps of my previous demo Testing with WireMock which (as you can expect) uses WireMock, another fantastic tool to mock APIs.

rogervinas / mockserver-testing

🎭 MockServer Testing

So in this demo we will use first MockServer JUnit5 extension:

And then we will use MockServer Docker image and Testcontainers:

BarClient
FooClient
AppUseCase
App

BarClient

BarClient interface

interface BarClient {
  fun call(name: String): String
}

BarKtorClient test

I will use a Ktor client for no other reason that I need an Http client and this seems interesting, as we are using Kotlin.

So a simple test with MockServerExtension for the BarKtorClient looks like:

@ExtendWith(MockServerExtension::class)
@TestInstance(PER_CLASS)
class BarKtorClientShould {

  private val name = "Sue"

  private lateinit var mockServerClient: MockServerClient
  private lateinit var mockServerUrl: String

  @BeforeAll
  fun beforeAll(mockServerClient: MockServerClient) {
    this.mockServerClient = mockServerClient
    this.mockServerUrl = "http://localhost:${mockServerClient.port}"
  }

  @BeforeEach
  fun beforeEach() {
    mockServerClient.reset()
  }

  @Test
  fun `call bar api`() {
    mockServerClient.`when`(request()
      .withMethod("GET").withPath("/bar/${name}")
    ).respond(response()
      .withStatusCode(200).withBody("Hello $name I am Bar!")
    )

    assertThat(BarKtorClient(mockServerUrl).call(name))
      .isEqualTo("Hello $name I am Bar!")
  }

  @Test
  fun `handle bar api server error`() {
    mockServerClient
      .`when`(request().withMethod("GET").withPath("/bar/.+"))
      .respond(response().withStatusCode(503))

    assertThat(BarKtorClient(mockServerUrl).call(name))
      .startsWith("Bar api error: Server error")
  }
}

Note that we can inject MockServerClient as a parameter in the test methods too.

BarKtorClient implementation

In order to make the test pass 🟩 we can implement the BarKtorClient this way:

class BarKtorClient(private val url: String) : BarClient {

  private val client = HttpClient(CIO) {
    expectSuccess = true
  }

  override fun call(name: String): String = runBlocking {
    try {
      client.get("$url/bar/$name").body<String>()
    } catch (e: Exception) {
      "Bar api error: ${e.message}"
    }
  }
}

FooClient

FooClient interface

interface FooClient {
  fun call(name: String): String
}

FooKtorClient test

For this test we will use MockServer's Mustache templates:

@TestInstance(PER_CLASS)
class FooKtorClientShould {

  private val name = "Joe"

  private lateinit var mockServerClient: MockServerClient
  private lateinit var mockServerUrl: String

  @BeforeAll
  fun beforeAll() {
    mockServerClient = ClientAndServer()
    mockServerUrl = "http://localhost:${mockServerClient.port}"
  }

  @BeforeEach
  fun beforeEach() {
    mockServerClient.reset()
  }

  @Test
  fun `call foo api`() {
    mockServerClient.`when`(request() 
     .withMethod("GET").withPath("/foo")
     .withQueryStringParameter("name", ".+")
    ).respond(template(
      MUSTACHE,
      """
      {
       statusCode: 200,
       body: 'Hello {{ request.queryStringParameters.name.0 }} I am Foo!'
      }
      """.trimIndent()
     ))

    assertThat(FooKtorClient(mockServerUrl).call(name))
      .isEqualTo("Hello $name I am Foo!")
  }

  @Test
  fun `handle foo api server error`() {
    mockServerClient.`when`(request()
      .withMethod("GET").withPath("/foo")
      .withQueryStringParameter("name", ".+")
    ).respond(response().withStatusCode(503))

    assertThat(FooKtorClient(mockServerUrl).call(name))
      .startsWith("Foo api error: Server error")
  }
}

Note that:

As in the previous test we can inject MockServerClient as a parameter in the test methods too.
MockServer provides also Velocity and Javascript templates that support more complex logic.

FooKtorClient implementation

Same as before in order to make the test pass 🟩 we can implement the FooKtorClient this way:

class FooKtorClient(private val url: String) : FooClient {

  private val client = HttpClient(CIO) {
    expectSuccess = true
  }

  override fun call(name: String): String = runBlocking {
    try {
      client.get("$url/foo") {
        parameter("name", name)
      }.body<String>()
    } catch (e: Exception) {
      "Foo api error: ${e.message}"
    }
  }
}

AppUseCase

Now we have to implement AppUseCase, which will use a FooClient to call the Foo API and then a BarClient to call the Bar API.

As it is not MockServer related because we can test first the implementation just using MockK JUnit5 extension we can skip the details and you can review the source code of AppUseCaseShould and AppUseCase.

App

App implementation

Let me introduce first the App implementation, as I will present later two different types of MockServer tests:

class App(
  private val name: String,
  private val fooApiUrl: String,
  private val barApiUrl: String
) {

  fun execute() = AppUseCase().execute(
    name,
    FooKtorClient(fooApiUrl),
    BarKtorClient(barApiUrl)
  )
}

App test with MockServerExtension

Since in this example Foo API and Bar API do not have conflicting endpoints, we can use MockServerExtension to mock both APIs:

@ExtendWith(MockServerExtension::class)
class AppShouldWithOneMockServer {

  private val name = "Ada"

  @Test
  fun `call foo and bar`(mockServerClient: MockServerClient) {
    mockServerClient
      .`when`(request().withMethod("GET").withPath("/foo").withQueryStringParameter("name", name))
      .respond(response().withStatusCode(200).withBody("Hello ${name} I am Foo!"))
    mockServerClient
      .`when`(request().withMethod("GET").withPath("/bar/${name}"))
      .respond(response().withStatusCode(200).withBody("Hello $name I am Bar!"))

    val mockServerUrl = "http://localhost:${mockServerClient.port}"
    val app = App(name, mockServerUrl, mockServerUrl)

    assertThat(app.execute()).isEqualTo(
      """
        Hi! I am $name
        I called Foo and its response is Hello $name I am Foo!
        I called Bar and its response is Hello $name I am Bar!
        Bye!
      """.trimIndent()
    )
  }
}

App test with two MockServers

But imagine a real scenario where Foo API and Bar API do have conflicting endpoints, or you just want to mock them separatedly for any reason. In this case you can use two MockServers instead of using MockServerExtension:

@TestInstance(PER_CLASS)
class AppShouldWithTwoMockServers {

  private val name = "Leo"

  private val mockServerClientFoo = ClientAndServer()
  private val mockServerClientBar = ClientAndServer()

  @Test
  fun `call foo and bar`() {
    mockServerClientFoo.`when`(request()
      .withMethod("GET")
      .withPath("/foo")
      .withQueryStringParameter("name", name)
    ).respond(response()
      .withStatusCode(200)
      .withBody("Hello ${name} I am Foo!")
    )
    mockServerClientBar.`when`(request()
      .withMethod("GET")
      .withPath("/bar/${name}")
    ).respond(response()
      .withStatusCode(200)
      .withBody("Hello $name I am Bar!")
    )

    val mockServerFooUrl = "http://localhost:${mockServerClientFoo.port}"
    val mockServerBarUrl = "http://localhost:${mockServerClientBar.port}"
    val app = App(name, mockServerFooUrl, mockServerBarUrl)

    assertThat(app.execute()).isEqualTo(
      """
        Hi! I am $name
        I called Foo and its response is Hello $name I am Foo!
        I called Bar and its response is Hello $name I am Bar!
        Bye!
      """.trimIndent()
    )
  }
}

App test with MockServer Docker

Static stubs

First we will use static stubs configured as json files:

In our docker-compose.yml:

We configure two MockServer containers, one for Foo API and one for Bar API.
We use dynamic ports for each container.
We load persisted expectations at startup setting MOCKSERVER_INITIALIZATION_JSON_PATH variable.
We mount as volumes the directories containing the MockServer expectations: foo-api/mockserver.json and bar-api/mockserver.json.

Finally we test the App using Testcontainers JUnit5 extension:

@Testcontainers
@TestInstance(PER_CLASS)
class AppShouldWithMockServerDocker {

  companion object {
    private const val name = "Ivy"

    private const val fooServiceName = "foo-api"
    private const val fooServicePort = 8080
    private const val barServiceName = "bar-api"
    private const val barServicePort = 8080

    private lateinit var fooApiHost: String
    private var fooApiPort: Int = 0
    private lateinit var barApiHost: String
    private var barApiPort: Int = 0

    val waitForMockServerLiveness = forHttp("/mockserver/status")
      .withMethod("PUT")
      .forStatusCode(200)

    @Container
    @JvmStatic
    val container = ComposeContainer(File("docker-compose.yml"))
      .withLocalCompose(true)
      .withExposedService(fooServiceName, fooServicePort, waitForMockServerLiveness)
      .withExposedService(barServiceName, barServicePort, waitForMockServerLiveness)

    @BeforeAll
    @JvmStatic
    fun beforeAll() {
      fooApiHost = container.getServiceHost(fooServiceName, fooServicePort)
      fooApiPort = container.getServicePort(fooServiceName, fooServicePort)
      barApiHost = container.getServiceHost(barServiceName, barServicePort)
      barApiPort = container.getServicePort(barServiceName, barServicePort)
    }
  }

  @Test
  fun `call foo and bar`() {
    val fooApiUrl = "http://${fooApiHost}:${fooApiPort}"
    val barApiUrl = "http://${barApiHost}:${barApiPort}"

    val app = App(name, fooApiUrl, barApiUrl)

    assertThat(app.execute()).isEqualTo(
      """
        Hi! I am $name
        I called Foo and its response is Hello $name I am Foo!
        I called Bar and its response is Hello $name I am Bar!
        Bye!
      """.trimIndent()
    )
  }
}

Dynamic stubs

We can also configure our stubs programmatically using the MockServerClient and connect it to each one of the two MockServer containers:

@Test
fun `call foo an bar with dynamic stubs`() {
  val fooApiUrl = "http://${fooApiHost}:${fooApiPort}/dynamic"
  val barApiUrl = "http://${barApiHost}:${barApiPort}/dynamic"

  MockServerClient(fooApiHost, fooApiPort)
    .`when`(
      request()
        .withMethod("GET")
        .withPath("/dynamic/foo")
        .withQueryStringParameter("name", name)
    )
    .respond(
      response()
        .withStatusCode(200)
        .withBody("Hi $name I am Foo, how are you?")
    )
  MockServerClient(barApiHost, barApiPort)
    .`when`(
      request()
        .withMethod("GET")
        .withPath("/dynamic/bar/$name")
    ).respond(
      response()
        .withStatusCode(200)
        .withBody("Hi $name I am Bar, nice to meet you!")
    )

  val app = App(name, fooApiUrl, barApiUrl)

  assertThat(app.execute()).isEqualTo(
    """
      Hi! I am $name
      I called Foo and its response is Hi $name I am Foo, how are you?
      I called Bar and its response is Hi $name I am Bar, nice to meet you!
      Bye!
    """.trimIndent()
  )
}

App run with MockServer Docker

MockServer with Docker has a cool advantage, we can use the same docker-compose used by the test to start the application and run/debug it locally:

In this case we only need to use fixed ports, configuring them in docker-compose.override.yml. This override does not affect @Testcontainers.

That was a good one! Happy coding! 💙

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