Manuel Perez on October 10, 2019

How to mock an external service for tests in GO

From my experience as a software developer, an issue that I have dealt when building an application are the dependencies for external services. A test with external dependencies where you don’t have much control can fail if there is a change in the service and the outcome is not the expected, with this in mind, we need to ensure the non-dependence of external services when running our tests. The most effective way to do this is mocking those dependencies.

In this post, I will focus on the functional tests of an application. I will also talk about how to mock an external service to not rely on it when running our functional tests.

To explain how to mock an external service, I’ll walk you step by step through an example.

Let’s say that we have an endpoint, that is using a third-party package called holidays that is making HTTP requests to an external service to get such holidays.

# /actions/holidays.go
package actions

import (
    "time"
    "github.com/somebody/holidays"
    "github.com/somebody/holidays/filters"
)

func Holidays(w http.ResponseWriter, r *http.Request) {
    currentMonth := int(time.Now().Month())

    filters := filters.Params{
        Months: []int{currentMonth}
    }
    
    message := "There are not holidays"
    
    days, _ := holidays.GetHolidays(filters)
    if len(days) > 0 {
        message = fmt.Sprintf("Holidays for this month: %v", days)
    }

    fmt.Fprintf(w, message)
}

Step 1: Separating the external service logic from our business logic.

The environment where we executed the tests should not depend on any external service. Since we’re only testing the functionalities of our application and how it behaves with any input, regarding external services, we only care about the outcome of it.

To achieve a higher level of control over our tests, we need to move all the holidays-service-logic to a separate package.

# /lib/holidays/holidays.go
package holidays

import (
    "github.com/somebody/holidays"
    "github.com/somebody/holidays/filters"
)

func Holidays(months []int) ([]int, error) {
    filters := filters.Params{
        Months: months
    }
    days, err := holidays.GetHolidays(filters)

    return days, err
}

Once the logic is separated we have to make sure we must call the external service using the new package from our actions package.

# /actions/holidays.go
import (
    "project/lib/holidays"
)

func Holidays(w http.ResponseWriter, r *http.Request) {
    ...
    days, _ := holidays.Holidays(months)
    ...
}

Step 2: Abstracting the use of the service through an interface.

So far, this solution doesn’t solve the problem, the tests are still executing all the holidays-service-logic so we are still making HTTP requests that we don’t want to be made. We have to do something else.

To mock the external service, we can abstract all its logic within an interface which we can implement elsewhere and use it by our functional tests, returning the outcome we need to test the behavior of our application.

Here is how such an interface would look like:

# /lib/holidays/holidays.go
package holidays

import (
    "github.com/somebody/holidays"
    "github.com/somebody/holidays/filters"
)

var Client ServiceFunctions = holidaysService{}

type ServiceFunctions interface {
    Holidays([]int) ([]int, error)
}

type holidaysService struct{}

func (hs holidaysService) Holidays(months []int) ([]int, error) {
    filters := filters.Params{
        Months: months
    }
    days, err := holidays.GetHolidays(filters)
    return days, err
}

Client is a global variable that receives as value any type that implements the ServiceFunctions interface, by default it uses holidaysService{} who communicate with the external service but on the side of the tests, we can update its value with another type that implements the same interface by returning the outcome we want.

var Client ServiceFunctions = holidaysService{}

We have to make a change in the way of calling the external service, now it would be through our Client global variable.

# /actions/holidays.go

func Holidays(w http.ResponseWriter, r *http.Request) {
    ...
    days, _ := holidays.Client.Holidays(months)
    ...
}

Step 3: Create the mock of our interface.

Once all the functions of the holidays-service are abstracted, we need to create another type that implements the same interface with the intent to use it as a dummy object for our tests. In this case, our new Holidays function we’ll return a list of hard-coded days and an error.

# /lib/holidays/holidays_mock.go
package holidays

var (
    Days []int{}
    HolidaysError error
)

type ServiceFuncMock struct{}

func (sfm ServiceFuncMock) Holidays(months []int) ([]int, error) {
    return Days, HolidaysError
}

We can make this interface as dynamic as we want, we can return different values in order to test how our application behaves.

Step 4: How to use the mock?

Given our test is still depending on external service. We will replace the holidays-service call by the simulation we created.

We assign an instance from our mock to the Client variable we defined before:

holidays.Client = holidays.ServiceFuncMock{}

Here we are setting the value for our Client, we’re using the ServiceFuncMock "test interface" as the default interface.

Here is an example of how to test our application with no external dependencies.

Test 1: In this test, we simulate the response from the holiday-service with an empty list of holidays and our application should display “There are not holidays”.

# /actions/holidays_test.go
package actions

import (
    ...
    "project/lib/holidays"
)

func Test_Holidays_Without_Days(t *testing.T) {
    holidays.Client = holidays.ServiceFuncMock{}

    req, _ := http.NewRequest("GET", "/holidays", nil)
    ...

    assert.Contains(t, res.Body.String(), "There are not holidays")
}

Test 2: In this case, we set a different response for the days that the Holidays function will return.

# /actions/holidays_test.go

...

func Test_Holidays_With_Days(t *testing.T) {
    holidays.Client = holidays.ServiceFuncMock{}
    holidays.Days = []int{1, 3, 5}

    req, _ := http.NewRequest("GET", "/holidays", nil)
    ...

    assert.Contains(t, res.Body.String(), "holidays for this month")
    assert.Contains(t, res.Body.String(), "1")
    assert.Contains(t, res.Body.String(), "3")
    assert.Contains(t, res.Body.String(), "5")
}

As you can see, mocking external services is an effective method when testing our application’s features that require the use of them. This method also serves us well to decrease the coupling of our application regarding external services.

Thanks a lot for reading! Keep on learning and coding!


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