A unit test checks one software component—the system under test (SUT)—in isolation. In a microservice, that can mean testing order placement without running the database or Payment service. Automated tests give faster feedback than manually running the full service flow. section 7.1 section 7.0
Test scenarios, not merely whether a function runs:
- Replace the database and payment dependencies with mocks that implement their interfaces. Configure Save and Charge to return the result needed for each scenario. section 7.2.4
- For the happy path, have both calls succeed and assert that PlaceOrder returns no error. section 7.2.4
- For persistence failure, make Save return an error and assert that PlaceOrder returns its message. For payment failure, assert the resulting gRPC status message and validation-error detail. section 7.2.4
func Test_Should_Return_Error_When_Payment_Fail(t *testing.T) {
payment := new(mockedPayment)
db := new(mockedDb)
payment.On("Charge", mock.Anything).Return(errors.New("insufficient
balance")) #1
db.On("Save", mock.Anything).Return(nil)
application := NewApplication(db, payment)
_, err := application.PlaceOrder(domain.Order{
CustomerID: 123,
OrderItems: []domain.OrderItem{
{
ProductCode: "bag",
UnitPrice: 2.5,
Quantity: 6,
},
},
CreatedAt: 0,
})
st, _ := status.FromError(err) #2
assert.Equal(t, st.Message(), "order creation failed") #3
assert.Equal(t,
st.Details()[0].(*errdetails.BadRequest).FieldViolations[0]
.Description, "insufficient balance") #4
assert.Equal(t, st.Code(), codes.InvalidArgument) #5
}
The payment-failure test shows why checking only for a non-nil error is insufficient: the assertion also inspects the status and field-violation detail returned to the caller. section 7.2.4
Run the Go tests with `go test ./... -cover -coverprofile=coverage.out` to generate a coverage report. Keep unit tests numerous because their isolation makes them fast and inexpensive, but use integration tests for interactions between modules and end-to-end tests for complete flows. Test results and coverage can also help verify pull requests in CI. section 7.2.4 section 7.1 section 7.6
