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Developing for Testability

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  • Best practices in development and architecture apply for OutSystems, as they would with any other programming language. Basically, you want to ensure that your developed code/module/application is reusable, maintainable, and scalable.

    When introducing Automated Tests in OutSystems (UI, Integration, or Component) the concept of developing code so that it is testable reinforces some of those best practices. It also introduces some new ones.

    Every application should be developed to facilitate tests that validate its correctness. This overarching concept is completely independent of the technology involved.

    Some of the best practices to consider when implementing testable OutSystems applications are presented in this section.

    Domain Isolation

    In OutSystems, business concepts and business logic are organized into modules. Each module “owns” any concept or logic defined within it. As a result of this organization, modules can ensure that any use of their concepts or logic is correct according to their own internal business rules. Domain-Driven Design (DDD) principles to promote code (and test) isolation are highly recommended. DDD drives the development of complex systems based on decoupled domains of technology artifacts. From a best practices perspective, in OutSystems a domain is mapped into a LifeTime team. This team owns a set of applications for a business domain with an independent lifecycle from other teams (domains). Domains can be horizontal or vertical. Depending on the scenario, the level of decoupling between domains needs to be carefully evaluated.

    If you are not too familiar with DDD in OutSystems and want to learn more about it, there’s already some great content describing its main concerns and principles. You should definitely read:

    With this introduction to the principles of DDD, you may observe some common pitfalls in implementing OutSystems applications. These will have an impact on application testability, so we’ll examine them further.

    Anti-Pattern #1: Validations at the UI Level Only

    In the following image, the screen in the UI module displays a form to edit a concept owned by the Core module. When submitting the changes, all validations take place in the corresponding Screen action, which then calls the action from the Core module to save the record. The Core action itself does not perform any validation on the given input and serves mostly as a wrapper for the Create action of the entity.

    Anti-Pattern #1 - Validations in the UI only

    When testing the Core action, we can input any values into the action, and it will always succeed, barring any problems communicating with the database. Any test that uses this action in its execution, therefore, loses its relevance. To solve this problem, move all business validations to the Core action to ensure the provided inputs are valid according to the business rules for that module.

    Keep in mind that the Core module action should be testable and take business validations/rules into account on its own. It should be able to be tested alone, without depending on a UI test, for the validation/rule check. On the UI level, validations should also be performed, but they should be focused more on user interaction and not so much on business rules - include simple input validation feedback.

    In summary, the focus at the UI level should be to validate only the inputs provided by the user. Consider these questions:

    • Did the user input a value that exists in the autocomplete input options?

    • Did the user fill in all mandatory inputs?

    Whereas, at the Core level, it should be to validate the business rules:

    • Is the option selected from the autocomplete a valid one in the current context?

    • Is the value submitted for the mandatory input valid in the current context?

    Anti Pattern #2: Business Logic at the UI Level

    The Screen action to submit the changes of the page has embedded business logic in its flow. In the example image, it creates a master record, then it creates or updates child records, and finally creates some notification.

    Anti-Pattern #2 - Business logic in the UI

    If this logic needs to be tested as a whole, then one of two things will happen:

    1. Either the test is forced into the UI level, which will make it potentially more expensive, especially because UI tests are generally harder to maintain and slower to run.

    2. That logic would have to be replicated in the test logic, which is of course not desirable because the test would need an update every time the screen action changes.

    The best way to handle this problem is to encapsulate the logic that needs to be tested in a public action at a Core module level, which will then be used in the screen action. This way, the business logic can be tested through component testing, without depending on the implementation of UI tests, which are way more expensive to produce and maintain.

    Anti Pattern #3: Unsupported Cross-Domain Referencing

    DDD recommends using Service Actions and Public Entities for dependencies between domains.

    Anti-Pattern #3 - Unsupported cross-domain dependencies

    This decision to directly reference Public entities or not between domains may impact the test data setup and therefore needs to be discussed here.

    There are a couple of things you can do to sort it out. If cross-domain entities are referenced directly, that means there’s no possibility to mock data from the other domain. As a result, it is acceptable for test setup activities in the local domain to create any required data directly in the external domain.

    However, if those entities are only referenced through REST/SOAP APIs or service actions, that opens up the possibility to use mock services to simulate data from the external domain. This will promote test independence and reliability. In this scenario, it isn’t acceptable for test setup activities in the local domain to directly create any required data in the external domain. Instead, use mock services to get the data.

    Service API Isolation

    Whenever a REST/SOAP API service external to the current domain needs to be consumed, it should be done so in a wrapper module. This wrapper module should then expose a set of public actions to use that REST API, and all modules needing to access the API must go through the wrapper module.

    Available since OutSystems 11, Service actions are essentially REST remote calls available only to other OutSystems applications running in the same environment. They are used to create loosely coupled dependencies between applications, allowing them to break monoliths inside the OutSystems context. Because at runtime they are REST services, it also makes sense to apply this isolation for service actions that are referenced across different domains.

    In this section, we will use API to refer to REST, SOAP, or service actions.

    Anti Pattern #1: Same API Consumed in Multiple Modules

    In an OutSystems environment, there are multiple modules that consume the same API. When we’re in the scope of a test execution and need to use mock services to isolate the application under test from the remote system (where the API is exposed), we’ll have to change every single module that is consuming this specific API. Needless to say, this becomes a maintenance nightmare while also being error-prone, as the potential to forget to change one module increases.

    Anti-Pattern #1 - Same API consumed in multiple Modules

    The best solution for you is to isolate the API consumption in a wrapper module that exposes the API methods through public actions. Core modules needing access to the API do it through the wrapper module. Later, in the scope of a test execution, when we need to point it to a mock service, that is done in one place only — the wrapper module

    In the case of REST APIs, the OnBeforeRequest event action available at the consumed REST definition can be used.

    • For SOAP APIs, use the SetWebReferenceURL action (from the EnhancedWebReferences extension) to override the target URL when in testing mode.

    • For service actions, because they behave like public actions at development time, the wrapper should implement logic to return the output expected for the test instead of executing the service action call.

    For more detail on mocking strategies, read Mock Services​ for Integration Points.

    Web UI Simulation

    Widget Naming

    The way automated UI testing works in web applications is by simulating the user interaction through application screens in order to fulfill a specific user journey or functionality provided by the application. This simulation is done through scripting the various steps a user needs to perform on the screen elements. An example would be: Follow the home menu link; Click on the submit button; Enter the “” into the email input.

    This means that the UI test script needs to uniquely identify the elements that the user is interacting with. The easiest way to uniquely identify an element in a screen is through its ID property. As such, and in the scope of OutSystems application screens, it implies that those widgets need to be properly identified in Service Studio.

    When developing a screen, the developer should ensure that all elements that have some degree of user interaction, such as inputs, buttons, and links, have the name property assigned with a meaningful value. This facilitates the element identification in the generated screen code, since the element’s ID attribute will always terminate with the widget’s name property value. This will also avoid additional effort later on if a UI test finds that a specific element on the screen is not properly identified.

    Eventually, it may be the case that a specific UI test will require additional elements on the screen, which will also need identifiers. However, by naming at least the widgets mentioned above, the developer minimizes the number of times that rework needs to be done. Saving some of this effort fosters a faster workflow when implementing UI testing.

    Using the name as the basis of the selector also allows the platform to detect name collision inside the same screen. This is an added benefit. It does not allow two different widgets to have the same Name property, and so the second will add the suffix of “2”, and so on.

    Still, because OutSystems allows composing a screen with multiple web blocks, if we use the same name in two different web blocks for the same screen, this name collision will not be automatically addressed by Service Studio. Taking that into account, we recommend that when naming a widget in a block, developers should include the web block name as a prefix to the name of the UI widget. This ensures that names will still be unique when a screen is composed by multiple, different web blocks.

    And what if there is a need to have multiple instances of the same web block on the same screen? In these scenarios, developers should wrap each web block in a container with a specific ID for the screen, and then use that container ID to anchor each specific web block instance in a unique way for that screen.

    Testing tools usually use XPath or CSS selectors to identify each UI element. When following the approach mentioned above for UI element identification, these UI widgets will all have IDs, but the OutSystems platform will generate a “composed” ID that will only end with the actual name given to the widget inside Service Studio.

    The following is a simple example of how you can identify these IDs that end with a given unique name. Remember, the name represents both CSS selectors and XPath selectors for an input with the name “UserNameInput” in Service Studio:

    CSS Selector
    XPath Selector (1.0)
    //input[substring(@id, string-length(@id) - string-length('UserNameInput') + 1) = 'UserNameInput']
    XPath Selector (2.0) not commonly supported by testing tools

    Extended Property Alternative

    As an alternative to using the name property to identify the elements, an extended property named “os-test-id” can be used as the identifier. For the same input in the previous example, add the “os-test-id” extended property with the value “UserNameInput” in Service Studio. In the generated HTML code, the platform will add an attribute with the mentioned name/value pair.

    Then, the following selectors can be used to identify the element on the screen:

    CSS Selector
    XPath Selector

    This approach allows for complete control identifying the elements in the screen, but it does come with some drawbacks.

    First of all, developers need to start adding these extended properties to their widgets in Service Studio, which introduces more development effort. Naming widgets may feel much more natural.

    In addition to that, the OutSystems platform doesn't provide any collision detection for extended properties, so it’s up to the developer to ensure it. This can be especially tricky to manage in complex screens.

    Mapping tests to the Architecture Canvas

    cd Architecture Canvas

    The Architecture Canvas is an OutSystems architecture tool to make the design of Service-Oriented Architectures simple.

    Architecture Canvas promotes the correct abstraction of reusable (micro)services and the correct isolation of distinct functional modules. It's useful in cases where you are developing and maintaining multiple applications that reuse common modules.

    Due to the nature of the functionalities implemented in modules at each layer, it is possible to map these layers to the types of functional tests that would make sense to implement at each level.

    Orchestration layer

    The Orchestration layer is applicable only to versions prior to OutSystems 11.


    Processes, dashboards, and portal home pages, mashing up information from different applications to provide a unified user experience.


    Automated UI (end-to-end) Testing (user journeys)

    Orchestrations modules often implement screens that are cross-application, which are more information-oriented and not so much form oriented. These provide links to access application screens built on the End-User Layer. This way, tests implemented on functionalities at this layer are full user journey UI tests that require moving between multiple applications. They validate both the navigation and how applications impact each other.


    1. Access the Intranet portal.
    2. Go to the Vacations app.
    3. Submit a week’s vacation.
    4. Go to the Allocation app.
    5. Verify that the submitted week is blocked for vacations.

    End User Layer


    User interfaces and processes, reusing Core and Library services to implement the user stories.


    Automated E2E UI Testing

    End-user modules implement screens specific for a single application with a very well defined business domain. They tend to be more form oriented, or at least a mix of information/form. They also focus on the business functionality for that application. As such, tests implemented on functionalities at this layer are UI tests that will validate the implemented business logic of that application.


    1. Login in the Vacations app.
    2. Go to the Calendar screen.
    3. Select a week in September and submit for approval.
    4. Logout/Login in the Vacations app with the manager’s account.
    5. Verify that the submitted request for vacations is in the “To Approve” list.

    Core Layer


    Services around business concepts, exporting reusable entities, business rules, and business widgets.

    Core modules implement business concepts and logic. Because horizontal references are allowed at the core layer, it actually establishes a hierarchy between core modules. Consequently, depending on how high or low a core module sits in that hierarchy, it is possible to implement different types of tests for it. The lower the level, the closer the test will be to a unit test, while higher levels promote end-to-end tests that may have dependencies on concepts from multiple modules and even integrate with external systems.


    Automated End-to-End Testing (non-UI)

    Non-UI end-to-end tests validate business logic by calling the OutSystems code directly without going through the application screens and asserting expected outputs. They actually test multiple components in the application. There are two dimensions that can be explored here:

    1. Explicitly calling multiple business actions in a logical sequence — simulating the sequence of actions that a user would follow through the UI — to assert a final outcome.

    2. Calling a single business action that encapsulates the orchestration of multiple concepts from different modules to achieve a full user interaction validation.

    Automated API Testing

    API tests are used to validate the correctness of exposed APIs, according to specification. These can either be APIs that are exposed from external systems and consumed inside the OutSystems platform, or APIs that are exposed by the OutSystems module to be consumed externally by other applications or systems. These tests are usually defined by a set of inputs for the API and the corresponding expected outputs.

    Automated Unit/Component Testing

    Unit/component tests validate business logic on the lower-level code modules. They are self-contained and have almost no dependencies (or none at all) to other core modules. For this reason, these modules typically contain the business logic specific for the set of concepts defined, with no context of any other concepts or higher level logic. This makes them the smallest testable business part of an OutSystems application.

    Library Layer


    Business-agnostic services to extend the framework with highly reusable assets, UI patterns, connectors to external systems, and integration of native code.


    Automated API Testing

    API tests validate the correctness of exposed APIs. These can be either APIs that are exposed from external systems and consumed inside the OutSystems platform, or APIs that are exposed by the OutSystems module to be consumed externally by other applications or systems. These tests are usually defined by a set of inputs for the API and the corresponding expected outputs.

    Automated Unit/Component Testing (non-business)

    Library modules are by definition business-agnostic. This way, they contain no business logic. But because they provide highly reusable assets that may be used throughout all OutSystems applications, it may be useful to test the functionalities provided by these modules. Examples of such libraries include auditing subsystems, where every action performed by a user on any application is logged in an Audit entity.

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