Outside-in TDD with Test Doubles

Slides I used to facilitate this month “TDD Hands-on session” at Cork Software Crafters.

A Test-Driven Development Refresher.
– Types of test doubles.
– How to create a test double manually or using a framework.
– Classic TDD vs Mockist TDD.


DDD – Aggregate Roots and Domain Events publication

Implementing DDD in a traditional object-oriented style *, when an action over an Aggregate succeeds the state of the aggregate changes and one or multiple domain events are published.
A Domain Event represents something that has happened in the domain. In the other hand if the action cannot be fulfilled a Domain Error is thrown as an exception by the aggregate.

Publishing Domain Events from an Aggregate
The question that arises here is Should the aggregates publish events directly?. We can find a multitude of articles and examples where the Aggregate implementation does exactly that. The very same Vaughn Vernon’s IDDD book shows this approach (page 300):

public class BacklogItem extends ConcurrencySafeEntity {
  public void commitTo(Sprint aSprint) {
      .publish(new BacklogItemCommitted(

the BacklogItem Aggregate creates and dispatches an event directly using a DomainEventPublisher instance before the database transaction is commited.

This approach can become quite complex to implement right, because of the transactional concerns. Think about it, if the aggregate publishes events directly, what happens if later on the persistence operation fails?
Unit testing of aggregates gets also complicated, the aggregate is coupled to a DomainEventPublisher that it instantiates directly. At the same time, we know that we should avoid dependency injection into an Aggregate.

An alternative approach is to use a Two-steps event publication

  • 1st) Aggregate root creates and registers domain events for publication.
  • 2nd) When the application Service persists the changes to the aggregate (calling the save method in a Repository) the infrastructure takes care of publishing the registered events only if the save transaction succeeds.

Let’s see this in action with some code. The domain example is for a Runbook which tracks Tasks, and the use case is assigning a task to an operator.

– Service retrieves and acts on the aggregate.

// RunbookService
fun assignTask(c: AssignTask) {
    val runbook = runbookRepository.findById(c.runbookId)

    runbook.assignTask(c.taskId, c.assigneeId, c.userId) // act on the aggregate

– Aggregate changes its state and then creates and registers domain events.

// Runbook
fun assignTask(taskId: String, assigneeId: String, userId: String) {


    registerEvent(TaskAssigned(runbookId, taskId, assigneeId)) // create and register event

Notice that Domain Events are only registered by the aggregate root because it is the one ensuring the integrity of the aggregate as a whole.

The way that the aggregate “registers” events is keeping track of them internally as a list of unpublished domain events.

private val domainEvents = ArrayList()

protected fun registerEvent(event: EventType) {

– Service calls repository to save changes
After calling the assignTask action on the aggregate the service saves the changes (the “changed” aggregate) using the repository.

// RunbookService
fun assignTask(c: AssignTask) {
    val runbook = runbookRepository.findById(c.runbookId)

    runbook.assignTask(c.taskId, c.assigneeId, c.userId)

    runbookRepository.save(runbook) // save aggregate

– Infrastructure publishes all the aggregate unpublished domain events.
This can be done directly with a custom repository implementation and some form of internal pub-subscribe mechanism or with the help of a framework. Later on, I describe how to do this for repositories based on Spring Data.

Unit Testing
Unit testing of the aggregate remains simple with this design, there is no need to mock an event publisher or anything else. Asserting that the aggregate created and registered the right events can be done directly checking the aggregate domain events collection.

    fun `can assign task`() {
        val runbook = Runbook(RUNBOOK_ID, RUNBOOK_NAME, OWNER_ID)
        runbook.addTask(TASK_ID, TASK_NAME, TASK_DESCRIPTION)

        runbook.assignTask(TASK_ID, TASK_ASSIGNEE_ID, OWNER_ID)

        assertThat(runbook.domainEvents()).contains(TaskAssigned(RUNBOOK_ID, TASK_ID, TASK_ASSIGNEE_ID))

Domain Events
Here is an example of how the Domain Events for an aggregate look using Kotlin sealed classes.

sealed class RunbookEvent

data class TaskAdded(
        val runbookId: String,
        val taskId: String) : RunbookEvent()

data class TaskAssigned(
        val runbookId: String,
        val taskId: String,
        val assigneeId: String) : RunbookEvent()

Using Spring Data infrastructure to Publishing Events from Aggregate Roots
Spring Data repositories have support for aggregate domain events publication. On a call to a repository save() method, Spring will look into the passed aggregate for any unpublish domain events and will automatically publish them via its ApplicationEventPublisher API.

To expose the events the aggregate root needs to provide an accessor method annotated with the @DomainEvents annotation.

private val domainEvents = ArrayList()
fun domainEvents(): Collection<EventType> {
    return Collections.unmodifiableList<EventType>(domainEvents)

Aggregate Root base class
We can reuse across all aggregates the same code for registering events placing it in an AggregateRoot base class.

abstract class AggregateRoot<EventType> {

    private val domainEvents = ArrayList<EventType>()

     * All domain events currently captured by the aggregate.
    fun domainEvents(): Collection<EventType> {
        return Collections.unmodifiableList<EventType>(domainEvents)

     * Registers the given event for publication.
    protected fun registerEvent(event: EventType): EventType {
        return event

     * Clears all domain events currently held. Invoked by the infrastructure in place in Spring Data
     * repositories.
    protected fun clearDomainEvents() {

Domain Events make explicit what changes within a domain. Use them not only to have a richer model but also a better design, implementation and tests. The different parts of the system can be decoupled much easily and prevent us from having large application services or aggregates.
It also simplifies future integration with other systems, translation from Domain Events to messaging Integration Events is trivial.

– Implementing Domain Driven Design, Vaughn Vernon.
– A better domain events pattern, Jimmy Bogard. https://lostechies.com/jimmybogard/2014/05/13/a-better-domain-events-pattern
– Spring – Domain event publication from aggregate roots, Mark Paluch. https://spring.io/blog/2017/01/30/what-s-new-in-spring-data-release-ingalls#domain-event-publication-from-aggregate-roots
– Domain Events vs. Integration Events in Domain-Driven Design and microservices architectures, Cesar de la Torre
– Domain events: design and implementation. https://docs.microsoft.com/en-us/dotnet/standard/microservices-architecture/microservice-ddd-cqrs-patterns/domain-events-design-implementation

1 – In a functional DDD style the aggregates: do not mutate state; do not publish or register events; do not throw exceptions. Instead, they return a result type containing the new version of the aggregate and domain events or in case of failure, the result contains the domain errors.

Angular pluggable architecture?

Pluggable Architecture – A Software Architecture that allows to plug functionality using PluggableModules. Ward Cunningham

One of the projects I am involved in these days is composed of multiple applications each of them with its own front-end implemented in Angular, all using consistent UX patterns and framework. The common user flows like managing projects or navigating to the applications is handled in a central application. At the same time, common components like navbar or sidebars are reused across.

This front-end separation by applications or large features has prevented us from creating a big monolithic web app and our teams have the kind of independence and control in the front-end we get in the back-end with micro-services.

All good so far, but let’s say that now we want to have a dashboard application composed of widgets and one of the requirements is that multiple teams can implement and deploy their own widgets independently. Hence, without the need of modifying the dashboard application code or redeploying it when a new widget comes or gets updated.

Can an Angular CLI application load and render external widgets at runtime?

The short answer is yes, it is possible. You can find a working demo in this repo https://github.com/paucls/angular-pluggable-architecture

The long answer is that there is not a straightforward solution ;-), it took me a little bit of research to find out how.

First of all, the application needs a way to load remote widget bundles at runtime. This sounds like lazy loading, but that is not going to help us, lazy loading is for loading modules linked to child routes, but the modules still need to belong to the same application and be known at build-time.

The solution is to use SystemJS, but only at runtime and only to load the widgets. We do not need to configure our CLI project to switch from Webpack to SystemJS.

Loading a bundle will look like this:

const module = await SystemJS.import('/path-to-a-module-bundle');

The next problem is that the bundle contains a Module and a Component for the widget but they have dependencies themselves to other modules. The smallest component has at least dependencies to @angular/core.

import { Component, OnInit } from '@angular/core';

To stop SystemJS from trying to find and fetch those vendor modules from the network we need to set SystemJS with the modules already available.

 * Set existing vendor modules into SystemJS registry.
 * This way SystemJS won't make HTTP requests to fetch imported modules
 * needed by the dynamicaly loaded Widgets.
import { System } from 'systemjs';
declare const SystemJS: System;

import * as angularCore from '@angular/core';
import * as angularCommon from '@angular/common';
import * as angularCommonHttp from '@angular/common/http';

SystemJS.set('@angular/core', SystemJS.newModule(angularCore));
SystemJS.set('@angular/common', SystemJS.newModule(angularCommon));
SystemJS.set('@angular/common/http', SystemJS.newModule(angularCommonHttp));

The second part is compiling and rendering each widget. For this, we need to make use of the Angular JIT compiler to compile the Angular module and component of the widget. And then create the widget component in ViewChield target div.

  selector: 'app-dashboard',
  template: '<div #content></div>'
export class DashboardComponent implements AfterViewInit {

  @ViewChild('content', { read: ViewContainerRef }) content: ViewContainerRef;

  constructor(private compiler: Compiler, private dashboardService: DashboardService,
    private injector: Injector) { }

  ngAfterViewInit() {

  private async loadWidgets() {
    const widgets = await this.dashboardService.getWidgetConfigs().toPromise();
    widgets.forEach((widget) => this.createWidget(widget));

  private async createWidget(widget: WidgetConfig) {
    // import external module bundle
    const module = await SystemJS.import(widget.moduleBundlePath);

    // compile module
    const moduleFactory = await this.compiler.compileModuleAsync(module[widget.moduleName]);

    // resolve component factory
    const moduleRef = moduleFactory.create(this.injector);
    const componentProvider = moduleRef.injector.get(widget.name);
    const componentFactory = moduleRef.componentFactoryResolver.resolveComponentFactory(componentProvider);

    // compile component


Note that to be able to use the JIT compiler our build needs to have AOT disabled what it is not ideal. There is an open issue (https://github.com/angular/angular/issues/20875) to allow the use of the compiler with AOT, I tried some of the temporal solutions suggested like defining custom providers for JIT in the app module but I could not make it work yet.

– Developing with Angular, Denys Vuika. https://github.com/DenisVuyka/developing-with-angular/tree/master/angular/plugins
– As busy as a bee — lazy loading in the Angular CLI, David Herges https://blog.angularindepth.com/as-busy-as-a-bee-lazy-loading-in-the-angular-cli-d2812141637f … how does lazy loading in the Angular CLI work under the hood?
– How to load dynamic external components into Angular application? https://stackoverflow.com/questions/45503497/how-to-load-dynamic-external-components-into-angular-application/45506470#45506470
– Here is what you need to know about dynamic components in Angular, Maxim Koretskyi https://blog.angularindepth.com/here-is-what-you-need-to-know-about-dynamic-components-in-angular-ac1e96167f9e
– Modules are not what you think they are, Maxim Koretskyi https://youtu.be/pERhnBBae2k
– Extension mechanism demo https://github.com/maximusk/extension-mechanism-demo
– JIT Compiler needed with AOT Build for Dynamic Component. https://github.com/angular/angular/issues/20875

DDD Building Blocks – Value Objects

Value objects are one of the foundational building blocks of Domain-Driven Design. They allow modelling concepts of the Ubiquitous Language that describe, measure or quantify other things in the domain.

It is crucial to understand their characteristics and make more use of them whenever possible instead of trying to model every concept as an Entity or worse, having an Anemic Domain Model where logic related to a value leaks and spreads around services or client code. Values help separating concerns and they are easier to implement, test and maintain.

Examples of Value Objects in a domain could be colour, money, address, e-mail, coordinate, etc. Note that a concept can be an Entity in one domain model and a Value Object in other.

Value Object Characteristics

– Inmutable Conceptual Whole
– Describes, Measures or Quantifies an Entity
– Does not have a unique identity
– Structural equality
– Replaceability
– Side-Effect-Free Behaviour

Implementing Value Objects


Value objects are inmutable, do not implement setter methods for its properties, instead provide a constructor with all parameters necessary to initialize its intrinsic state. Parameters may be other objects used to derive internal properties on the new Value.

public class Money {
    private BigDecimal amount;
    private String currency;

    public Money(BigDecimal amount, String currency) {
        this.amount = amount;
        this.currency = currency;

    public BigDecimal getAmount() {
        return amount;

    public String getCurrency() {
        return currency;

You can find useful, especially for testing immutability, to provide a copy constructor.

    public Money(Money money) {
        this.amount = money.amount;
        this.currency = money.currency;

Use Guard Clauses in the constructor to ensure that the parameters passed are not only valid individually also as a whole initialize a valid Value.

public class FullName {
    private String firstName;
    private String lastName;

    public FullName(String firstName, String lastName) {
        if (firstName == null || lastName == null) {
            throw new IllegalArgumentException("First and last name are required.");

        this.firstName = firstName;
        this.lastName = lastName;


Value objects must not expose setter methods, but internally, we may use and delegate to them to simplify constructors.

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