Modelling your Domain in Sequent

A tour of the Post Aggregate

The app we generated in Getting Started comes with an example Post aggregate. In this guide we will take a quick look at Sequent’s directory structure, go over some of the concepts used in Sequent by expanding on Post, and create our very own Author aggregate.

Directory structure

Let’s have a look at the general directory structure of a generated Sequent project. If something doesn’t make sense right away, bear with us because we will walk through these folders one by one in the rest of this article.

app/           # Non-domain application logic
  projectors/  # Subscribe to events and write to records
  records/     # Ephemeral view tables (ActiveRecord models)
config/        # Configurations to glue everything together
db/            # Database management and configuration
lib/           # Contains your domain logic
  post/        # Aggregate roots define the namespaces
spec/          # Tests for your application

Zooming in on the lib folder reveals the domain and most important concepts of the app:

post/                      # Files are grouped by aggregate root
  commands.rb              # All post command go here
  events.rb                # All post events go here
  post_command_handler.rb  # Subscribes to post commands and dispatches post events
  post.rb                  # The aggregate root
post.rb                    # Requires the entire aggregate root

Adding a command

Changes to state start by executing a command. Commands are quite simple classes containing some attributes and attribute validations. Looking at lib/post/commands.rb, we have one command:

class AddPost < Sequent::Command
  attrs author: String, title: String, content: String
  validates_presence_of :author, :title, :content
end

Let’s add a PublishPost command to take our Post from draft to published. The command will look like this:

class PublishPost < Sequent::Command
  attrs publication_date: DateTime
  validates_presence_of :publication_date
end

We only need the publication_date attribute. Commands always target an aggregate, so we already know what to change by its aggregate_id. We could set a publish flag, but the event already communicates this intent.

Learn all about commands in the Command Reference Guide.

Handling our new command

The PostCommandHandler in lib/post/post_command_handler.rb subscribes to Post commands and calls the domain (i.e. the Post aggregate root). We can see this happening for AddPost:

class PostCommandHandler < Sequent::CommandHandler
  on AddPost do |command|
    repository.add_aggregate Post.new(command)
  end
end

Because we are adding a new aggregate, add_aggregate is called with Post.new(command) as its argument. The actual business logic of what Post looks like is contained in the aggregate root.

You are free to define your own signature of the constructor. In the example we chose to pass the command as argument, but nothing prevents you to define it using the separate attributes.

We can add our own on block below the AddPost block in lib/post/post_command_handler.rb:

on PublishPost do |command|
  do_with_aggregate(command, Post) do |post|
    post.publish(command.publication_date)
  end
end

Sequent retrieves the post for us and we call the (to be defined) publish method on the returned Post instance.

Learn all about command handlers in the CommandHandler Reference Guide.

The Aggregate Root

In lib/post/post.rb we find the aggregate root. This class encapsulates your business logic. Events are applied to instances of Post to give it its current state. We can see here that create a new Post will apply multiple events. Besides PostAdded we’re also applying events to change the author, title and content. You might be tempted to group all those fields in one event, which can be a good idea if those fields always change together. We’re using multiple events to emphasise that a single command does not always correlate to a single event.

class Post < Sequent::AggregateRoot
  def initialize(command)
    super(command.aggregate_id)
    apply PostAdded
    apply PostAuthorChanged, author: command.author
    apply PostTitleChanged, title: command.title
    apply PostContentChanged, content: command.content
  end

  # ...
end

Let’s define how the domain should behave when receiving our new PublishPost command. Below the initialize method add:

def publish(publication_date)
  fail PostAlreadyPubishedError if @publication_date.any?
  apply PostPublished, publication_date: publication_date
end

In Sequent you execute / enforce your business rules in these methods before applying events.

Learn all about aggregate roots in the AggregateRoot Reference Guide.

Adding the event

In lib/post/post.rb we just applied the PostPublished event. We need to define this event in lib/post/events.rb:

class PostPublished < Sequent::Event
  attrs publication_date: Date
end

Events contain all the state changes on an AggregateRoot. These events are stored in the database in the event_records table as JSON.

Learn all about events in the Event Reference Guide.

Adding an Author

We have now gone through the generated example files.

Before we can add a Post we need to add an Author. In this guide, we will ‘upgrade’ Author to its own Aggregate Root. This means we need to add new files defining the Author Aggregate Root, and make some changes to the Post Commands and Events. i.e. using the author aggregate_id instead of an author String.

In lib/author/author.rb add:

class Author < Sequent::AggregateRoot
end

Let’s create the command to add an Author.

In lib/author/commands.rb add:

class AddAuthor < Sequent::Command
  attrs name: String, email: String
  validates_presence_of :name, :email
end

One of the things we need to do, is to check the uniqueness of the Author’s email. Since we only store the events in the event store, we can not simply add a unique constraint to ensure uniqueness. A common solution to this problem is to create yet another Aggregate responsible for maintaining all usernames. We will name this Aggregate Usernames. Since it needs to ensure uniqueness, there can be only one instance of this. In order to achieve that we create the Usernames class as a Singleton.

In lib/usernames/usernames.rb add:

class Usernames < Sequent::AggregateRoot
  class UsernameAlreadyRegistered < StandardError; end

  # We can generate and hardcode the UUID since there is only one instance
  ID = "85507d60-8645-4a8a-bdb8-3a9c86a0c635"

  def self.instance(id = ID)
    Sequent.configuration.aggregate_repository.load_aggregate(id)
  rescue Sequent::Core::AggregateRepository::AggregateNotFound
    usernames = Usernames.new(id)
    Sequent.aggregate_repository.add_aggregate(usernames)
    usernames
  end
end

We can now obtain the Usernames Aggregate by invoking Usernames.instance. Next thing we want to do is create the AuthorCommandHandler and add an Author. To ensure everything will work, we start by defining our tests.

In spec/lib/author/author_command_handler_spec.rb

require_relative '../../spec_helper'
require_relative '../../../lib/author'
require_relative '../../../lib/usernames'

describe AuthorCommandHandler do
  before :each do
    Sequent.configuration.command_handlers = [AuthorCommandHandler.new]
  end

  context AddAuthor do
    it 'creates a user when valid input'
    it 'fails if the username already exists'
    it 'ignores case in usernames'
  end
end

There might be more edge cases, but for now this is sufficient.

Let’s create the necessary classes in order to get the test to ‘green’.

We will stick to Sequent’s suggested directory structure, so we will end up with something like this:

blog.rb
lib/           # Contains your domain logic
  author.rb    # Requires all author/*.rb
  usernames.rb # Requires all usernames/*.rb
  author/      # Contains the author related domain classes
    author.rb
    events.rb
    command.rb
    author_command_handler.rb
  usernames/   # Contains the usernames related domain classes
    usernames.rb
    events.rb

The following files need to be created / edited:

1. Require the new Author and Usernames aggregates in blog.rb:

require_relative 'lib/author'
require_relative 'lib/usernames'

1. Add the new AuthorCommandHandler class in lib/author/author_command_handler.rb:

class AuthorCommandHandler < Sequent::CommandHandler

end

1. Add the new AddAuthor command in lib/author/commands.rb

class AddAuthor < Sequent::Command
  attrs name: String, email: String
  validates_presence_of :name, :email
end

1. Include the AuthorCommandHandler in config/initializers/sequent.rb

Sequent.configure do |config|
  # ...

  config.command_handlers = [
    PostCommandHandler,
    AuthorCommandHandler
  ].map(&:new)

  # ...
end

When we run the tests in spec/lib/author/author_command_handler_spec.rb, all are marked as Pending: Not yet implemented. Before we can go any further, we need to think about what kind of Events we are interested in. What do we want to know in this case? When registering our very first Author, it will not only create the Author, but also create our Usernames Aggregate to ensure uniqueness of the usernames.

The test will read something like:

When i add an Author for the first time
Then the Usernames registry is created
And the username is checked for uniqueness and added to the Usernames
And the Author is created with the given name and email

By leveraging Sequent’s test DSL we can modify the test we have created for this in spec/lib/author/author_command_handler_spec.rb as follows:

context AddAuthor do
  let(:user_aggregate_id) { Sequent.new_uuid }
  let(:email) { 'ben@sequent.io' }

  it "creates a user when valid input" do
    when_command AddAuthor.new(aggregate_id: user_aggregate_id, name: "Ben", email: email)
    then_events UsernamesCreated.new(aggregate_id: Usernames::ID, sequence_number: 1),
                UsernameAdded.new(aggregate_id: Usernames::ID, username: email, sequence_number: 2),
                AuthorCreated.new(aggregate_id: user_aggregate_id, sequence_number: 1),
                AuthorNameSet,
                AuthorEmailSet.new(aggregate_id: user_aggregate_id, email: email, sequence_number: 3)
  end
  it "fails if the username already exists"
  it "ignores case in usernames"
end

In Sequent (or other event sourcing libraries) you test your code by checking the applied events, and which order they were run in. In this case we modelled the AuthorNameSet and AuthorEmailSet as separate events, since they probably don’t change together.

In more comprehensive cases we can imagine triggering other events, e.g. when the email changes, a confirmation is sent. You should take these considerations into account when modelling your domain and defining your Events.

Let’s create the necessary code to make the test pass.

In lib/usernames/events.rb

class UsernamesCreated < Sequent::Event

end

class UsernameAdded < Sequent::Event

end

As you can see the events have no attributes yet. This is not necessary to make this test pass. Sequent only looks at the defined attributes and sets those as values on the event, in the event store. Therefore all attributes you want to include on an event, need to be explicitly declared.

In lib/usernames/usernames.rb

class Usernames < Sequent::AggregateRoot
  class UsernameAlreadyRegistered < StandardError; end

  # We can generate and hardcode the UUID since there is only one instance
  ID = "85507d60-8645-4a8a-bdb8-3a9c86a0c635"

  def self.instance(id = ID)
    Sequent.aggregate_repository.load_aggregate(id)
  rescue Sequent::Core::AggregateRepository::AggregateNotFound
    usernames = Usernames.new(id)
    Sequent.aggregate_repository.add_aggregate(usernames)
    usernames
  end

  def initialize(id)
    super(id)
    apply UsernamesCreated
  end

  def add(username)
    apply UsernameAdded, username: username
  end
end

In lib/author/events.rb

class AuthorCreated < Sequent::Event

end

class AuthorNameSet < Sequent::Event
  attrs name: String
end

class AuthorEmailSet < Sequent::Event
  attrs email: String
end

In lib/author/commands.rb

class AddAuthor < Sequent::Command
  attrs name: String, email: String
  validates_presence_of :name, :email
end

In lib/author/author.rb

class Author < Sequent::AggregateRoot
  def initialize(command)
    super(command.aggregate_id)
    apply AuthorCreated
    apply AuthorNameSet, name: command.name
    apply AuthorEmailSet, email: command.email
  end
end

In lib/author/author_command_handler.rb

class AuthorCommandHandler < Sequent::CommandHandler
  on AddAuthor do |command|
    Usernames.instance.add(command.email)
    repository.add_aggregate(Author.new(command))
  end
end

For the next test case we want to assert the following:

Given an Author with email 'ben@sequent.io'
When I try to add another author with email 'ben@sequent.io'
Then it should fail

This translate to the following rspec test:

it 'fails if the username already exists' do
  given_events UsernamesCreated.new(aggregate_id: Usernames::ID, sequence_number: 1),
    UsernameAdded.new(aggregate_id: Usernames::ID, username: email, sequence_number: 2)
  expect {
    when_command AddAuthor.new(
      aggregate_id: Sequent.new_uuid,
      name: 'kim',
      email: 'ben@sequent.io'
    )
  }.to raise_error Usernames::UsernameAlreadyRegistered
end

When we run this spec we get the following error message:

RuntimeError: cannot find aggregate type associated with creation event {UsernamesCreated: @aggregate_id=[85507d60-8645-4a8a-bdb8-3a9c86a0c635], @sequence_number=[1], @created_at=[2018-09-21T14:17:23+02:00]}, did you include an event handler in your aggregate for this event?

Sequent requires us to define an event handler in the Aggregate for at least the creation event, otherwise Sequent is not able to find an Aggregate in the repository.

So let’s change our aggregates to satisfy this demand.

Add to Usernames in lib/usernames/usernames.rb

class Usernames < Sequent::AggregateRoot
  ...

  on UsernamesCreated do

  end
end

Add to Author in lib/author/author.rb

class Author < Sequent::AggregateRoot
  ...

  on AuthorCreated do

  end
end

Running the spec again results in the following error: expected Usernames::UsernameAlreadyRegistered but nothing was raised

This is as expected, since we didn’t implement anything. Let’s start by enforcing uniqueness.

Change the Usernames aggregate as follows:

class Usernames < Sequent::AggregateRoot
  def add(username)
    fail UsernameAlreadyRegistered if @usernames.include?(username)

    apply UsernameAdded, username: username
  end

  on UsernamesCreated do
    @usernames = Set.new
  end

  on UsernameAdded do |event|
    @usernames << event.username
  end
end

The UsernameAdded event in lib/usernames/events.rb needs to contain the username attribute to make the test pass.

class UsernameAdded < Sequent::Event
  attrs username: String
end

The event handlers UsernamesCreated and UsernameAdded will keep track of the current usernames in a Set. Whenever a new name is being added, we first check if the name does not yet exist. If it does not exist, a new event is applied.

Let’s finish up by implementing our last test to check that string case is ignored when registering an Author.

The test:

it 'ignores case in usernames' do
  given_events UsernamesCreated.new(aggregate_id: Usernames::ID, sequence_number: 1),
    UsernameAdded.new(aggregate_id: Usernames::ID, username: email, sequence_number: 2)
  expect {
    when_command AddAuthor.new(
      aggregate_id: Sequent.new_uuid,
      name: 'kim',
      email: 'BeN@SeQuEnT.io'
    )
  }.to raise_error Usernames::UsernameAlreadyRegistered
end

We change our Usernames class to satisfy this requirement as follows:

class Usernames < Sequent::AggregateRoot
  def add(username)
    fail UsernameAlreadyRegistered if @usernames.include?(username.downcase)

    apply UsernameAdded, username: username
  end

  on UsernameAdded do |event|
    @usernames << event.username.downcase
  end
end

Adding a Post using the new Author Aggregate Root

The last thing we need to do to successfully add a post, is refactor out Author name, and instead use the Author aggregate_id. This requires a few changes:

1. Change the passed command values and event attributes in spec/lib/post/post_command_handler_spec.rb:

let(:aggregate_id) { Sequent.new_uuid }
let(:author_aggregate_id) { Sequent.new_uuid }

# ...

it 'creates a post' do
  when_command AddPost.new(aggregate_id: aggregate_id, author_aggregate_id: author_aggregate_id, title: 'My first blogpost', content: 'Hello World!')
  then_events(
    PostAdded.new(aggregate_id: aggregate_id, sequence_number: 1),
    PostAuthorChanged.new(aggregate_id: aggregate_id, sequence_number: 2, author_aggregate_id: author_aggregate_id),
    PostTitleChanged,
    PostContentChanged
  )
end

1. Update the attribute presence validation of AddPost in lib/post/commands.rb:

class AddPost < Sequent::Command
  attrs author_aggregate_id: String, title: String, content: String
  validates_presence_of :author_aggregate_id, :title, :content
end

1. Update the initialize method and on PostAuthorChanged handler in lib/post/post.rb

class Post < Sequent::AggregateRoot
  def initialize(command)
    super(command.aggregate_id)
    apply PostAdded
    apply PostAuthorChanged, author_aggregate_id: command.author_aggregate_id
    apply PostTitleChanged, title: command.title
    apply PostContentChanged, content: command.content
  end

  # omitted ...
  
  on PostAuthorChanged do |event|
    @author_aggregate_id = event.author_aggregate_id
  end
end

When running the tests, they should now all pass.

Summary

In this guide we learned:

1. Explored the generated Post AggregateRoot.
2. Added new functionality to publish a Post
3. Added a new Aggregate Author and Usernames and showed how Aggregates can depend on each other
4. Explored how to add tests in Sequent in order to test the domain

In this guide we mainly focussed on the domain. In the next guide we will take it a step further and see how can actually build a web application that our Authors can use. We will learn how to initialize and set up Sequent with Sinatra, learn about Projectors and see how Sequent deals with migrations.