Given at DogFoodCon 2016 in Columbus, Ohio Domain Events, a Domain-Driven Design (DDD) pattern, provide a way to decouple your code. Rather than hard-coding a series of steps that must be completed in response to a particular action a user takes, that action can raise an event. Handlers for an event can be added without changing the code responsible for raising the event, resulting in a more extensible design. Coupled with SignalR or push notifications in the UI layer, domain events can trigger live updates, even to web-based applications and mobile apps.

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Decoupling with
Domain Events
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Steve Smith

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Pluralsight Courses
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Repositories ✓
Factories ✓
Services ✓
Entities ✓
Value Objects ✓
Aggregates ✓
Oh yeah, Domain Events
Usually the last kid picked from
the DDD patterns I’m right
here!

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Domain Events not covered in original DDD book (2004)
Covered by Martin Fowler in 2005
Evans published article on them in 2010
Domain-Driven Design

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Application Events
Page Load, Button Click, Window Scroll
System Events
Restart, Backup Completed
Domain Events
Appointment Confirmed, Checkout Completed, Analysis Finished
Kinds of Events

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Given a customer has created an order
When the customer completes their purchase
Then the customer receives an email confirmation
An example scenario

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OrderService
Checkout(Order order)
- Save pending order in database
- Send confirmation email
An example solution

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So far, so good

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Given a customer has created an order
When the customer completes their purchase
Then the customer’s card is charged
And if it fails, send a different message to the customer
Then inventory is checked to ensure the order can be fulfilled
And if not a different message is sent to the customer
More requirements

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OrderService
Checkout(Order order)
oAttempt to process payment
oIf payment fails, send notification email; exit
oSave pending order in database
oConfirm inventory is available
oIf insufficient email, send notification email; exit
oSend successful order confirmation email
An example solution

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Complexity is growing
Must change with each new requirement
Open/Closed Principle
Checkout’s responsibilities are growing
Single Responsibility Principle
Potentially different abstraction levels (emails, order processing rules)
Analysis

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OrderService
Checkout(Order order)
- Dispatch new OrderCompletedEvent(order)
Event Handlers:
OrderPaymentHandler
OrderInventoryHandler
OrderNotificationHandler
An event-driven solution

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An Event
Something that happened
…that other parts of the application may need to know about
It’s a message
…which should be immutable, since it represents the past
Usually asynchronous
…especially across process boundaries
Event-Driven Programming

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Model something that happens which is of interest to a domain expert
May lead to (or result from) a state change in a domain object
Are part of the domain model
Are usually handled synchronously within the application (but may themselves raise
events outside of the application)
Domain Events

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Related concept – not required to take advantage of domain events
Store sequences of events rather than directly mutating object state
Determine current state by replaying events
Learn more: http://martinfowler.com/eaaDev/EventSourcing.html
Event Sourcing

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An event is raised
Handlers within the current process handle the event
If external applications need to be notified, specific handlers can adapt and send
events to these applications as well
If unknown or future external applications will need to respond to events, a service
bus can be implemented
Event Processing

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How do you add logic to entities that affects multiple entities?
Example:
When a customer’s total amount purchased exceeds $1000, notify a salesperson to
contact them.
Common Scenario

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How do you add logic to entities that affects multiple entities?
Don’t solve this by using Dependency Injection on your entities. You should be able to
easily instantiate entities without dependencies.
Avoid: Injecting Dependencies into Entities

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How do you add logic to entities that affects multiple entities?
Don’t move logic that belongs within an entity into a service just because other
entities are interested in what’s happening. This leads to the anemic domain model
antipattern.
Avoid: Shifting Entity logic to Services

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How do you add logic to entities that affects multiple entities?
Raise a domain event to represent the action that took place.
Handle the domain event in a handler that collaborates with other entities.
Alternately, have the Aggregate Root register for events raised by members of its
Aggregate.
Use Domain Events (and perhaps Aggregates)

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C# events
Static DomainEvents helper (Udi Dahan, 2009)
Return, don’t publish, domain events (2013)
Entity-specific event log (Jimmy Bogard, 2014)
Implementations

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Demonstration
Implementing
Domain Events

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Pluralsight http://bit.ly/PS-ssmith
Domain Events – Fowler http://martinfowler.com/eaaDev/DomainEvent.html
Domain Events – Evans https://domainlanguage.com/newsletter/2010-03/
Domain Events – Bogard https://lostechies.com/jimmybogard/2010/04/08/strengthening-your-
domain-domain-events/
Implementations
Udi Dahan – http://www.udidahan.com/2009/06/14/domain-events-salvation/
Jimmy Bogard – https://lostechies.com/jimmybogard/2014/05/13/a-better-domain-events-pattern/
Jay Kronquist - http://www.jayway.com/2013/06/20/dont-publish-domain-events-return-them/
References

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