Decoupling with Domain Events

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Decoupling with Domain Events

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Steve Smith

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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!


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


Application EventsPage Load, Button Click, Window Scroll

System EventsRestart, Backup Completed

Domain EventsAppointment Confirmed, Checkout Completed, Analysis Finished

Kinds of Events


Given a customer has created an orderWhen the customer completes their purchaseThen the customer receives an email confirmation

An example scenario


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

An example solution


So far, so good


Given a customer has created an orderWhen the customer completes their purchaseThen 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 fulfilledAnd if not a different message is sent to the customer

More requirements


OrderServiceCheckout(Order order)oAttempt to process paymento If payment fails, send notification email; exitoSave pending order in databaseoConfirm inventory is availableo If insufficient email, send notification email; exitoSend successful order confirmation email

An example solution


Complexity is growing

Must change with each new requirementOpen/Closed Principle

Checkout’s responsibilities are growingSingle Responsibility Principle

Potentially different abstraction levels (emails, order processing rules)

Analysis


OrderService Checkout(Order order) - Dispatch new OrderCompletedEvent(order)

Event Handlers:OrderPaymentHandlerOrderInventoryHandlerOrderNotificationHandler

An event-driven solution


An EventSomething 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


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


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

http://martinfowler.com/eaaDev/EventSourcing.html


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


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



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



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



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)


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

Implementations


Demonstration

Implementing Domain Events


Pluralsight http://bit.ly/PS-ssmithDomain 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/

ImplementationsUdi 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


http://martinfowler.com/eaaDev/DomainEvent.html

https://domainlanguage.com/newsletter/2010-03/

https://lostechies.com/jimmybogard/2010/04/08/strengthening-your-domain-domain-events/

https://lostechies.com/jimmybogard/2010/04/08/strengthening-your-domain-domain-events/

http://www.udidahan.com/2009/06/14/domain-events-salvation/

https://lostechies.com/jimmybogard/2014/05/13/a-better-domain-events-pattern/

http://www.jayway.com/2013/06/20/dont-publish-domain-events-return-them/


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Steve Smith