Events

What is an Event?

In DDD meaning

Domain Event: a data structure representing one domain change

http://blog.sapiensworks.com/post/2015/05/25/DDD-Concepts-As-One-Liners

An event is something that has happened in the past

https://docs.microsoft.com/en-us/dotnet/standard/microservices-architecture/microservice-ddd-cqrs-patterns/domain-events-design-implementation

• It's a fact
• It's a struct
• It's a mutation

Issues: naming

• Past tense
  • ItemAdded
  • ProductOrdered
  • InvoiceEmailChanged
• Noun(s) + verb
  • Mutated thing + action
• Use Domain Language
  • Or Ubiquitous language

Issues: granularity

TL;DR

It's tricky

Small

• AddressChanged
• EmailChanged
• FirstNameChanged

Big

• ProfileChanged

• More flexibility
• Less "ifs" in event handlers but more "ifs" in command handlers
• Better language
• Possibly many events to handle after a single request

• Requires routing for handling
• Complex actions = heavy methods
• Fragile for changes and nulls
• Redundant data on small changes

Might help: http://blog.sapiensworks.com/post/2014/03/27/Domain-Event-Design-Principles.aspx

Essential details

• every related aggregate id
• timestamp
• version number
• mutated data

Might help: http://verraes.net/2014/11/domain-events/

Bonus thoughts

• You don't need DDD to start emitting events
• You don't need CQRS to start emitting events
• You don't need handlers to start emitting events

• audit log of changes
• force to think about domain
  • language
  • transactions
  • triggers

Questions?