Events 2.0

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

I recommend using natural language and make small sentences, such as “Stock was depleted”, instead of newspaper-style shortcuts such as “Depleted stock”. In my experience, this greatly improves the communication and understanding.

https://verraes.net/2014/11/domain-events/

• Past tense
  • ItemWasAdded
  • ProductWasOrdered
  • InvoiceEmailWasChanged
  • ToggleWasSwitchedOff
• Noun(s) + verb
  • Mutated thing + was + action
• Use Domain Language
  • Or Ubiquitous language

Issues: granularity

TL;DR

It's tricky

Small

• AddressWasChanged
• EmailWasChanged
• FirstNameWasChanged

Big

• ProfileWasChanged

• 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

Heuristics for grouping

• significance
• subscribers
• mutuality

Might help:

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

https://open.spotify.com/episode/7eXMu7rPTBqbylf5E9LFOm?si=e197d401546b4c1a

Essential details

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

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

weak schema

vs

strict schema

Must read

https://leanpub.com/esversioning

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

domain events vs integration events

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

Questions?