Events on DRUGs
Disclaimer
What is an Event?
In DDD meaning
Domain Event: a data structure representing one domain change
An event is something that has happened in the past
- 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.
- Past tense
- ItemWasAdded
- ProductWasOrdered
- InvoiceEmailWasChanged
- ToggleWasSwitchedOff
- Noun(s) + verb
- Mutated thing + was + action
- Use Domain 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 struct changes and nulls
- Redundant data on small changes
Heuristics for grouping
- significance
- subscribers
- mutuality
Essential details
- every related aggregate id
- timestamp
- struct version number
- mutated data
Might help: http://verraes.net/2014/11/domain-events/
weak schema
vs
strict schema
Must read
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
- You don't need to save emitted events
- audit log of changes
- force to think about domain
- language
- transactions
- triggers
domain events vs integration events
GDPR
Last but not least
What we already know is not boring
Number of software developers double every five years
Questions?