Two styles of event
When a service publishes an event, it picks how much to include. The two ends of that choice are event notification and event carried state transfer.
Event notification
A notification is a thin announcement that something happened, such as an order was placed with just an order id. The consumer that cares must call back to the source to fetch details.
- Small payloads keep the bus light.
- Fresh reads because the consumer fetches current data.
- Tighter runtime coupling since consumers depend on the source being reachable.
Event carried state transfer
Here the event includes the full data the consumer needs, such as the whole order with line items. Consumers store their own copy and never call back.
- No callback so the source can be offline later.
- Looser runtime coupling at the cost of duplicated data.
- Staleness risk because copies can drift until the next event.
Choosing
Use notification when data changes fast or is large. Use carried state when you want consumers to keep working even when the source is down.
Key idea
Notification trades small payloads for runtime coupling, while carried state trades duplication for independence.