Event-carried state transfer (ECST)

Principle

Messages contain the state associated with the event in such a way that they don’t need to contact the source system in order to do further work.

Description

This principle helps to avoid query-back (“skinny”) messages, which contain too little information to be useful. Messages should be enriched (“fat”) with enough data to provide all the details about the event not the domain object that it’s related to.

Consequences

• A message does not contain the full state of impacted entities, rather just the data associated with the event.
• Where a consumer requires the latest state, there must also be guaranteed order of delivery. (E.g. Messages may arrive out of order resulting in processing using stale data)

Example

A message that is sent after the domain event of a vehicle’s registration number being updated, would contain enough information for the receiver. * Skinny: “reg updated, here’s an ARN” * Too fat: “reg updated, here’s the whole agreement, dealer information and information about the vehicle which happens to include a reg” * Correct: “reg updated, here’s an ARN as a unique identifier, here’s the registration number that it was changed to”

References

See for reference stateful-event-pattern and event-driven systems. In domain-driven design, enriched messages are considered conducive to the autonomy of teams.