[ivan_gammel]

I‘d pick neither and just let the system in possession of data send with the event only the part of data it owns (i.e. something in between fat and thin). Saves API call back, the body doesn’t have to be fully deserialized, so no format coupling, the rest can be fetched from other services on demand (coherent state though is not guaranteed, but that’s usually not critical with well designed bounded contexts).

[SideburnsOfDoom]

> just let the system in possession of data send with the event only the part of data it owns (i.e. something in between fat and thin).

Is that really different from a fat event?

[ivan_gammel]

Yes. The owned part of saga can be as small as an acknowledgment of something that happened. Basically, you do not create the pattern of fat event in your architecture and then stick to it, instead you send both fat, thin and in between depending on context.