John Frizelle Wojciech Trocki I'm not sure if Wei Li wrote this down, but he mentioned in passing there was some discussion about a 'messaging' api in the Core SDK.

I believe the idea was any service SDK could send/publish a message e.g. 'METRIC::something::somevalue' using a Core api, and another service SDK could subscribe to those types of messages and do something.
This may be more about inter service dependencies, but sounds like it has an impact on how service SDKs depend on the Core too.