Workspace microservice can say to Channels microservice that a new member was added to a workspace.
Workspace microservice can say to Channels microservice that a member was removed from a workspace.
Messages microservice can say to Channels microservice that a message was added.
Channel microservice can send a system message to Message microservice
Channel microservice sends notification preferences updates to Notification microservice
Channel microservice sends "mark channel as read" update to Notification microservice