If you build a whole (micro) service universe you will soon notice a need for events triggered from one system and sent to another.

This can - and is - often handled as Rest-api's on two systems that call each other. While this strategy may get the job done you have an issue with the two systems must know about the other - and this is quickly turns into a spaghettis tangled web of services that cannot really change without breaking something else.

A much better approach is to have the more generic of the two systems expose a generic callback - an event.

Such events best implemented by an open socket that can be used by the generic system to signal events - that in turn may cause action in the caller.

In .net SignalR is implementing this infrastructure.

In MDrivenServer we use SignalR to enable internet signaling and event sending to subscribers

realtimesubscribetoevent

realtimetriggerevent