Every path or flow stops in a terminus event. Those are the filled circles. Often, we call them end event, too!

The “error path” or “failure track” is the lower path going the the failure terminus.

The “happy path” or “success track” is the straight path from start to the terminus named success.

If anything here did fail in “validate omniauth”, all other steps in the chain would be skipped as the flow would follow the path leading to the failure terminus.

The adapter is where authentication, policy checks, and eventually your domain logic happen. All termini of the protocol’s activity are standardized end events - that’s how protocol and adapter communicate.