External Triggers (Magic xpi 4.9)
External triggers do not run in a trigger worker thread in a managed engine. Instead, they run in their own processes and create flow invocation requests originating from other applications.
The HTTP trigger, for example, is a module running under a web server (IIS or Apache). The SOAP Web services trigger is another example of an external trigger invoked by the Systinet server.
External triggers are typically synchronous.
For more information about trigger architecture, see this image.