Using Interjob communication (IJC)

This is a coding example working on Caché 2018.1.3 and IRIS 2020.2 
It will not be kept in sync with new versions 
It is also NOT serviced by InterSystems Support !

Since the very beginning of Caché and most of its predecessors
there exists a way of in-memory communication of processes.
It's called InterJob Communication - IJC.

I have the feeling it's forgotten by most developers now.
here is the documentation: Communication Between Caché Processes

Earlier I've written about command pipes.
This is the internal variant of a PIPE. 
 
To make this more tangible and visible for you I prepared a small example
 
The scenario is to run a monitoring process that receives
input from an unknown number of sensors. 
(Could be Lab equipment or similar.) 
The monitor should not poll his sensors nor run
in a hang loop to scan a common ^global 
and work independently of any disk access. 
 
To try it log into a terminal 
    DO ##class(IJC.Demo).%Start() 
The Highlander principle applies for this example: There can only be one 
  
Next open a new terminal and run
    DO ##class(IJC.Demo).Sensor("mytext")
and see what happens.

It's clear that for real applications:
- the monitor might run in a background tasks
- the sensors will do something useful.

But this reduces visibility.