This was a late trick in Keykos; a pattern that often uses a particular implementation.
It is one of the simplest Keykos objects. (Its domain's address space has no writable RAM. Its per instance cost is one domain—three nodes.)
X asks Y for something whenever it might become available. Another facet, Z, to Y that says: “now is the time”. In other words Y should respond to X now perhaps with a exceptional value so that X may take unusual steps in this case. At least X is in the best context to know what to do in these cases. Sometimes X need not be cognizant of such events. Of course X is not in a position to invoke Z when needed for X is waiting on a response from Y, but a friend of X can do so. The Keykos timers were of this form. Some I/O keys in some ports of Keykos were like this when the hardware allowed such behavior. The Keykos join object was almost a pure version of this pattern.