Client systems could benefit from an event notifier system

[Jason Stephenson]

Systems built on OpenSRF could benefit from an event notifier system, such as that described by the Observer pattern: https://en.wikipedia.org/wiki/Observer_pattern. Such a system would allow interested modules to register their interest in certain events, and then they would be notified when such events occurred so that they could act upon them.

For example, in the case of Evergreen, circulation drones could register their interest in various circulation-related settings with a settings handler service. They would in turn be notified when these settings changed and would consequently update their local values. This would allow the drones to keep their view of the current state of the settings up to date without requiring database hits for every single circulation.