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This might be best done using triggers. I've had good experiences with the Hair Trigger gem for managing these in rails.

Trigger would work. But going the pg_notify approach could allow for asynchronous updating whereas the trigger approach could slow writes.

Real-time means latency is predictably within some deadline. But the system he describes is returning wrong answers quickly rather than invalidating stale cache hits and recomputing right answers too slowly.

You could do something similar with built-in ActiveRecord callbacks as well.