Lesson 5 showed what the board holds — hypotheses at several levels, linked bottom-up and top-down. This lesson opens control: the part deciding, cycle by cycle, which of the many things a blackboard system could do, it actually does next.
Whenever a hypothesis changes, every knowledge source (KS) whose trigger condition now matches gets recorded as a KSAR — a knowledge-source activation record: “KS X could fire, targeting hypothesis Y, if scheduled.” Unlike lesson 3’s production system, a KSAR firing doesn’t happen immediately — it’s placed on an agenda, a queue of pending, not-yet-executed opportunities. Because dozens of KSARs can be pending at once and only one (or a few) can run per cycle, the scheduler needs a way to rank them. That ranking is focus of attention: where the system’s limited effort goes right now.
A simple scoring heuristic — call it priority = credibility ÷ cost — favors KSARs that are both likely to be right and cheap to try, over KSARs that are expensive long shots. Three KSARs are currently pending:
| KSAR | What it would do | Credibility | Cost |
|---|---|---|---|
| K1 | Extend the confident “seven” island upward into a phrase hypothesis | 0.9 | 2 |
| K2 | Re-examine the weak, noisy segment data at the very start of the utterance | 0.4 | 5 |
| K3 | Try an unlikely competing word hypothesis (“heaven” instead of “seven”) | 0.3 | 1 |
Score each KSAR with priority = credibility ÷ cost, and report the highest score — that KSAR is the one the scheduler runs next.
(One more piece worth knowing before you compute: Hearsay-II’s successor architecture, BB1, took this idea one level further — it put the scheduling knowledge itself on a second, meta-level control blackboard, so the strategy for choosing what to do next could be inspected, reasoned about, and revised by knowledge sources, exactly like any other hypothesis. Control stopped being hard-coded plumbing and became a first-class object the system reasoned about — the architectural move lesson 3 flagged as “conflict resolution is not plumbing, it’s behavior,” now given its own blackboard.)