What is the Protocol State Tracker, and how does it drive a recurring workflow like periodic interest payments?

The Protocol State Tracker watches protocol instances for state changes and, when one reaches a "ready" state, invokes the permission that does the real work — creating a self-repeating cycle.

For periodic payments the states are modelled explicitly, e.g. waiting_for_payment and ready_for_payment. The cycle runs like this:

1. A time-based obligation fires at the payout deadline and flips the state to ready_for_payment.
2. The Protocol State Tracker monitors instances and notices the change.
3. It invokes a permission that executes the business logic — distribute the interest, update the next payout date, and transition back to waiting_for_payment.

Then the loop repeats. This cyclical pattern gives recurring payments with full control and traceability. It is resilient because the invocation is idempotent (only the first valid call proceeds) — so designers just need to keep the tracker running via ordinary monitoring and alerting.