ADR 0119 — Delivery policy is ordered or replaceable

Context

An event bus invokes callbacks; it is not a queue. The application snapshot sink already has a different and useful rule: one write plus one replaceable pending revision. Applications otherwise copy count limits, byte accounting and close semantics while calling both behaviours “events”.

Decision

createBoundedChannel offers values without a hidden writer queue. ordered retains every accepted value in order and refuses count/byte overflow. latest retains exactly one pending replaceable value and reports each coalescing event. The caller supplies sizeOf; the channel accounts the exact retained byte value it returns.

Only one next() may be parked. Close either drains accepted values or discards them explicitly; abort uses discard semantics; failure rejects the reader and all later reads. Every offer reports delivered, queued, coalesced or a reasoned refused result.

createCreditWindow is a separate exact byte-permission primitive. Its leases cannot overdraw the window and replenish once. Credit is not a durable acknowledgement.

The application snapshot sink reuses the same latest-value channel mechanics while retaining its revision filter and public status contract.

Consequences

• Slow-reader policy is visible at construction and at every offer.
• Memory and waiter counts are finite; there is no implicit loss in ordered mode and no false history claim in latest mode.
• Protocol adapters decide how refusal, gaps and resynchronisation appear on their wire.
• No broker, persistence, distributed acknowledgement or boot-epoch policy enters the application core.