Examples

From the explanation to the engine

Pseudocode using the Mainframe Engine API. Application helpers are illustrative, not a complete implementation. Click any row to follow it in the diagram.

1. WHEN a buyer submits an orderasync fn purchase(order) {
2. Assign a request ID; reuse it on retry. entry = encode(order.with_stable_request_id());
3. APPEND the purchase; wait for confirmation. p = stack.propose(entry).await?;
4. FOR EACH saved entry, in log order// Engine::apply(entry, ctx), called in log order
5. IF its request ID already has a result, reuse it. if results.contains(order.id) { /* keep result */ }
6. ELSE IF stock is sufficient, record the sale. else if stock >= order.quantity { record_sale(order); }
7. ELSE record an out-of-stock result. else { save_result(order.id, OutOfStock); }
8. Save stock, result, and progress together. commit_atomically(stock, results, ctx.position.next());
9. RETURN the result after this service applies it. stack.sync_and_apply(p.next()).await?; return results[order.id]; }

Your engine checks state during apply and saves state, request results, and progress atomically. The handler waits with sync_and_apply(p.next()) before reading the result; p.next() includes entry p. Apply callbacks must tolerate retries after failure or cancellation.

The receipt does not need to wait for every replica. Payment and shipping require separate handling.