Auction (eBay)

01What makes a bid the new high bid?

A bid is accepted only if it is strictly higher than the current high; equal or lower bids get a clean rejection that carries the current price and the minimum next bid.

02Do we support max bids, or only direct bids?

Proxy bidding: a bidder sets a private ceiling and the system auto-raises on their behalf by the minimum increment; two ceilings resolve deterministically — the earlier one wins a tie.

03Who ends the auction — a clock on whose machine?

The server ends it: one close worker performs the open → closed transition exactly once, names the winner, and every bid arriving after that gets a definitive no.

04What happens to last-second snipers?

Policy decides: the design supports both a hard close and an auto-extend window (a bid inside the final N minutes pushes the close out, capped), switchable per marketplace.

05How does the outbid loser find out?

Promptly and asynchronously: an outbid event fans out to push/email right after the accepting write commits — never inside it.

06A buyer disputes the final price — what can we show?

The full bid history: every bid attempt is an append-only ledger row with server timestamp and outcome, so any close can be replayed and audited in a dispute.

01How fast must a bid answer in the last ten seconds?

Tens of milliseconds to accept or reject, even in the closing seconds — the decision is one conditional write, not a workflow.

02Two bidders race — can the price ever go backwards?

The price never goes backwards and never forks: the accept is an atomic compare on the current-high row, so exactly one bidder holds the high at any instant.

03The close timer fires twice — do we get two winners?

Close fires exactly once: the open → closed transition is guarded by current state, so a duplicate or retried timer fire is a no-op.

04How stale can the price on a watcher’s screen be?

Watchers see the price within seconds: accepted bids push versioned updates, and the monotonic version lets clients drop out-of-order events.

05The hottest listing melts — does the site go with it?

One hot listing degrades only itself: contention is confined to that row, retries back off with jitter, and the rest of the site never notices.