Bidify

The idea

Listings in the Shopify ecosystem compete for attention through opaque algorithms. Bidify makes it transparent: anyone can list instantly with no account, and position is set only by confirmed bids, with ties going to whoever reached the amount first.

Under the hood

• Concurrency-safe ranking: a Postgres advisory lock per category is taken before row locks, which removed a deadlock between two confirmations in the same category. Ranks are recomputed in one transaction.
• Pay first, rank second: only a confirmed payment changes position. Webhooks are verified and deduplicated, and for the provider without signed webhooks the server re-fetches the payment result.
• Three payment providers, picked by the visitor's location.
• SSRF-safe link previews: user-submitted URLs are resolved and private ranges are blocked before anything is fetched or screenshotted.

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