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Eight concurrent workers sharing one idempotency key turned a $49 order into $392 of real charges on PostgreSQL 17.6, and adding a FOR UPDATE row lock changed nothing at all. This walks through what actually prevents the double charge, measured at every step, and what Stripe's and Shopify's own documentation says rather than what gets repeated about them.

A client-side router watches the URL, matches it against patterns, and calls a handler. Build one in 110 lines — params, wildcards, back/forward — and see the four decisions every router makes for you.