Idempotency Keys: The One API Pattern That Prevents Duplicate Payments (and Wors

<h1> Idempotency Keys: The API Pattern That Saves You From Duplicate Payments and Phantom Records </h1> <p>Your user clicks "Pay" and nothing happens. So they click again. Your

<p>A scheduled post fires. The request to publish it goes out. The network hangs. After 30 seconds, our client times out. We retry. The tweet publishes. Then the original request

<p>Two things quietly decide whether a small application survives being used: whether its reads<br /> are indexed, and whether its writes can be repeated safely. They are usually

<p>In distributed systems, failure is not an exception—it’s the default.</p> <p>Network calls fail. Services timeout. APIs return 500s. The real question isn’t <em>“Will things

<p>Your HTTP client raises <code>ETIMEDOUT</code> on <code>POST /charges</code>. Something has to be written to the database, the status column has two useful values, and so the

<p>In this guide, we explore system. Your transaction is pending. A timeout occurs. Now you're staring at a screen wondering if you just paid $1,000 twice or if your money

<p>A user clicks “Pay”, the network drops, the client retries… and your backend creates <strong>two</strong> payments.</p> <p>This isn’t a “frontend bug”. Retries happen

<h1> API layer pattern (HTTP) </h1> <h2> 1) Contract </h2> <ul> <li>Require <code>Idempotency-Key</code> on <strong>POST</strong> that creates resources (optional on GET, safe