How would you implement idempotency-key based duplicate request detection in a Spring REST API?

The client generates a unique key - typically a UUID - once per logical action, and sends it as a header (for example, Idempotency-Key) on the request, reusing the same key if it needs to retry.

@Component
public class IdempotencyInterceptor implements HandlerInterceptor {
    private final StringRedisTemplate redis;

    public boolean preHandle(HttpServletRequest req, HttpServletResponse res, Object handler) {
        String key = req.getHeader("Idempotency-Key");
        if (key == null) return true;
        Boolean isNew = redis.opsForValue()
            .setIfAbsent("idem:" + key, "processing", Duration.ofMinutes(10));
        if (Boolean.FALSE.equals(isNew)) {
            res.setStatus(409); // already seen - reject or return cached result
            return false;
        }
        return true;
    }
}

A Redis SETNX-style atomic check-and-set is a natural fit: it's fast, and the built-in expiry means stale keys are cleaned up automatically without a background job. For stronger guarantees than "reject the duplicate," the handler can instead store the first request's actual response body against the key once processing completes, so a genuine retry (not just a rejected duplicate) gets back the exact same result rather than an error - which matters for clients that legitimately need to know the outcome of a request they're not sure succeeded.

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