The article dissects a common interview question about automatically canceling unpaid orders, explains why a simple cron job fails at massive scale, and presents three robust designs—Redis expiration, Redis ZSet polling, and MQ/time‑wheel approaches—plus pitfalls and a ready‑to‑use answer template.
The article explains the concept of delayed tasks versus scheduled tasks and evaluates several backend solutions—including database polling with Quartz, JDK DelayQueue, Netty's HashedWheelTimer, Redis ZSET, Redis key‑space notifications, and RabbitMQ delayed queues—detailing their implementations, code samples, advantages, and drawbacks for automatically cancelling orders that remain unpaid for a set period.
The article dissects a common interview question about automatically canceling unpaid orders after 30 minutes, explains why naïve cron‑based scans are unsuitable for tens of millions of rows, and presents three progressively robust solutions using Redis expiration, Redis ZSet polling, and message‑queue or time‑wheel architectures.
