Push Notification Fan-Out at Scale

Mobile apps rarely fall over because of raw traffic — they fall over at the fan-out layer, when one campaign needs to reach millions of devices through a push gateway that enforces a hard per-connection rate limit. This simulator models that exact bottleneck: a dispatcher splits a campaign across a configurable number of worker shards, each throttled to a realistic APNs/FCM-style messages-per-second cap, while a tunable failure rate re-queues a share of every batch as retries. Watch the backlog bars drain — or stall under retry pressure — while live readouts track delivered count, current backlog, effective throughput and a formula-driven ETA, making the difference between horizontal fan-out (more shards) and retry-storm risk (rising failure rate) directly visible.