FCM主题通知技术疑问：4周过期通知的设备接收问题

Let’s break down each of your scenarios clearly, based on how Firebase Cloud Messaging (FCM) handles topic notifications in practice:

Scenario 1: Non-existent topic with no subscribers, device subscribes 1 hour later #

• Result: The device will not receive the notification.
• Why? FCM topic notifications are built for immediate delivery to the active subscriber pool at the time of sending. If the topic has zero subscribers (or doesn’t exist at all) when you send the message, FCM discards the notification right away—there’s no system to store it and deliver it to future subscribers, even if the topic is created later. The 4-week expiration only applies to messages sent to active subscribers who are offline; FCM will hold those until the device comes back online or the expiration window closes.

Scenario 2: Topic existed (no current subscribers), former subscriber re-subscribes 1 hour later #

• Result: The device will not receive the notification.
• Why? Same core logic applies here. When you sent the notification, the topic had no active subscribers, so FCM discarded it immediately. Re-subscribing to the topic later doesn’t trigger delivery of past messages—FCM doesn’t retain topic notifications for future subscriber additions, regardless of the expiration timeline.

Scenario 3: Existing subscribers received the notification, new device subscribes 1 hour later #

• Result: The new device will not receive the notification.
• Why? Topic notifications target the set of devices subscribed to the topic exactly when the message is sent. Any devices that subscribe after the message is dispatched won’t get retroactively delivered that message. The 4-week expiration only affects offline devices that were already subscribed when the message was sent—FCM will wait for those devices to come online (up to 4 weeks) to deliver, but new subscribers aren’t included in this.

内容的提问来源于stack exchange，提问作者Nickname0222022022