G24 · Design Android Push Notifications (FCM) G24 · 设计 Android 推送（FCM）
Verified source经核实出处
FCM is public (docs). Credibility A.
Key decisions关键决策
- **Persistent device connection** to FCM; device authenticated by registration token.**设备常连 FCM**，用注册 token 认证。
- **Priority classes**: high (immediate) vs normal (batched for battery).**优先级**：high 即时；normal 按电量批处理。
- **Fanout sharding**: topic message explodes into per-device tasks; idempotent retry.**Fanout 分片**：topic 消息展开为设备级任务；幂等重试。
- **QoS**: at-least-once; collapse key dedups stale notifications.**QoS**：at-least-once；collapse key 去陈旧。
Follow-ups追问
- Device offline for days? store-and-forward up to 28 days per FCM policy.设备离线数日？FCM 策略下最长存转 28 天。