GCM（现在是 FCM）是否有任何限制？
Is GCM (now FCM) free for any limit?
我想知道 Firebase 云消息传递是否对无限用户免费？
来自Firebase Cloud Messaging Docs：
Firebase Cloud Messaging (FCM) is a cross-platform messaging solution that lets you reliably deliver messages at no cost.
可以参考官方Firebase Pricings页面。从那里您将能够看到可用的项目类型：SPARK、FLAME、BLAZE，它们都具有 FCM（ 连同分析、应用索引、身份验证、动态链接、邀请、通知、崩溃报告和远程配置) 免费提供。
除了 from AL.
来自 Pricing page Faqs:
Which products are paid? Which are free?
Firebase's paid infrastructure products are the Realtime Database,
Firebase Storage, Hosting, and Test Lab. We offer a free tier for all
of these products except Test Lab.
Firebase also has many free products: Analytics, App Indexing,
Authentication, Dynamic Links, Cloud Messaging, Notifications,
Invites, Crash Reporting, &, Remote Config. You can use an unlimited
amount of these in all plans, including our free Spark Plan.
所以，它是免费的。文档中的任何地方都没有提到限制，但是 Firebase 实时数据库有限制，如常见问题解答中所述：
Firebase imposes hard limits on the number of connections to your app's database at the same time. These limits are in place to protect both Firebase and our users from abuse.
The free plan limit is 100, and cannot be raised. The Flame and Blaze
Plans have an initial limit of 10,000 simultaneous database
connections. This is to prevent abuse and can be raised by contacting
Firebase support with 24 hours notice.
This limit isn't the same as the total number of users of your app,
because your users don't all connect at once. We encourage you to
monitor your peak simultaneous database connections and upgrade if
needed.
We're working hard to remove the initial 10,000 simultaneous
connection cap on the Flame and Blaze plans.
因此，如果您使用 Firebase 数据库保存用户数据，并希望使用存储在 Firebase 数据库中的 FCM 注册令牌向用户设备发送大量推送通知，则可能会达到免费限制Spark 计划，但是要达到 10k 的限制真的很难。
Firebase 云消息传递 (FCM) 连接服务器 (CCS) 位于设备和应用服务器之间。以下限制适用
- 从文档 here 来看，FCM 似乎确实强加了 1000 个并行连接的限制。
For each sender ID, FCM allows 1000 connections in parallel.
Sender Id 是您在创建 Firebase 项目时创建的唯一数值，可在 Firebase 控制台设置窗格的云消息传递选项卡中找到。发件人 ID 用于标识可以向客户端应用程序发送消息的每个应用程序服务器。
注意：如果此限制不是关于从应用服务器到 CCS 的最大连接数，请纠正我
- 其次，如果您正在使用 XMPP 连接服务器协议（如果使用设备到云的消息传递，则必须使用该协议），您还应该阅读 Flow Control。在任何一个时间点，发送给 CCS 的未确认消息不应超过 100 条。
If the pending message count reaches 100, the app server should stop sending new messages and wait for CCS to acknowledge some of the existing pending messages
我想知道 Firebase 云消息传递是否对无限用户免费？
来自Firebase Cloud Messaging Docs：
Firebase Cloud Messaging (FCM) is a cross-platform messaging solution that lets you reliably deliver messages at no cost.
可以参考官方Firebase Pricings页面。从那里您将能够看到可用的项目类型：SPARK、FLAME、BLAZE，它们都具有 FCM（ 连同分析、应用索引、身份验证、动态链接、邀请、通知、崩溃报告和远程配置) 免费提供。
除了
来自 Pricing page Faqs:
Which products are paid? Which are free?
Firebase's paid infrastructure products are the Realtime Database, Firebase Storage, Hosting, and Test Lab. We offer a free tier for all of these products except Test Lab.
Firebase also has many free products: Analytics, App Indexing, Authentication, Dynamic Links, Cloud Messaging, Notifications, Invites, Crash Reporting, &, Remote Config. You can use an unlimited amount of these in all plans, including our free Spark Plan.
所以，它是免费的。文档中的任何地方都没有提到限制，但是 Firebase 实时数据库有限制，如常见问题解答中所述：
Firebase imposes hard limits on the number of connections to your app's database at the same time. These limits are in place to protect both Firebase and our users from abuse.
The free plan limit is 100, and cannot be raised. The Flame and Blaze Plans have an initial limit of 10,000 simultaneous database connections. This is to prevent abuse and can be raised by contacting Firebase support with 24 hours notice.
This limit isn't the same as the total number of users of your app, because your users don't all connect at once. We encourage you to monitor your peak simultaneous database connections and upgrade if needed.
We're working hard to remove the initial 10,000 simultaneous connection cap on the Flame and Blaze plans.
因此，如果您使用 Firebase 数据库保存用户数据，并希望使用存储在 Firebase 数据库中的 FCM 注册令牌向用户设备发送大量推送通知，则可能会达到免费限制Spark 计划，但是要达到 10k 的限制真的很难。
Firebase 云消息传递 (FCM) 连接服务器 (CCS) 位于设备和应用服务器之间。以下限制适用
- 从文档 here 来看，FCM 似乎确实强加了 1000 个并行连接的限制。
For each sender ID, FCM allows 1000 connections in parallel.
Sender Id 是您在创建 Firebase 项目时创建的唯一数值，可在 Firebase 控制台设置窗格的云消息传递选项卡中找到。发件人 ID 用于标识可以向客户端应用程序发送消息的每个应用程序服务器。
注意：如果此限制不是关于从应用服务器到 CCS 的最大连接数，请纠正我
- 其次，如果您正在使用 XMPP 连接服务器协议（如果使用设备到云的消息传递，则必须使用该协议），您还应该阅读 Flow Control。在任何一个时间点，发送给 CCS 的未确认消息不应超过 100 条。
If the pending message count reaches 100, the app server should stop sending new messages and wait for CCS to acknowledge some of the existing pending messages