问题描述
我一直在研究与来自 FCM Messenger 的 Web 推送通知相关的材料.虽然我遇到了下游消息和上游消息，但显然 HTTP 服务器只允许下游消息，而 XMPP 服务器允许上游消息.
I have been studying materials related to Web push notification from FCM Messenger. While I came across downstream message and upstream message, apparently HTTP server only allows downstream message while XMPP server allows upstream message.
我无法理解下游和上游消息之间的区别，并且 FCM 上没有很好地解释细节.我试图在网上搜索，但我没有运气.请帮助我，因为我是编码新手.
I have trouble understanding the difference between downstream and upstream message and the details are not well explained on FCM. I tried to search online but I have no luck. Please do help me as I am new to coding.
推荐答案
用外行的话来说，下游消息是指您从应用服务器向客户端应用发送推送通知.但是对于 FCM 的情况，您可以在没有应用服务器的情况下发送下游消息，或者通过使用 Firebase 控制台.
To put it on Layman's terms, Downstream Messaging is when you are sending a push notification from your App Server towards the Client App. However for FCM's case, you may send a downstream message without an App Server or by sending a usual curl request by making use of the Firebase Console.
虽然上游消息是相反的，但客户端应用程序正在向您的应用程序服务器发送消息.为此，需要应用服务器.
While Upstream Messaging is the reverse, the Client App is sending a message towards your App Server. For this, the App Server is a requirement.
请记住，FCM 服务器仍在处理下游和上游消息排队的流程中(中间人).
Do keep in mind that the FCM server is still in the flow (the middle man) that handles the queuing of your messages for both downstream and upstream.
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