问题
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.
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.
回答1:
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.
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.
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.
来源：https://stackoverflow.com/questions/39979416/fcm-difference-between-downstream-message-and-upstream-message