Google App Engine Channel Token and Client ID relationship

Question

In Google App Engine documentation for Channel API, it is stated that "Only one client at a time can connect to a channel using a given Client ID".

In contradiction, when I try to create a channel with the same Client ID, a new channel is consumed. In other words, when I call creat channel api for ClientID "user_one" channelService.createChannel("user_one"); for ten times, 10 different channels with 10 different token strings would be created, and when I send a message to "user_one" channelService.sendMessage(new ChannelMessage("user_one", message));, all of the ten clients on different devices, would receive the message!

How would you describe this scenario ? How one can benefit from this mechanism (one clientID with different channel, all receiving same message)? how to prevent it from creating multiple channel for single user ?

Answer 1

The documentation is not complete but it describes its intent pretty well. It says:

Only one client at a time can connect to a channel using a given Client ID, so an application cannot use a Client ID for fan-out. In other words, it's not possible to create a central Client ID for connections to multiple clients (For example, you can't create a Client ID for something like a "global-high-scores" channel and use it to broadcast to multiple game clients.)

It's up to you to keep track of the of the number of channels you create for each ClientID. From my experience, what you describe works, but not reliably. The newest channel created with the ClientID works, the older ones, sometimes work, sometimes don't receive messages.

Note that if you're trying this out on the dev_appserver, all the clients receive messages, but on the real server this behavior is not the same.