I use granite ds in my java server , but , when 100 user they're connected to flex application , granite return this error "Could not get channel id for message: flex.messaging.messages.RemotingMessage" Thank for the help.
This is not an error but just a warning issued by the AMF3Serializer class. It can happen, for example, when the server is sending data without explicitly using a declared channel.
You can safely ignore this warning: channel identification is only used in order to handle legacy (ie: old-fashion) serialization options (legacyCollection / legacyXMLDocument), see Adobe documentation about them here (the "Configuring AMF serialization on a channel" section).
It is very unlikely that you need to use these very specific serialization options. So, again, just ignore or disable this warning.