google cloud messaging (or gcm) sends two types of messages:
- collapsible, “send-to-sync” messages, where new messages replace older ones in the sending queue. (i.e. the older messages are “collapsed”).
- non-collapsible messages with payload, where every single message is delivere d.
each payload in non-collapsible messages is a unique content that has to be delivered and can’t be just replaced with a more recent message in the server sending queue. on the other hand, a collapsible message can be a simple ping from the server to ask its mobile clients to sync their data.