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.

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