The downstream messages allow us to send send short (4KB) messages to alert the user of new updates, new content or even reminders.
A downstream message is a one-way communication channel where the users can receive messages, but they cannot respond to them directly or take any immediate action.
To build interactive experiences, such as a chat system, we will have to support a bidirectional communication where the user can receive downstream messages and also send upstream messages to other devices or groups of devices.
In our next example, we will build a simple group messaging system based on the GCM upstream messaging and topic messaging features. The group messaging system will allow multiple devices to publish text messages to a shared message channel.
GCM topic messaging allows your backend server to send a message to devices that have a particular topic. Once the GCM receives a message to a particular topic, it will route and deliver the message to the...
