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When the app server first starts up, it is responsible for establishing the connection to the CCS. During this connection process, the app server uses the Firebase project server key and sender ID used in previous examples to authenticate with the Firebase messaging system. Once authenticated and connected, the app server simply waits to process incoming messages.
== Implementing the App Server ==
Once the message is sent it will arrive at the CCS where it will be passed to the app server. Any downstream messages sent to the client app will need to be handled within the onMessageReceived() method of the FirebaseMessagingService subclass as documented in the chapter entitled [[Firebase Cloud Messaging]].
If the onMessageSent() and onMessageError() callback methods are also overridden in the FirebaseMessagingService subclass, these will be called to notify the app of the status of the message transmission: