This cluster provides an interface for sending targeted commands to an Observer of a Content App on a Video Player device such as a Streaming Media Player, Smart TV or Smart Screen. The cluster server for Content App Observer is implemented by an endpoint that communicates with a Content App, such as a Casting Video Client. The cluster client for Content App Observer is implemented by a Content App endpoint. A Content App is informed of the NodeId of an Observer when a binding is set on the Content App. The Content App can then send the ContentAppMessage to the Observer (server cluster), and the Observer responds with a ContentAppMessageResponse.