Abstract: The subject matter of this specification can be embodied in, among other things, a system that includes a container document, a plurality of portable program modules residing in the container document, and a repeater residing in the container document. In addition, at least one of the plurality of portable program modules may be configured to define a communication channel and to identify itself to the repeater as a publisher to the communication channel. Furthermore, at least one of the plurality of portable program modules may be configured to identify itself as a subscriber to the communications channel.