It starts and monitors the execution of channels. Each channel carries an update schedule provided by its developer. When it's time to update a channel, or you direct the tuner to so so, the tuner connects to the channel's transmitter. The tuner and transmitter compare the files constituting the channel. Channel updates are performed under the protection of a transaction so that channel files are always consistent with one another, even if power fails in the midst of an update.
Tuners are themselves channels, and automatically update themselves once a week.
Transmitters Castanet transmitters serve channel files in response to tuner requests, roughly as HTTP servers serve Web pages to Web browsers. In fact, transmitters can be configured as extensions to popular HTTP servers, or they can run as stand-alone processes. Unlike HTTP servers, transmitters send tuners only the files that are out of date on the tuner's machine, and they send all of the files in a single connection. Transmitters are, accordingly, much more efficient than HTTP servers. Developers use the Castanet publisher application to interact with transmitters.