Discovery of resources is a frequent strategy in various protocols. Discovery is using requests to figure out the state or status of things. It is necessary whenever an information isn't available already. XMPP calls it disco.

PSYC tries to avoid these approaches and pushes information out to possibly interested recipients in advance. This usually comes with the additional benefit of being multicastable. A logical enhancement of this is to keep distributed state.

While traditional approaches give you a search box where you have to think of something you want, then see if it's there, PSYC is more like a toy store where you see all the offerings with colourful pictures presented to you and you just use what you need. This makes sense especially for file sharing between friends.

See also Directory services.