xmpp/design/28_disco.md

Proposal: Implement service discovery

Author(s): Sam Whited
Last updated: 2021-08-10
Discussion: https://mellium.im/issue/28

Abstract

An API should be designed to handle responding to service discovery requests that integrates with the mux package.

Background

Even the simplest client or server needs to discover information about other entities on the network. Because of this adding an API for responding to service discovery improves our user experience for almost all XMPP related projects including clients, servers, bots, and embedded devices. In the XMPP world service discovery is handled by XEP-0030: Service Discovery and augmented by XEP-0115: Entity Capabilities, but only the first will be targeted by this design.

Requirements

• Ability to register handlers on a multiplexer and have a registry of features automatically created from the set of handlers
• Implementation must not preclude the addition of items or XEP-0115 support at a later date

Proposal

A new package, disco/info will be created and disco.Feature will be moved to info.Feature to avoid import loops between the mux and disco packages. This is a breaking API change, but is acceptable as we are currently before version 1.0.

The info package will contain interfaces that can be implemented by handlers providing features:

// FeatureIter is the interface implemented by types that implement disco
// features.
type FeatureIter interface{
  ForFeatures(node string, f func(Feature) error) error
}

To make responding to service discovery requests easier, the mux package will also be modified to implement this new interface.

// ForFeatures implements info.FeatureIter for the mux by iterating over all
// child features.
func (m *ServeMux) ForFeatures(node string, f func(info.Feature) error) error

Finally, the disco package will be given a new muc.Option that responds to disco requests by calling m.ForFeatures method:

// Handle returns an option that configures a multiplexer to handle service
// discovery requests by iterating over its own handlers and checking if they
// implement info.FeatureIter.
func Handle() mux.Option

Overall this will result in 1 new type, one new method, and one new function that will need to remain backwards compatible once we reach 1.0.