Real time capable and bidirectional (do we need this?)
Modern
Used by Mastodon and other “fediverse” projects
Complex
Not widely adopted (outside of “fediverse” circles)
Harder to deploy as a “static” file
RSS 2.0 Specification (version 2.0.11)
Podcast RSS feed requirements - Apple Podcasts for Creators
Widely used (for podcasting and blogs)
XML is extensible by design
Can be served up as a static file
XML is falling out of favour
Badly specified
Atom Publishing Format and Protocol (atompub) - Documents
Tries to fix the short-comings of RSS
Can be served as a static file
Can be bi-directional (do we need this?)
XML
Not as widely used as RSS
More complex than RSS
Minimal Standard (like RSS)
Replaces XML with JSON
Not widely used
Not standardised by a recognized standards body (e.g. W3C)
(note: this adds push notification protocol to Atom)
Simpler than ActivityPub
Is this being used anywhere?
http://searsia.org/protocol.html
Made for federated search
Not standardised, tied to a particular implementation