Podping is a global message bus for podcast feed events: a shared channel where publishers post a short notice when a feed changes, and anyone can read the notices. Podcast Index (Podcastindex.org) developed it as an alternative to WebSub and rssCloud, two older protocols that push feed updates to subscribers one feed at a time.
A podcast hosting company, or a podcaster who hosts the feed alone, sends a plain GET request with the updated feed URL to podping.cloud, the hosted front end. The server checks the publisher's identity and writes the URL, with optional "reason" and "medium" codes, as a custom JSON object into the Hive blockchain, an open blockchain that adds a new block every 3 seconds. Apps, directories and aggregators run a watcher script that reads the blockchain and see the update within about 20 seconds, with no subscription to each feed.
Podping is an open-source project on GitHub with separate parts: podping.cloud, a backend writer for Hive, watcher scripts, and Cap'n Proto schema definitions in Python and Rust. Podcast Index runs the first servers; anyone can watch the events.