Manual:Echo

Extension:Echo provides a central notification system for MediaWiki.

Mentions
Echo can send notifications to users mentioned in discussions; this feature is often referred to as "pinging". Linking to a user's page in a signed message, either directly or through a template, will trigger a mention notification for them. The technical details of this are laid out below.

Technical details
When generating mention events, Echo processes revision diffs chunk by chunk, to check when a user has added a new comment which should be searched for mentioned users to ping. As of this writing, for user mention notifications to work:
 * The diff hunk must be recognised as an addition of new content, not a change to existing content.
 * The user must sign their message; the message cannot contain any other signatures.
 * The signature must contain a plain wiki link to the user's page, user talk page, or contributions page, on the local wiki; it cannot be embedded in a template. (However, links to mentioned users' pages can be embedded in templates, because the diff is template-expanded. The   variable is unavailable and expands to the empty string when Echo parses the diff.)
 * The signature must be timestamped.
 * The comment must either belong in its entirety to an existing section (the diff chunk of added lines must not contain new section headers), or start a new section (diff chunk starts with a new section header).  Only sections of level 2 and above count.
 * If the number of detected to-be-pinged users exceeds 20, no notifications will be delivered.

Also, all markup is parsed with regular expressions; sufficiently tricky markup can trigger bogus results.