watcher

Definition

an entity or component that keeps a copy of a KERL for an identifier but that is not designated by the controller of the identifier as one of its witnesses. See annex watcher.
Source: Dr. S.Smith

Explanation

KERI alternative to total global ordering and consensus protocols is a mechanism called duplicity detection. In the verification and validation watchers are all that matter; they guarantee that logs are immutable by one very simple rule: "first seen wins".

KERI operational

This would be a set of watchers (that the validators trust) that record any and all copies of key event logs (KEL) that they see. Because these watchers can be anyone and anywhere, any controller of a public identifier is at peril should they choose to publish inconsistent copies of their KEL. This removes the incentive to be duplicitous.