watcher

A watcher is someone or something that observes or monitors, often for information, safety, or response. The term derives from the verb watch, with the agentive suffix -er. Usage ranges from people who observe nature or events to automated systems designed to detect changes in data or state.

Human watchers include birdwatchers, sports spectators, surveillance personnel, and on-duty guards who maintain a watch or patrol. In many organizations, a watch duty refers to a period of surveillance, vigilance, and reporting. The concept also appears in roles such as night watch, duty watch, or watch committees that monitor specific conditions and respond to incidents or anomalies.

In computing, a watcher is a component that monitors resources for changes and triggers actions. File system watchers notify applications when files or directories are created, modified, or deleted. Common implementations include Linux inotify, macOS FSEvents, and Windows FileSystemWatcher. Programming environments offer libraries such as Node.js fs.watch and Python watchdog. The watcher pattern is closely related to the observer pattern in software design, where observers respond to events produced by subjects.

Cultural and historical uses of the term appear in phrases like watch and ward, security watches, and narratives that emphasize vigilance. In many domains, watchers serve to provide timely information, enabling proactive response to evolving conditions.