I'm looking for a word for a computer that monitors a group of things, and computer always notices when a member of the group misbehaves. So the group cannot escape notice.

Example: The monitor noticed that a node went offline.

  • 4
    What's wrong with monitor?
    – DCShannon
    Nov 16, 2015 at 18:22
  • 7
    Monitor is indeed the accepted term in software. Watchdog can also be used, but is more specific and implies something that notices when other things fail and restarts them. If you're going for fiction rather than technical accuracy, then watcher, tracker or possibly surveyor might work.
    – Avish
    Nov 16, 2015 at 21:42
  • 1
    It's called a "monitor", as others have said.
    – Hot Licks
    Nov 17, 2015 at 3:21
  • 1
    "and computer always notices when a member of the group misbehaves." This is in fact impossible, and is a very deep result in computer science. Nov 17, 2015 at 15:11
  • 1
    In this context (computers and computer networks), a monitor very much implies active, continuous watching. On the other hand, a sentinel is passive. To use the word sentinel in this context, when you really mean monitor, is almost as incorrect as saying that someone who cleans school buildings is a superintendent of schools (using superintendent in the sense of "a janitor or custodian in a building").
    – David K
    Nov 17, 2015 at 23:24

20 Answers 20



"a soldier or guard whose job is to stand and keep watch."

It doesn't imply infallibility, but it implies vigilance. (And really, your watcher program can't be infallible anyway, can it?)

  • 11
    I don't think I've seen "sentinel" used that way in computer programming, though. Usually, it's a special value in memory, used passively to indicate a no-go zone. Nov 16, 2015 at 22:33
  • I agree with @200_success -- I don't ever recall seeing "sentinel" used in this sense. A "sentinel", in computerese, is an object which literally or figuratively "stands guard" at the edge or perimeter of something.
    – Hot Licks
    Nov 17, 2015 at 3:23
  • @200_success Interesting. Maybe it depends on your background? I'm a programmer and haven't seen "sentinel" used at all; it just seemed like a good metaphor - probably better for a thing that's actively watching than for something that functions more like a label. And lots of words do get re-used in different domains. A "node" in a graph and a "node" in a database cluster are different things. Nov 17, 2015 at 12:05
  • 4
    Programmers often use sentinel values and sentinel nodes, which are quite unrelated to your proposed usage. On the other hand, it's easy to extend nodes in graphs to apply to nodes in a database cluster — the correspondence is obvious if you try to sketch out your architecture on a whiteboard. Since terms like monitoring and watchdog are already well established, it makes little sense to repurpose a less-appropriate word plucked out of a thesaurus. Nov 17, 2015 at 20:19
  • I agree with 200_success... If I saw a program called "sentinel", I would be confused... but if I saw a program called "monitor" or "watchdog" I would instantly understand its purpose. Nov 17, 2015 at 21:07

It is a... monitor.

An instrument or device for continuously measuring some quantity or property (in early use chiefly levels of radioactivity). [OED]

It is usually used for medical devices but not necessarily; and monitors don't only measure quantities but they can monitor certain conditions or properties also.

The term also applies to a person who advises or monitors, and extended uses.

  • Indeed, a simple operating system is sometimes referred to as a "monitor", and the term is used in several other contexts in computing.
    – Hot Licks
    Nov 17, 2015 at 8:33

A software or hardware device whose specific purpose is to constantly monitor and restart unresponsive components is called a watchdog. For example, the Linux kernel has support for such devices.


Computers are operated by human-made programs, not on their own. That's why it is not easy to put all those adjectives or phrases before computer.

You could consider using "computer with surveillance program(s)" or "computer with surveillance solution(s)".

"Surveillance" means:

the act of carefully watching someone or something especially in order to prevent or detect a crime




"The overseer noticed that a node went offline."


It sounds like you're describing a computerized panopticon.

The central observer in a panopticon is sometimes called a watchman.

From Wiktionary:

One set to watch; a person who keeps guard, especially one who guards a building, or the streets of a city, by night.

  • I've never seen either word used in computer systems.
    – Hot Licks
    Nov 17, 2015 at 3:24
  • Watchdog is fairly common, actually. linux.die.net/man/8/watchdog Watchman isn't far removed. Never heard panopticon in that context, though.
    – Kal Zekdor
    Nov 17, 2015 at 16:03
  • @KalZekdor Never heard panopticon in what context? The question describes a computer program that watches everybody to enforce their behavior. I guess the "always watching" part of it is a departure from not knowing whether you're being watched, but close enough. Same effect of self-policing.
    – DCShannon
    Nov 17, 2015 at 22:51
  • In the context of computer programs/systems. Not that it's an invalid descriptor, just one that's not in common use, like watchdog is.
    – Kal Zekdor
    Nov 20, 2015 at 16:40
  • @KalZekdor If you/re talking about 'watchman', I agree that's not as common as watchdog, but I'm not sure if watchdog is what they're looking for. As far as panopticon, I've mostly encountered it in the context of computers. My Ethics of Technology class in college had a whole unit on panopticism.
    – DCShannon
    Nov 20, 2015 at 23:03

"Big Brother" comes to mind and is used for an all-powerful organization monitoring people's actions. It might as well be used for the monitoring device you have in mind. The term was first used as a personification of the power of the state in Orwell's "1984".

a person or organization that watches people all the time and tries to control everything they say or do. - macmillan

  • Big Brother is good for this, but for what I want, it has too negative a meaning in society these days.
    – adeady
    Nov 16, 2015 at 15:00



: an alert or watchful person; a guardian AHD


: (French) watchman The Free Dictionary


One that looks for, locates, and reports something, as:

a. A military or civil defense lookout.

b. Informal A person hired to detect dishonest acts by employees, as in a bank AHD


: one that snoops

snoop: pry into the private affairs of others, especially by prowling about. American Heritage® Dictionary



  1. a person who is regarded as underhanded and furtive and contemptible disagreeable person, unpleasant person - a person who is not pleasant or agreeable

  2. someone who prowls or sneaks about

  3. someone acting as an informer or decoy for the police

verb to go stealthily or furtively WordNet



v.i. 1. to rove or go about stealthily, as in search of prey or something to steal.

v.t. 2. to rove over or through in search of what may be found: to prowl the streets.

n. 3. the act of prowling.

Idioms: on the prowl, in the act of prowling; searching stealthily. Random House Kernerman Webster's College Dictionary


I'd say that everybody's under an eagle eye, or that nothing can dodge/evade the eagle eye(s) (of the monitoring system).


Consider the idiom "under the watchful eye of someone":

watched by someone who has power or authority over you

(Macmillan English Dictionary)

Also, under observation:

(esp. of a patient or a suspected criminal) being closely and constantly watched or monitored

(Oxford American Dictionary)


Sampler or Data logger . Actually it is an in-process quality checker or controller as monitoring is rarely done open ended without corrections.



) ... Alarm


However, assuming this is fiction, how about inventing a word that sounds menacing or combining terms?

The tryptaminicator will shock anyone whose 5-hydroxytryptamine levels fall below ten micrograms per decilitre.

  • Serotonin has nothing to do with technological "misbehaving"
    – Centaurus
    Nov 16, 2015 at 14:45
  • Ha ha! Good for you for recognizing the reference. My point is that in spy or science fiction you can invent an item of interest, e.g., flux capacitor from Back to the Future or pick an item from Q from James Bond movies
    – Stu W
    Nov 16, 2015 at 22:33

I'm looking for a word for a computer that monitors a group of things, and computer always notices when a member of the group misbehaves. So the group cannot escape notice.

If you're describing a computer, then you might want to use Warden. In some security contexts (e.g., encryption algorithms), authors often use names to describe the parties involved (e.g., Eve might listen to a conversation, and the name evokes eavesdropper). In many of these narratives, a monitor system might be called Walter or Warren, to evoke the sound of Warden. See the Wikipedia article, Alice and Bob, for a bit more information, and the example:

Walter, a warden, may be needed to guard Alice and Bob in some respect, depending on the protocol being discussed.


It sounds like your monitor is designed to detect certain activities/ behaviors/uses of the members of the group, so maybe you could call it

the/an “[Abnormal] Activity/Behavior/Use Detector”

or the/an “[Abnormal] Activity/Behavior/Use Detection System/Program.”

(“Abnormal Activity Detector” described in 3rd full paragraph at the first link as designed “to sniff out suspicious behavior …” from ‘CMS Wire’)

(example usage of “Abnormal Use Detection” shown in the second link, from ‘Advanced Multimedia and Ubiquitous Engineering: Future Information ..., Volume 2’ via Google Books)

Although I don’t think it would be necessary (or advisable), you could add the adjective “Fail-safe” in front of the above suggestions to try to capture the bolded notion of “always” that is in your question.

(fail–safe: adjective: “certain not to fail” … “[definition] 3: having no chance of failure : infallibly problem-free ” from Merriam-Webster)


Something that conducts surveillance keeps close watch of something. Per Oxford Dictionaries:

continuous observation of a place, person, group, or ongoing activity in order to gather information: video cameras used for covert surveillance.


In computer science there is a pattern for when a code object is in charge of monitoring the behavior of other objects. That object is known as an Observer.




I'm not savvy enough to say if the computer can take action on log events, but a logger captures all activity.

  • It captures whatever it captures. And it takes no actions, other than to capture the info for later examination.
    – Hot Licks
    Nov 17, 2015 at 8:29

Also could be called a Daemon

In multitasking computer operating systems, a daemon (/ˈdiːmən/ or /ˈdeɪmən/)[1] is a computer program that runs as a background process,

  • 2
    But a daemon doesn't necessarily monitor anything. It might simply send an event every 15 minutes.
    – Hot Licks
    Nov 17, 2015 at 8:28
  • True @HotLicks - it's a general purpose term for a background task, one that could also be monitoring {insert resource here}.
    – kolossus
    Nov 19, 2015 at 15:03

You can also consider calling it Hypervisor. Though it usually defines as

a piece of computer software, firmware or hardware that creates and runs virtual machines.

If you have, say, 100 worker processes and one process that watches, restarts them and creates new if needed, you can call it hypervisor... I guess


'sentry' is the first that comes to my mind. I'm not assuming that it is for computer/programming uses anyway.

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