If we have some critical servers that have to be running 7 days a week, 24 hours a day and they can't be down even for one second, what is this particular type of server called in English?

Example sentence:

Our servers must be flawless; they must be _____.

Does the term productive suit?

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    @EnglishLearner, edit your question to include a sample sentence about how you want to use the phrase. For example: "Our servers must be flawless; they must be _____." Commented May 17, 2017 at 20:19
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    In this case I think closing because of no sample sentence is following the 'letter' not the 'spirit' of the rules. The question in its current form seems unambiguous.
    – k1eran
    Commented May 19, 2017 at 0:49
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    As the suggested sample sentence proposed by @BrockAdams seems uncontroversial would anyone object if someone other that the OP made the edit?
    – k1eran
    Commented May 19, 2017 at 0:50
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    @k1eran go ahead. I voted to reopen the question because it is not off topic. Only one reopen vote needed.
    – Mari-Lou A
    Commented May 20, 2017 at 7:37
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    What is wrong with "always on"?
    – Spencer
    Commented May 20, 2017 at 18:29

10 Answers 10


Critical services have their own terminology for this that lessens the ability to weasel around phrases like "24/7", "round-the-clock", "Mission critical", etc.

It's the "Nines" system and is common in legal contracts for "High availability" services.

Calculate "Nines" by taking the percentage of uptime over total time. For example, if a server was up for only 9 out of 10 seconds, that's 90% or "One nine".

"Five nines":

The common "Five nines" is 99.999% which still allows 6 seconds of downtime per week.

To get to less than 1 second per week, you need to guarantee "Six nines" or more -- which is a herculean task in the long term. (Even more-so if you don't use the standard "Unplanned" weasel word.)
For example, Google and Microsoft typically only promise "3 nines" (and frequently fail even that), which is a whopping 10 minutes down per week.

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    +1. This answer is indeed the terminology used in I.T. in SLAs- service level agreements.
    – k1eran
    Commented May 14, 2017 at 20:34
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    +1 for the expression that allows to communicate factual and precise information, as compared to marketing promises.
    – svavil
    Commented May 14, 2017 at 22:19
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    @Mari-LouA, I know that "5 nines" is used by many Engineers and Sales-types in Europe and India too -- at least within the international companies where I first heard the term. ... Few would say the phrase: "the Nines system", however. It's just something we use when formally/legally/technically specifying "zero downtime", etc. Commented May 15, 2017 at 7:11
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    @Mari-LouA I am a British IT engineer, and I use the "n nines" terminology to describe different levels of availability.
    – Mike Scott
    Commented May 15, 2017 at 19:55
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    Thank you for the quote. "Countless examples" are hard to wade through for the uninitiated; one or two well-chosen typical examples are far more helpful.
    – 1006a
    Commented May 15, 2017 at 21:58


lasting all day and all night.

  • ‘round-the-clock surveillance’
  • ‘He has hired 500 more workers and this month is adding a third shift for
    round-the-clock production - a first in auto manufacturing history.’

Oxford Dictionaries

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    The original title was: What is another word for 24/7 in English? I am not an IT expert, I know next to nothing about servers, I would not have answered if the original title corresponded precisely with the body. Please see the first comments/suggestions posted below the OP, and the first answer that was posted too. The OP had plenty of time to clarify, but he chose not to. Perhaps servers can be called round-the-clock to the layperson, someone like me. I do not know because this is not my field of expertise.
    – Mari-Lou A
    Commented May 15, 2017 at 6:47
  • The original title was "what does it call in English?" (^_^) ... And it was NOT a [single-word-request]. Commented May 15, 2017 at 19:55
  • @BrockAdams yes, I know that now, the situation became clearer when I visited meta yesterday. Nevertheless, the OP suggested the term productive, and if we were to correct the grammar in the title it would read: What is it called in English? so perhaps he thought a single-word was needed which is why the first editor (Perry) included that tag. Can we both agree that the question has nothing at all to do with grammar?
    – Mari-Lou A
    Commented May 16, 2017 at 5:17
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    Agreed, [grammar] does not apply. Submitted a suggested edit. If the OP ever comes back will he recognize his post? ;) Commented May 16, 2017 at 5:40
  • "Full-time" or "full time" used to be perfectly acceptable to mean this. It's clear and does not rely on use (or misuse) of jargon. See the Merriam Webster dictionary meaning no 2 and Concise Oxford (1996) p 548. I recall people using it to mean a service including a computer or online service that had to be on all the time.
    – Flynn
    Commented May 16, 2017 at 18:43

If the server cannot be down for even one second, the downtime it experiences must be zero seconds.

In other words, it is a zero-downtime server.

The term zero downtime seems to be widely used in the industry, for example here and here.

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    Linguistically, I support this answer (+1, hence). However, as an experienced developer (and we're talking the sad kind of experience here), I claim that there's no such thing. Every system will go down. Instead of talking about ETF (expected time to failure), we talk about ETR (expected time to restore). But it could be specific for my field. Still, no such think as 100% uptime over a longer period. A-ah! :) Commented May 15, 2017 at 8:55
  • @KonradViltersten I agree, anyone who promises a service with absolutely zero downtime is making unrealistic hype. Here is (I think) a more realistic viewpoint, saying what they will do when (inevitably) they fail to achieve the zero-downtime goal. (And even there I see they are talking about a network, not an individual server or even a service.) I think "24/7" (from the question) also is generally not completely accurate; it might be more realistic to ask whether you can achieve 23.9 x 7, 23.99 x 7, or 23.999 x 7 on average.
    – David K
    Commented May 15, 2017 at 12:29

I would go with high-availability.

Most, if not all servers, need some down-time when updates and patches are installed.

  • "Highly available" was already mentioned, but this is a different formation of the words that one might use.
    – David K
    Commented May 15, 2017 at 12:21
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    No. High availability normally means redundant. That is, if something fails, there is a backup ready to take over. High availability tends to go with high uptime, but they are different. Commented May 15, 2017 at 18:06
  • @BenAveling point taken. You are right that, within a system, this would imply redundancy. I still think that 'High availability' can be used in connection to a single server to describe its' likelihood of being available at any time.
    – paul
    Commented May 16, 2017 at 5:00
  • @BenAveling I agree that high availability generally requires redundancy, but I think this is a weakness in the question, not this answer. The question seems to imply that no single server in the installation can ever have any downtime, but frankly, before Gary edited the title the requested term could have been interpreted to apply to the servers collectively rather than individually. The original intent of the question is unclear at this point.
    – David K
    Commented May 16, 2017 at 17:53
  • Yeah, it's not a sensible question. The way to get high-uptime is high availability. They've missed the difference between a service and a server. But this is english.stackexchange, not it.stackexchange. Probably a better answer would be "bullet proof" or "magical unicorn powered". Commented May 23, 2017 at 12:31

"what is this particular type of server called in English"

I would go with either continuously available or mission critical. These terms better suit the specific context of a system or set of systems that support or underpin a goal or task. Personally I think the first specifically describes the type of system you are asking about.

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    I think mission critical is something different: A redundant server is available 24/7, but isn't mission critical (business is just switched to a different server in case of failure). On the other hand, if your business is only open 9 to 5, your server might only be available 9 to 5, and still be mission critical.
    – nikie
    Commented May 15, 2017 at 8:30
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    @nikie Yes -- OP's title emphasizes passively being on 24/7, but the actual question seems to focus more on 100% availability. If it's mission critical then it's required to be available whenever people need it... which as you say could be 9-5, but applied to an online asset that's expected to possibly be used 24/7 by people around the world, then being "mission critical" implies 24/7 availability. It's a good distinction, though -- it's technically OK for a mission critical resource to be unavailable whenever there's absolutely zero chance someone could need it.
    – A C
    Commented May 15, 2017 at 16:37
  • Yes to continuously available, and yes to mission critical. Continuously available means 24.7. Mission critical systems are often 24x7, but not always. And plenty of 24x7 systems are not mission critical. But OP specified both, so yes and yes. Commented May 15, 2017 at 18:07
  • @Ben Aveling - I think you are perhaps bring too literal. I suggested misson critical because that could very well describe "critical servers..." which "can't be down even for one second". It would depend on the context really but it general it could be applicable.
    – Fraser
    Commented May 15, 2017 at 18:13

A 100% uptime server:


Time during which a piece of equipment (such as a computer) is functioning or able to function

-- Merriam-Webster

Here are some examples of the phrase in use: -

Uptime is never 100%. A world of factors conspire against 100% uptime, and can potentially disrupt the flow of bits from the server to your browser. But despite the number of factors, most hosting companies are at or above 99.9% uptime.

-- WP Engine

What we mean when we say 100% uptime is that there will never be any downtime in the time period starting now and extending to infinity.


  • Yes it does Mari-Lou A I happen to know this as I work in IT and am intimately familiar with the technical terms. I'll edit my question to cite references that explain this.
    – Gary
    Commented May 15, 2017 at 6:28
  • Deleted my previous comment, it was obsolete.
    – Mari-Lou A
    Commented May 21, 2017 at 5:53

Usually such computers nodes are a group of machines in such a way that either one of these could be down, the group as a whole keeps running and the intended service is delivered. Such systems are highly available systems.


The answer in your title is rock solid.

The answer in your question is mission critical.

The Free Dictionary:

rock-solid or rock solid adj
2. extremely reliable

The Free Dictionary:

Mission critical
Mission critical refers to any factor of a system (equipment, process, procedure, software, etc.) whose failure will result in the failure of business operations. That is, it is critical to the organization's 'mission'.

  • mission critical is one of a set of related criticalities. A 24/7 server might be business critical, when its failure doesn't just threaten the mission/goal of the server but of the whole business. Even more critical are life critical servers, where a loss of functionality could be lethal.
    – MSalters
    Commented May 16, 2017 at 12:21
  • A company that rents snow skis, for example, could easily have a mission critical system that only operates a few months out of a year.
    – barbecue
    Commented May 16, 2017 at 14:55

If you depend on a continuously running server which must not be down for even one second, the term is time bomb.

Sooner or later, it will be down for more than one second.

Machines that cannot be down for any reason for more than a second require special hardware with redundancy for fault tolerance, a special operating system, running in dedicated data center.

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    Good advice, wrong venue. Commented May 16, 2017 at 19:15

I suppose you could use "Continuous", but I believe the word you're looking for is


Which means:

endless or apparently endless

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    "Interminable" doesn't mean something is running or working continuously. It means something that seems to never end, like a bad movie, or a really boring lesson.
    – Mari-Lou A
    Commented May 14, 2017 at 11:48
  • I was working on an interminable coding problem for my high-availability service. Literally in- +*term*; without a set term or time period.
    – flith
    Commented May 15, 2017 at 6:05
  • You want "uninterruptible". Commented Jul 30, 2018 at 10:26

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