I was reading another thread and I really don't understand what's being said in the +5 upvoted answer here, where the user explains that "comments are ephemeral":

Can I view a list of my "great comments", perhaps in my profile?

The user "Yahoo Answers enthusiast" speaks of himself at the third-person (I can already see some more of this in the comments/answers here but that's not the point):

...you'll soon understand why no one really cares about comments, they are pure noise. And let's not forget that comments are ephemeral... I know of at least one moderator who absolutely hates them and misses no opportunity to delete them.

And I don't get this: does it mean that mods on SO can delete comments at will, simply because they do not like the very idea of someone writing a comment? If that's the case, shouldn't the very ability of leaving a comment be simply deleted from SO?

It's an honest question: I've received very good comments to questions I've asked, like explanation about the side-effects of certain Clojure constructs vs the side-effect-free of other constructs. I've also read a lot of very informative comments from questions I didn't ask.

Meanwhile, what should be done with either:

  • informative comments if comments are indeed ephemeral and can be deleted at will by any mod? (should these comments be merged into an answer before a mod flips tail or heads or the inverse and hence decide because it's raining or not to delete to maybe or not delete the comment?)

  • a mod deleting comment just because he thinks comments are ephemeral and can be deleted simply because he doesn't like them?

tl;dr: are comments ephemeral or not and if they are what steps needs to be done to protect informative comments?

  • Btw this user thinks that there are some informative and that if SO offered the ability to add comments informative comments shouldn't be deleted because someone doesn't like them: stackoverflow.com/users/986890/cedric-martin – Cedric Martin May 1 '12 at 21:53
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    e·phem·er·al [ih-fem-er-uhl] adjective lasting a very short time; short-lived; transitory: the ephemeral joys of childhood. – animuson May 1 '12 at 21:56
  • @animuson: what is your point exactly? I'm specifically asking if comments should be seen as lasting a very short-time, because they can be deleted at will by moody mods. Or are you talking sarcastically about some grammatical mistake? If that's the case I think you have enough rep to edit my question no!? – Cedric Martin May 12 '12 at 11:27
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    I'm defining the word for those who don't know what it means. – animuson May 12 '12 at 15:59

Comments are ephemeral by nature, which means:

  • The user who posted them or a moderator can delete them at any time, that's just stating a fact, I'm not discussing intent,
  • Regular users have no way of seeing deleted comments, they are only available to moderators, in contrast with deleted questions and answers which can still be accessed by regular users with more than 10,000 reputation,
  • Moderators don't have access to a central list of deleted comments, if they don't know that a comment was deleted they might never find out.

Strictly speaking, you should only post comments to:

  • Request clarification from the author;
  • Leave constructive criticism that guides the author in improving the post;
  • Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the question has been updated).

Any other comment is subject to deletion, and it's up to the moderator who responds to a comment flag or happens to see the comment by chance to decide whether to delete or not. You can safely assume that comments that follow the above guidelines will not be deleted, other than by mistake (we are only human, after all).

Now to put my other answer in perspective, it was answering a question regarding popular comments, and I think I provided enough evidence that popular comments usually don't follow the aforementioned guidelines, and thus are more noise than signal. You have nothing to fear when informative comments on your posts, however that doesn't mean that you shouldn't really try and work actually informative comments into your posts, other than being ephemeral comments are also less visible, and I think you'll agree with me that useful information deserves to be where it will be more easily discovered, and that's the question or the answer itself.

Some examples of actual comments that don't follow the guidelines and can be deleted at any time:

And if I go through my comments, I'm sure I'll find quite a few that should be deleted, certainly more than yours.

Lastly, one quite valid use for comments would be a courtesy ping. You went through all the trouble to call me out using a rather ironic tone, I would have appreciated a comment pointing me to this question, I just happened to find it by accident and that's not very nice. But I guess nice wasn't exactly what you were aiming for.

  • Psh. What do you know about this? – Cody Gray May 2 '12 at 1:03
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    @TheEstablishment Nothing really, I just make stuff up as I go (but don't tell anyone ;) – yannis May 2 '12 at 1:04
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    Another comment example: -1 not enough jQuery – Manishearth May 2 '12 at 7:38
  • "You went through all the trouble to call me out using a rather ironic tone"... I didn't call you out. I simply re-used your "talking in the third person, putting links pointing to myself" technique. Apparently you don't like that kind of behavior. Well, if you think it's ironic that I reuse such kind of (sarcastic ?) behavior and shouldn't be done, then maybe that this kind of behavior shouldn't be used in the first place. And certainly not by mods no!? – Cedric Martin May 12 '12 at 11:44

In general, informative comments that are related to the post are never deleted.

The problem occurs with certain tags that are very chatty (you know who you are); noisy comments become interspersed with the useful comments, and the comment thread veers off-topic. In such cases, it can be easier for the moderator to just hit the nuke all the comments button, rather than trying to sift through all the comments and delete the useless ones.

So do yourself a favor: stay on topic with your comments, and they will generally stay around until the next ice age (or when the server drives crash, whichever happens first).

As always, if there is useful information in the comments, it should generally be incorporated into the post. Don't make people sift through mounds of comments to find the nuggets of gold; put those nuggets into your question or answer.

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    "informative comments that are related to the post are never deleted." - except when they are. – BlueRaja May 2 '12 at 6:15
  • @BlueRaja "in general"... – nhinkle May 2 '12 at 8:53
  • One man's trash is another man's treasure. – user7116 May 2 '12 at 14:27
  • @BlueRaja: The moderator comment at the question you linked indicated that the comments were mostly expressions of gratitude. In other words, they were not informative. – Robert Harvey May 2 '12 at 14:46
  • @Robert: The highest-voted comment was from an engineer who has personal experience timing USB inputs and was sharing his findings in response to John's statement, "there is no direct way to measure the latency on the input path." That's about as on-topic as you could possibly get, but that comment is gone now. There were several others which were also highly on-topic. I don't understand why ALL the comments had to be deleted, and not just the "expressions of gratitude" – BlueRaja May 2 '12 at 14:55
  • @BlueRaja: Read my answer; it already explains why. For fsck sake, if something needs to be preserved, put it in the post! Anyone can suggest an edit, so there's no excuse now. – Robert Harvey May 2 '12 at 14:56
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    I read your post. You said informative comments are never deleted, but then imply all comments are taboo because the informative ones should have been edited into the post to begin with. Why even have comments then? Also, isn't this what the comment upvoting system is for, to help sift through the noise!? I really don't think the "nuke all comments" buttons is ever called for, especially when you are deleting extremely useful comments!! – BlueRaja May 2 '12 at 15:05
  • I never implied all comments are taboo. – Robert Harvey May 2 '12 at 15:10
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    @RobertHarvey With 2000 reputation ,can I suggest edits? – Jose Antonio Dura Olmos Feb 19 '17 at 7:43

does it mean that mods on SO can delete comments at will, simply because they do not like the very idea of someone writing a comment?

Oh absolutely, no! I'm fairly sure this is about deleting comments like

  • "Thanks! This helped!" or
  • "I have the same problem" or
  • "That question is stupid" or
  • "Ask this question on supueruzer!" or
  • … you name it

By no means an informative comment would be deleted like that, especially since comments that once have been deleted can't be restored (and I guess this is the ephemeral part about them). They can only be viewed by moderators and developers, not even by users with 10k reputation.

If comment threads drift to a chatty discussion, they could be deleted, since comments are not chat and shouldn't be abused as such (even if there were a tiny bit of useful information contained). Just keep comments on-topic and to the point.

what steps needs to be done to protect informative comments?

Work them into the post they belong to. If a comment really adds value or is an absolutely necessary part of the answer, then edit it and include what the comment says.

Imagine you're a random visitor from a search engine. Would you want the interesting/helpful stuff to stay somewhere buried in the comments? As an example, see this answer and house9's comment.

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