There is definitely at least a bit of misunderstanding related to this aspect of memcached, though. Even if people don't have this exact issue, it seems that people come on here every month or so and post something related to this functionality (or lack thereof) of memcached. Usually, it's along the lines of "I figured out a scenario where memcached could get inconsisent," wherein they proceed to jump through a lot of hoops explaining the situation where a server gets a value, goes offline, the value gets updated elsewhere and then the server comes back online and they could possibly have a bad value.
Obviously, these things are not really a problem in the real world, since all of us run large instances of memcached doing hundreds of thousands of requests per-second or more and have little, if any, problem with it, but it maybe is something that should be explicitly spelled out. It's covered a bit in the wiki and the documentation (c.f. the "Persisent Storage" section of the Overview in the wiki), but I almost feel like there needs to be a very explicit: - memcached is not replicated. servers are unaware of each other-- they don't speak to each other in any way. - memcached is not persistent. there are many excellent, persisent K/V stores available. memcached is not one of them. - memcached is fast. it is fast precisely because it does one thing and does it very well. - memcached is a cache. don't put data in it that you can't recreate and that you can't afford to lose. On Fri, Jun 18, 2010 at 1:03 PM, Marc Bollinger <[email protected]>wrote: > It didn't seem to me that Brian was talking about a quorum of people > thinking that, but rather that some small portion of people conflate the > two, one of whom started the long thread about memcached not being > distributed. I agree that most people probably wouldn't trip over that > wording. > > > - Marc > > On Fri, Jun 18, 2010 at 6:04 AM, Simon Riggs <[email protected]>wrote: > >> On Fri, 2010-06-11 at 11:54 -0500, Brian Moon wrote: >> > After the recent thread and reading some comments on the memcached wiki >> > I think I know what is wrong. People see the word distributed and think >> > it means replicated. >> >> I'm not really sure there's a quorum of people that think that. I didn't >> worry too much when I read what that previously. >> >> -- >> Simon Riggs www.2ndQuadrant.com >> PostgreSQL Development, 24x7 Support, Training and Services >> >> > -- awl
