How about max update sequence of current view range? 2011/9/13 Robert Newson <[email protected]>
> CRC32 should be good enough but there are better hash algorithms out > there (not completely sure they're commutative though). Will update. > > On 13 September 2011 14:09, Paul Davis <[email protected]> > wrote: > > Yeah, that's the basic idea. I walked through the idea of using > > something more familiar like SHA's or what not, but unless someone > > knows how to combine SHA hash states commutatively then I think that > > idea is shot because it'd cause a stampeding herd effect after > > compaction. > > > > On Tue, Sep 13, 2011 at 8:46 AM, Robert Newson <[email protected]> > wrote: > >> My joke about bloom filters was apparently misunderstood but the > >> notion above, which sounds a lot like a Merkle tree, seems lucid to > >> me. > >> > >> As for the strong vs. weak ETag variants, I think views need strong > >> ETags in all cases, given the declared semantics for them in 13.3.3 > >> > >> B. > >> > >> On 12 September 2011 23:28, Paul Davis <[email protected]> > wrote: > >>> In general the idea is intriguing. Using a combining hash would allow > >>> you to get a specific hash value for a given range. Unfortunately, > >>> bloom filters are not a good solution here because they require an a > >>> priori guess of the number of keys that are going to be stored. On the > >>> other hand, CRC32 appears to be combinable.There are a couple issues > >>> though. The first of which is whether this is a strong enough hash to > >>> use for an ETag. There are two types of ETags with slightly different > >>> semantics, so we'd have to figure out what we can do and where this > >>> falls on that spectrum. Secondly, computing the range ETag would > >>> require the equivalent of a reduce=false view call in addition to > >>> streaming the output if validation matched which has performance > >>> implications as well. > >>> > >>> On Mon, Sep 12, 2011 at 6:50 PM, Alon Keren <[email protected]> > wrote: > >>>> Disclosure: I don't know much about e-tags, CouchDB internals (or > bloom > >>>> filters). > >>>> > >>>> How about maintaining an e-tag for each sub-tree in the view, similar > to the > >>>> way (I think) reduce works? > >>>> When a row gets updated, its e-tag would be recalculated, and then its > >>>> parent's e-tag would be recalculated, and so on. The e-tag of an > internal > >>>> node could be the hash of all its children's hashes. > >>>> The actual e-tag that a view-query receives: the e-tag of the common > >>>> ancestor of all involved rows. > >>>> > >>>> The next time you query the same keys, you would supply the e-tag > you've > >>>> just received. > >>>> > >>>> Alon > >>>> > >>>> > >>>> On 10 September 2011 16:41, Andreas Lind Petersen < > >>>> [email protected]> wrote: > >>>> > >>>>> Hi! > >>>>> > >>>>> Background: I'm working on a web app that uses a single CouchDB > database > >>>>> for > >>>>> storing data belong to 400000+ users. Each user has an average of > about 40 > >>>>> documents that need to be fetched in one go when the frontend is > launched. > >>>>> I > >>>>> have accomplished this by querying a simple view with ?key=ownerID > (with a > >>>>> fallback to /_alldocs?startkey=<ownerID>_...&endkey=<ownerID>~ if the > view > >>>>> isn't built). Since the data for each user rarely changes, there's a > >>>>> potential to save resources by supporting conditional GET with > >>>>> If-None-Match, which would amount having the web app backend copy the > >>>>> CouchDB-generated ETag into the response sent to the browser. > >>>>> > >>>>> However, I just learned that CouchDB only maintains a single ETag for > the > >>>>> entire view, so every time one of my users changes something, the > ETag for > >>>>> everyone else's query result also changes. This makes conditional > GETs > >>>>> useless with this usage pattern. > >>>>> > >>>>> I asked about this on #couchdb and had a brief talk with rnewson, who > was > >>>>> sympathetic to the idea. Unfortunately we weren't able to come up > with an > >>>>> idea that didn't involve traversing all docs in the result just for > >>>>> computing the ETag (my suggestion was a hash of the _revs of all docs > >>>>> contributing to the result). That would be a bad default, but might > still > >>>>> work as an opt-in thing per request, eg. slowetag=true. > >>>>> > >>>>> Newson said I should try raising the discussion here in case someone > else > >>>>> had an idea for a cheaper way to calculate a good ETag. So what does > >>>>> everyone else think about this? Is my use case too rare, or would it > be > >>>>> worthwhile to implement it? > >>>>> > >>>>> Best regards, > >>>>> Andreas Lind Petersen (papandreou) > >>>>> > >>>>> Here's our chat transcript: > >>>>> > >>>>> [11:46] <papandreou> Does anyone know if there are plans for issuing > even > >>>>> more granular etags for view lookups? When you only look up a small > range > >>>>> or > >>>>> a specific key it would be really great if the ETag only changed when > that > >>>>> subset changes rather than the entire view. > >>>>> [11:47] <papandreou> In the application I'm working on I'll hardly > ever be > >>>>> able to get a 304 response because of this. > >>>>> [...] > >>>>> [13:51] <+rnewson> papandreou: unlikely. > >>>>> [13:52] <papandreou> rnewson: So the best thing I can do is to fetch > the > >>>>> data and compute a better etag myself? (My use case is a backend for > a web > >>>>> app) > >>>>> [13:53] <+rnewson> papandreou: You might be able to set ETag in a > list > >>>>> function? If you can't, I'll gladly change CouchDB so you can. > >>>>> [13:54] <papandreou> rnewson: I thought about that, too, but that > would > >>>>> cause a big overhead for every request, right? > >>>>> [13:55] <papandreou> rnewson: (Last time I tried views were slooow) > >>>>> [13:55] <papandreou> I mean lists > >>>>> [13:55] <+rnewson> papandreou: slower, yes, because couch needs to > evaluate > >>>>> the javascript in an external process. > >>>>> [13:55] <+rnewson> how will you calculate the fine-grained ETag? > >>>>> [13:56] <+rnewson> Also we did recently make it slightly finer, > before it > >>>>> was view group scope and now it's the view itself (I think) > >>>>> [13:56] <papandreou> rnewson: Maybe something like a hash of the > _revs of > >>>>> all the documents contributing to the result? > >>>>> [13:56] <+rnewson> hm, that makes no sense actually. but we did > refine it > >>>>> recently. > >>>>> [13:57] <+rnewson> papandreou: that doesn't sound cheap at all, and > it > >>>>> would > >>>>> need to be cheaper than doing the view query itself to make sense. > >>>>> [13:58] <papandreou> rnewson: There's still the bandwidth thing > >>>>> [13:58] <+rnewson> oh, you're working with restricted bandwidth > and/or have > >>>>> huge view responses? > >>>>> [13:59] <papandreou> rnewson: And it would be really nice to have > something > >>>>> like this completely handled by the database instead of inventing a > bunch > >>>>> of > >>>>> workarounds. > >>>>> [14:01] <+rnewson> If there's a correct and efficient algorithm for > doing > >>>>> it, I'm sure it would be applied. > >>>>> [14:02] <papandreou> rnewson: I guess it depends on the use case. If > the > >>>>> database is rarely updated I suppose the current tradeoff is better. > >>>>> [14:03] <+rnewson> I'm sure the only reason we have ETags at the > current > >>>>> granularity is because it's very quick to calculate. A finer-grain > would be > >>>>> committed if a viable approach was proposed. > >>>>> [14:04] <papandreou> rnewson: I have a huge database with data > belonging to > >>>>> 400000+ different users, and I'm using a view to enable a > lookup-by-owner > >>>>> thing. But every time a single piece of data is inserted, the ETag > for the > >>>>> view changes > >>>>> [14:04] == case_ [~ > [email protected]] > >>>>> has quit [Read error: Connection reset by peer] > >>>>> [14:04] <+rnewson> yes, I've completely understood the problem you > stated > >>>>> earlier. > >>>>> [14:05] <+rnewson> I can't think of a way to improve this right now > but I > >>>>> would spend the time to implement it if you had one. > >>>>> [14:06] <papandreou> rnewson: So right now the code path that sends a > 304 > >>>>> only needs to look at a single piece of metadata for the view to make > its > >>>>> decision? That'll be hard to beat :) > >>>>> [14:07] <+rnewson> doesn't need to beat it, it just needs to be fast. > >>>>> [14:07] <+rnewson> but I don't see any current possible solutions, > let > >>>>> alone > >>>>> fast ones. > >>>>> [14:07] <papandreou> rnewson: Well, thanks anyway for considering my > >>>>> suggestion. I'll let you know of I get an idea :) > >>>>> [14:08] <+rnewson> and it is now per-view and not per-viewgroup. so > it's > >>>>> what I said first before I thought it was silly > >>>>> [14:08] <+benoitc> query + last seq returned maybe .... > >>>>> [14:08] <+rnewson> but obviously a change could affect one view in a > group > >>>>> but not others > >>>>> [14:09] <papandreou> benoitc: The query is already sort of included > since > >>>>> it's in the url. > >>>>> [14:09] <+rnewson> benoitc: ? > >>>>> [14:10] <+benoitc> i was meaning last committed seq,but it won't > change > >>>>> anything ... > >>>>> [14:10] <papandreou> benoitc: I guess you'd also need to make sure > that the > >>>>> ETag changes if a document is deleted? > >>>>> [14:10] <papandreou> ah > >>>>> [14:10] <+rnewson> benoitc: we already use the update_seq of the > #view, > >>>>> which is finer-grained that db's last committed seq > >>>>> [14:11] <+benoitc> rnewson: commited seq in the view group but anyway > it > >>>>> won't work > >>>>> [14:12] <+rnewson> benoitc: right, that would be the pre-1.1.0 > behavior, I > >>>>> think. > >>>>> [14:12] <+rnewson> which is coarser > >>>>> [14:12] <+rnewson> we simply don't record the info that papandreou's > >>>>> suggestion would need to work. > >>>>> [14:12] <+benoitc> papandreou: easier solution would be to request > each > >>>>> time > >>>>> on on stale view > >>>>> [14:13] <papandreou> rnewson: Another reason why my suggestion sucks > is > >>>>> that > >>>>> it would require two traversals of the range, right? I'm guessing it > starts > >>>>> streaming as soon as it has found the first doc now? > >>>>> [14:13] <+benoitc> and update after, think it would work. except if > you > >>>>> want > >>>>> something strict > >>>>> [14:13] <+rnewson> papandreou: yes, we stream the results as we read > them, > >>>>> we don't buffer. > >>>>> [14:14] <papandreou> benoitc: Hmm, so the theory is that stale=ok > would > >>>>> increase the percentage of 304 responses? > >>>>> [14:14] <papandreou> rnewson: Right, yes, then it would take a > serious hit. > >>>>> [14:14] <+rnewson> papandreou: but we could add an option that reads > the > >>>>> thing, builds an etag, and then streams the result. it would be > slower, but > >>>>> for the times that we can send 304 we'd save bandwidth. It sounds a > bit too > >>>>> niche to me, but you could raise it on user@ > >>>>> [14:15] == Frippe [~Frippe@unaffiliated/frippe] has quit [Ping > timeout: > >>>>> 240 > >>>>> seconds] > >>>>> [14:15] <papandreou> rnewson: Would be awesome to have that as a > >>>>> configuration option > >>>>> [14:15] <+rnewson> papandreou: the view would not change, so neither > would > >>>>> the ETag (with stale=ok) > >>>>> [14:15] <+rnewson> papandreou: I think it would be a runtime option > >>>>> ?slow_etag=true > >>>>> [14:15] <papandreou> rnewson: That would also be fine > >>>>> [14:16] <+rnewson> a better solution would not require two passes, > though. > >>>>> [14:16] <+benoitc> papandreou: i would use stale=ok, then query the > view > >>>>> async, save new etag & ... > >>>>> [14:16] <papandreou> rnewson: I really don't think it's that niche > :). But > >>>>> maybe ETag-nerds are rarer than I think, hehe > >>>>> [14:16] <+benoitc> rnewson: that could encourage pretty dangerous > things > >>>>> [14:16] <+rnewson> benoitc: ? > >>>>> [14:17] <+benoitc> rnewson: cpu intensives tasks eacht time the call > is > >>>>> done, > >>>>> [14:17] <+benoitc> rather than encouraging something async > >>>>> [14:18] <+benoitc> rahh I hate osx, it introduce be bad unicode chars > in > >>>>> vim > >>>>> :@ > >>>>> [14:23] == Frippe_ has changed nick to Frippe > >>>>> [14:23] <papandreou> benoitc: I'm not sure exactly how that would > work? I'm > >>>>> working on the backend for a web app, so the requests will be coming > from > >>>>> multiple machines > >>>>> [14:24] <+benoitc> papandreou: call with stale==ok and have a process > >>>>> asking > >>>>> your deb for refresh from time to time > >>>>> [14:24] <+benoitc> s/deb/view > >>>>> [14:25] <+rnewson> benoitc: not sure I follow. doubling the number of > view > >>>>> requests to achieve a finer etag is an ok solution, but shouldn't be > the > >>>>> default, but I do think we'd need a better solution than that. > >>>>> [14:25] <+rnewson> benoitc: and you might be forgetting all the md5 > >>>>> verification we do all the time. > >>>>> [14:27] <+benoitc> rnewson: you don't need to call each views though > >>>>> [14:27] <+benoitc> I don't see the arg about last one > >>>>> [14:27] <papandreou> benoitc: Ah, ok, I understand now. Won't work > very > >>>>> well > >>>>> for me, though, the web app is a single page thing that only asks for > this > >>>>> particular chunk of data once per session, so the ETag will probably > have > >>>>> changed anyway unless we accept day-old data. > >>>>> [14:27] <+benoitc> anyway enotime to discuss about that , i'm on > >>>>> anotherthing > >>>>> [14:32] <papandreou> rnewson: But next step would be for me to raise > the > >>>>> issue on the user mailing list? > >>>>> [14:33] <+rnewson> papandreou: on reflection, it's more a dev@thing, but > >>>>> yes. > >>>>> [14:33] <+rnewson> post the suggestion about calculating an etag over > the > >>>>> results and then streaming them, with the caveat that a better > solution > >>>>> should be found. > >>>>> [14:34] <papandreou> rnewson: Ok, I will, thanks :). Btw. do you > think > >>>>> there's a chance that this will be easier for key=... queries than > >>>>> arbitrary > >>>>> startkey=...&endkey=... ones? > >>>>> [14:35] <+rnewson> papandreou: yes. for key= we could use a bloom > filter. > >>>>> [14:38] <papandreou> rnewson: Man, I've got some reading up to do :). > >>>>> Thanks! So dev@ it is? > >>>>> [14:39] <+rnewson> papandreou: yes. > >>>>> [14:40] <+rnewson> papandreou: 'bloom filter' is just how we handwave > >>>>> solutions these days, it just sounds vaguely plausible to for the > keys= > >>>>> variant > >>>>> [14:40] <+rnewson> but doesn't make sense at all for startkey/endkey > >>>>> [14:40] <+jan____> haha, I'm sitting in an ""HTTP Architecture" > session, > >>>>> and > >>>>> all the two speakers do is tell the audience how CouchDB gets it all > right. > >>>>> [14:41] <+rnewson> at base, we'd want some cheap way to invalidate a > range > >>>>> of keys in memory. > >>>>> [14:49] <+jan____> the answer must include bloom filters. > >>>>> > >>>> > >>> > >> > > > -- - sleepnova
