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. >>>> >>> >> >
