this pretty much requires us to write it.
On 13 September 2011 09:50, Max Ogden <[email protected]> wrote: > At this time I would like to suggest that when someone writes a Merkle tree > in Erlang that they call it Erkel. > > On Tue, Sep 13, 2011 at 1:46 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. >> >>> >> >> >> > >> >
