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. > >>> > >> > > >
