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From Alon Keren <alon.ke...@gmail.com>
Subject Re: Even more fine-grained ETag support when querying views?
Date Mon, 12 Sep 2011 18:50:22 GMT
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 <
andreaslindpetersen@gmail.com> 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_ [~case@AMontsouris-651-1-123-169.w83-202.abo.wanadoo.fr]
> 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.
>

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