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From Paul Davis <paul.joseph.da...@gmail.com>
Subject Re: Even more fine-grained ETag support when querying views?
Date Tue, 13 Sep 2011 13:09:35 GMT
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 <rnewson@apache.org> 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 <paul.joseph.davis@gmail.com> 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 <alon.keren@gmail.com> 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 <
>>> 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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