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From sleepnova <wanpee...@gmail.com>
Subject Re: Even more fine-grained ETag support when querying views?
Date Thu, 15 Sep 2011 14:51:23 GMT
How about max update sequence of current view range?

2011/9/13 Robert Newson <rnewson@apache.org>

> CRC32 should be good enough but there are better hash algorithms out
> there (not completely sure they're commutative though). Will update.
>
> On 13 September 2011 14:09, Paul Davis <paul.joseph.davis@gmail.com>
> wrote:
> > 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.
> >>>>>
> >>>>
> >>>
> >>
> >
>



-- 
- sleepnova

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