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From anil gupta <anilgupt...@gmail.com>
Subject Re: Pagination with HBase - getting previous page of data
Date Sat, 02 Feb 2013 08:02:17 GMT
Hi Anoop,

Please find my reply inline.

Thanks,
Anil

On Wed, Jan 30, 2013 at 3:31 AM, Anoop Sam John <anoopsj@huawei.com> wrote:

> @Anil
>
> >I could not understand that why it goes to multiple regionservers in
> parallel. Why it cannot guarantee results <= page size( my guess: due to
> multiple RS scans)? If you have used it then maybe you can explain the
> behaviour?
>
> Scan from client side never go to multiple RS in parallel. Scan from
> HTable API will be sequential with one region after the other. For every
> region it will open up scanner in the RS and do next() calls. The filter
> will be instantiated at server side per region level ...
>
> When u need 100 rows in the page and you created a Scan at client side
> with the filter and suppose there are 2 regions, 1st the scanner is opened
> at for region1 and scan is happening. It will ensure that max 100 rows will
> be retrieved from that region.  But when the region boundary crosses and
> client automatically open up scanner for the region2, there also it will
> pass filter with max 100 rows and so from there also max 100 rows can
> come..  So over all at the client side we can not guartee that the scan
> created will only scan 100 rows as a whole from the table.
>

I agree with other people on this email chain that the 2nd region should
only return (100 - no. of rows returned by Region1), if possible.

When the region boundary crosses and client automatically open up scanner
for the region2, why doesnt the scanner in Region2 knows that some of the
rows are already fetched by region1. Do you mean to say that by default,
for a scan spanning multiple regions, every region has it's own count of
no.of rows that its going to return? i.e. lets say for a scan setCaching is
10 and scan is done across two regions. 9 Results(satisfying the filter)
are in Region1 and 10 Results(satisfying the filter) are in Region2. Then
will this scan return 19 (9+10) results?

>
> I think I am making it clear.   I have not PageFilter at all.. I am just
> explaining as per the knowledge on scan flow and the general filter usage.
>
> "This is because the filter is applied separately on different region
> servers. It does however optimize the scan of individual HRegions by making
> sure that the page size is never exceeded locally. "
>
> I guess it need to be saying that   "This is because the filter is applied
> separately on different regions".
>
> -Anoop-
>
> ________________________________________
> From: anil gupta [anilgupta84@gmail.com]
> Sent: Wednesday, January 30, 2013 1:33 PM
> To: user@hbase.apache.org
> Subject: Re: Pagination with HBase - getting previous page of data
>
> Hi Mohammad,
>
> You are most welcome to join the discussion. I have never used PageFilter
> so i don't really have concrete input.
> I had a look at
>
> http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/filter/PageFilter.html
> I could not understand that why it goes to multiple regionservers in
> parallel. Why it cannot guarantee results <= page size( my guess: due to
> multiple RS scans)? If you have used it then maybe you can explain the
> behaviour?
>
> Thanks,
> Anil
>
> On Tue, Jan 29, 2013 at 7:32 PM, Mohammad Tariq <dontariq@gmail.com>
> wrote:
>
> > I'm kinda hesitant to put my leg in between the pros ;)But, does it sound
> > sane to use PageFilter for both rows and columns and having some
> additional
> > logic to handle the 'nth' page logic?It'll help us in both kind of
> paging.
> >
> > On Wednesday, January 30, 2013, Jean-Marc Spaggiari <
> > jean-marc@spaggiari.org>
> > wrote:
> > > Hi Anil,
> > >
> > > I think it really depend on the way you want to use the pagination.
> > >
> > > Do you need to be able to jump to page X? Are you ok if you miss a
> > > line or 2? Is your data growing fastly? Or slowly? Is it ok if your
> > > page indexes are a day old? Do you need to paginate over 300 colums?
> > > Or just 1? Do you need to always have the exact same number of entries
> > > in each page?
> > >
> > > For my usecase I need to be able to jump to the page X and I don't
> > > have any content. I have hundred of millions lines. Only the rowkey
> > > matter for me and I'm fine if sometime I have 50 entries displayed,
> > > and sometime only 45. So I'm thinking about calculating which row is
> > > the first one for each page, and store that separatly. Then I just
> > > need to run the MR daily.
> > >
> > > It's not a perfect solution I agree, but this might do the job for me.
> > > I'm totally open to all other idea which might do the job to.
> > >
> > > JM
> > >
> > > 2013/1/29, anil gupta <anilgupta84@gmail.com>:
> > >> Yes, your suggested solution only works on RowKey based pagination. It
> > will
> > >> fail when you start filtering on the basis of columns.
> > >>
> > >> Still, i would say it's comparatively easier to maintain this at
> > >> Application level rather than creating tables for pagination.
> > >>
> > >> What if you have 300 columns in your schema. Will you create 300
> tables?
> > >> What about handling of pagination when filtering is done based on
> > multiple
> > >> columns ("and" and "or" conditions)?
> > >>
> > >> On Tue, Jan 29, 2013 at 1:08 PM, Jean-Marc Spaggiari <
> > >> jean-marc@spaggiari.org> wrote:
> > >>
> > >>> No, no killer solution here ;)
> > >>>
> > >>> But I'm still thinking about that because I might have to implement
> > >>> some pagination options soon...
> > >>>
> > >>> As you are saying, it's only working on the row-key, but if you want
> > >>> to do the same-thing on non-rowkey, you might have to create a
> > >>> secondary index table...
> > >>>
> > >>> JM
> > >>>
> > >>> 2013/1/27, anil gupta <anilgupta84@gmail.com>:
> > >>> > That's alright..I thought that you have come-up with a killer
> > solution.
> > >>> So,
> > >>> > got curious to hear your ideas. ;)
> > >>> > It seems like your below mentioned solution will not work on
> > filtering
> > >>> > on
> > >>> > non row-key columns since when you are deciding the page numbers
> you
> > >>> > are
> > >>> > only considering rowkey.
> > >>> >
> > >>> > Thanks,
> > >>> > Anil
> > >>> >
> > >>> > On Fri, Jan 25, 2013 at 6:58 PM, Jean-Marc Spaggiari <
> > >>> > jean-marc@spaggiari.org> wrote:
> > >>> >
> > >>> >> Hi Anil,
> > >>> >>
> > >>> >> I don't have a solution. I never tought about that ;) But
I was
> > >>> >> thinking about something like you create a 2nd table where
you
> place
> > >>> >> the raw number (4 bytes) then the raw key. You go directly
to a
> > >>> >> specific page, you query by the number, found the key, and
you
> know
> > >>> >> where to start you scan in the main table.
> > >>> >>
> > >>> >> The issue is properly the number for each lines since with
a MR
> you
> > >>> >> don't know where you are from the beginning. But you can built
> > >>> >> something where you store the line number from the beginning
of
> the
> > >>> >> region, then when all regions are parsed you can reconstruct
the
> > total
> > >>> >> numbering... That should work...
> > >>> >>
> > >>> >> JM
> > >>> >>
> > >>> >> 2013/1/25, anil gupta <anilgupta84@gmail.com>:
> > >>> >> > Inline...
> > >>> >> >
> > >>> >> > On Fri, Jan 25, 2013 at 9:17 AM, Jean-Marc Spaggiari
<
> > >>> >> > jean-marc@spaggiari.org> wrote:
> > >>> >> >
> > >>> >> >> Hi Anil,
> > >>> >> >>
> > >>> >> >> The issue is that all the other sub-sequent page
start should
> be
> > >>> moved
> > >>> >> >> too...
> > >>> >> >>
> > >>> >> > Yes, this is a possibility. Hence the Developer has to
take care
> > of
> > >>> >> > this
> > >>> >> > case. It might also be possible that the pageSize is
not a hard
> > >>> >> > limit
> > >>> >> > on
> > >>> >> > number of results(more like a hint or suggestion on size).
I
> would
> > >>> >> > say
> > >>> >> > it
> > >>> >> > varies by use case.
> > >>> >> >
> > >>> >> >>
> > >>> >> >> so if you want to jump directly to page n, you might
be totally
> > >>> >> >> shifted because of all the data inserted in the meantime...
> > >>> >> >>
> > >>> >> >> If you want a real complete pagination feature, you
might want
> to
> > >>> have
> > >>> >> >> a coproccessor or a MR updating another table refering
to the
> > >>> >> >> pages....
> > >>> >> >>
> > >>> >> > Well, the solution depends on the use case. I will be
doing
> > >>> >> > pagination
> > >
> >
> > --
> > Warm Regards,
> > Tariq
> > https://mtariq.jux.com/
> > cloudfront.blogspot.com
> >
>
>
>
> --
> Thanks & Regards,
> Anil Gupta
>



-- 
Thanks & Regards,
Anil Gupta

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