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Subject: Re: Using Scans in parallel
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From: Bryan Keller <bryanck@gmail.com>
In-Reply-To: <2B36CF16-70D7-4C20-AB4B-556BBF954F4A@gmail.com>
Date: Sun, 9 Oct 2011 11:03:39 -0700
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To: Bryan Keller <bryanck@gmail.com>

On further thought, it seems this might be a serious issue, as two =
unrelated processes within an application may be scanning the same table =
at the same time.

On Oct 9, 2011, at 10:59 AM, Bryan Keller wrote:

> I was not able to get consistent results using multiple scanners in =
parallel on a table. I implemented a counter test that used 8 scanners =
in parallel on a table with 2m rows with 2k+ columns each, and the =
results were not consistent. There were no errors thrown, but the count =
was off by as much as 2%. Using a single thread gave the same (correct) =
result every run.
>=20
> I tried various approaches, such as creating an HTable and opening a =
connection per thread, but I was not able to get stable results. I would =
do some testing before using parallel scanners as described here.
>=20
>=20
> On Oct 5, 2011, at 10:11 PM, lars hofhansl wrote:
>=20
>> That's part of it, the other part is to get the region demarcations.
>> You can also just get the smallest and largest key of the table and =
pick other demarcations for your scans. Then your individual scans will =
likely cover multiple regions and regionservers.
>>=20
>>=20
>> Your threading model depends on your needs. If you interested in =
lowest latency you want to keep your regionservers busy for each query.
>> What exactly that means depends on your setup. Maybe you split up the =
overall scan so that no more than N scans are active at any =
regionserver.
>>=20
>> If you're more interested in overall predictability, you might not =
want parallelize each scan too much.
>>=20
>>=20
>>=20
>> ----- Original Message -----
>> From: Sam Seigal <selekt86@yahoo.com>
>> To: user@hbase.apache.org; lars hofhansl <lhofhansl@yahoo.com>
>> Cc: "hbase-user@hadoop.apache.org" <hbase-user@hadoop.apache.org>
>> Sent: Wednesday, October 5, 2011 6:18 PM
>> Subject: Re: Using Scans in parallel
>>=20
>> So the whole point of getting the region locations is to ensure that
>> there is one thread per region server ?
>>=20
>>=20
>> On Wed, Oct 5, 2011 at 4:42 PM, lars hofhansl <lhofhansl@yahoo.com> =
wrote:
>>> Hi Sam,
>>>=20
>>>=20
>>> There were some attempts to build this in. In the end I think the =
exact patterns are different based on what one is trying to achieve.
>>> Currently what you can do is getting all the region locations =
(HTable.getRegionLocations). =46rom the HRegionInfos you can
>>> get the regions start and end keys.
>>> Now you can issue parallel scan for as many regions as you want (by =
create a Scan object with start and row set to the region's
>>> start and end key).
>>> You probably want to group the regions by regionserver and have one =
thread per region server, or something.
>>>=20
>>>=20
>>> -- Lars
>>> ________________________________
>>> From: Sam Seigal <selekt86@yahoo.com>
>>> To: hbase-user@hadoop.apache.org
>>> Sent: Wednesday, October 5, 2011 4:29 PM
>>> Subject: Using Scans in parallel
>>>=20
>>> Hi ,
>>>=20
>>> Is there a known way to be able to do Scan's in parallel (in =
different
>>> threads even) and then sort/combine the output ?
>>>=20
>>> For a row key like:
>>>=20
>>> prefix-event_type-event_id
>>> prefix-event_type-event_id
>>>=20
>>> I want to declare two scan objects (for say event_id_type foo)
>>>=20
>>> Scan 1 =3D>  0-foo
>>> Scan 2 =3D>  1-foo
>>>=20
>>> execute the scans in parallel (maybe even in different threads) and
>>> then merge the results ?
>>>=20
>>> Thank you,
>>>=20
>>> Sam
>>>=20
>>=20
>=20