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From Sam Seigal <selek...@yahoo.com>
Subject Re: hbase hashing algorithm and schema design
Date Fri, 10 Jun 2011 02:51:45 GMT
Thanks for the reply Joey. This sounds better than using uuids .. I will
give it a shot.

One more thing, with such a setup, will it still be possible to do map
reduce jobs ? Is it possible to create a single scanner that will look at
all the prefixes ? if not, is it possible to map reduce with multiple
scanners ?

Thanks a lot for your help.

*From:* Joey Echeverria <joey@cloudera.com>
*To:* Sam Seigal <selekt86@yahoo.com>
*Sent:* Wed, June 8, 2011 5:08:32 PM
*Subject:* Re: hbase hashing algorithm and schema design

A better option than a uuid would be to take a hash of the
eventid-timestamp, modulo some value (maybe 4x #regionservers) and add that
to the front of the key. If you need to scan, create a scanner per prefix
and merge the results.

On Jun 8, 2011 4:54 PM, "Sam Seigal" <selekt86@yahoo.com> wrote:
> On Wed, Jun 8, 2011 at 12:40 AM, tsuna <tsunanet@gmail.com> wrote:
>> On Tue, Jun 7, 2011 at 7:56 PM, Kjew Jned <selekt86@yahoo.com> wrote:
>> > I was studying the OpenTSDB example, where they also prefix the row
>> with
>> > event id.
>> >
>> > I further modified my row keys to have this ->
>> >
>> > <eventid> <uuid> <yyyy-mm-dd>
>> >
>> > The uuid is fairly unique and random.
>> > Is appending a uuid to the event id help the distribution ?
>> Yes it will help the distribution, but it will also make certain query
>> patterns harder. You can no longer scan for a time range, for a given
>> eventid. How to solve this problem depends on how you generate the
>> UUIDs.
>> I wouldn't recommend doing this unless you've already tried simpler
>> approaches and reached the conclusion that they don't work. Many
>> people seem to be afraid of creating hot spots in their tables without
>> having first-hand evidence that the hot spots would actually be a
>> problem.
> Can I not use regex row filters to query for date ranges ? There is an
> overhead for the client to
> order them, and it is not an efficient query, but it is certainly possible
> to do so ? Am I wrong ?
>> > Let us say if I have 4 region servers to start off with and I start the
>> If you have only 4 region servers, your goal should be to have roughly
>> 25% of writes going to each server. It doesn't matter if the 25%
>> slice of one server is going to a single region or not. As long as
>> all the writes don't go to the same row (which would cause lock
>> contention on that row), you'll get the same kind of performance.
> I am worried about the following scenario, hence putting a lot of thought
> into how to design this schema.
> For example, for simplicity sake, I only have two event Ids A and B, and
> traffic is equally distributed
> between them i.e. 50% of my traffic is event A and 50% is event B. I have
> two region servers running, on
> two physical nodes with the following schema -
> <eventid> <timestamp>
> Ideally, I now have all of A traffic going into regionServerA and all of B
> traffic going into regionserver B.
> The cluster is able to hold this traffic, and the write load is
> 50-50.
> However, now I reach a point where I need to scale, since the two clusters
> are not being able to
> cope with the write traffic. Adding extra regionservers to the cluster is
> not going to make any difference
> , since only the physical machine holding the tail end of the region is
> one that will receive
> the traffic. Most of my other cluster is going to be idle.
> To generalize, if I want to scale where the # of machines is greater than
> the # of unique event ids, I have no way to
> distribute the load in an efficient manner, since I cannot distribute the
> load of a single event id across multiple machines
> (without adding a uuid somewhere in the middle and sacrificing data
> on ordered timestamps).
> Do you think my concern is genuine ? Thanks a lot for your help.
>> > workload, how does HBase decide how many regions is it going to create,
>> and what
>> > key is going to go into what region ?
>> Your table starts as a single region. As this region fills up, it'll
>> split. Where it split is chosen by HBase. HBase tries to spit the
>> region "in the middle", so that roughly the number of keys ends up in
>> each new daughter region.
>> You can also manually pre-split a table (from the shell). This can be
>> advantageous in certain situations where you know what your table will
>> look like and you have a very high write volume coupled with
>> aggressive latency requirements for >95th percentile.
>> > I could have gone with something like
>> >
>> > <uuid><eventid><yyyy-mm-dd> , but would not like to, since
my queries
>> always
>> > going to be against a particular event id type, and i would like them
>> be
>> > spatially located.
>> If you have a lot of data per <eventid>, then putting the <uuid> in
>> between the <eventid> and the <yyyy-mm-dd> will screw up data locality
>> anyway. But the exact details depend on how you pick the <uuid>.
>> --
>> Benoit "tsuna" Sigoure
>> Software Engineer @ www.StumbleUpon.com <http://www.stumbleupon.com/>

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