Re: HBase region assignment by range?

[Michael Segel <mi...@hotmail.com>]

Sorry, but ‘local’ secondary indexes are a joke. 

If anyone is going to the Big Data TechCon in Boston, find me and I’ll give you a nice lecture on why this not a good idea. 
But for those who can’t make it… 

1) You’re fighting the system.  
2) Unless you have at least 10K rows in the region, you’re going to probably be faster doing a filtered scan.
3) Think about table joins against indexed columns as your use case.  The minute you do this… you’ll understand why its not a good idea.
4) Sparse indexes. (Uniqueness of values and their distribution throughout the data set…) 
5) What are you actually using as your index… ;-) [hint:  Lucene or a modified Lucene could work for a secondary index… 
	you are only thinking in terms of inverted tables. ]
6) Look at your index. 
	A) You’re combining all indexes for the region in to a single table. 
	B) You have to now synchronize splits and management of the two tables so that splits and locality are maintained. (Fighting the underlying HDFS system too.) 
	C) You’re adding to the overhead on managing the region which means longer splits and compactions.
	D) Look at your key to value size ratio. In fact if you’re doing all of the data in a single table, you will end up with a ratio of one row in the index to one row in the table
	     and will not even need to see the value stored because everything would be in the index.  (column name, attribute, value) 
	E) Increased complexity during splits.  (You will end up having to rebuild your index during every split.) 



The minute you actually spend a quick NY minute on thinking about these things, you’ll realize that its a dumb idea. 

HTH

-Mike


> On Apr 8, 2015, at 6:11 PM, Nick Dimiduk <nd...@gmail.com> wrote:
> 
> Your needs (and use case?) looks a lot like the local secondary index work
> happening around Phoenix.
> 
> On Wed, Apr 8, 2015 at 11:50 AM, Anoop John <an...@gmail.com> wrote:
> 
>> bq.while the region can surely split when more data added-on, but can HBase
>> keep the new regions still on the same regionServer according to the
>> predefined bounary?
>> 
>> You need custom LB for that.. If there, it is possible to restrict
>> 
>> -Anoop-
>> 
>> 
>> On Thu, Apr 9, 2015 at 12:09 AM, Demai Ni <ni...@gmail.com> wrote:
>> 
>>> hi, Guys,
>>> 
>>> many thanks for your quick response.
>>> 
>>> First, Let me share what I am looking at, which may help to clarify the
>>> intention and answer a few of questions. I am working on a POC to bring
>> in
>>> MPP style of OLAP on Hadoop, and looking for whether it is feasible to
>> have
>>> HBase as Datastore. With HBase, I'd like to take advantage of 1) OLTP
>>> capability ; 2) many filters ; 3) in-cluster replica and between-clusters
>>> replication. I am currently using TPCH schema for this POC, and also
>>> consider star-schema. Since it is a POC, I can pretty much define my
>> rules
>>> and set limitations as it fits. :-)
>>> 
>>> Why doesn't this(presplit) work for you?
>>> 
>>> The reason is that presplit won't guarantee the regions stay at the
>>> pre-assigned regionServer. Let's say I have a very large table and a very
>>> small table with different data distribution, even with the same presplit
>>> value. HBase won't ensure the same range of data located on the same
>>> physical node. Unless we have a custom LB mentioned by @Anoop and
>> @esteban.
>>> Is my understanding correct? BTW, I will look into HBASE-10576 to see
>>> whether it fits my needs.
>>> 
>>> Is your table staic?
>>>> 
>>> while I can make it static for POC purpose, but I will use this
>> limitation,
>>> as I'd like the HBase for its OLTP feature. So besides the 'static'
>> HFile,
>>> need HLOGs on the same local node too. But again, I would worry about the
>>> 'static' HFile for now
>>> 
>>> However as you add data to the table, those regions will eventually
>> split.
>>> 
>>> while the region can surely split when more data added-on, but can HBase
>>> keep the new regions still on the same regionServer according to the
>>> predefined bounary? I will worry about hotspot-issue late. that is the
>>> beauty of doing POC instead of production. :-)
>>> 
>>> What you’re suggesting is that as you do a region scan, you’re going to
>> the
>>>> other table and then try to fetch a row if it exists.
>>>> 
>>> Yes, something like that. I am currently using the client API: scan()
>> with
>>> start and end key.  Since I know my start and end keys, and with the
>>> local-read feature, the scan should be local-READ. With some
>>> statistics(such as which one is larger table) and  a hash join
>>> operation(which I need to implement), the join will work with not-too-bad
>>> performance. Again, it is POC, so I won't worry about the situation that
>> a
>>> regionServer hosts too much data(hotspot). But surely, a LB should be
>> used
>>> before putting into production if it ever occurs.
>>> 
>>> either the second table should be part of the first table in the same CF
>> or
>>>> as a separate CF
>>>> 
>>> I am not sure whether it will work for a situation of a large table vs a
>>> small table. The data of the small table has to be duplicated in many
>>> places, and a update of the small table can be costly.
>>> 
>>> Demai
>>> 
>>> 
>>> On Wed, Apr 8, 2015 at 10:24 AM, Esteban Gutierrez <esteban@cloudera.com
>>> 
>>> wrote:
>>> 
>>>> +1 Anoop.
>>>> 
>>>> Thats pretty much the only way right now if you need a custom
>> balancing.
>>>> This balancer doesn't have to live in the HMaster and can be invoked
>>>> externally (there are caveats of doing that, when a RS die but works ok
>>> so
>>>> far). A long term solution for your the problem you are trying to solve
>>> is
>>>> HBASE-10576 by tweaking it a little.
>>>> 
>>>> cheers,
>>>> esteban.
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> --
>>>> Cloudera, Inc.
>>>> 
>>>> 
>>>> On Wed, Apr 8, 2015 at 4:41 AM, Michael Segel <
>> michael_segel@hotmail.com
>>>> 
>>>> wrote:
>>>> 
>>>>> Is your table staic?
>>>>> 
>>>>> If you know your data and your ranges, you can do it. However as you
>>> add
>>>>> data to the table, those regions will eventually split.
>>>>> 
>>>>> The other issue that you brought up is that you want to do ‘local’
>>> joins.
>>>>> 
>>>>> Simple single word response… don’t.
>>>>> 
>>>>> Longer response..
>>>>> 
>>>>> You’re suggesting that the tables in question share the row key in
>>>>> common.  Ok… why? Are they part of the same record?
>>>>> How is the data normally being used?
>>>>> 
>>>>> Have you looked at column families?
>>>>> 
>>>>> The issue is that joins are expensive. What you’re suggesting is that
>>> as
>>>>> you do a region scan, you’re going to the other table and then try to
>>>> fetch
>>>>> a row if it exists.
>>>>> So its essentially for each row in the scan, try a get() which will
>>>> almost
>>>>> double the cost of your fetch. Then you have to decide how to do it
>>>>> locally. Are you really going to write a coprocessor for this?
>> (Hint:
>>> If
>>>>> this is a common thing. Then either the second table should be part
>> of
>>>> the
>>>>> first table in the same CF or as a separate CF. You need to rethink
>>> your
>>>>> schema.)
>>>>> 
>>>>> Does this make sense?
>>>>> 
>>>>>> On Apr 7, 2015, at 7:05 PM, Demai Ni <ni...@gmail.com> wrote:
>>>>>> 
>>>>>> hi, folks,
>>>>>> 
>>>>>> I have a question about region assignment and like to clarify some
>>>>> through.
>>>>>> 
>>>>>> Let's say I have a table with rowkey as "row00000 ~ row30000" on a
>> 4
>>>> node
>>>>>> hbase cluster, is there a way to keep data partitioned by range on
>>> each
>>>>>> node? for example:
>>>>>> 
>>>>>> node1:  <=row10000
>>>>>> node2:  row10001~row20000
>>>>>> node3:  row20001~row30000
>>>>>> node4:  >row30000
>>>>>> 
>>>>>> And even when one of the node become hotspot, the boundary won't be
>>>>> crossed
>>>>>> unless manually doing a load balancing?
>>>>>> 
>>>>>> I looked at presplit: { SPLITS => ['row100','row200','row300'] } ,
>>> but
>>>>>> don't think it serves this purpose.
>>>>>> 
>>>>>> BTW, a bit background. I am thinking to do a local join between two
>>>>> tables
>>>>>> if both have same rowkey, and partitioned by range (or same hash
>>>>>> algorithm). If I can keep the join-key on the same node(aka
>>>>> regionServer),
>>>>>> the join can be handled locally instead of broadcast to all other
>>>> nodes.
>>>>>> 
>>>>>> Thanks for your input. A couple pointers to blog/presentation would
>>> be
>>>>>> appreciated.
>>>>>> 
>>>>>> Demai
>>>>> 
>>>>> The opinions expressed here are mine, while they may reflect a
>>> cognitive
>>>>> thought, that is purely accidental.
>>>>> Use at your own risk.
>>>>> Michael Segel
>>>>> michael_segel (AT) hotmail.com
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>> 
>>> 
>> 

The opinions expressed here are mine, while they may reflect a cognitive thought, that is purely accidental. 
Use at your own risk. 
Michael Segel
michael_segel (AT) hotmail.com