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From Ryan Rawson <ryano...@gmail.com>
Subject Re: incremental counters and a global String->Long Dictionary
Date Wed, 01 Dec 2010 03:03:48 GMT
CheckAndPut interprets a 'null' value argument as a check for
existence.  That is if you set the expected value to null it will only
succeed if the value does not exist.

Would that help?


On Tue, Nov 30, 2010 at 6:07 AM, Claudio Martella
<claudio.martella@tis.bz.it> wrote:
> Hi Dave,
> thanks for you idea. I also considered this possibility. Although the
> possibility of a collision is very small, what scares me is the fact
> that i don't think the corruption can be corrected.
> I can for sure detect it afterwards in O(NlogN) time by scanning the
> table, but correcting my long-based corpus is impossible. Once the
> database is converted, the information is lost.
> On 11/30/10 1:43 AM, Buttler, David wrote:
>> A while back I had a strange idea to bypass this problem: create a 64-bit hash code
for the word.  Your word space should be significantly smaller than 64 bits, so a good hash
algorithm (the top 64 bits of sha1 say) should make collisions extremely rare.  And, if you
can always check your dictionary later for collisions if this feels wrong.
>> This should be a good deal simpler than trying to keep around an order dependent
integer mapping for your dictionary.  And, it is somewhat recoverable if you ever lose your
dictionary for some reason.
>> Dave
>> -----Original Message-----
>> From: Claudio Martella [mailto:claudio.martella@tis.bz.it]
>> Sent: Monday, November 29, 2010 7:13 AM
>> To: user@hbase.apache.org
>> Subject: incremental counters and a global String->Long Dictionary
>> Hello list,
>> I'm kind of new to HBase, so I'll post this email with a request for
>> comment.
>> Very briefly, I do a lot of text processing with mapreduce, so it's very
>> useful for me to convert string to longs, so i can make my computations
>> faster.
>> My corpus keeps on growing and I want this String->Long mapping to be
>> persistent and dynamical (i want to add new mappings when i find new words).
>> At the moment i'm tackling the problem this way (pseudo-code):
>> longvalue = convert(word) # gets from hbase
>> if longvalue == -1:
>>     longvalue = insert(word) # puts in hbase
>> longvalue now contains the new mapped value. This approach requires a
>> global counter that saves the latest mapped long and increments at every
>> insert. I can easily do this two ways. A special row in hbase "_counter"
>> that I increment through IncrementColumnValue, or creating a sequential
>> non-ephemeral znode in zookeeper and use the version as my counter. The
>> first one is of course faster. So the solution would be:
>> insert(word):
>>     longvalue = hbase.incrementColumnValue("_counter", "v")
>>     hbase.put(word, longvalue)
>>     return longvalue
>> The problem is that between the time i realize there's no mapping for my
>> word and the time i insert the new longvalue, somebody else might have
>> done the same for me, so I have a corrupted dictionary.
>> One possible solution would be to acquire a lock on the "_counter" row,
>> recheck for the presence of the mapping and then insert my new value:
>> safe_insert(word):
>>     lock("_counter")
>>     longvalue = convert(word)
>>     if longvalue == -1: #nobody inserted the mapping in the meantime
>>         longvalue = insert(word)
>>     unlock("_counter")
>>     return longvalue
>> This way the counter row, with its lock, would behave as a global lock.
>> This would solve my problems but would create a bottleneck (although
>> with time my inserts tend to get very rare as the dictionary grows). A
>> solution to this problem would be to have locks on zookeeper based on words.
>> ZKsafe_insert(word):
>>     ZKlock("/words/"+ word)
>>     longvalue = convert(word)
>>     if longvalue == -1: #nobody inserted the mapping in the meantime
>>         longvalue = insert(word)
>>     ZKunlock("/words/"+word)
>>     return longvalue
>> This of course would allow me to have more finegrained locks and better
>> scalability, but I'd relay on a system with higher latency (ZK).
>> Does anybody have a better solution with hbase? I guess using
>> hbase_transational would also be a possibility, but again, what about
>> speed and the actual issues with the package (like recovering in the
>> face of hregion failure).
>> Thank you,
>> Claudio
> --
> Claudio Martella
> Digital Technologies
> Unit Research & Development - Analyst
> TIS innovation park
> Via Siemens 19 | Siemensstr. 19
> 39100 Bolzano | 39100 Bozen
> Tel. +39 0471 068 123
> Fax  +39 0471 068 129
> claudio.martella@tis.bz.it http://www.tis.bz.it
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