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From Matthias Pfau <p...@l3s.de>
Subject Re: Storing pre-sorted data
Date Thu, 13 Oct 2011 14:03:13 GMT
Hi Zach,
thanks for that good idea. Unfortunately, our list needs to be rewritten 
often because our data is far away from being evenly distributed.

However, we could get this under control but there is a more severe 
problem: Random access is very hard to implement on a structure with 
undefined distances between two following index numbers. We absolutely 
need random access because the lists are too big to do this on the 
application side :-(

Kind regards
Matthias

On 10/13/2011 02:30 PM, Zach Richardson wrote:
> Matthias,
>
> This is an interesting problem.
>
> I would consider using long's as the column type, where your column
> names are evenly distributed longs in sort order when you first write
> your list out.  So if you have items A and C with the long column
> names 1000 and 2000, and then you have to insert B, it gets inserted
> at 1500.  Once you run out of room between any two column name
> entries, i.e 1000, 1001, 1002 entries are all taken at any spot in the
> list, go ahead and re-write the list.
>
> If your unencrypted data is uniformly distributed, you will have very
> few collisions on your column names and should not have to re-write
> the list to often.
>
> If your lists are small enough, then you could use ints to save space,
> but will then have to re-write the list more often.
>
> Thanks,
>
> Zach
>
> On Thu, Oct 13, 2011 at 2:47 AM, Matthias Pfau<pfau@l3s.de>  wrote:
>> Hi Stephen,
>> this is a great idea but unfortunately doesn't work for us either as we can
>> not store the data in an unencrypted form.
>>
>> Kind regards
>> Matthias
>>
>> On 10/12/2011 07:42 PM, Stephen Connolly wrote:
>>>
>>> could you prefix the data with 3-4 bytes of a linear hash of the
>>> unencypted data? it wouldn't be a perfect sort, but you'd have less of a
>>> range to query to get the sorted values?
>>>
>>> - Stephen
>>>
>>> ---
>>> Sent from my Android phone, so random spelling mistakes, random nonsense
>>> words and other nonsense are a direct result of using swype to type on
>>> the screen
>>>
>>> On 12 Oct 2011 17:57, "Matthias Pfau"<pfau@l3s.de<mailto:pfau@l3s.de>>
>>> wrote:
>>>
>>>     Unfortunately, that is not an option as we have to store the data in
>>>     an compressed and encrypted and therefore binary and non-sortable form.
>>>
>>>     On 10/12/2011 06:39 PM, David McNelis wrote:
>>>
>>>         Is it an option to not convert the data to binary prior to
>>> inserting
>>>         into Cassandra?  Also, how large are the strings you're sorting?
>>>           If its
>>>         viable to not convert to binary before writing to Cassandra, and
>>>         you use
>>>         one of the string based column ordering techniques (utf8, ascii,
>>> for
>>>         example), then the data would be sorted without you  needing to
>>>         specifically worry about that.  Of course, if the strings are
>>>         lengthy
>>>         you could run into  additional issues.
>>>
>>>         On Wed, Oct 12, 2011 at 11:34 AM, Matthias Pfau<pfau@l3s.de
>>>         <mailto:pfau@l3s.de>
>>>         <mailto:pfau@l3s.de<mailto:pfau@l3s.de>>>  wrote:
>>>
>>>             Hi there,
>>>             we are currently building a prototype based on cassandra and
>>>         came
>>>             into problems on implementing sorted lists containing
>>>         millions of items.
>>>
>>>             The special thing about the items of our lists is, that
>>>         cassandra is
>>>             not able to sort them as the data is stored in a binary
>>>         format which
>>>             is not sortable. However, we are able to sort the data
>>>         before the
>>>             plain data gets encoded (our application is responsible for
>>>         the order).
>>>
>>>             First Approach: Storing Lists in ColumnFamilies
>>>             ***
>>>             We first tried to map the list to a single row of a
>>>         ColumnFamily in
>>>             a way that the index of the list is mapped to the column
>>>         names and
>>>             the items of the list to the column values. The column names
>>> are
>>>             increasing numbers which define the sort order.
>>>             This has the major drawback that big parts of the list have
>>>         to be
>>>             rewritten on inserts (because the column names are numbered
>>>         by their
>>>             index), which are quite common.
>>>
>>>
>>>             Second Approach: Storing the whole List as Binary Data:
>>>             ***
>>>             We tried to store the compressed list in a single column.
>>>         However,
>>>             this is only feasible for smaller lists. Our lists are far
>>>         to big
>>>             leading to multi megabyte reads and writes. As we need to
>>>         read and
>>>             update the lists quite often, this would put our Cassandra
>>>         cluster
>>>             under a lot of pressure.
>>>
>>>             Ideal Solution: Native support for storing lists
>>>             ***
>>>             We would be very happy with a way to store a list of sorted
>>>         values
>>>             without making improper use of column names for the list
>>>         index. This
>>>             implies that we would need a possibility to insert values at
>>>         defined
>>>             positions. We know that this could lead to problems with
>>>         concurrent
>>>             inserts in a distributed environment, but this is handled by
>>> our
>>>             application logic.
>>>
>>>
>>>             What are your ideas on that?
>>>
>>>             Thanks
>>>             Matthias
>>>
>>>
>>>
>>>
>>>         --
>>>         *David McNelis*
>>>         Lead Software Engineer
>>>         Agentis Energy
>>>         www.agentisenergy.com<http://www.agentisenergy.com>
>>>         <http://www.agentisenergy.com>
>>>         c: 219.384.5143<tel:219.384.5143>
>>>
>>>         /A Smart Grid technology company focused on helping consumers of
>>>         energy
>>>         control an often under-managed resource./
>>>
>>>
>>>
>>
>>


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