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From Felipe Schmidt <felipef...@gmail.com>
Subject Re: Retrieving old data version for a given row
Date Wed, 16 May 2012 14:16:26 GMT
That was very helpfull, thank you very much!

I still have some questions:
-it is possible to make Cassandra keep old value data after flushing?
The same question for the memTable, before flushing. Seems to me that
when I update some tuple, the old data will be overwrited in memTable,
even before flushing.
-it is possible to scan values from the memtable, maybe using the
so-called Thrift API? Using the client-api I can just see the newest
data version, I can't see what's really happening with the memTable.

I ask that cause what I'll try to do is a Change Data Capture to
Cassandra and the answers will define what kind of aproaches I'm able
to use.

Thanks in advance.

Regards,
Felipe Mathias Schmidt
(Computer Science UFRGS, RS, Brazil)


2012/5/14 aaron morton <aaron@thelastpickle.com>:
> Cassandra does not provide access to multiple versions of the same column.
> It is essentially implementation detail.
>
> All mutations are written to the commit log in a binary format, see the
> o.a.c.db.RowMutation.getSerializedBuffer() (If you want to tail it for
> analysis you may want to change commitlog_sync in cassandra.yaml)
>
> Here is post about looking at multiple versions columns in an
> sstable http://thelastpickle.com/2011/05/15/Deletes-and-Tombstones/
>
> Remember that not all "versions" of a column are written to disk
>  (see http://thelastpickle.com/2011/04/28/Forces-of-Write-and-Read/). Also
> compaction will compress multiple versions of the same column from multiple
> files into a single version in a single file .
>
> Hope that helps.
>
>
> -----------------
> Aaron Morton
> Freelance Developer
> @aaronmorton
> http://www.thelastpickle.com
>
> On 14/05/2012, at 9:50 PM, Felipe Schmidt wrote:
>
> Yes, I need this information just for academic purposes.
>
> So, to read old data values, I tried to open the Commitlog using tail
> -f and also the log files viewer of Ubuntu, but I can not see many
> informations inside of the log!
> Is there any other way to open this log? I didn't find any Cassandra
> API for this purpose.
>
> Thanks averybody in advance.
>
> Regards,
> Felipe Mathias Schmidt
> (Computer Science UFRGS, RS, Brazil)
>
>
>
>
> 2012/5/14 zhangcheng2 <zhangcheng2@software.ict.ac.cn>:
>
> After compaciton, the old version data will gone!
>
>
> ________________________________
>
> zhangcheng2
>
>
> From: Felipe Schmidt
>
> Date: 2012-05-14 05:33
>
> To: user
>
> Subject: Retrieving old data version for a given row
>
> I'm trying to retrieve old data version for some row but it seems not
>
> be possible. I'm a beginner  with Cassandra and the unique aproach I
>
> know is looking to the SSTable in the storage folder, but if I insert
>
> some column and right after insert another value to the same row,
>
> after flushing, I only get the last value.
>
> Is there any way to get the old data version? Obviously, before compaction.
>
>
> Regards,
>
> Felipe Mathias Schmidt
>
> (Computer Science UFRGS, RS, Brazil)
>
>
>

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