Mailing-List: contact user-help@cassandra.apache.org; run by ezmlm
Precedence: bulk
Reply-To: user@cassandra.apache.org
Received-SPF: pass (nike.apache.org: domain of jiangc@gmail.com designates
 209.85.220.172 as permitted sender)
MIME-Version: 1.0
Date: Mon, 29 Aug 2011 17:03:14 -0400
Message-ID: 
 <CAKK9U-AgP+CzaxOxUDeXfQ8ycN_8YSsufj00p=O-Le=92BD__w@mail.gmail.com>
Subject: Updates lost
From: Jiang Chen <jiangc@gmail.com>
To: user@cassandra.apache.org
Content-Type: text/plain; charset=ISO-8859-1

Hi,

Just started developing using Cassandra (0.8.4). I noticed when
updating the same row and column repeatedly, say, in a test case,
updates may get lost. I found it in a Java client but the following
python script also exhibits the same problem.

******************************************************************
import pycassa
import time

pool = pycassa.ConnectionPool('TestKeySpace', server_list=['localhost'])

cf = pycassa.ColumnFamily(pool, 'blah')

for i in range(1, 1000):
	cf.insert('foo', {'body': 'aaa'})
	assert 'aaa'==cf.get('foo')['body']	
	cf.insert('foo', {'body': 'bbb'})
	assert 'bbb'==cf.get('foo')['body']
	#time.sleep(0.015)

	print i
******************************************************************

The script errors out at assertion, when the value is supposed to be
updated back to 'aaa' from 'bbb'

******************************************************************
$ python test.py
1
Traceback (most recent call last):
  File "test.py", line 10, in <module>
    assert 'aaa'==cf.get('foo')['body']
AssertionError
******************************************************************

If the client sleeps for a few ms at each loop, the success rate
increases. At 15 ms, the script always succeeds so far. Interestingly,
the problem seems to be sensitive to alphabetical order. Updating the
value from 'aaa' to 'bbb' never has problem. No pause needed.

Is this a real problem or I'm missing something about how Cassandra
works? Only one Cassandra instance is used in the test.

Thanks.

Jiang