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From William Oberman <ober...@civicscience.com>
Subject Re: Consistency model
Date Sun, 17 Apr 2011 16:15:08 GMT
Cool, that is exactly what I was thinking/expecting, and I feel better about
the assumptions I've used in my refactor to move from MySQL -> cassandra.

I basically gave up on trying to force my code to assume (even at quorum):
-read value(s) for key from cassandra
-update/add/delete columns for key in memory based on value(s)
-write new value(s) for key to cassandra
That the next read for key in the same process would be equal to the new
value(s).  But I do assume I read value(s) >= the new value(s). Of course,
in another process, I don't assume anything (e.g. my application is ok
reading the same state A in two processes and updating the state to either B
or C.  I figured that's the best a distributed algorithm could do without a
"test and set" operation).

For a second, I thought this thread was saying I could see value(s) < new
value(s) within the same process on the second read at quorum, which would
have been bad for me.  But this thread is not saying that, so I'm still
happy with cassandra :-)

will

On Sun, Apr 17, 2011 at 11:24 AM, Milind Parikh <milindparikh@gmail.com>wrote:

> William
> The issue is regarding whether you will see A or B; with any guarantee of
> either. The discussion implies no; until the QW is complete.
>
> /***********************
> sent from my android...please pardon occasional typos as I respond @ the
> speed of thought
> ************************/
>
> On Apr 17, 2011 7:48 AM, "William Oberman" <oberman@civicscience.com>
> wrote:
>
> I'm pretty new to all of this, and I'm in the process of building my mental
> model of Cassandra, but I'm still feeling better about this thread. The way
> I figure it:
>  1. I'm trying to mutate the state of a key's column from A to B from a
> thread somewhere (quorum)
> 2. I'm trying to read the state of a key from a thread somewhere else
> (quorum)
>
> If #1 succeeds I'm guaranteed to see B. If #1 fails (with an exception)
> I'll see either A or B. I think I was concerned about that, and wanted to
> see A in #2 until success in #1.  But, I wanted to get to state B, and if #1
> retries until guaranteed success, do I care if I set B earlier than I
> expected?  I'm thinking no.
>
> I guess in terms of distributed algorithms/reasoning about systems, I'm
> feeling ok with this level of guarantee (again, given the failed write tells
> the client code of the undefined state).
>
>
>
> On Apr 17, 2011, at 10:10 AM, James Cipar <jcipar@cmu.edu> wrote:
>
> > I'm pretty new to Cassandra, ...
>
>


-- 
Will Oberman
Civic Science, Inc.
3030 Penn Avenue., First Floor
Pittsburgh, PA 15201
(M) 412-480-7835
(E) oberman@civicscience.com

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