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From Antony Blakey <antony.bla...@gmail.com>
Subject Re: single node atomic bulk_docs operations
Date Tue, 17 Mar 2009 14:18:24 GMT

On 17/03/2009, at 11:46 PM, Dean Landolt wrote:

> Interesting read -- and good examples. But I would argue there are  
> more than
> philosophical drawbacks. As I understand it, it would mean giving up  
> the
> replication feature entirely. Forever...or at least as long as bulk- 
> docs are
> relied upon. There's more to replication than scaling (fault  
> tolerance, for
> one). If your application absolutely needs transactions, and you can't
> design around it (e.g. doc-level transactions), you may need another  
> tool
> for the job -- one not named for a "cluster of unreliable commodity
> hardware".

You can do atomic local commits, and even replicate those commits, on  
a cluster of unreliable commodity hardware. This issue is not one of  
technical impossibility, rather it is a choice for CouchDB to pursue  
one particular model in this problem space. CouchDB is at one extreme  
of the spectrum, and that extreme is certainly worth pursuing. There  
are different choices with different trade-offs that have most of the  
CouchDB flavour.

The issue as I see it (I've been arguing/considering this particular  
issue for some time now) is that CouchDB is also intended to represent  
one consistent model, in the sense that it is a product, not a  
toolkit, and so a configuration option to modify those semantics isn't  
in consideration for the product branded 'CouchDB'.

I think that keeping these two points explicit results in simpler and  
more transparent reasoning around issues *such as* atomicity - in  
particular, why it will never happen.

As far as rollback is concerned, the problem is that concurrent  
accessors can see the intermediate state. There is no way to fake ACID  
without serializing all access (not always feasible), and even then  
there are failure modes in a clustered situation equivalent to  
transaction monitor lockup.

Antony Blakey
--------------------------
CTO, Linkuistics Pty Ltd
Ph: 0438 840 787

The trouble with the world is that the stupid are cocksure and the  
intelligent are full of doubt.
   -- Bertrand Russell



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