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From Antony Blakey <>
Subject Re: Replication and forms of weak consistency
Date Mon, 16 Feb 2009 23:10:09 GMT

On 17/02/2009, at 9:26 AM, Robert Newson wrote:

>> The point of doing it in the server is that every node guarantees  
>> Monotonic
>> Writes, and you never have to worry about failing because you can't  
>> achieve
>> it. Given the current server implementation, IMO it's more likely  
>> than not
>> that you would fail.
> This comment leaves me puzzled which is entirely the fault of my poor,
> tired brain.

More likely my poor explanation. Current CouchDB replication semantics  
are very unfriendly to Monotonic Writes, because they only replicate  
the data of the head revision (and conflict sources). Compaction makes  
it theoretically worse by deleting data that wouldn't get replicated,  
but I say theoretically because compaction is only a barrier to fixing  
the problem - replication already acts as though constant compaction  
was occurring. The proposed revision stemming does however make it  
practically worse because all records of writes disappear from a  
prefix of the document revisions, and such removal occurs at multiple  
unaligned points in the linear write log. There's no equivalent to  
Bayou's handling of log truncation.

I haven't described an alternative to the current multi-node proposal  
(which isn't a formal proposal). My comments purely concern the  
current implementation of single-node operation and replication.

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

He who would make his own liberty secure, must guard even his enemy  
from repression.
   -- Thomas Paine

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