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From Damien Katz <dam...@apache.org>
Subject Re: couchdb transactions changes
Date Sun, 08 Feb 2009 05:22:04 GMT

On Feb 7, 2009, at 11:59 PM, Antony Blakey wrote:

>
> On 08/02/2009, at 3:07 PM, Damien Katz wrote:
>
>>> I can't see why this needs to be the case. The fact that there are  
>>> peers that have an incomplete replication doesn't stop the master  
>>> taking updates - iff the updates to the master are multi-document  
>>> ACID.
>>
>> Because during downstream replication it may get just some of the  
>> documents from a commit that happens during replication.  
>> Replication isn't all or nothing, and it doesn't know about  
>> transactions
>
> OK, I would have thought that a) replication was relative to a  
> specific MVCC commit; and b) a given user-level transaction was all  
> within a specific MVCC commit. So the replicator sees at least an  
> MVCC consistent view of the updates. Actually, I can't see how  
> replication that doesn't respect MVCC commits can work at all,  
> because the resultant data would be inconsistent with even the  
> weaker ACID that you've proposed.

No, CouchDB replication doesn't support replicating the transactions.  
Never has, never will. That's more like transaction log replication  
that's in traditonal dbs, a different beast.

For the new bulk transaction model, I'm only proposing supporting  
eventual consistency. All changes are safe to disk, but the db may not  
be in a consistent state right away.

>
>
>> The thing is, to avoid all conflicts and inconsistent states, you  
>> must read lock the source db during active replication and write  
>> lock the target database pretty much until the replication is  
>> complete. Otherwise, you'll will get inconsistent states.
>
> Locking for writes is obvious, but read locking the source isn't.

As it works now, if the source is updated while replicating, the  
target may get some of the documents from those updates. Transactions  
don't replicate.

>
>
>> Does each user get their own local instance of the database?
>
> Not necessarily, and even if they did, it's likely that they'll have  
> multiple browser windows open.

What's the front end written in?

-Damien

>
>
> Antony Blakey
> -------------
> CTO, Linkuistics Pty Ltd
> Ph: 0438 840 787
>
> The ultimate measure of a man is not where he stands in moments of  
> comfort and convenience, but where he stands at times of challenge  
> and controversy.
>  -- Martin Luther King
>
>


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