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From Paul Davis <paul.joseph.da...@gmail.com>
Subject Re: couchdb transactions changes
Date Mon, 09 Feb 2009 07:55:46 GMT
On Mon, Feb 9, 2009 at 2:40 AM, Antony Blakey <antony.blakey@gmail.com> wrote:
>
> On 09/02/2009, at 5:45 PM, Paul Davis wrote:
>
>> A write takes the most recent status of the database. It performs the
>> write using the append only semantics of editing btrees. When the
>> write completes it uses an atomic write to the db header. This means
>> that no matter what, new readers get a consistent view of the entire
>> database.
>
> The atomic write of the root is the commit. A reader, by virtue of an atomic
> read of the header, sees a commit point.
>
>> As I read your emails you seem to be assuming that CouchDB could walk
>> back through the valid database commits. As far as I understand, this
>> is not possible given the current database format. Furthermore, making
>> it possible would require a large amount of engineering to accomplish.
>
> No, you don't have to walk the commits. There is no record of commits,
> except in as much as you might have a number of different roots in use by
> concurrent processes at any given time e.g. multiple commit points are an
> ephemeral thing. There is only every *one* durable commit point.
>
>> AFAIK, we supported inter-document consistency to a single node. Now
>> that we're more seriously contemplating multi-node setups its becoming
>> apparent that the single the atomicity was a special case when it can
>> be violated by something as simple as a replication.
>
> Well, I believe I've shown that a simple change can make replication
> (optionally) respect MVCC commit points, involves very little change to the
> source algorithm, doesn't impact the current semantics at all unless you
> wish it to, and works on a per-replication request basis.
>
> This is orthogonal to the problem of cluster-ACID, which is also do-able,
> but I'm trying to work through this replication issue right now.
>
>> I'm uncertain by what you mean by 'replication model'.
>
> According to my use-case list e.g whether replication is exclusive with
> normal operation, and whether it can result in conflict (i.e. Single master
> deployments).
>
>> My current
>> understanding of replication is that it violates the promises of
>> _bulk_docs. As Damien mentions further down, to support what you're
>> asking for, you more or less need to repeat all _bulk_docs calls to
>> your central server in app code. This is quite possible. If enough
>> other people chimed in and voiced an opinion that this is something
>> they are interested in, I can see it as a valid reason for supporting
>> _bulk_docs like functionality in the future.
>
> I don't want to replicate reified transactions. The current state of the
> source wrt. an MVCC commit point is all that is required iff your MVCC
> commit point is exposed to the user. You can build local transactions and a
> useful (NOT generic) form of distributed transactional consistency on top of
> that.
>
>> If it's trivial, then post a patch to JIRA.
>
> We're discussing a proposed patch, against which this idea would be a patch
> :)
>
> I'm just addressing the idea that you can't compact the source database
> while replication occurs if replication is made MVCC aware.
>
>> The thing is, your interpretation is asking CouchDB to prove the CAP
>> theorem incorrect.
>
> Not at all. I'm saying that there are application/deployment models and
> use-cases that
>
> a) distinguish between replication and normal operation e.g. thesystem moves
> from normal, conflict free operation, to replication, to
> conflict-resolution, back to normal operation; and/or
>
> b) have a model that doesn't generate replication conflicts e.g.
> single-master replication doesn't fall under CAP.
>
> Antony Blakey
> -------------
> CTO, Linkuistics Pty Ltd
> Ph: 0438 840 787
>
> One should respect public opinion insofar as is necessary to avoid
> starvation and keep out of prison, but anything that goes beyond this is
> voluntary submission to an unnecessary tyranny.
>  -- Bertrand Russell
>
>
>

I would be convinced by an implementation. Until then I'll remain skeptical.

HTH,
Paul Davis

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