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From Mahesh Paolini-Subramanya <>
Subject Re: Proposal: Extending immutability
Date Mon, 05 Jan 2009 18:05:05 GMT
Mahesh Paolini-Subramanya
CTO,  Aptela Inc.
(703.386.1500 x9100)

On Jan 5, 2009, at 8:26 AM, Antony Blakey wrote:

> I've cc'd this to couchdb-user, because I think this discussion
> belongs on -dev, but everyone watches -user.
> One of the great features of Couch is the use of optimistic locking
> i.e. rev as a bedrock mechanism, and the way this is permeated through
> the API. The combination of id + rev is a reference to an immutable
> value (with some caveats, one subject of this proposal). This means
> that you get caching for free. By keying off id + rev, you can cache
> the document along with any (functional) derived values. Additionally
> you can trivially memoize functions of multiple documents using that
> mechanism.
> I use this to good effect in my application, where I aggressively
> cache the documents (which are sometimes large) and therefore don't
> need the document content in queries. To take advantage of this
> however this means that my views need to include the _rev as the
> value, and transformation that would normally happen in the map
> happens in the client.
> It would be very useful to have the rev returned wherever an id is
> returned, specifically in view results. You could then use a view
> without include_docs, and get the ids and revs. You can keep a cache
> (per view, pre db) of the results. The actual view results only need
> to be fetched on a cache miss, which can be driven by the cache
> machinery.
> The nice thing is that all of this caching machinery can be
> transparently interposed. Except when the view definition is changed.
> So I also propose to have the rev and id of the design doc returned in
> the view results. And for completeness, every database should be
> assigned a UUID when it is created. This UUID should be provided in
> the dbinfo, and for every view and view-like result.
> This means that from every view result you can construct a list of
> universally unique references to immutable values e.g. DB UUID + (View
> id + rev) + (Document id + rev). A form of referential transparency -
> and with a cache and a little bit of 100% generic machinery, it can be
> true referential transparency. Clients don't have to watch/be notified
> about changes to design docs, or even database creation/deletion.
> Systemwide transparent caching in particular becomes trivial.
> So, in summary I propose:
> 1. Provide the document rev whenever the id is returned, such as view
> results i.e. not in the document, but in the per-row hash.
> 2. Provide the design document id and rev in view results i.e. in the
> top level hash.
> 3. Add a UUID to databases, and provide that in view results i.e. in
> the top level hash, and all other database operation results.
> I think you could do this even with reduce results, but I haven't
> though a lot about it.
> I think this generalised the current API in a very useful way, that
> will greatly simplify, and hence 'robustify' client code. Although I
> haven't checked the implementation code, my experience so far suggest
> this isn't difficult.
> Antony Blakey
> -------------
> CTO, Linkuistics Pty Ltd
> Ph: 0438 840 787
> Every task involves constraint,
> Solve the thing without complaint;
> There are magic links and chains
> Forged to loose our rigid brains.
> Structures, structures, though they bind,
> Strangely liberate the mind.
>   -- James Fallen

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