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From "Nick North (JIRA)" <>
Subject [jira] [Commented] (COUCHDB-1495) Allow the UUID algorithm to vary by database
Date Mon, 11 Jun 2012 18:10:43 GMT


Nick North commented on COUCHDB-1495:

I don't want to waste a lot of other people's time flushing out my misunderstandings but,
as far as I can tell, all requests to create document ids go through couch_uuids:new, which
uses gen_server:call, regardless of the chosen algorithm. You can call random and utc_random
directly, but the latter seems never to be called externally, and the former is only used
externally for other purposes, like salt generation, and I would keep that functionality in
my proposal.
> Allow the UUID algorithm to vary by database
> --------------------------------------------
>                 Key: COUCHDB-1495
>                 URL:
>             Project: CouchDB
>          Issue Type: Improvement
>          Components: Database Core
>    Affects Versions: 1.2
>            Reporter: Nick North
>            Priority: Minor
>              Labels: uuid
> The UUID algorithm for generating document ids is currently a global setting, affecting
all databases. When returning to after
a long hiatus, I realised it might be useful to allow a default algorithm, with per-database
> I propose that the uuids section of the config be of the following form, modelled somewhat
on the compactions section:
> algorithm = AlgorithmName
> overrides = [ {DatabaseName, AlgorithmName, [ { ParamName, ParamValue}, {ParamName, ParamValue},
>                    {DatabaseName, AlgorithmName, [ { ParamName, ParamValue}, {ParamName,
ParamValue}, ...]}, ...]
> The "algorithm" element is exactly as it now is, specifying the default UUID algorithm,
to be used when the algorithm is not overridden and for responding to the _uuids HTTP request.
It also gives backwards compatibility.
> The optional "overrides" element specifies algorithms for named databases, each with
a list of algorithm-specific parameters. The list may be empty (or potentially absent altogether,
which would have the same meaning as an empty list).
> The special database name _default is used to specify a default algorithm that needs
parameters or instead or the "algorithm" element.
> Implementation would be by giving the couch_uuids gen_server a state consisting of a
dictionary mapping database names to an {AlgorithmName, AlgorithmState} pair, where AlgorithmState
is an algorithm-dependent value and the default algorithm is associated with database name
_default. Requests for a UUID for a specific database would pass in the database name for
use in determining the right algorithm. [This gives rise to a potential objection to the proposal,
as there is a performance hit: every new document id requires a dictionary lookup of the database
name, and possibly a second lookup of the _default entry if the name is not found; there may
also be a dictionary insertion of the new AlgorithmState value. However, I suggest that is
small compared to everything else that goes on when creating a document.]
> If people like this idea, I am happy to put forward some code, and wrap in the implementation
of, which I have neglected for far too
long. The two proposals are not necessarily tied together, but implementing this one changes
the implementation of the other so it is more convenient to do both together.

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