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From "Paul Joseph Davis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (COUCHDB-1495) Allow the UUID algorithm to vary by database
Date Mon, 11 Jun 2012 16:38:43 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-1495?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13292866#comment-13292866
] 

Paul Joseph Davis commented on COUCHDB-1495:
--------------------------------------------

More important than the dictionary lookup is the message passes between the uuid gen_server
and the requesting processes. Also important is that the obvious gen_server:call implementation
would eventually blow that gen_server out of the water.

I think the idea is good and there are a number of other settings that people have requested
be database specific as well. The issue is that our config system was never designed to be
parameterized by the database name. There are only one or two db specific options and those
are stored in the database header which means its not going to be a good way to scale out
the db specific stuff.

So while there's been a decent number of requests for such a thing there hasn't been a very
good discussion on actually implementing generalized db specific options. I'd think that figuring
that out would be the larger priority so that we don't have to have a specialized implementation
for individual options.
                
> Allow the UUID algorithm to vary by database
> --------------------------------------------
>
>                 Key: COUCHDB-1495
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-1495
>             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 https://issues.apache.org/jira/browse/COUCHDB-1373 after
a long hiatus, I realised it might be useful to allow a default algorithm, with per-database
overrides.
> 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 https://issues.apache.org/jira/browse/COUCHDB-1373, 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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