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Date: Mon, 11 Jun 2012 09:17:42 +0000 (UTC)
From: "Nick North (JIRA)" <jira@apache.org>
To: dev@couchdb.apache.org
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Subject: [jira] [Created] (COUCHDB-1495) Allow the UUID algorithm to vary by
 database
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Nick North created COUCHDB-1495:
-----------------------------------

             Summary: 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


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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