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From "Jukka Zitting (JIRA)" <j...@apache.org>
Subject [jira] Resolved: (JCR-2760) Use hash codes instead of sequence numbers for string indexes
Date Tue, 05 Oct 2010 08:39:32 GMT

     [ https://issues.apache.org/jira/browse/JCR-2760?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Jukka Zitting resolved JCR-2760.

       Resolution: Fixed
    Fix Version/s: 2.2.0
         Assignee: Jukka Zitting

Patch committed in revision 1004568.

The restriction to 24 bits is an unfortunate side-effect of the way the bundle serialization
format chops off the first eight bits from the namespace index of the name of the primary
type of a node. As mentioned by Thomas, this has a notable effect on the likelihood of collisions,
but that seems OK since this only affects the non-standard use case of copying raw workspace
data between repositories and even there the chance of problems is pretty low for reasonably
sized repositories.

A more complete solution would be to drop the use of namespace and name indexes in favour
of a more efficient serialization format.

> Use hash codes instead of sequence numbers for string indexes
> -------------------------------------------------------------
>                 Key: JCR-2760
>                 URL: https://issues.apache.org/jira/browse/JCR-2760
>             Project: Jackrabbit Content Repository
>          Issue Type: Improvement
>          Components: jackrabbit-core
>            Reporter: Jukka Zitting
>            Assignee: Jukka Zitting
>            Priority: Minor
>             Fix For: 2.2.0
>         Attachments: 0001-JCR-2760-Use-hash-codes-instead-of-sequence-numbers.patch
> We use index numbers instead of namespace URIs or other strings in many places. The two-way
mapping between namespace URIs and index numbers is by default stored in the repository-global
ns_idx.properties file, and the index numbers are allocated using a linear sequence. The problem
with this approach is that two repositories will easily end up with different string index
mappings, which makes it practically impossible to make low-level copies of workspace content
across repositories.
> The ultimate solution for this problem would be to store the namespace URIs closer to
the stored content, ideally as an implementation detail of a persistence manager.
> An easier short-term solution would be to decrease the chances of two repositories having
different string index mappings. A simple (and backwards-compatible) way to do this is to
use the hash code of a namespace URI as the basis of allocating a new index number. Hash collisions
are fairly unlikely, and can be handled by incrementing the intial hash code until the collision
is avoided. In the common case of no collisions (with a uniform hash function the chance of
a collision is less than 1% even with tousands of registered namespaces) this solution allows
workspaces to be copied between repositories without worrying about the namespace index mappings.

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