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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-393) Allow multiple JVMs to have read-only access to the same directory-based database
Date Wed, 07 Apr 2010 17:46:34 GMT

     [ https://issues.apache.org/jira/browse/DERBY-393?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Mike Matrigali updated DERBY-393:

While these hacks may allow the access to procede the results are not going to be consistent
and may cause corruption.

Here are some of the problems:

1) When a database is opened in write mode, not all of the data is actually on disk.
    Some of the dirty data is in memory (in the database cache) only accessible to the jvm
that has opened the database for write.  So another db opening for write if allowed would
see inconsistent data.

2) a second db opening in read only mode still has to boot.  This boot process has to read
the log file and may need to write stuff into the log.  This can cause corruption when it
conflicts with the write db.  

> Allow multiple JVMs to have read-only access to the same directory-based database
> ---------------------------------------------------------------------------------
>                 Key: DERBY-393
>                 URL: https://issues.apache.org/jira/browse/DERBY-393
>             Project: Derby
>          Issue Type: Improvement
>          Components: Store
>            Reporter: Trejkaz
>         Attachments: readonly.patch
> For an application I'm building, we needed to permit multiple JVMs to access the same
> We couldn't easily use a network server configuration, as it would be difficult to figure
out who to connect to since either user might want to view the database while the other database
is offline.
> We couldn't just dump all the data in a JAR file, as our databases often end up being
several gigabytes in size.
> So what we really need is a version of the directory store which is treated as if it
were read-only.

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