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https://issues.apache.org/jira/browse/JENA-327?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15091142#comment-15091142
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Andy Seaborne commented on JENA-327:
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If the application really, really wants to get exclusive access, it can evict
the database with `StoreConnection`. It must ensure that no use is being made
of the database. This is an "at your own risk" feature.
The requirement for an exclusive lock for TDB is noted and is provided in the
TDB2 work (not part of Jena at this time).
Otherwise the ability to copy files underneath a live instance of TDB is "won't
fix".
> TDB Tx transaction lock to permit backups
> -----------------------------------------
>
> Key: JENA-327
> URL: https://issues.apache.org/jira/browse/JENA-327
> Project: Apache Jena
> Issue Type: Improvement
> Components: TDB
> Affects Versions: TDB 0.9.4
> Reporter: Simon Helsen
>
> With large repositories, it is important to be able to create backups once in
> a while. This is because recreating an rdf store with millions of triples can
> be forbiddingly expensive. Moreover, it should be possible to take those
> backups while still allowing read activity on the store as in many cases, a
> complete shutdown is usually not possible. Before the introduction of tx, it
> was relatively straightforward to provide the right locks on the client-side
> to safely suspend any disk activity for a period of time enough to make a
> backup of the index.
> However, since tx, things have become slightly more complicated because TDB
> Tx touches the disk at other times than when performing write/sync
> activities. Right now, because of some understanding of how TDB Tx is
> implemented, it is still possible for clients to avoid disk activities to
> implement a backup process, but this dependency on TDB Tx implementation
> details is not very good. Moreover, we anticipate that in the future, the
> merging process from the journal into the main index may become entirely
> asynchornous for performance reasons. The moment that happens, client have no
> control anymore as to when the disk is being touched.
> For this reason, we are requesting the following feature: a "backup" lock (by
> lack of a better name). Its semantics is that when the lock is taken, TDB Tx
> guarantees that no disk activity takes place and if necessary pauses
> activities. In other words, no write transaction should be able to complete
> and read transactions will not attempt to merge the journal. The idea would
> be that regular read activities can still continue. The API could be as
> simple as something like this:
> try {
> dataset.begin(ReadWrite.BACKUP) ;
> <do whatever is necessary to backup the index>
> } finally {
> dataset.end()
> }
> As for the implementation, we suspect you currently have locks in place which
> could be used to guarantee this behavior. E.g. could
> txn.getBaseDataset().getLock().enterCriticalSection(Lock.WRITE) be sufficient?
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