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https://issues.apache.org/jira/browse/JENA-327?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13464008#comment-13464008
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Andy Seaborne commented on JENA-327:
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Re: READ transaction backup.
This is the most stable and it means the disk format, or even implementing
system, can be changed as the backup is in a standard format, N-Quads.
And the system can keep running at the same time.
It's not as fast as a disk copy, but then a disk copy requires locking out
writers and also needs to stop the system doing any write-back. A WRITE
transaction currently does that, but that is knowing the internal details.
> 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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