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https://issues.apache.org/jira/browse/LUCENE-4731?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13565414#comment-13565414
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Shai Erera commented on LUCENE-4731:
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Why do you need such a Directory implementation? HDFS already does replication
(unless you turn it off), so I wonder what does that replication give you, that
HDFS replication doesn't?
> New ReplicatingDirectory mirrors index files to HDFS
> ----------------------------------------------------
>
> Key: LUCENE-4731
> URL: https://issues.apache.org/jira/browse/LUCENE-4731
> Project: Lucene - Core
> Issue Type: New Feature
> Components: core/store
> Reporter: David Arthur
> Fix For: 4.2, 5.0
>
> Attachments: ReplicatingDirectory.java
>
>
> I've been working on a Directory implementation that mirrors the index files
> to HDFS (or other Hadoop supported FileSystem).
> A ReplicatingDirectory delegates all calls to an underlying Directory
> (supplied in the constructor). The only hooks are the deleteFile and sync
> calls. We submit deletes and replications to a single scheduler thread to
> keep things serializer. During a sync call, if "segments.gen" is seen in the
> list of files, we know a commit is finishing. After calling the deletage's
> sync method, we initialize an asynchronous replication as follows.
> * Read segments.gen (before leaving ReplicatingDirectory#sync), save the
> values for later
> * Get a list of local files from ReplicatingDirectory#listAll before leaving
> ReplicatingDirectory#sync
> * Submit replication task (DirectoryReplicator) to scheduler thread
> * Compare local files to remote files, determine which remote files get
> deleted, and which need to get copied
> * Submit a thread to copy each file (one thead per file)
> * Submit a thread to delete each file (one thead per file)
> * Submit a "finalizer" thread. This thread waits on the previous two batches
> of threads to finish. Once finished, this thread generates a new
> "segments.gen" remotely (using the version and generation number previously
> read in).
> I have no idea where this would belong in the Lucene project, so i'll just
> attach the standalone class instead of a patch. It introduces dependencies on
> Hadoop core (and all the deps that brings with it).
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