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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2373) Change StandardTermsDictWriter to work with streaming and append-only filesystems
Date Wed, 07 Apr 2010 10:10:33 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2373?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12854409#action_12854409
] 

Michael McCandless commented on LUCENE-2373:
--------------------------------------------

I would love to make Lucene truly write once (and moreve IndexOutput.seek), but... this approach
makes me a little nervous...

In some environments, relying on the length of the file to be accurate might be risky: it's
metadata, that can be subject to different client-side caching than the file's contents. 
EG on NFS I've seen issues where the file length was stale yet the file contents were not.

Maybe we could offer a separate codec that takes this approach, for use on filesystems like
HDFS that can't seek during write?  We should refactor standard codec so that "where this
long gets stored" can be easily overridden by a subclass.

Or, alternatively, we could write this "index of the index" to a separate file?

> Change StandardTermsDictWriter to work with streaming and append-only filesystems
> ---------------------------------------------------------------------------------
>
>                 Key: LUCENE-2373
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2373
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Andrzej Bialecki 
>             Fix For: 3.1
>
>
> Since early 2.x times Lucene used a skip/seek/write trick to patch the length of the
terms dict into a place near the start of the output data file. This however made it impossible
to use Lucene with append-only filesystems such as HDFS.
> In the post-flex trunk the following code in StandardTermsDictWriter initiates this:
> {code}
>     // Count indexed fields up front
>     CodecUtil.writeHeader(out, CODEC_NAME, VERSION_CURRENT); 
>     out.writeLong(0);                             // leave space for end index pointer
> {code}
> and completes this in close():
> {code}
>       out.seek(CodecUtil.headerLength(CODEC_NAME));
>       out.writeLong(dirStart);
> {code}
> I propose to change this layout so that this pointer is stored simply at the end of the
file. It's always 8 bytes long, and we known the final length of the file from Directory,
so it's a single additional seek(length - 8) to read it, which is not much considering the
benefits.

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