Portable index format [LUCENE-3491]

This issue presents the idea of a special-purpose Lucene index format that is suitable for cross-version compatibility and resilient to changes in Lucene implementations. Please see the attached document for more details.

The code needs branch lucene-2621 to compile and run. It's still incomplete, in a roughly the same way as the Codec api is still incomplete ;) However, a separate tool PortableIndexExporter was implemented to illustrate how serialization of all index parts could look like.

Comments and suggestions are welcome!

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Migrated from [LUCENE-3491](https://issues.apache.org/jira/browse/LUCENE-3491) by Andrzej Bialecki (@sigram), 1 vote, updated Dec 09 2011
Attachments: [lucene-portable.tgz](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3491/lucene-portable.tgz), [portable.pdf](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3491/portable.pdf)