add XA transaction support [LUCENE-1149]

Need to add XA transaction support to Lucene.

Without XA support, it is difficult to keep disparate resources (e.g. database) in sync with the Lucene index.

A review of the XA support added to Hibernate might be a good start (although Hibernate almost always uses a XA capable backing store database).

It would be ideal to have a combined IndexReaderWriter instance, then create a XAIndexReaderWriter which wraps it.

The implementation might be as simple as a XA log file which lists the XA transaction id, and the segments XXX number(s), since Lucene already allows you to rollback to a previous version (??? for sure, or does it only allow you to abort the current commit).

If operating under a XA transaction, the no explicit commits or rollbacks should be allowed on the instance.

The index would be committed during XA prepare(), and then if needed rolledback when requested. The XA commit() would be a no-op.

There is a lot more to this but this should get the ball rolling.

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Migrated from [LUCENE-1149](https://issues.apache.org/jira/browse/LUCENE-1149) by robert engels, 2 votes, updated May 16 2011

No implementation files or tests are named. Start by reviewing the XA support added to Hibernate and Lucene's existing commit and rollback behavior; completion would require an XA-aware IndexReaderWriter design with prepare, commit, rollback, and restrictions on explicit commits or rollbacks during XA transactions.

Written by the indexing model from the issue text.