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From Nicolas Lalevée (JIRA) <>
Subject [jira] Created: (LUCENE-662) Extendable writer and reader of field data
Date Tue, 22 Aug 2006 16:40:13 GMT
Extendable writer and reader of field data

                 Key: LUCENE-662
             Project: Lucene - Java
          Issue Type: Improvement
          Components: Store
            Reporter: Nicolas Lalevée
            Priority: Minor
         Attachments: generic-fieldIO.patch

As discussed on the dev mailing list, I have modified Lucene to allow to define how the data
of a field is writen and read in the index.

Basically, I have introduced the notion of IndexFormat. It is in fact a factory of FieldsWriter
and FieldsReader. So the IndexReader, the indexWriter and the SegmentMerger are using this
factory and not doing a "new FieldsReader/Writer()".

I have also introduced the notion of FieldData. It handles every data of a field, and also
the writing and the reading in a stream. I have done this way because in the current design
of Lucene, Fiedable is an interface, so methods with a protected or package visibility cannot
be defined.

A FieldsWriter just writes data into a stream via the FieldData of the field.
A FieldsReader instanciates a FieldData depending on the field name. Then it use the field
data to read the stream. And finnaly it instanciates a Field with the field data.

About compatibility, I think it is kept, as I have writen a DefaultIndexFormat that provides
some DefaultFieldsWriter and DefaultFieldsReader. These implementations do the exact job that
is done today.
To acheive this modification, some classes and methods had to be moved from private and/or
final to public or protected.

About the lazy fields, I have implemented them in a more general way in the implementation
of the abstract class FieldData, so it will be totally transparent for the Lucene user that
will extends FieldData. The stream is kept in the fieldData and used as soon as the stringValue
(or something else) is called. Implementing this way allowed me to handle the recently introduced
LOAD_FOR_MERGE; it is just a lazy field data, and when read() is called on this lazy field
data, the saved input stream is directly copied in the output stream.

I have a last issue with this patch. The current design allow to read an index in an old format,
and just do a writer.addIndexes() into a new format. With the new design, you cannot, because
the writer will use the FieldData.write provided by the reader.

enjoy !

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