The goal of this page is to design/describe a more flexible indexing scheme for Lucene, as first described in #11 of Lucene2Whiteboard.
Since this is a significant change to Lucene and will require some planning, this page is intended to be the starting point of the design until some patches can be worked out.
There are currently two main areas of interest in regards to flexible indexing:
1. API for flexible Posting (indexing) options. Preliminary options, as detailed in ConversationsBetweenDougMarvinAndGrant, are:
b. <doc, boost>+
c. <doc, freq, <position>+ >+
d. <doc, freq, <position, boost>+ >+
These suggest the following booleans per field:
2. document boost
3. position (requires freq)
4. position boost (requires position)
2. Storing Index level metadata. This can be useful for storing information about the index, such as display name, internal name, name of analyzer used (if appropriate), collection statistics outside the scope of the index itself.
Flexible indexing implemented in 4.0-dev (trunk)
The overall goal is to make Lucene extensible, even at its lowest levels, on what it records into the index, and how. Your app should be able to easily store new things into the index, or, alter how existing things (doc IDs, positions, payloads, etc.) are encoded. To accomplish this, a Codec class was introduced. The Codec currently covers the postings API (fields, terms, docs, positions+payloads enumerators); other elements in the index (norms, deleted docs, stored docs/fields, term vectors) are not covered.
Changes in how postings are consumed
The first big change in flexible indexing is the consumption of the postings enumerators APIs:
A term is now an arbitrary byte, represented by a BytesRef (which references an offset + length "slice" into an existing byte). By default terms will be UTF8 encoded character string, created during indexing, but your analysis chain can produce a term that is not UTF8 bytes.
Fields are separately enumerated (via FieldsEnum) from term text. Consumers of the flex API no longer need to check Term.field() on each .next() call; instead, they obtain a TermsEnum for the specific field they need and iterate it until exhaustion.
TermsEnum iterates and seeks to all terms (returned as BytesRef) in the index. A TermsEnum is optionally able to seek to the ordinal (long) for the term, and return the ordinal for the current term. SegmentReader implements this but MultiReader does not because working with ords is far too costly (requires merging).
Deleted documents are no longer implicitly filtered by DocsEnum (previously TermDocs). Instead, you provide an arbitrary skipDocs bit set (Bits) stating which documents should be skipped during enumeration. For example, this could be used with a cached filter to enforce your own deletions. IndexReader.getDeletedDocs returns a Bits for the current deleted docs of this reader.
Seeking to a term is no longer done by the docs/positions enums; instead, you must use TermsEnum.seek and then TermsEnum.docs or .docsAndPositions to obtain the enumerator (there are also sugar APIs to accomplish this). TermsEnum's seek method has three return values: FOUND (the exact term matched), NOT_FOUND (another term matched) and END (you seek'd past the end of the enum).
Composite readers (currently MultiReader or DirectoryReader) are not able to provide these postings enumerators directly; instead, one must use the static methods on MultiFields to obtain the enumerators.
The second big change in flexible indexing is the Codec and CodecProvider APIs that enables apps to plug in different implementations for writing and reading postings data in the index. When you obtain an IndexWriter or IndexReader, you can optionally pass in a CodecProvider, which knows 1) which Codec should be used when writing a new segment, and 2) how to resolve the codec name (String) to a Codec instance, when reading from the index.
The default codec is StandardCodec, whose format is similar to the pre-4.0 index format, but introduces sizable improvements to how the terms index is stored. In particular the RAM required by the terms index when reading a segment has been substantially reduced. The on-disk format of the .tis/.tii files is also slightly smaller.
There are some experimental core codecs:
PulsingCodec stores rare terms directly into the terms dicts. This is an excellent match for primary key fields (see here for details), and should also help even "normal" fields (so this may become the default codec at some point).
SepCodec and IntBlockCodec are available for block-based codecs. These codes are not useful themselves, rather, they serve as the base for block-based codecs. These codecs separately store the docs, freqs, positions and payloads data, allowing for int block codecs to encode the docs, freqs and positions.
Apps can also create custom Codecs. Please report back if you do! All of these APIs are very new and need some good baking in time.