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From "Marvin Humphrey (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1458) Further steps towards flexible indexing
Date Tue, 25 Nov 2008 20:27:44 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1458?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12650722#action_12650722
] 

Marvin Humphrey commented on LUCENE-1458:
-----------------------------------------

> How about the caller provides a codec instance which when asked will
> return a TermsDictReader "matching" the codec that had been used to
> write the index?

OK, it makes sense to have the user access these capabilities via a single
handle at both index-time and search-time.  However, for Lucy/KS, the handle
should definitely be specified via the Schema subclass rather than via
constructor argument.

"Codec" isn't really the right name for this, though.  "IndexComponent",
maybe?  Lucy would have three main index components by default:
LexiconComponent, PostingsComponent, StorageComponent.  

{code}
// If Lucy's Schema class were implemented in Java instead of C...
abstract class Schema extends Obj {
  LexiconComponent lexiconComponent() { return new LexiconComponent(); }
  PostingsComponent postingsComponent() { return new PostingsComponent(); }
  StorageComponent storageComponent() { return new StorageComponent(); }
  ...
}
{code}

Auxiliary IndexComponents might include TermVectorsComponent,
SortCacheComponent, ColumnStrideComponent, RTreeComponent, etc.

Here's example code for overriding the default LexiconComponent:

{code}
// Implements term dictionary as a hash table with term texts as keys.
class HashLexiconComponent extends LexiconComponent {
  LexiconReader makeReader(InvIndex invindex, Snapshot snapshot) {
    SegInfos segInfos = Snapshot.getSegInfos();
    if (segInfos.size == 1) { 
      return (LexiconReader) new SegHashLexiconReader(invindex, snapshot);
    }
    else {
      return (LexiconReader) new MultiHashLexiconReader(invindex, snapshot);
    }
  }

  LexiconWriter makeWriter(InvIndex invindex, SegInfo segInfo) {
    return (LexiconWriter) new HashLexiconWriter(invindex, segInfo);
  }
}

// [User code]
class MySchema extends Schema {
  LexiconComponent lexiconComponent() {
    return (LexiconComponent) new HashLexiconComponent();
  }
  ...
}
{code}


> Further steps towards flexible indexing
> ---------------------------------------
>
>                 Key: LUCENE-1458
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1458
>             Project: Lucene - Java
>          Issue Type: New Feature
>          Components: Index
>    Affects Versions: 2.9
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>            Priority: Minor
>             Fix For: 2.9
>
>         Attachments: LUCENE-1458.patch, LUCENE-1458.patch, LUCENE-1458.patch, LUCENE-1458.patch
>
>
> I attached a very rough checkpoint of my current patch, to get early
> feedback.  All tests pass, though back compat tests don't pass due to
> changes to package-private APIs plus certain bugs in tests that
> happened to work (eg call TermPostions.nextPosition() too many times,
> which the new API asserts against).
> [Aside: I think, when we commit changes to package-private APIs such
> that back-compat tests don't pass, we could go back, make a branch on
> the back-compat tag, commit changes to the tests to use the new
> package private APIs on that branch, then fix nightly build to use the
> tip of that branch?o]
> There's still plenty to do before this is committable! This is a
> rather large change:
>   * Switches to a new more efficient terms dict format.  This still
>     uses tii/tis files, but the tii only stores term & long offset
>     (not a TermInfo).  At seek points, tis encodes term & freq/prox
>     offsets absolutely instead of with deltas delta.  Also, tis/tii
>     are structured by field, so we don't have to record field number
>     in every term.
> .
>     On first 1 M docs of Wikipedia, tii file is 36% smaller (0.99 MB
>     -> 0.64 MB) and tis file is 9% smaller (75.5 MB -> 68.5 MB).
> .
>     RAM usage when loading terms dict index is significantly less
>     since we only load an array of offsets and an array of String (no
>     more TermInfo array).  It should be faster to init too.
> .
>     This part is basically done.
>   * Introduces modular reader codec that strongly decouples terms dict
>     from docs/positions readers.  EG there is no more TermInfo used
>     when reading the new format.
> .
>     There's nice symmetry now between reading & writing in the codec
>     chain -- the current docs/prox format is captured in:
> {code}
> FormatPostingsTermsDictWriter/Reader
> FormatPostingsDocsWriter/Reader (.frq file) and
> FormatPostingsPositionsWriter/Reader (.prx file).
> {code}
>     This part is basically done.
>   * Introduces a new "flex" API for iterating through the fields,
>     terms, docs and positions:
> {code}
> FieldProducer -> TermsEnum -> DocsEnum -> PostingsEnum
> {code}
>     This replaces TermEnum/Docs/Positions.  SegmentReader emulates the
>     old API on top of the new API to keep back-compat.
>     
> Next steps:
>   * Plug in new codecs (pulsing, pfor) to exercise the modularity /
>     fix any hidden assumptions.
>   * Expose new API out of IndexReader, deprecate old API but emulate
>     old API on top of new one, switch all core/contrib users to the
>     new API.
>   * Maybe switch to AttributeSources as the base class for TermsEnum,
>     DocsEnum, PostingsEnum -- this would give readers API flexibility
>     (not just index-file-format flexibility).  EG if someone wanted
>     to store payload at the term-doc level instead of
>     term-doc-position level, you could just add a new attribute.
>   * Test performance & iterate.

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