OpenSearch Lucene Study Group Meeting - Monday, January 8th, 2024

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Link to previous meeting’s post: OpenSearch Lucene Study Group Meeting - Monday, December 18th, 2023

Welcome to the OpenSearch Lucene Study Group!

Apache Lucene is the open-sourced search library that powers OpenSearch and many search applications large and small.

We start the meeting with a Lucene learning topic or Q&A session. In the second half of the meeting, we review recent developments in Apache Lucene and discuss their potential impact to OpenSearch, with a particular focus on new and exciting Lucene features that we can (and should) expose through OpenSearch. Since some changes require a deep dive to fully understand, we sometimes ask participants to volunteer for “homework” to dig deeper into changes and report back for the next meeting.

Standing Agenda:

  • Welcome / introduction (5 minutes)
  • Lucene learning series - someone will either present a Lucene-related talk or we will do Lucene Q&A (20 minutes, recorded)
  • Review assigned issues from last time (10 minutes)
  • Review new Lucene changes and assign homework (20 minutes)

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I’m not going to bother with the fancy HTML table for changes this week, because there is only one change list entry for this week:

It’s in the “Optimizations” section for Lucene 9.10.

For the learning part of the meeting, @reta followed up on his homework from the previous meeting, to review He opened [Feature Request] Explore the use of hidden classes (JDK-15 and above) for Painless script classes generation. · Issue #11800 · opensearch-project/OpenSearch · GitHub to investigate how we can apply a similar approach to compilation of Painless scripts in OpenSearch.

I then did a quick code dive, looking at how Lucene layers abstractions to get from files on disk to structures that help process search queries. As a quick recap, those abstractions are:

  • Directory: Models a (platform-independent) file system directory, with simple operations like listing available files, deleting files, and opening files for read/write.
  • IndexInput / IndexOutput: Abstraction for a file reading/writing, returned from a Directory. IndexInput supports "slicing", where you can get an IndexInput corresponding to a byte range from another IndexInput. This is how "compound file segments" (.cfs) files work. Essentially, they're a bunch of segment files concatenated together, with a table of contents (with byte offsets) at the end of the file. When reading, Lucene passes a slice from the CFS index input to a reader for a particular data structure, and it's as though that byte range were its own file. Note that IndexInput and IndexOutput also provide (via inheritance from DataInput/DataOutput) methods to read/write frequently-used primitives as bytes (e.g. VInt, ZInt, string maps, arrays of numeric types).
  • Codecs: Codecs are the (Lucene version-specific) accumulation of readers/writers for various data structures used by the higher-level query logic. Each data structure tends to have an associated "[Version][Type]Format" class that either directly implements reading/writing or can return readers and writers.
  • Tying it together: Many Lucene data structures are stored off-heap, which is achieved by making the reader implement the data structure's interface, so navigating the data structure makes the (Codec-specific) reader move through the IndexInput, which is usually a memory-mapped file supplied by MMapDirectory.