Recent Additions to Lucene Arsenal
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The document discusses advancements in Lucene and Solr, focusing on replication components such as the replicator, replication client, and various handlers for index and taxonomy replication. It explains the anatomy of a Lucene index, highlighting the importance of segment order and strategies for optimizing retrieval speeds, including early termination and sorting techniques. Future work includes exploring parallel replication and strategies for handling index speed versus search speed.
Recent Additions to Lucene Arsenal
- 2. Recent Additions to Lucene’s Arsenal Shai Erera, Researcher, IBM Adrien Grand, ElasticSearch
- 3. Who We Are • Shai Erera – – – – • Working at IBM – Information Retrieval Research Lucene/Solr committer and PMC member http://shaierera.blogspot.com [email protected] Adrien Grand – – – @jpountz Lucene/Solr committer and PMC member Software engineer at Elasticsearch
- 6. Failover
- 7. Index Backup
- 9. Replication Components • Replicator – – – • Revision – – • Describes a list of files and metadata Responsible to ensure the files are available as long as clients replicate it ReplicationClient – – – • Mediates between the client and server Manages the published Revisions Implementation for replication over HTTP Performs the replication operation on the replica side Copies delta files and invokes ReplicationHandler upon successful copy Always replicates latest revision ReplicationHandler – Acts on the copied files
- 10. Index Replication • IndexRevision – – • IndexReplicationHandler – – – • Obtains a snapshot on the last commit through SnapshotDeletionPolicy Released when revision is released by Replicator Copies the files to the index directory and fsync them Aborts (rollback) on any error Upon successful completion, invokes a callback (e.g. SearcherManager.maybeRefresh()) Similar extensions for faceted index replication – – IndexAndTaxonomyRevision: obtains snapshots on both the search and taxonomy indexes IndexAndTaxonomyReplicationHandler: copies the files to the respective directories, keeping both in sync
- 11. Sample Code // Server-side: publish a new Revision Replicator replicator = new LocalReplicator(); replicator.publish(new IndexRevision(indexWriter)); // Client-side: replicate a Revision Replicator replicator; // either LocalReplicator or HttpReplicator // refresh SearcherManager after index is updated Callable<Boolean> callback = new Callable<Boolean>() { public Boolean call() throws Exception { // index was updated, refresh manager searcherManager.maybeRefresh(); } } ReplicationHandler handler = new IndexReplicationHandler(indexDir, callback); SourceDirectoryFactory factory = new PerSessionDirectoryFactory(workDir); ReplicationClient client = new ReplicationClient(replicator, handler, factory); client.updateNow(); // invoke client manually // -- OR -client.startUpdateThread(30000); // check for updates every 30 seconds
- 12. Future Work • Resume – – • Parallel Replication – • Session level: don’t copy files that were already successfully copied File level: don’t copy file parts that were already successfully copied Copy revision files in parallel Other replication strategies – Peer-to-peer
- 13. Index Sorting How to trade index speed for search speed
- 14. Anatomy of a Lucene index Index = collection of immutable segments Segments store documents sequentially on disk Add data = create a new segment Segments get eventually merged together Order of segments / documents in segments doesn’t matter – the following segments are equivalent Id 1 3 10 4 7 20 42 11 9 8 15 18 30 31 99 5 12 Price 9 0 7 8 2 2 1 8 10 3 4 4 6 10 1 1 13 Id 12 9 31 1 4 11 10 30 15 18 8 7 20 42 99 5 3 Price 13 10 10 9 8 8 7 6 4 4 3 2 2 1 1 1 0
- 15. Anatomy of a Lucene index ordinal of a doc in a segment = doc id used in the inverted index to refer to docs shoe 1, 3, 5, 8, 11, 13, 15 doc id 0 1 2 3 4 5 7 8 9 10 11 12 13 14 15 16 Id 1 3 10 4 7 20 42 11 9 8 15 18 30 31 99 5 12 Price 9 0 7 8 2 2 10 3 4 1 13 6 1 8 4 6 10 1
- 16. Top hits Get top N=2 results: – Create a priority queue of size N – Accumulate matching docs Id 1 3 10 4 7 20 42 11 9 8 15 18 30 31 99 5 12 Price 9 0 7 8 2 2 1 8 10 3 4 4 6 10 1 1 13 () (3) Create an empty priority queue (3,4) (4,20) (4,9) Automatic overflow of the priority queue to remove the least one (4,9) (9,31) (9,31) Top hits
- 17. Early termination Let’s do the same on a sorted index Id 12 9 31 1 4 11 10 30 15 18 8 7 20 42 99 5 3 Price 13 10 10 9 8 8 7 6 4 4 3 2 2 1 1 1 0 () (9) (9,31) (9,31) Priority queue never changes after this document (9,31) (9,31) (9,31) (9,31)
- 18. Early termination Pros – makes finding the top hits much faster – file-system cache-friendly Cons – only works for static ranks – not if the sort order depends on the query – requires the index to be sorted – doesn’t work for tasks that require visiting every doc: – total number of matches – faceting
- 19. Static ranks Not uncommon! Graph-based ranks – Google’s PageRank Facebook social search / Unicorn – https://www.facebook.com/publications/219621248185635 Many more... Doesn’t need to be the exact sort order – heuristics when score is only a function of the static rank
- 20. Offline sorting A live index can’t be kept sorted – would require inserting docs between existing docs! – segments are immutable Offline sorting to the rescue: – index as usual – sort into a new index – search! Pros/cons – super fast to search, the whole index is fully sorted – but only works for static content
- 21. Offline Sorting // open a reader on the unsorted index and create a sorted (but slow) view DirectoryReader reader = DirectoryReader.open(in); boolean ascending = false; Sorter sorter = new NumericDocValuesSorter("price", ascending); AtomicReader sortedReader = SortingAtomicReader.wrap( SlowCompositeReaderWrapper.wrap(reader), sorter); // copy the content of the sorted reader to the new dir IndexWriter writer = new IndexWriter(out, iwConf); writer.addIndexes(sortedReader); writer.close(); reader.close();
- 22. Online sorting? Sort segments independently – wouldn’t require inserting data into existing segments – collection could still be early-terminated on a per-segment basis But segments are immutable – must be sorted before starting writing them
- 23. Online sorting? 2 sources of segments – flush – merge flushed segments can’t be sorted – Lucene writes stored fields to disk on the fly – could be buffered but this would require a lot of memory merged segments can be sorted – create a sorted view over the segments to merge – pass this view to SegmentMerger instead of the original segments not a bad trade-off – flushed segments are usually small & fast to collect
- 24. Online sorting? Merged segments can easily take 99+% of the size of the index Merged segments Flushed segments - NRT reopens - RAM buffer size limit hit http://blog.mikemccandless.com/2011/02/visualizing-lucenes-segment-merges.html
- 25. Online Sorting IndexWriterConfig iwConf = new IndexWriterConfig(...); // original MergePolicy finds the segments to merge MergePolicy origMP = iwConf.getMergePolicy(); // SortingMergePolicy wraps the segments with a sorted view boolean ascending = false; Sorter sorter = new NumericDocValuesSorter("price", ascending); MergePolicy sortingMP = new SortingMergePolicy(origMP, sorter); // setup IndexWriter to use SortingMergePolicy iwConf.setMergePolicy(sortingMP); IndexWriter writer = new IndexWriter(dir, iwConf); // index as usual
- 26. Early termination Collect top N matches Offline sorting – index sorted globally – early terminate after N matches have been collected – no priority queue needed! Online sorting – no early termination on flushed segments – early termination on merged segments – if N matches have been collected – or if current match is less than the top of the PQ
- 27. Early Termination class MyCollector extends Collector { @Override public void setNextReader(AtomicReaderContext context) throws IOException { readerIsSorted = SortingMergePolicy.isSorted(context.reader(), sorter); collected = 0; } @Override public void collect(int doc) throws IOException { if (readerIsSorted && (++collected >= maxDocsToCollect || curVal <= pq.top()) { // Special exception that tells IndexSearcher to terminate // collection of the current segment throw new CollectionTerminatedException(); } else { // collect hit } } }
- 28. Questions?