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Luwak - stored query engine from Flax

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Luwak is now part of the Apache Lucene library as of the v8.2.0 release - see the details here.

What is Luwak?

Based on the open source Lucene search library, Luwak is a high performance stored query engine. Simply put, it allows you to define a set of search queries and then monitor a stream of documents for any that might match these queries: a function also known as 'reverse search' and 'document routing'. Flax developed Luwak for clients who monitor high volumes of news using often extremely complex Boolean expressions. Luwak is being used by companies including Infomedia, Bloomberg and Booz Allen Hamilton.

You can find out a bit more about how Flax use Luwak for media monitoring applications in this video from Lucene Revolution 2013 and this video from Berlin Buzzwords 2014 and how we combined it with Apache Samza (including a great illustration of how Luwak internals work)

Here's some tests we did to compare Luwak to Elasticsearch Percolator:

Scott Stults of Open Source Connections wrote "How to use Luwak to run preset queries against incoming documents": and Ryan Walker wrote a complete streaming search implementation with Luwak

Get the artifacts


Using the monitor

Basic usage looks like this:

Monitor monitor = new Monitor(new LuceneQueryParser("field"), new TermFilteredPresearcher());

MonitorQuery mq = new MonitorQuery("query1", "field:text");
List<QueryError> errors = monitor.update(mq);

// match one document at a time
InputDocument doc = InputDocument.builder("doc1")
                        .addField(textfield, document, new StandardAnalyzer())
Matches<QueryMatch> matches = monitor.match(doc, SimpleMatcher.FACTORY);

// or match a batch of documents
Matches<QueryMatch> matches = monitor.match(DocumentBatch.of(listOfDocuments), SimpleMatcher.FACTORY);

Adding queries

The monitor is updated using MonitorQuery objects, which consist of an id, a query string, and an optional metadata map. The monitor uses its provided MonitorQueryParser to parse the query strings and cache query objects.

In Luwak 1.5.0, errors thrown when adding queries (from query parsing, for example) cause an UpdateException to be thrown, detailing which queries could not be added.

In Luwak 1.4 and below, Monitor.update() returns a list of QueryError objects, which should be checked for parse errors. The list will be empty if all queries were added successfully.

Matching documents

Queries selected by the monitor to run against an InputDocument are passed to a CandidateMatcher class. Four basic implementations are provided:

  • SimpleMatcher - reports which queries matched the InputDocument
  • ScoringMatcher - reports which queries matched, with their scores
  • ExplainingMatcher - reports which queries matched, with an explanation for their scores
  • HighlightingMatcher - reports which queries matched, with the individual matches for each query

In addition, luwak has two multithreaded matchers which wrap the simpler matchers:

  • ParallelMatcher - runs queries in multiple threads as they are collected from the Monitor
  • PartioningMatcher - collects queries, partitions them into groups, and then runs each group in its own thread

Running the demo

A small demo program is included in the distribution that will run queries provided in a text file over a small corpus of documents from project gutenberg (via nltk).


Filtering out queries

The monitor uses a Presearcher implementation to reduce the number of queries it runs during a match run. Luwak comes with three presearcher implementations.


This Presearcher does no filtering whatsoever, so the monitor will run all its registered queries against every document passed to match.


This Presearcher extracts terms from each registered query and indexes the queries against them in the Monitor's internal index. At match-time, the passed-in InputDocument is tokenized and converted to a disjunction query. All queries that match this query in the monitor's index are then run against the document.


An extension of TermFilteredPresearcher that tries to improve filtering on phrase queries by indexing different combinations of terms.

The TermFilteredPresearcher can be configured with different PresearcherComponent implementations - for example, you can ignore certain fields with a FieldFilterPresearcherComponent, or get accurate filtering on wildcard queries with an WildcardNGramPresearcherComponent.

Adding new query types

TermFilteredPresearcher extracts terms from queries by using a QueryAnalyzer to build a tree representation of the query, and then selecting the best possible set of terms from that tree that uniquely identify the query. The tree is built using a set of specialized QueryTreeBuilder implementations, one for each lucene Query class.

This will not be appropriate for all custom Query types. You can create your own custom tree builder by subclassing QueryTreeBuilder, and then pass it to the TermFilteredPresearcher in a PresearcherComponent.

public class CustomQueryTreeBuilder extends QueryTreeBuilder<CustomQuery> {

    public CustomQueryTreeBuilder() {

    public QueryTree buildTree(QueryAnalyzer builder, CustomQuery query) {
        return new TermNode(getYourTermFromCustomQuery(query));



Presearcher presearcher = new TermFilteredPresearcher(new PresearcherComponent(new CustomerQueryTreeBuilder()));

Customizing the existing presearchers

Not all terms extracted from a query need to be indexed, and the fewer terms indexed, the more performant the presearcher filter will be. For example, a BooleanQuery with many SHOULD clauses but only a single MUST clause only needs to index the terms extracted from the MUST clause. Terms in a parsed query tree are given weights and the QueryAnalyzer uses these weights to decide which terms to extract and index. The weighting is done by a TreeWeightor.

Weighting is configured by a WeightPolicy, which will contain a set of WeightNorms and a CombinePolicy. A query term will be run through all the WeightNorm objects to determine its overall weighting, and a parent query will then calculate its weight with the CombinePolicy, passing in all child weights.

The following WeightNorm implementations are provided:

  • FieldWeightNorm - weight all terms in a given field
  • FieldSpecificTermWeightNorm - weight specific terms in specific fields
  • TermTypeNorm - weight terms according to their type (EXACT terms, ANY terms, CUSTOM terms)
  • TermWeightNorm - weight a specific set of terms with a given value
  • TokenLengthNorm - weight a term according to its length
  • TermFrequencyWeightNorm - weight a term by its term frequency

A single CombinePolicy is provided:

  • MinWeightCombiner - a parent node's weight is set to the minimum weight of its children

You can create your own rules, or combine existing ones

WeightPolicy weightPolicy = WeightPolicy.Default(new FieldWeightNorm("category", 0.01f));
CombinePolicy combinePolicy = new MinWeightCombiner();

TreeWeightor weightor = new TreeWeightor(weightPolicy, combinePolicy);
Presearcher presearcher = new TermFilteredPresearcher(weightor);

You can debug the output of any weightor by using a ReportingWeightor. QueryTreeViewer is a convenience class that may help here.

Creating an entirely new type of Presearcher

You can implement your own query filtering code by subclassing Presearcher. You will need to implement buildQuery(InputDocument, QueryTermFilter) which converts incoming documents into queries to be run against the Monitor's query index, and indexQuery(Query, Map<String,String>) which converts registered queries into a form that can be indexed.

Note that indexQuery(Query, Map<String,String>) may not create fields named '_id' or '_query', as these are reserved by the Monitor's internal index.


A java library for stored queries







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