Make an object be searchable is difficult? Taking control of search result's ranking is even more challenging. In BarCamp Bangkhen 2015 (Nov 22, 2015), Suparit Krityakien, Co-founder & Software Architecture of Wongnai (http://www.wongnai.com) shared his experiences on applying Wongnai ranking algorithm to take control of our search engine's scoring systematically as well as what can be tuned to make it smarter. This presentation is to furnish you with fundamental knowledge required when tweaking ranking on your search.

1. Solr’s Search Relevancy
Understand Solr’s query debug
By Suparit Krityakien
Co-founder and Software Architect at Wongnai Media
Barcamp Bangkhen 2015

2. Agenda
● Introduction to Apache Lucene and Apache Solr
● Relevancy Problem
● Scoring Model
● Solutions

3. Apache Lucene
● Full-text Search
● Inverted Index
● Fast!!!!!
● Java Library (pure)
● Apache License, Version 2.0. Privacy Policy

4. Inverted Index

5. Basic API Usage
Directory directory = new RAMDirectory();
Analyzer analyzer = new StandardAnalyzer();
IndexWriterConfig config = new IndexWriterConfig(analyzer);
IndexWriter iwriter = new IndexWriter(directory, config);
Document doc = new Document();
doc.add(new Field("title", “title”, TextField.TYPE_STORED));
doc.add(new Field("content", "content", TextField.TYPE_STORED));
iwriter.addDocument(doc);
iwriter.close();
DirectoryReader ireader = DirectoryReader.open(directory);
IndexSearcher isearcher = new IndexSearcher(ireader);
QueryParser parser = new QueryParser("content", analyzer);
Query query = parser.parse("text");
ScoreDoc[] hits = isearcher.search(query, null, 1000).scoreDocs;
for (int i = 0; i < hits.length; i++) {
Document hitDoc = isearcher.doc(hits[i].doc);
hitDoc.get("title"));
}
ireader.close();
directory.close();

6. Query Syntax (Introduction)
● Grammar
○ Query ::= ( Clause )*
Clause ::= ["+", "-"] [<FIELD> ":"] ( <TERM> | "(" Query ")" )
● Example - name:kasetsart event:barcamp
● Wildcard - m?n or sofite*
● Fuzzy - chicalecious~2
● Proximity - “after you cafe”~2
● Boosting - wongnai^10

7. Apache Solr
● Index Server
● Built on top of Luceue
● REST API / XML / Binary
● Configurable without programming
○ Schema, Type
○ Analyzer
○ Query .. etc
● Still be Java
○ Deployed to servlet container
○ or Standalone (jetty)

8. Components
http://butchiso.com/assets/posts/tim-hieu-ve-apache-solr/solr-achitecture.png

9. Example of configuration files
<?xml version="1.0" encoding="UTF-8" ?>
<!-- solrconfig.xml -->
<config>
<luceneMatchVersion>LUCENE_42</luceneMatchVersion>
<dataDir>${solr.data.dir:}</dataDir>
<directoryFactory name="DirectoryFactory" class="${solr.directoryFactory:solr.
NRTCachingDirectoryFactory}" />
<abortOnConfigurationError>${solr.abortOnConfigurationError:true}
</abortOnConfigurationError>
<lib dir="../../../contrib/extraction/lib" regex=".*.jar" />
<lib dir="../../../dist/" regex="solr-cell-d.*.jar" />
<indexConfig>...</indexConfig>
<updateHandler class="solr.DirectUpdateHandler2">...</updateHandler>
<query>...</query>
<requestDispatcher handleSelect="false">...</requestDispatcher>
<requestHandler name="/select" class="solr.SearchHandler">...</requestHandler>
<requestHandler name="/update" class="solr.UpdateRequestHandler" />
<requestHandler name="/admin/" class="solr.admin.AdminHandlers" />
<requestHandler name="/replication" class="solr.ReplicationHandler" />
<admin>
<defaultQuery>*:*</defaultQuery>
</admin>
</config>
<?xml version="1.0" ?>
<!-- schema.xml -->
<schema name="example core zero" version="1.1">
<types>
<fieldType name="string" class="solr.StrField" sortMissingLast="true" />
<fieldType name="string_ci" class="solr.TextField" sortMissingLast="true" omitNorms="true">
<analyzer>
<tokenizer class="solr.KeywordTokenizerFactory" />
<filter class="solr.LowerCaseFilterFactory" />
</analyzer>
</fieldType>
.
.
.
</types>
<fields>
<field name="id" type="string" indexed="true" stored="true" multiValued="false" required="true" />
<field name="_version_" type="long" indexed="true" stored="true" />
<field name="name" type="text_general" indexed="true" stored="false" multiValued="false" required="
true" />
<field name="coordinate" type="location" indexed="true" stored="false" multiValued="false" required="
false" />
<dynamicField name="*_coordinate" type="tdouble" indexed="true" stored="false" />
</fields>
<uniqueKey>id</uniqueKey>
<defaultSearchField>name</defaultSearchField>
<solrQueryParser defaultOperator="OR" />
</schema>

10. Web UI

11. Relevancy Problem
Why does the result look like this?
● Why document id:xxx was found?
● Why document id:yyy was not found (top 5)?
● Why doesn't document id:xxx appear before
id:yyy?

12. TF-IDF -- the default scoring model
● tf (term frequency)
● idf (inverse document frequency)
● coord (coordination factor)
● norm (normalization)
○ fieldNorm - index-time
○ queryNorm - not effect ranking!
● query-time boosting

13. Search with debug query

14. Search with debug query (cont.)
<str name="123686">
0.20652547 = (MATCH) product of:
0.4646823 = (MATCH) sum of:
0.20399374 = (MATCH) weight(name:think in 169148) [DefaultSimilarity], result of:
0.20399374 = score(doc=169148,freq=2.0 = termFreq=2.0
), product of:
0.041768804 = queryWeight, product of:
11.050954 = idf(docFreq=7, maxDocs=185423)
0.003779656 = queryNorm
4.8838778 = fieldWeight in 169148, product of:
1.4142135 = tf(freq=2.0), with freq of:
2.0 = termFreq=2.0
11.050954 = idf(docFreq=7, maxDocs=185423)
0.3125 = fieldNorm(doc=169148)
0.03779656 = (MATCH) ConstantScore(name_notok:think cafe name_notok:think cafe @ the bloc name_notok:think cafe siam center name_notok:
think cafe siamcenter name_notok:think cafe สยามเซ็นเตอร name_notok:think tank name_notok:think tank (third place) name_notok:thinkcafe
name_notok:thinkcafe siam center name_notok:thinkcafe siamcenter name_notok:thinkcafe สยามเซ็นเตอร name_notok:thinkcafe@thebloc name_notok:
thinktank name_notok:thinktank(thirdplace))^10.0, product of:
10.0 = boost
0.003779656 = queryNorm
0.01889828 = (MATCH) ConstantScore(name_notok:[ka-fe-vi-no] think park name_notok:[ka-fe-vi-no] thinkpark name_notok:[kafevino] think park
name_notok:[kafevino] thinkpark name_notok:dogaholic cafe think park name_notok:dogaholic cafe thinkpark name_notok:dogaholiccafe think park
name_notok:dogaholiccafe thinkpark name_notok:hamburg think park name_notok:hamburg thinkpark name_notok:satophlatthinkaraoke name_notok:
surathinkaimaranoodle(kaitun) name_notok:think cafe name_notok:think cafe @ the bloc name_notok:think cafe siam center name_notok:think cafe
siamcenter name_notok:think cafe สยามเซ็นเตอร name_notok:think tank name_notok:think tank (third place) name_notok:thinkcafe name_notok:
thinkcafe siam center name_notok:thinkcafe siamcenter name_notok:thinkcafe สยามเซ็นเตอร name_notok:thinkcafe@thebloc name_notok:thinktank
name_notok:thinktank(thirdplace) name_notok:tom n toms coffee think park name_notok:tom n toms coffee thinkpark name_notok:tomntomscoffee
think park name_notok:tomntomscoffee thinkpark name_notok:คาเฟวิโน think park name_notok:คาเฟวิโน thinkpark name_notok:ทัม แอนด ทัมส think park

15. Search with debug query (cont.)
0.01889828 = (MATCH) ConstantScore(name_notok:[ka-fe-vi-no] think park name_notok:[ka-fe-vi-no] thinkpark name_notok:[kafevino] think park
name_notok:[kafevino] thinkpark name_notok:dogaholic cafe think park name_notok:dogaholic cafe thinkpark name_notok:dogaholiccafe think park
name_notok:dogaholiccafe thinkpark name_notok:hamburg think park name_notok:hamburg thinkpark name_notok:satophlatthinkaraoke name_notok:
surathinkaimaranoodle(kaitun) name_notok:think cafe name_notok:think cafe @ the bloc name_notok:think cafe siam center name_notok:think cafe
siamcenter name_notok:think cafe สยามเซ็นเตอร name_notok:think tank name_notok:think tank (third place) name_notok:thinkcafe name_notok:
thinkcafe siam center name_notok:thinkcafe siamcenter name_notok:thinkcafe สยามเซ็นเตอร name_notok:thinkcafe@thebloc name_notok:thinktank
name_notok:thinktank(thirdplace) name_notok:tom n toms coffee think park name_notok:tom n toms coffee thinkpark name_notok:tomntomscoffee
think park name_notok:tomntomscoffee thinkpark name_notok:คาเฟวิโน think park name_notok:คาเฟวิโน thinkpark name_notok:ทัม แอนด ทัมส think park
name_notok:ทัม แอนด ทัมส thinkpark name_notok:ทัมแอนดทัมส think park name_notok:ทัมแอนดทัมส thinkpark)^5.0, product of:
5.0 = boost
0.003779656 = queryNorm
0.20399374 = (MATCH) sum of:
0.20399374 = (MATCH) weight(name:think in 169148) [DefaultSimilarity], result of:
0.20399374 = score(doc=169148,freq=2.0 = termFreq=2.0
), product of:
0.041768804 = queryWeight, product of:
11.050954 = idf(docFreq=7, maxDocs=185423)
0.003779656 = queryNorm
4.8838778 = fieldWeight in 169148, product of:
1.4142135 = tf(freq=2.0), with freq of:
2.0 = termFreq=2.0
11.050954 = idf(docFreq=7, maxDocs=185423)
0.3125 = fieldNorm(doc=169148)
0.44444445 = coord(4/9)
</str>

16. Tuning
● Adding different type field (analyzer) eg. no tokenizer
● omitNorm
● Document & field boosting
● More query terms + term boosting
● Use boost function
○ {!boost b=numberOfReviews}
● Using filter instead of normal query

17. Reference
● https://lucene.apache.org/core/5_0_0/core/org/apache/lucene/search/similarities/TFIDFSimilarity.
html
● http://butchiso.com/assets/posts/tim-hieu-ve-apache-solr/solr-achitecture.png