Three exciting Lucene features in one day

Yesterday was a productive day: suddenly, there are three exciting new features coming to Lucene.

Expressions module

The first feature, committed yesterday, is the new expressions module. This allows you to define a dynamic field for sorting, using an arbitrary String expression. There is builtin support for parsing JavaScript, but the parser is pluggable if you want to create your own syntax.

For example, you could define a sort field using the expression

sqrt(_score) + ln(popularity)

if you want to offer a blended sort primarily by relevance and boosting by a popularity field.

The code is very easy to use; there are some nice examples in the TestDemoExpressions.java unit test case, and this will be available in Lucene’s next stable release (4.6).

Updateable numeric doc-values fields

The second feature, also committed yesterday, is updateable numeric doc-values fields, letting you change previously indexed numeric values using the new updateNumericDocValue method on IndexWriter. It works fine with near-real-time readers, so you can update the numeric values for a few documents and then re-open a new near-real-time reader to see the changes.

The feature is currently trunk only as we work out a few remaining issues involving an particularly controversial boolean. It also currently does not work on sparse fields, i.e. you can only update a document’s value if that document had already indexed that field in the first place.

Combined, these two features enable powerful use-cases where you want to sort by a blended field that is changing over time. For example, perhaps you measure how often your users click through each document in the search results, and then use that to update the popularity field, which is then used for a blended sort. This way the rankings of the search results change over time as you learn from users which documents are popular and which are not.

Of course such a feature was always possible before, using custom external code, but with both expressions and updateable doc-values now available it becomes trivial to implement!

Free text suggestions

Finally, the third feature is a new suggester implementation, FreeTextSuggester. It is a very different suggester than the existing ones: rather than suggest from a finite universe of pre-built suggestions, it uses a simple ngram language model to predict the “long tail” of possible suggestions based on the 1 or 2 previous tokens.

Under the hood, it uses ShingleFilter to create the ngrams, and an FST to store and lookup the resulting ngram models. While multiple ngram models are stored compactly in a single FST, the FST can still get quite large; the 3-gram, 2-gram and 1-gram model built on the AOL query logs is 19.4 MB (the queries themselves are 25.4 MB). This was inspired by Google’s approach.

Likely this suggester would not be used by itself, but rather as a fallback when your primary suggester failed to find any suggestions; you can see this behavior with Google. Try searching for “the fast and the “, and you will see the suggestions are still full queries. But if the next word you type is “burning” then suddenly google (so far!) does not have a full suggestion and falls back to their free text approach.
 

Reference: Three exciting Lucene features in one day from our JCG partner Michael Mc Candless at the Changing Bits blog.

Do you want to know how to develop your skillset to become a Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you two of our best selling eBooks for FREE!

JPA Mini Book

Learn how to leverage the power of JPA in order to create robust and flexible Java applications. With this Mini Book, you will get introduced to JPA and smoothly transition to more advanced concepts.

JVM Troubleshooting Guide

The Java virtual machine is really the foundation of any Java EE platform. Learn how to master it with this advanced guide!

Given email address is already subscribed, thank you!
Oops. Something went wrong. Please try again later.
Please provide a valid email address.
Thank you, your sign-up request was successful! Please check your e-mail inbox.
Please complete the CAPTCHA.
Please fill in the required fields.

Leave a Reply


four − = 2



Java Code Geeks and all content copyright © 2010-2014, Exelixis Media Ltd | Terms of Use | Privacy Policy | Contact
All trademarks and registered trademarks appearing on Java Code Geeks are the property of their respective owners.
Java is a trademark or registered trademark of Oracle Corporation in the United States and other countries.
Java Code Geeks is not connected to Oracle Corporation and is not sponsored by Oracle Corporation.
Do you want to know how to develop your skillset and become a ...
Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you two of our best selling eBooks for FREE!

Get ready to Rock!
You can download the complementary eBooks using the links below:
Close