The great improvements in the capabilities of Lucene and Solr open source search technology have created rapidly growing interest in using them as alternatives to other search applications. As is often the case with open-source technology, online community documentation provides rich details on features and variations, but does little to provide explicit direction on which technologies would be the best choice. So when is Lucene preferable to Solr and vice versa?

The great improvements in the capabilities of Lucene and Solr open source search technology have created rapidly growing interest in using them as alternatives to other search applications. As is often the case with open-source technology, online community documentation provides rich details on features and variations, but does little to provide explicit direction on which technologies would be the best choice. So when is Lucene preferable to Solr and vice versa?

There is in fact no single answer, as Lucene and Solr are complementary technologies that bring very similar underlying capabilities to bear on somewhat distinct problems. Solr is versatile and powerful, a full featured, production-ready search application server requiring relatively less formal software programming. Lucene presents a collection of directly callable Java libraries, with fine-grained control of machine functions and independence from higher-level protocols.

In choosing which might be best for your search solution, the key questions to consider are application scope, deployment environment, and software development preferences. If you are new to developing search applications, you should start with Solr. Solr provides scalable search power out of the box, whereas Lucene requires relatively more information retrieval experience and some meaningful heavy lifting in Java to take advantage of its capabilities.

Solr is essentially the “serverization” of Lucene, and many of its abstract functions are highly similar, if not just the same. If you are building an app for the enterprise sector, for instance, you will find Solr an almost 100% match to your business requirements: it comes ready to run in a servlet container such as Tomcat or Jetty, and ready to scale in a production Java environment. Its RESTful interfaces and XML-based configuration files can greatly accelerate development and maintenance. In fact, Lucene programmers have often reported that they find Solr to contain “the same features I was going to build myself as a framework for Lucene, but already very-well implemented.” Once you start with Solr, and you find yourself using a lot of the features Solr provides out of the box, you will likely be better off using Solr’s well-organized extension mechanisms instead of starting from scratch using Apache Lucene.

If, on the other hand, you don’t want to make any calls via HTTP, and want to have all of your resources controlled exclusively by Java API calls that you write, Lucene may be a better choice. Lucene works best when constructing and embedding a state-of-the-art search engine, allowing programmers to assemble and compile inside a native Java application. Some  programmers set aside the convenience of Solr in order to more directly control the large set of sophisticated features with low-level access, data, or state manipulation, and choose Lucene instead, for example with byte-level manipulation of segments or intervention in data I/O. Investment at the lower level enables development of extremely sophisticated, cutting edge text search and retrieval capabilities.

As for features, the latest version of Solr generally encapsulates the latest version of Lucene. As the two are in many ways functional siblings, spending time on gaining a solid understanding how Lucene works internally can help you understand Apache Solr and its extension of Lucene's workings.

No matter which you choose, the power of open source search is yours to harness. More information on both Lucene and Solr can be found at http://www.lucidimagination.com.