MG4J (big): Managing Gigabytes for Java

MG4J (Managing Gigabytes for Java) is a free full-text search engine for large document collections written in Java. The big version is a fork of the original MG4J that can handle more than 2³¹ terms and documents.

MG4J is distributed under the GNU Lesser General Public License.

Warning

MG4J 5.0 brings several new features, but also source and binary incompatibilities with previous releases.

MG4J is no longer based on gap-based indices. Classical interleaved indices are used for incremental index construction and high-performance indices are still supported for historical reasons, but all new indices are by default built using the new {@linkplain it.unimi.di.big.mg4j.index.QuasiSuccinctIndexWriter quasi-succinct format}, which brings unprecedented performance and improves compression.
The package prefix of MG4J is now it.unimi.di.*, following the change of name of our department, so to ease the transition and making coexistence with previous versions possible.
{@link it.unimi.di.big.mg4j.search.DocumentIterator#nextDocument()} now returns {@link it.unimi.di.big.mg4j.search.DocumentIterator#END_OF_LIST} instead of -1 to denote list exhaustion.
The plethora of methods that accessed the positions of a term in an {@link it.unimi.di.big.mg4j.index.IndexIterator} have been replaced by the single lazy {@link it.unimi.di.big.mg4j.index.IndexIterator#nextPosition()} call, which returns {@link it.unimi.di.big.mg4j.index.IndexIterator#END_OF_POSITIONS} when the positions are exhausted. Some static methods in {@link it.unimi.di.big.mg4j.index.IndexIterators} should help with the transition.

Roadmap

MG4J is vast. Some of its component are the result of longtime research efforts, and are not easy to describe in full detail. Here we give a roadmap to the documentation, so that you do not have to wander recklessly through dozens of package descriptions.

First of all, MG4J comes with a manual that describes how to build indices, and how to access them from the command line or from the web. It is a good idea to start from the manual, build and play with a few indices, and then come back to package documentation, as the latter often refers to artifacts created by index construction.

If you want to interface MG4J with your own data, you must read the package documentation of {@link it.unimi.di.big.mg4j.document}, which describes document sequences, collections and factories.

If you want to load and query an index, you must read the package documentation of {@link it.unimi.di.big.mg4j.index}, which describes indices and index readers. The package contains also the documentation about {@linkplain it.unimi.di.big.mg4j.index.TermProcessor term processors}, which transform terms before they are actually indexed; they are fundamental to customise the indexing process.

If you want to have a look at your index, the package {@link it.unimi.di.big.mg4j.query} contains many useful classes that can help. In particular, a simple {@linkplain it.unimi.di.big.mg4j.query.Query command-line tool} let you query an index using a standard syntax. The tool makes it also possible to query the index using a browser (if you plan on using the command-line frequency, we suggest a utility such as rlwrap to provide command-line history and editing).

In a real applications, you might want to customise the index querying process. First of all, you must decide which syntax you want to use. A good starting point is described in the package {@link it.unimi.di.big.mg4j.query.parser}, which contains a simple parser generated with JavaCC. The parser generates an abstract query describe by a composite object whose description is given in {@link it.unimi.di.big.mg4j.query.nodes}. The query can then be turned into a {@link it.unimi.di.big.mg4j.search.DocumentIterator}, which will return the documents matching the query and also the document intervals satisfying the query: the minimal-interval semantics used by MG4J is described in detail in {@link it.unimi.di.big.mg4j.search}, which also contains a description of the syntax used by the {@linkplain it.unimi.di.big.mg4j.query.Query command-line tool}.

Once a document iterator returning the matching documents is available, it is usually necessary to rank the documents. MG4J provides an abstract notion of {@link it.unimi.di.big.mg4j.search.score.Scorer} and provides several examples. Scoring is a very sophisticated issue, and a lot of research has been devoted to this subject. MG4J provides implementation for some state-of-the-art scorers such as {@linkplain it.unimi.di.big.mg4j.search.score.BM25Scorer BM25}, and also new scorers based on minimal-interval semantics such as {@link it.unimi.di.big.mg4j.search.score.VignaScorer}.

All these pieces come together in the {@link it.unimi.di.big.mg4j.query.QueryEngine}, which takes one or more queries, scores their results using one or more scorers, and returns only a certain part of the results themselves, decorated with suitably selected intervals that can be used to generate snippets. The query engine has several tunable parameters, so you can adapt it to your application. We suggest that you play with the {@linkplain it.unimi.di.big.mg4j.query.Query command-line tool} and the associated web interface to become familiar with the query-engine inner workings.