Monday, April 30, 2012

Lucene's TokenStreams are actually graphs!

Lucene's TokenStream class produces the sequence of tokens to be indexed for a document's fields. The API is an iterator: you call incrementToken to advance to the next token, and then query specific attributes to obtain the details for that token. For example, CharTermAttribute holds the text of the token; OffsetAttribute has the character start and end offset into the original string corresponding to this token, for highlighting purposes. There are a number of standard token attributes, and some tokenizers add their own attributes.

The TokenStream is actually a chain, starting with a Tokenizer that splits characters into initial tokens, followed by any number of TokenFilters that modify the tokens. You can also use a CharFilter to pre-process the characters before tokenization, for example to strip out HTML markup, remap character sequences or replace characters according to a regular expression, while preserving the proper offsets back into the original input string. Analyzer is the factory class that creates TokenStreams when needed.

Lucene and Solr have a wide variety of Tokenizers and TokenFilters, including support for at least 34 languages.

Let's tokenize a simple example: fast wi fi network is down. Assume we preserve stop words. When viewed as a graph, the tokens look like this:

Each node is a position, and each arc is a token. The TokenStream enumerates a directed acyclic graph, one arc at a time.

Next, let's add SynoynmFilter into our analysis chain, applying these synonyms:
  • fastspeedy
  • wi fiwifi
  • wi fi networkhotspot
resulting in this graph:

Now the graph is more interesting! For each token (arc), the PositionIncrementAttribute tells us how many positions (nodes) ahead this arc starts from, while the new (as of 3.6.0) PositionLengthAttribute tells us how many positions (nodes) ahead the arc arrives to.

Besides SynonymFilter, several other analysis components now produce token graphs. Kuromoji's JapaneseTokenizer outputs the decompounded form for compound tokens. For example, tokens like ショッピングセンター (shopping center) will also have an alternate path with ショッピング (shopping) followed by センター (center). Both ShingleFilter and CommonGramsFilter set the position length to 2 when they merge two input tokens.

Other analysis components should produce a graph but don't yet (patches welcome!): WordDelimiterFilter, DictionaryCompoundWordTokenFilter, HyphenationCompoundWordTokenFilter, NGramTokenFilter, EdgeNGramTokenFilter, and likely others.


There are unfortunately several hard-to-fix problems with token graphs. One problem is that the indexer completely ignores PositionLengthAttribute; it only pays attention to PositionIncrementAttribute. This means the indexer acts as if all arcs always arrive at the very next position, so for the above graph we actually index this:

This means certain phrase queries should match but don't (e.g.: "hotspot is down"), and other phrase queries shouldn't match but do (e.g.: "fast hotspot fi"). Other cases do work correctly (e.g.: "fast hotspot"). We refer to this "lossy serialization" as sausagization, because the incoming graph is unexpectedly turned from a correct word lattice into an incorrect sausage. This limitation is challenging to fix: it requires changing the index format (and Codec APIs) to store an additional int position length per position, and then fixing positional queries to respect this value.

QueryParser also ignores position length, however this should be easier to fix. This would mean you can run graph analyzers at query time (i.e., query time expansion) and get the correct results.

Another problem is that SynonymFilter also unexpectedly performs its own form of sausagization when the injected synonym is more than one token. For example if you have this rule:
  • dnsdomain name service
it results in graphs like this:

Notice how name was overlapped onto is, and service was overlapped onto up. It's an odd word salad!

This of course also messes up phrase queries ("domain name service is up" should match but doesn't, while "dns name up" shouldn't match but does). To work around this problem you should ensure all of your injected synonyms are single tokens! For this case, you could run the reverse mapping (domain name servicedns) at query time (as well as indexing time) and then both queries dns and domain name service will match any document containing either variant.

This happens because SynonymFilter never creates new positions; if it did so, it could make new positions for tokens in domain name service, and then change dns to position length 3.

Another problem is that SynonymFilter, like the indexer, also ignores the position length of the incoming tokens: it cannot properly consume a token graph. So if you added a second SynonymFilter it would fail to match hotspot is down.

We've only just started but bit by bit our token streams are producing graphs!

Saturday, April 28, 2012

Lucene has two Google Summer of Code students!

I'm happy to announce that two Lucene Google Summer of Code projects were accepted for this summer!

The first project (LUCENE-3312), proposed by Nikola Tanković, will separate StorableField out of IndexableField, and also fix the longstanding confusing trap that one can retrieve a Document at search time and re-index it, without losing anything. It's unfortunately not true!

The second project (LUCENE-3892), proposed by Han Jiang, will create a postings format using PForDelta compression (PDF). Long ago we saw great performance from PForDelta, but with a hacked up prototype patch that couldn't be committed. The goal of this project is to implement PForDelta "for real" and characterize the resulting performance.

Welcome aboard Nikola and Han!