How MoreLikeThis Works in Lucene

We created a ‘related items’ feature way back in Clearspace 1.0 (I mocked some of it out here just to prove that it worked) which shows related content based on the document, thread or blog post that you’re currently viewing. It was built using the MoreLikeThis class, a contribution made to the Lucene project by David Spencer, Mark Harwood and my favorite Canadian coworker Bruce Ritchie.

The really interesting thing about MoreLikeThis (at least to me) is that my first inclination was probably to think that Lucene goes and spends some cycles looking for content related to the source document: the focus being on the search and the other content. The reality is that the majority of the work is done by analyzing content the source document relative to the aggregate of all the other content that exists in the index. Said another way, MoreLikeThis doesn’t work by running some special search or query, it works by comparing the document that you’re asking about and the entire index as a whole (and then by running a query).

Anyway, in general I think it works pretty well, but for some odd reason, people keep asking the question “how does it work?” and since the documentation doesn’t go into much detail, I thought I’d try writing it up. So here goes nothing…

The first thing you need to do if you’re going to find related content is to tell Lucene what you want to find related content for. The MoreLikeThis class gives you five options: you can either tell it to look at an existing document in the Lucene index or let it try to parse an external resource from a file, an inputstream, a reader or a URL. In Clearspace we actually have a way of getting the Lucene document ID given the Clearspace object type and object ID, the code for our ‘related items’ feature looks something like this:

Query q = buildObjectTypeAndIDQuery(String.valueOf(messageID), JiveConstants.MESSAGE);
Hits hits =;
int docNum =;
MoreLikeThis mlt = new MoreLikeThis(reader);
mlt.setFieldNames(new String[]{IndexField.subject.toString(), IndexField.body.toString(),
q =;

So in Clearspace we first fetch the Lucene document, construct a MoreLikeThis instance, tell it that we want to match on the subject, body and tag fields and we only want to calculate ‘relatedness’ based on words whose length is two characters or more. That’s the easy part, now it gets interesting.

Given that you already have a Lucene document, the MoreLikeThis instance loops over the field names (fields are kind of like the Lucene equivalent of database columns) that we specified (or all the field names available in the Lucene index if you don’t specify any) and retrieves a term vector for each of the fields in the document we’re analyzing. A term vector is a data structure that holds a list of all the words that were in the field and the number of times each word was used (excluding words that it considers to be ‘stop words’, you can see a list of common stop words here).

Since I’m sure some of you are tuning out at this point, let’s look at something concrete: assume you’ve created a document that looks like this (by the way, what a thorough entry!):

Subject: Twinkle Twinkle Little Star
Body: Twinkle Twinkle little star, How I wonder what you are! Up above the world so high, Like a diamond in the sky, Twinkle twinkle little star, How I wonder what you are!

The term vector for the subject would look like this:
terms: twinkle, little, star
termFrequencies: twinkle[2], little[1], star[1]

and for the body:
terms: [above, diamond, high, how, like, little, sky, so, star, twinkle, up, what, wonder, world]
termFrequencies: above[1], diamond[1], high[1], how[2], like[1], little[2], sky[1], so[1], star[2], [twinkle[4], up[1], what[2], wonder[2], world[1]

Meanwhile, back at the ranch.. so we get a term vector for each of the fields available in the document we’re looking for related items for. After we’ve got those, each of the term vectors is merged into a map: the key being the term and the value being the number of times the word was used in the document. The map is then handed to a method (called createQueue) that calculates a reasonably complex score for each word in the map. Since this is really the meat of the entire class, I’ll step through this in a little more detail and I’ll season the meat with data from the index on my blog.

The createQueue method first retrieves the total number of documents that exist in the index that we’re dealing with.

int numDocs = ir.numDocs();

In the index I maintain for my personal blog, there are 998 documents in the index. The second thing it does is create an instance of a class called FreqQ, which extends the PriorityQueue class. This object will maintain an object array whose elements are ordered according to their score, which we’ll cover in a second.

Now the createQueue method iterates over each word in the term frequency map, throwing out words if they don’t occur enough times (see setMinTermFreq) and then testing to find out which field across the entire the Lucene index contains the term the most. Next it calculates the Inverse Document Frequency, which, according to the JavaDoc, is:

… a score factor based on a term’s document frequency (the number of documents which contain the term)… Terms that occur in fewer documents are better indicators of topic, so implementations of this method usually return larger values for rare terms, and smaller values for common terms.

and finally, a score, which is a product of the IDF score and the number of times the word existed in the source document.

Again, I’m guessing you’re getting really sleepy, here’s some real data to look at. Like I mentioned above, I have 998 documents in my Lucene index and if we take a document like this one, you’ll end up with a term vector that looks like this:

a[20], you[20], pre[18], the[17], to[13], of[11], this[10], username[10], column[9], oracle[9], and[8], if[8], null[8], alter[7], appuser[7], into[7], on[7], that[7], href[6], http[6], i[6], sql[6], an[5], com[5], is[5], not[5], password[5], table[5], values[5], varchar[5], www[5], be[4], but[4], case[4], clearspace[4], empty[4], get[4], insert[4], modify[4], which[4], administrator[3], are[3], can[3], database[3], db[3], in[3], jivesoftware[3], mysql[3], nullability[3], server[3], string[3], t[3], thing[3], with[3], about[2], all[2], assume[2], been[2], blogged[2], d[2], different[2], feature[2], for[2], from[2], has[2], have[2], hsqldb[2], jive[2], make[2], modified[2], nvarchar[2], out[2], plan[2], postgres[2], products[2], ran[2], requirements[2], sensitivity[2], so[2], space[2], store[2], strike[2], support[2], where[2], will[2], above[1], again[1], against[1], already[1], andrew[1], any[1], appears[1], application[1], at[1], attempt[1], bennett[1], bit[1], blog[1], borrowed[1], both[1], bottom[1], by[1], cannot[1], changing[1], chars[1], classes[1], cmu[1], code[1], columns[1], consider[1], contrib[1], converted[1], cool[1], couple[1], crazy[1], day[1], decided[1], default[1], detail[1], didn[1], discuss[1], do[1], doing[1], edu[1], error[1], example[1], finally[1], first[1], flushed[1], forums[1], further[1], going[1], good[1], goodness[1], heads[1], helpful[1], his[1], insensitive[1], instead[1], interesting[1], involved[1], issue[1], issues[1], it[1], itself[1], jist[1], job[1], jsp[1], kb[1], kind[1], lately[1], least[1], like[1], line[1], ll[1], look[1], looks[1], lot[1], mcelwee[1], microsoft[1], more[1], my[1], needed[1], nice[1], no[1], notice[1], nugget[1], number[1], only[1], or[1], ora[1], over[1], page[1], past[1], platforms[1], probably[1], product[1], re[1], readers[1], reading[1], reason[1], respect[1], results[1], retrieve[1], ridiculous[1], sample[1], second[1], see[1], select[1], semicolon[1], sensitive[1], servers[1], shadow[1], similar[1], six[1], software[1], something[1], specific[1], specification[1], specify[1], standard[1], statement[1], supporting[1], sure[1], system[1], tails[1], than[1], then[1], there[1], they[1], things[1], those[1], thought[1], thunderguy[1], tolowercase[1], touppercase[1], tried[1], try[1], two[1], txt[1], unless[1], ups[1], used[1], user[1], variety[1], ve[1], very[1], want[1], warned[1], was[1], we[1], week[1], whatever[1], wonderful[1], word[1], work[1], write[1], yes[1], your[1], yourself[1]

and stop words would get stripped out and words that occurred less than two times would get stripped out so you’d be left with a term frequency map that looked like this:

pre[18], username[10], column[9], oracle[9], alter[7], appuser[7], href[6], http[6], sql[6], com[5], password[5], table[5], values[5], varchar[5], www[5], case[4], clearspace[4], empty[4], get[4], insert[4], modify[4], which[4], administrator[3], can[3], database[3], db[3], jivesoftware[3], mysql[3], nullability[3], server[3], string[3], thing[3], about[2], all[2], assume[2], been[2], blogged[2], d[2], different[2], feature[2], from[2], has[2], have[2], hsqldb[2], jive[2], make[2], modified[2], nvarchar[2], out[2], plan[2], postgres[2], products[2], ran[2], requirements[2], sensitivity[2], so[2], space[2], store[2], strike[2], support[2], where[2],

So for each of these terms, Lucene finds the field that contains the most instances of the given term and then calculates the idf value and the score. The default implementation of the idf value looks like this:

return (float)(Math.log(numDocs/(double)(docFreq+1)) + 1.0)

So given the term frequency map above and the document frequency noted below, we get the following idf values and scores for the most popular terms in this document:

Term Number of Instances of Term in Document Number of Documents Matching Term IDF value Score
pre 18 26 4.609916 82.978
username 10 23 4.7276993 47.276
column 9 13 5.266696 47.400264
oracle 9 8 5.7085285 51.376
alter 7 1 7.212606 50.488

Finally, after all the terms have been added to the priority queue, we create a Lucene query, looping over the first 25 terms (this is the default and can be changed via setMaxQueryTerms) in the queue

TermQuery tq = new TermQuery(new Term("body", "oracle"));

and optionally boosting each term according to the score:

tq.setBoost(51.376 / 82.978);

The resulting Lucene query (in string format) looks something like this:

body:pre body:username^.56974 body:column^.57123 body:oracle^.61915 ...

Now I’ll be curious to see what the related item is for this post!

I hope you learned as much as I did. If you’re extremely curious about this whole Lucene thing and you like the in-depth stuff, you should definitely check out Luke, it’ll open a whole new world to you.

Links: 3-22-2008

Links: 3-13-2008