How to use Lucene to query ElasticSearch index

Question

Can I use Lucene to query an ElasticSearch index?

Using ElasticSearch I created an index and inserted these three documents:

$ curl -XPOST localhost:9200/index1/type1 -d '{"f1":"dog"}'
$ curl -XPOST localhost:9200/index1/type2 -d '{"f2":"cat"}'
$ curl -XPOST localhost:9200/index1/type2 -d '{"f3":"horse"}'

So, I have one index, two types, and three documents. Now, I would like to search for these using standard Lucene. Using a hex editor, I identified which shard has the indexed documents, and I can successfully query that index. I can't figure out though, how to retrieve the field values from the matching document(s).

The following program successfully searches but is unable to retrieve results.

import org.apache.lucene.analysis.standard.StandardAnalyzer;
import org.apache.lucene.document.Document;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.queryparser.classic.QueryParser;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.Query;
import org.apache.lucene.search.ScoreDoc;
import org.apache.lucene.search.TopScoreDocCollector;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;
import org.apache.lucene.util.Version;

import java.io.File;

public class TestES {

void doWork(String[] args) throws Exception {
    // Index reader for already created ElasticSearch index
    String indx1 = "/path-to-index/elasticsearch-0.90.0.RC2-SNAPSHOT/data/elasticsearch/nodes/0/indices/index1/1/index";
    Directory index = FSDirectory.open(new File(indx1));
    IndexReader reader = DirectoryReader.open(index);
    IndexSearcher searcher = new IndexSearcher(reader);

    // Looks like the query is correct since we do get a hit
    StandardAnalyzer analyzer = new StandardAnalyzer(Version.LUCENE_41);
    Query q = new QueryParser(Version.LUCENE_41, "f2", analyzer).parse("cat");
    TopScoreDocCollector collector = TopScoreDocCollector.create(10, true);
    searcher.search(q, collector);
    ScoreDoc[] hits = collector.topDocs().scoreDocs;

    // We do get a hit, but results always displayed as null except for "_uid"
    if (hits.length > 0) {
        int docId = hits[0].doc;
        Document d = searcher.doc(docId);
        System.out.println("DocID " + docId + ", _uid: " + d.get("_uid") );
        System.out.println("DocID " + docId + ", f2: " + d.get("f2") );
    }
    reader.close();
}

public static void main(String[] args) throws Exception {
  TestES hl = new TestES();
  hl.doWork(args);
}
}

Results:
DocID 0, _uid: type2#3K5QXeZhQnit9UXM9_4bng
DocID 0, f2: null

The _uid value above is correct.

Eclipse shows me that the variable Document d does have two fields:

stored,indexed,tokenized,omitNorms<_uid:type2#3K5QXeZhQnit9UXM9_4bng>
stored<_source:[7b 22 66 32 22 3a 22 63 61 74 22 7d]>

Unfortunately, d.get("_source") also returns null.

How can I retrieve the document fields for a matching query?

Thank you.

Well, first off I would ask you why you're making your like harder than it should be :) Anyways, you're doing it right, the _source field is stored by default and contains the whole document you sent to elasticsearch. You have to retrieve it and parse it as a json document. Don't know why you get null. Did you make sure you're using the right lucene version? — javanna
– javanna, Commented May 30, 2013 at 8:37
I was afraid someone would ask that question :) Yes, I verified that I am running elasticsearch-0.90.0.RC2-SNAPSHOT/bin and the Lucene jars are in elasticsearch-0.90.0.RC2-SNAPSHOT/lib. I still cannot retrieve "_source" — user2434291
– user2434291, Commented May 30, 2013 at 11:06
Ah, I got it. Interestingly, I needed to retrieve the field "_source" as a binary value. So this worked: d.getBinaryValue("_source") and it retrieved [7b 22 66 32 22 3a 22 63 61 74 22 7d] which is {"f2":"cat"} — user2434291
– user2434291, Commented May 30, 2013 at 11:36
Right, sure! Missed that at first glance. Maybe you can post it as your own answer since you solved! — javanna
– javanna, Commented May 30, 2013 at 14:27
I wonder if there is a way to scope the Lucene query by ElasticSearch type. For example, is something like QueryParser(Version.LUCENE_41, "type1/f2", analyzer).parse("cat") ? — user2434291
– user2434291, Commented May 31, 2013 at 14:32

user2434291 · Accepted Answer · 2013-05-31 02:36:41Z

2

As stated in the comment, I needed to retrieve the field "_source" as a binary value. So this worked: d.getBinaryValue("_source") and it retrieved [7b 22 66 32 22 3a 22 63 61 74 22 7d] which is {"f2":"cat"}. Javanna, thanks for helping.

answered May 31, 2013 at 2:36

user2434291

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