Renames "default" similarity into "classic".

Replaces deprecated DefaultSimilarity by ClassicSimilarity.
Fixes #15102

core/src/main/java/org/apache/lucene/queries/BlendedTermQuery.java

@@ -54,7 +54,7 @@ import java.util.Objects;
  * While aggregating the total term frequency is trivial since it
  * can be summed up not every {@link org.apache.lucene.search.similarities.Similarity}
  * makes use of this statistic. The document frequency which is used in the
- * {@link org.apache.lucene.search.similarities.DefaultSimilarity}
+ * {@link org.apache.lucene.search.similarities.ClassicSimilarity}
  * can only be estimated as an lower-bound since it is a document based statistic. For
  * the document frequency the maximum frequency across all fields per term is used
  * which is the minimum number of documents the terms occurs in.

core/src/main/java/org/elasticsearch/action/termvectors/TermVectorsFilter.java

@@ -24,7 +24,7 @@ import org.apache.lucene.index.Term;
 import org.apache.lucene.index.Terms;
 import org.apache.lucene.index.TermsEnum;
 import org.apache.lucene.search.TermStatistics;
-import org.apache.lucene.search.similarities.DefaultSimilarity;
+import org.apache.lucene.search.similarities.ClassicSimilarity;
 import org.apache.lucene.search.similarities.TFIDFSimilarity;
 import org.apache.lucene.util.BytesRef;
 import org.elasticsearch.common.Nullable;
@@ -67,7 +67,7 @@ public class TermVectorsFilter {
 
         this.dfs = dfs;
         this.scoreTerms = new HashMap<>();
-        this.similarity = new DefaultSimilarity();
+        this.similarity = new ClassicSimilarity();
     }
 
     public void setSettings(TermVectorsRequest.FilterSettings settings) {

core/src/main/java/org/elasticsearch/common/lucene/search/MoreLikeThisQuery.java

@@ -30,7 +30,7 @@ import org.apache.lucene.index.TermsEnum;
 import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.BooleanQuery;
 import org.apache.lucene.search.Query;
-import org.apache.lucene.search.similarities.DefaultSimilarity;
+import org.apache.lucene.search.similarities.ClassicSimilarity;
 import org.apache.lucene.search.similarities.Similarity;
 import org.apache.lucene.search.similarities.TFIDFSimilarity;
 import org.apache.lucene.util.BytesRef;
@@ -138,7 +138,7 @@ public class MoreLikeThisQuery extends Query {
         if (rewritten != this) {
             return rewritten;
         }
-        XMoreLikeThis mlt = new XMoreLikeThis(reader, similarity == null ? new DefaultSimilarity() : similarity);
+        XMoreLikeThis mlt = new XMoreLikeThis(reader, similarity == null ? new ClassicSimilarity() : similarity);
 
         mlt.setFieldNames(moreLikeFields);
         mlt.setAnalyzer(analyzer);

core/src/main/java/org/elasticsearch/common/lucene/search/XMoreLikeThis.java

@@ -52,7 +52,7 @@ import org.apache.lucene.search.BoostQuery;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.TermQuery;
-import org.apache.lucene.search.similarities.DefaultSimilarity;
+import org.apache.lucene.search.similarities.ClassicSimilarity;
 import org.apache.lucene.search.similarities.TFIDFSimilarity;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.CharsRefBuilder;
@@ -304,7 +304,7 @@ public final class XMoreLikeThis {
     /**
      * For idf() calculations.
      */
-    private TFIDFSimilarity similarity;// = new DefaultSimilarity();
+    private TFIDFSimilarity similarity;// = new ClassicSimilarity();
 
     /**
      * IndexReader to use
@@ -346,7 +346,7 @@ public final class XMoreLikeThis {
      * Constructor requiring an IndexReader.
      */
     public XMoreLikeThis(IndexReader ir) {
-        this(ir, new DefaultSimilarity());
+        this(ir, new ClassicSimilarity());
     }
 
     public XMoreLikeThis(IndexReader ir, TFIDFSimilarity sim) {

core/src/main/java/org/elasticsearch/index/mapper/core/TypeParsers.java

@@ -35,6 +35,8 @@ import org.elasticsearch.index.mapper.MappedFieldType.Loading;
 import org.elasticsearch.index.mapper.Mapper;
 import org.elasticsearch.index.mapper.MapperParsingException;
 import org.elasticsearch.index.mapper.object.ObjectMapper;
+import org.elasticsearch.index.similarity.SimilarityProvider;
+import org.elasticsearch.index.similarity.SimilarityService;
 
 import java.util.ArrayList;
 import java.util.Collections;
@@ -162,7 +164,8 @@ public class TypeParsers {
                 builder.omitNorms(nodeBooleanValue(propNode));
                 iterator.remove();
             } else if (propName.equals("similarity")) {
-                builder.similarity(parserContext.getSimilarity(propNode.toString()));
+                SimilarityProvider similarityProvider = resolveSimilarity(parserContext, name, propNode.toString());
+                builder.similarity(similarityProvider);
                 iterator.remove();
             } else if (parseMultiField(builder, name, parserContext, propName, propNode)) {
                 iterator.remove();
@@ -296,7 +299,8 @@ public class TypeParsers {
                 // ignore for old indexes
                 iterator.remove();
             } else if (propName.equals("similarity")) {
-                builder.similarity(parserContext.getSimilarity(propNode.toString()));
+                SimilarityProvider similarityProvider = resolveSimilarity(parserContext, name, propNode.toString());
+                builder.similarity(similarityProvider);
                 iterator.remove();
             } else if (propName.equals("fielddata")) {
                 final Settings settings = Settings.builder().put(SettingsLoader.Helper.loadNestedFromMap(nodeMapValue(propNode, "fielddata"))).build();
@@ -455,4 +459,16 @@ public class TypeParsers {
         builder.copyTo(copyToBuilder.build());
     }
 
+    private static SimilarityProvider resolveSimilarity(Mapper.TypeParser.ParserContext parserContext, String name, String value) {
+        if (parserContext.indexVersionCreated().before(Version.V_3_0_0) &&
+            "default".equals(value) && parserContext.getSimilarity(value) == null) {
+            // "default" similarity has been renamed into "classic" in 3.x.
+            value = SimilarityService.DEFAULT_SIMILARITY;
+        }
+        SimilarityProvider similarityProvider = parserContext.getSimilarity(value);
+        if (similarityProvider == null) {
+            throw new MapperParsingException("Unknown Similarity type [" + value + "] for [" + name + "]");
+        }
+        return similarityProvider;
+    }
 }

core/src/main/java/org/elasticsearch/index/similarity/DefaultSimilarityProvider.java → core/src/main/java/org/elasticsearch/index/similarity/ClassicSimilarityProvider.java

@@ -19,23 +19,23 @@
 
 package org.elasticsearch.index.similarity;
 
-import org.apache.lucene.search.similarities.DefaultSimilarity;
+import org.apache.lucene.search.similarities.ClassicSimilarity;
 import org.elasticsearch.common.settings.Settings;
 
 /**
- * {@link SimilarityProvider} for {@link DefaultSimilarity}.
+ * {@link SimilarityProvider} for {@link ClassicSimilarity}.
  * <p>
  * Configuration options available:
  * <ul>
  *     <li>discount_overlaps</li>
  * </ul>
- * @see DefaultSimilarity For more information about configuration
+ * @see ClassicSimilarity For more information about configuration
  */
-public class DefaultSimilarityProvider extends AbstractSimilarityProvider {
+public class ClassicSimilarityProvider extends AbstractSimilarityProvider {
 
-    private final DefaultSimilarity similarity = new DefaultSimilarity();
+    private final ClassicSimilarity similarity = new ClassicSimilarity();
 
-    public DefaultSimilarityProvider(String name, Settings settings) {
+    public ClassicSimilarityProvider(String name, Settings settings) {
         super(name);
         boolean discountOverlaps = settings.getAsBoolean("discount_overlaps", true);
         this.similarity.setDiscountOverlaps(discountOverlaps);
@@ -45,7 +45,7 @@ public class DefaultSimilarityProvider extends AbstractSimilarityProvider {
      * {@inheritDoc}
      */
     @Override
-    public DefaultSimilarity get() {
+    public ClassicSimilarity get() {
         return similarity;
     }
 }

core/src/main/java/org/elasticsearch/index/similarity/SimilarityService.java

@@ -35,7 +35,7 @@ import java.util.function.BiFunction;
 
 public final class SimilarityService extends AbstractIndexComponent {
 
-    public final static String DEFAULT_SIMILARITY = "default";
+    public final static String DEFAULT_SIMILARITY = "classic";
     private final Similarity defaultSimilarity;
     private final Similarity baseSimilarity;
     private final Map<String, SimilarityProvider> similarities;
@@ -44,9 +44,9 @@ public final class SimilarityService extends AbstractIndexComponent {
     static {
         Map<String, BiFunction<String, Settings, SimilarityProvider>> defaults = new HashMap<>();
         Map<String, BiFunction<String, Settings, SimilarityProvider>> buildIn = new HashMap<>();
-        defaults.put("default", DefaultSimilarityProvider::new);
+        defaults.put("classic", ClassicSimilarityProvider::new);
         defaults.put("BM25", BM25SimilarityProvider::new);
-        buildIn.put("default", DefaultSimilarityProvider::new);
+        buildIn.put("classic", ClassicSimilarityProvider::new);
         buildIn.put("BM25", BM25SimilarityProvider::new);
         buildIn.put("DFR", DFRSimilarityProvider::new);
         buildIn.put("IB", IBSimilarityProvider::new);

core/src/test/java/org/apache/lucene/queries/BlendedTermQueryTests.java

@@ -37,7 +37,7 @@ import org.apache.lucene.search.ScoreDoc;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.search.TopDocs;
 import org.apache.lucene.search.similarities.BM25Similarity;
-import org.apache.lucene.search.similarities.DefaultSimilarity;
+import org.apache.lucene.search.similarities.ClassicSimilarity;
 import org.apache.lucene.search.similarities.Similarity;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.util.TestUtil;
@@ -214,7 +214,7 @@ public class BlendedTermQueryTests extends ESTestCase {
     }
 
     public IndexSearcher setSimilarity(IndexSearcher searcher) {
-        Similarity similarity = random().nextBoolean() ? new BM25Similarity() : new DefaultSimilarity();
+        Similarity similarity = random().nextBoolean() ? new BM25Similarity() : new ClassicSimilarity();
         searcher.setSimilarity(similarity);
         return searcher;
     }

core/src/test/java/org/elasticsearch/index/similarity/SimilarityTests.java

@@ -19,11 +19,11 @@
 
 package org.elasticsearch.index.similarity;
 
+import org.apache.lucene.search.similarities.ClassicSimilarity;
 import org.apache.lucene.search.similarities.AfterEffectL;
 import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.BasicModelG;
 import org.apache.lucene.search.similarities.DFRSimilarity;
-import org.apache.lucene.search.similarities.DefaultSimilarity;
 import org.apache.lucene.search.similarities.DistributionSPL;
 import org.apache.lucene.search.similarities.IBSimilarity;
 import org.apache.lucene.search.similarities.LMDirichletSimilarity;
@@ -31,11 +31,16 @@ import org.apache.lucene.search.similarities.LMJelinekMercerSimilarity;
 import org.apache.lucene.search.similarities.LambdaTTF;
 import org.apache.lucene.search.similarities.NormalizationH2;
 import org.elasticsearch.common.compress.CompressedXContent;
+import org.elasticsearch.Version;
+import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.xcontent.XContentFactory;
 import org.elasticsearch.index.IndexService;
 import org.elasticsearch.index.mapper.DocumentMapper;
+import org.elasticsearch.index.mapper.DocumentMapperParser;
+import org.elasticsearch.index.mapper.MapperParsingException;
 import org.elasticsearch.test.ESSingleNodeTestCase;
+import org.elasticsearch.test.VersionUtils;
 
 import java.io.IOException;
 
@@ -45,42 +50,43 @@ import static org.hamcrest.CoreMatchers.instanceOf;
 public class SimilarityTests extends ESSingleNodeTestCase {
     public void testResolveDefaultSimilarities() {
         SimilarityService similarityService = createIndex("foo").similarityService();
-        assertThat(similarityService.getSimilarity("default").get(), instanceOf(DefaultSimilarity.class));
+        assertThat(similarityService.getSimilarity("classic").get(), instanceOf(ClassicSimilarity.class));
         assertThat(similarityService.getSimilarity("BM25").get(), instanceOf(BM25Similarity.class));
+        assertThat(similarityService.getSimilarity("default"), equalTo(null));
     }
 
-    public void testResolveSimilaritiesFromMapping_default() throws IOException {
+    public void testResolveSimilaritiesFromMapping_classic() throws IOException {
         String mapping = XContentFactory.jsonBuilder().startObject().startObject("type")
-                .startObject("properties")
-                .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
-                .endObject()
-                .endObject().endObject().string();
+            .startObject("properties")
+            .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
+            .endObject()
+            .endObject().endObject().string();
 
         Settings indexSettings = Settings.settingsBuilder()
-                .put("index.similarity.my_similarity.type", "default")
-                .put("index.similarity.my_similarity.discount_overlaps", false)
-                .build();
+            .put("index.similarity.my_similarity.type", "classic")
+            .put("index.similarity.my_similarity.discount_overlaps", false)
+            .build();
         IndexService indexService = createIndex("foo", indexSettings);
         DocumentMapper documentMapper = indexService.mapperService().documentMapperParser().parse("type", new CompressedXContent(mapping));
-        assertThat(documentMapper.mappers().getMapper("field1").fieldType().similarity(), instanceOf(DefaultSimilarityProvider.class));
+        assertThat(documentMapper.mappers().getMapper("field1").fieldType().similarity(), instanceOf(ClassicSimilarityProvider.class));
 
-        DefaultSimilarity similarity = (DefaultSimilarity) documentMapper.mappers().getMapper("field1").fieldType().similarity().get();
+        ClassicSimilarity similarity = (ClassicSimilarity) documentMapper.mappers().getMapper("field1").fieldType().similarity().get();
         assertThat(similarity.getDiscountOverlaps(), equalTo(false));
     }
 
     public void testResolveSimilaritiesFromMapping_bm25() throws IOException {
         String mapping = XContentFactory.jsonBuilder().startObject().startObject("type")
-                .startObject("properties")
-                .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
-                .endObject()
-                .endObject().endObject().string();
+            .startObject("properties")
+            .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
+            .endObject()
+            .endObject().endObject().string();
 
         Settings indexSettings = Settings.settingsBuilder()
-                .put("index.similarity.my_similarity.type", "BM25")
-                .put("index.similarity.my_similarity.k1", 2.0f)
-                .put("index.similarity.my_similarity.b", 1.5f)
-                .put("index.similarity.my_similarity.discount_overlaps", false)
-                .build();
+            .put("index.similarity.my_similarity.type", "BM25")
+            .put("index.similarity.my_similarity.k1", 2.0f)
+            .put("index.similarity.my_similarity.b", 1.5f)
+            .put("index.similarity.my_similarity.discount_overlaps", false)
+            .build();
         IndexService indexService = createIndex("foo", indexSettings);
         DocumentMapper documentMapper = indexService.mapperService().documentMapperParser().parse("type", new CompressedXContent(mapping));
         assertThat(documentMapper.mappers().getMapper("field1").fieldType().similarity(), instanceOf(BM25SimilarityProvider.class));
@@ -93,18 +99,18 @@ public class SimilarityTests extends ESSingleNodeTestCase {
 
     public void testResolveSimilaritiesFromMapping_DFR() throws IOException {
         String mapping = XContentFactory.jsonBuilder().startObject().startObject("type")
-                .startObject("properties")
-                .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
-                .endObject()
-                .endObject().endObject().string();
+            .startObject("properties")
+            .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
+            .endObject()
+            .endObject().endObject().string();
 
         Settings indexSettings = Settings.settingsBuilder()
-                .put("index.similarity.my_similarity.type", "DFR")
-                .put("index.similarity.my_similarity.basic_model", "g")
-                .put("index.similarity.my_similarity.after_effect", "l")
-                .put("index.similarity.my_similarity.normalization", "h2")
-                .put("index.similarity.my_similarity.normalization.h2.c", 3f)
-                .build();
+            .put("index.similarity.my_similarity.type", "DFR")
+            .put("index.similarity.my_similarity.basic_model", "g")
+            .put("index.similarity.my_similarity.after_effect", "l")
+            .put("index.similarity.my_similarity.normalization", "h2")
+            .put("index.similarity.my_similarity.normalization.h2.c", 3f)
+            .build();
         IndexService indexService = createIndex("foo", indexSettings);
         DocumentMapper documentMapper = indexService.mapperService().documentMapperParser().parse("type", new CompressedXContent(mapping));
         assertThat(documentMapper.mappers().getMapper("field1").fieldType().similarity(), instanceOf(DFRSimilarityProvider.class));
@@ -118,18 +124,18 @@ public class SimilarityTests extends ESSingleNodeTestCase {
 
     public void testResolveSimilaritiesFromMapping_IB() throws IOException {
         String mapping = XContentFactory.jsonBuilder().startObject().startObject("type")
-                .startObject("properties")
-                .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
-                .endObject()
-                .endObject().endObject().string();
+            .startObject("properties")
+            .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
+            .endObject()
+            .endObject().endObject().string();
 
         Settings indexSettings = Settings.settingsBuilder()
-                .put("index.similarity.my_similarity.type", "IB")
-                .put("index.similarity.my_similarity.distribution", "spl")
-                .put("index.similarity.my_similarity.lambda", "ttf")
-                .put("index.similarity.my_similarity.normalization", "h2")
-                .put("index.similarity.my_similarity.normalization.h2.c", 3f)
-                .build();
+            .put("index.similarity.my_similarity.type", "IB")
+            .put("index.similarity.my_similarity.distribution", "spl")
+            .put("index.similarity.my_similarity.lambda", "ttf")
+            .put("index.similarity.my_similarity.normalization", "h2")
+            .put("index.similarity.my_similarity.normalization.h2.c", 3f)
+            .build();
         IndexService indexService = createIndex("foo", indexSettings);
         DocumentMapper documentMapper = indexService.mapperService().documentMapperParser().parse("type", new CompressedXContent(mapping));
         assertThat(documentMapper.mappers().getMapper("field1").fieldType().similarity(), instanceOf(IBSimilarityProvider.class));
@@ -143,15 +149,15 @@ public class SimilarityTests extends ESSingleNodeTestCase {
 
     public void testResolveSimilaritiesFromMapping_LMDirichlet() throws IOException {
         String mapping = XContentFactory.jsonBuilder().startObject().startObject("type")
-                .startObject("properties")
-                .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
-                .endObject()
-                .endObject().endObject().string();
+            .startObject("properties")
+            .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
+            .endObject()
+            .endObject().endObject().string();
 
         Settings indexSettings = Settings.settingsBuilder()
-                .put("index.similarity.my_similarity.type", "LMDirichlet")
-                .put("index.similarity.my_similarity.mu", 3000f)
-                .build();
+            .put("index.similarity.my_similarity.type", "LMDirichlet")
+            .put("index.similarity.my_similarity.mu", 3000f)
+            .build();
         IndexService indexService = createIndex("foo", indexSettings);
         DocumentMapper documentMapper = indexService.mapperService().documentMapperParser().parse("type", new CompressedXContent(mapping));
         assertThat(documentMapper.mappers().getMapper("field1").fieldType().similarity(), instanceOf(LMDirichletSimilarityProvider.class));
@@ -162,15 +168,15 @@ public class SimilarityTests extends ESSingleNodeTestCase {
 
     public void testResolveSimilaritiesFromMapping_LMJelinekMercer() throws IOException {
         String mapping = XContentFactory.jsonBuilder().startObject().startObject("type")
-                .startObject("properties")
-                .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
-                .endObject()
-                .endObject().endObject().string();
+            .startObject("properties")
+            .startObject("field1").field("type", "string").field("similarity", "my_similarity").endObject()
+            .endObject()
+            .endObject().endObject().string();
 
         Settings indexSettings = Settings.settingsBuilder()
-                .put("index.similarity.my_similarity.type", "LMJelinekMercer")
-                .put("index.similarity.my_similarity.lambda", 0.7f)
-                .build();
+            .put("index.similarity.my_similarity.type", "LMJelinekMercer")
+            .put("index.similarity.my_similarity.lambda", 0.7f)
+            .build();
         IndexService indexService = createIndex("foo", indexSettings);
         DocumentMapper documentMapper = indexService.mapperService().documentMapperParser().parse("type", new CompressedXContent(mapping));
         assertThat(documentMapper.mappers().getMapper("field1").fieldType().similarity(), instanceOf(LMJelinekMercerSimilarityProvider.class));
@@ -178,4 +184,47 @@ public class SimilarityTests extends ESSingleNodeTestCase {
         LMJelinekMercerSimilarity similarity = (LMJelinekMercerSimilarity) documentMapper.mappers().getMapper("field1").fieldType().similarity().get();
         assertThat(similarity.getLambda(), equalTo(0.7f));
     }
+
+    public void testResolveSimilaritiesFromMapping_Unknown() throws IOException {
+        String mapping = XContentFactory.jsonBuilder().startObject().startObject("type")
+            .startObject("properties")
+            .startObject("field1").field("type", "string").field("similarity", "unknown_similarity").endObject()
+            .endObject()
+            .endObject().endObject().string();
+
+        IndexService indexService = createIndex("foo");
+        try {
+            indexService.mapperService().documentMapperParser().parse("type", new CompressedXContent(mapping));
+            fail("Expected MappingParsingException");
+        } catch (MapperParsingException e) {
+            assertThat(e.getMessage(), equalTo("Unknown Similarity type [unknown_similarity] for [field1]"));
+        }
+    }
+
+    public void testSimilarityDefaultBackCompat() throws IOException {
+        String mapping = XContentFactory.jsonBuilder().startObject().startObject("type")
+            .startObject("properties")
+            .startObject("field1")
+            .field("similarity", "default")
+            .field("type", "string")
+            .endObject()
+            .endObject()
+            .endObject().string();
+        Settings settings = Settings.builder()
+            .put(IndexMetaData.SETTING_VERSION_CREATED, VersionUtils.randomVersionBetween(random(), Version.V_2_0_0, Version.V_2_2_0))
+            .build();
+
+        DocumentMapperParser parser = createIndex("test_v2.x", settings).mapperService().documentMapperParser();
+        DocumentMapper documentMapper = parser.parse("type", new CompressedXContent(mapping));
+        assertThat(documentMapper.mappers().getMapper("field1").fieldType().similarity(), instanceOf(ClassicSimilarityProvider.class));
+        assertThat(documentMapper.mappers().getMapper("field1").fieldType().similarity().name(), equalTo("classic"));
+
+        parser = createIndex("test_v3.x").mapperService().documentMapperParser();
+        try {
+            parser.parse("type", new CompressedXContent(mapping));
+            fail("Expected MappingParsingException");
+        } catch (MapperParsingException e) {
+            assertThat(e.getMessage(), equalTo("Unknown Similarity type [default] for [field1]"));
+        }
+    }
 }

core/src/test/java/org/elasticsearch/similarity/SimilarityIT.java

@@ -28,7 +28,7 @@ import static org.elasticsearch.index.query.QueryBuilders.matchQuery;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.not;
 
-public class SimilarityIT  extends ESIntegTestCase {
+public class SimilarityIT extends ESIntegTestCase {
     public void testCustomBM25Similarity() throws Exception {
         try {
             client().admin().indices().prepareDelete("test").execute().actionGet();
@@ -45,7 +45,7 @@ public class SimilarityIT  extends ESIntegTestCase {
                                     .field("type", "string")
                                 .endObject()
                                 .startObject("field2")
-                                    .field("similarity", "default")
+                                    .field("similarity", "classic")
                                     .field("type", "string")
                             .endObject()
                         .endObject()

docs/reference/index-modules/similarity.asciidoc

@@ -48,10 +48,10 @@ Here we configure the DFRSimilarity so it can be referenced as
 === Available similarities
 
 [float]
-[[default-similarity]]
-==== Default similarity
+[[classic-similarity]]
+==== Classic similarity
 
-The default similarity that is based on the TF/IDF model. This
+The classic similarity that is based on the TF/IDF model. This
 similarity has the following option:
 
 `discount_overlaps`::
@@ -59,7 +59,7 @@ similarity has the following option:
     0 position increment) are ignored when computing norm. By default this
     is true, meaning overlap tokens do not count when computing norms.
 
-Type name: `default`
+Type name: `classic`
 
 [float]
 [[bm25]]

docs/reference/mapping/params/similarity.asciidoc

@@ -15,7 +15,7 @@ similarities. For more details about this expert options, see the
 The only similarities which can be used out of the box, without any further
 configuration are:
 
-`default`::
+`classic`::
         The Default TF/IDF algorithm used by Elasticsearch and
         Lucene. See {defguide}/practical-scoring-function.html[Lucene’s Practical Scoring Function]
         for more information.
@@ -49,6 +49,6 @@ PUT my_index
 }
 --------------------------------------------------
 // AUTOSENSE
-<1> The `default_field` uses the `default` similarity (ie TF/IDF).
+<1> The `default_field` uses the `classic` similarity (ie TF/IDF).
 <2> The `bm25_field` uses the `BM25` similarity.
 

docs/reference/mapping/types/string.asciidoc

@@ -166,7 +166,7 @@ Defaults depend on the <<mapping-index,`index`>> setting:
 <<similarity,`similarity`>>::
 
     Which scoring algorithm or _similarity_ should be used. Defaults
-    to `default`, which uses TF/IDF.
+    to `classic`, which uses TF/IDF.
 
 <<term-vector,`term_vector`>>::
 

docs/reference/migration/migrate_3_0.asciidoc

@@ -205,6 +205,10 @@ cluster settings please use the settings update API and set their superseded key
 The `index.translog.flush_threshold_ops` setting is not supported anymore. In order to control flushes based on the transaction log
 growth use `index.translog.flush_threshold_size` instead.
 
+==== Similarity settings
+
+The 'default' similarity has been renamed to 'classic'.
+
 [[breaking_30_mapping_changes]]
 === Mapping changes