Add per-index setting to limit number of nested fields

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

@@ -95,6 +95,7 @@ public enum MergeReason {
 
     public static final String DEFAULT_MAPPING = "_default_";
     public static final String INDEX_MAPPER_DYNAMIC_SETTING = "index.mapper.dynamic";
+    public static final String INDEX_MAPPING_NESTED_FIELDS_LIMIT_SETTING = "index.mapping.nested_fields.limit";
     public static final boolean INDEX_MAPPER_DYNAMIC_DEFAULT = true;
     private static ObjectHashSet<String> META_FIELDS = ObjectHashSet.from(
             "_uid", "_id", "_type", "_all", "_parent", "_routing", "_index",
@@ -242,12 +243,12 @@ public DocumentMapper merge(String type, CompressedXContent mappingSource, Merge
                         // only apply the default mapping if we don't have the type yet
                         && mappers.containsKey(type) == false;
                 DocumentMapper mergeWith = parse(type, mappingSource, applyDefault);
-                return merge(mergeWith, updateAllTypes);
+                return merge(mergeWith, reason, updateAllTypes);
             }
         }
     }
 
-    private synchronized DocumentMapper merge(DocumentMapper mapper, boolean updateAllTypes) {
+    private synchronized DocumentMapper merge(DocumentMapper mapper, MergeReason reason, boolean updateAllTypes) {
         if (mapper.type().length() == 0) {
             throw new InvalidTypeNameException("mapping type name is empty");
         }
@@ -300,6 +301,11 @@ private synchronized DocumentMapper merge(DocumentMapper mapper, boolean updateA
             }
         }
         fullPathObjectMappers = Collections.unmodifiableMap(fullPathObjectMappers);
+
+        if (reason == MergeReason.MAPPING_UPDATE) {
+            checkNestedFieldsLimit(fullPathObjectMappers);
+        }
+
         Set<String> parentTypes = this.parentTypes;
         if (oldMapper == null && newMapper.parentFieldMapper().active()) {
             parentTypes = new HashSet<>(parentTypes.size() + 1);
@@ -412,6 +418,19 @@ private void checkObjectsCompatibility(String type, Collection<ObjectMapper> obj
         }
     }
 
+    private void checkNestedFieldsLimit(Map<String, ObjectMapper> fullPathObjectMappers) {
+        long allowedNestedFields = indexSettings.getSettings().getAsLong(INDEX_MAPPING_NESTED_FIELDS_LIMIT_SETTING, 50L);
+        long actualNestedFields = 0;
+        for (ObjectMapper objectMapper : fullPathObjectMappers.values()) {
+            if (objectMapper.nested().isNested()) {
+                actualNestedFields++;
+            }
+        }
+        if (allowedNestedFields >= 0 && actualNestedFields > allowedNestedFields) {
+            throw new IllegalArgumentException("Limit of nested fields [" + allowedNestedFields + "] in index [" + index().name() + "] has been exceeded");
+        }
+    }
+
     public DocumentMapper parse(String mappingType, CompressedXContent mappingSource, boolean applyDefault) throws MapperParsingException {
         String defaultMappingSource;
         if (PercolatorService.TYPE_NAME.equals(mappingType)) {

core/src/test/java/org/elasticsearch/index/mapper/nested/NestedMappingTests.java

@@ -20,14 +20,22 @@
 package org.elasticsearch.index.mapper.nested;
 
 import org.elasticsearch.common.compress.CompressedXContent;
+import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.xcontent.XContentFactory;
 import org.elasticsearch.index.mapper.DocumentMapper;
+import org.elasticsearch.index.mapper.MapperService;
+import org.elasticsearch.index.mapper.MapperService.MergeReason;
 import org.elasticsearch.index.mapper.ParsedDocument;
 import org.elasticsearch.index.mapper.internal.TypeFieldMapper;
 import org.elasticsearch.index.mapper.object.ObjectMapper;
 import org.elasticsearch.index.mapper.object.ObjectMapper.Dynamic;
 import org.elasticsearch.test.ESSingleNodeTestCase;
 
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.function.Function;
+
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.nullValue;
 
@@ -340,4 +348,58 @@ public void testNestedArrayStrict() throws Exception {
         assertThat(doc.docs().get(1).get("field"), nullValue());
         assertThat(doc.docs().get(2).get("field"), equalTo("value"));
     }
-}
+
+    public void testLimitOfNestedFieldsPerIndex() throws Exception {
+        Function<String, String> mapping = type -> {
+            try {
+                return XContentFactory.jsonBuilder().startObject().startObject(type).startObject("properties")
+                    .startObject("nested1").field("type", "nested").startObject("properties")
+                    .startObject("nested2").field("type", "nested")
+                    .endObject().endObject()
+                    .endObject().endObject().endObject().string();
+            } catch (IOException e) {
+                throw new UncheckedIOException(e);
+            }
+        };
+
+        // default limit allows at least two nested fields
+        createIndex("test1").mapperService().merge("type", new CompressedXContent(mapping.apply("type")), MergeReason.MAPPING_UPDATE, false);
+
+        // explicitly setting limit to 0 prevents nested fields
+        try {
+            createIndex("test2", Settings.builder().put(MapperService.INDEX_MAPPING_NESTED_FIELDS_LIMIT_SETTING, 0).build())
+                .mapperService().merge("type", new CompressedXContent(mapping.apply("type")), MergeReason.MAPPING_UPDATE, false);
+            fail("Expected IllegalArgumentException");
+        } catch (IllegalArgumentException e) {
+            assertThat(e.getMessage(), containsString("Limit of nested fields [0] in index [test2] has been exceeded"));
+        }
+
+        // setting limit to 1 with 2 nested fields fails
+        try {
+            createIndex("test3", Settings.builder().put(MapperService.INDEX_MAPPING_NESTED_FIELDS_LIMIT_SETTING, 1).build())
+                .mapperService().merge("type", new CompressedXContent(mapping.apply("type")), MergeReason.MAPPING_UPDATE, false);
+            fail("Expected IllegalArgumentException");
+        } catch (IllegalArgumentException e) {
+            assertThat(e.getMessage(), containsString("Limit of nested fields [1] in index [test3] has been exceeded"));
+        }
+
+        MapperService mapperService = createIndex("test4", Settings.builder().put(MapperService.INDEX_MAPPING_NESTED_FIELDS_LIMIT_SETTING, 2)
+            .build()).mapperService();
+        mapperService.merge("type1", new CompressedXContent(mapping.apply("type1")), MergeReason.MAPPING_UPDATE, false);
+        // merging same fields, but different type is ok
+        mapperService.merge("type2", new CompressedXContent(mapping.apply("type2")), MergeReason.MAPPING_UPDATE, false);
+        // adding new fields from different type is not ok
+        String mapping2 = XContentFactory.jsonBuilder().startObject().startObject("type3").startObject("properties").startObject("nested3")
+            .field("type", "nested").startObject("properties").endObject().endObject().endObject().endObject().string();
+        try {
+            mapperService.merge("type3", new CompressedXContent(mapping2), MergeReason.MAPPING_UPDATE, false);
+            fail("Expected IllegalArgumentException");
+        } catch (IllegalArgumentException e) {
+            assertThat(e.getMessage(), containsString("Limit of nested fields [2] in index [test4] has been exceeded"));
+        }
+
+        // do not check nested fields limit if mapping is not updated
+        createIndex("test5", Settings.builder().put(MapperService.INDEX_MAPPING_NESTED_FIELDS_LIMIT_SETTING, 0).build())
+            .mapperService().merge("type", new CompressedXContent(mapping.apply("type")), MergeReason.MAPPING_RECOVERY, false);
+    }
+}

docs/reference/mapping/types/nested.asciidoc

@@ -199,3 +199,10 @@ phase.  Instead, highlighting needs to be performed via
 
 =============================================
 
+
+==== Limiting the number of `nested` fields
+
+Indexing a document with 100 nested fields actually indexes 101 documents as each nested
+document is indexed as a separate document. To safeguard against ill-defined mappings
+the number of nested fields that can be defined per index has been limited to 50. This
+default limit can be changed with the index setting `index.mapping.nested_fields.limit`.

docs/reference/migration/index.asciidoc

@@ -18,6 +18,8 @@ See <<setup-upgrade>> for more info.
 --
 include::migrate_3_0.asciidoc[]
 
+include::migrate_2_3.asciidoc[]
+
 include::migrate_2_2.asciidoc[]
 
 include::migrate_2_1.asciidoc[]

docs/reference/migration/migrate_2_3.asciidoc

@@ -0,0 +1,19 @@
+[[breaking-changes-2.3]]
+== Breaking changes in 2.3
+
+This section discusses the changes that you need to be aware of when migrating
+your application to Elasticsearch 2.3.
+
+* <<breaking_23_index_apis>>
+
+[[breaking_23_index_apis]]
+=== Mappings
+
+==== Limit to the number of `nested` fields
+
+Indexing a document with 100 nested fields actually indexes 101 documents as each nested
+document is indexed as a separate document. To safeguard against ill-defined mappings
+the number of nested fields that can be defined per index has been limited to 50.
+This default limit can be changed with the index setting `index.mapping.nested_fields.limit`.
+Note that the limit is only checked when new indices are created or mappings are updated. It
+will thus only affect existing pre-2.3 indices if their mapping is changed.