Allow removing index.number_of_replicas setting

server/src/internalClusterTest/java/org/elasticsearch/indices/settings/UpdateSettingsIT.java

@@ -646,4 +646,28 @@ public void testNumberOfReplicasSettingsVersion() {
         assertThat(newSettingsVersion, equalTo(1 + settingsVersion));
     }
 
+    /*
+     * Test that we are able to set the setting index.number_of_replicas to the default.
+     */
+    public void testDefaultNumberOfReplicas() {
+        if (randomBoolean()) {
+            assertAcked(client().admin()
+                .indices()
+                .prepareCreate("test")
+                .setSettings(Settings.builder().put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, randomIntBetween(1, 8))));
+        } else {
+            assertAcked(client().admin().indices().prepareCreate("test"));
+        }
+
+        /*
+         * Previous versions of Elasticsearch would throw an exception that the number of replicas had to have a value, and could not be
+         * null. In the update settings logic, we ensure this by providing an explicit default value if the setting is set to null.
+         */
+        assertAcked(client().admin()
+            .indices()
+            .prepareUpdateSettings("test")
+            .setSettings(Settings.builder().putNull(IndexMetadata.SETTING_NUMBER_OF_REPLICAS)));
+
+    }
+
 }

server/src/main/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsService.java

@@ -190,6 +190,16 @@ public ClusterState execute(ClusterState currentState) {
                             if (preserveExisting) {
                                 indexSettings.put(indexMetadata.getSettings());
                             }
+                            /*
+                             * The setting index.number_of_replicas is special; we require that this setting has a value in the index. When
+                             * creating the index, we ensure this by explicitly providing a value for the setting to the default (one) if
+                             * there is a not value provided on the source of the index creation. A user can update this setting though,
+                             * including updating it to null, indicating that they want to use the default value. In this case, we again
+                             * have to provide an explicit value for the setting to the default (one).
+                             */
+                            if (indexSettings.get(IndexMetadata.SETTING_NUMBER_OF_REPLICAS) == null) {
+                                indexSettings.put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 1);
+                            }
                             Settings finalSettings = indexSettings.build();
                             indexScopedSettings.validate(
                                 finalSettings.filter(k -> indexScopedSettings.isPrivateSetting(k) == false), true);