IndexMetaData: Introduce internal format index setting

core/src/main/java/org/elasticsearch/cluster/metadata/IndexMetaData.java

@@ -23,7 +23,6 @@
 import com.carrotsearch.hppc.cursors.IntObjectCursor;
 import com.carrotsearch.hppc.cursors.ObjectCursor;
 import com.carrotsearch.hppc.cursors.ObjectObjectCursor;
-
 import org.elasticsearch.Version;
 import org.elasticsearch.action.support.ActiveShardCount;
 import org.elasticsearch.cluster.Diff;
@@ -259,6 +258,13 @@ static Setting<Integer> buildNumberOfShardsSetting() {
                       Setting.Property.Dynamic,
                       Setting.Property.IndexScope);
 
+    /**
+     * an internal index format description, allowing us to find out if this index is upgraded or needs upgrading
+     */
+    private static final String INDEX_FORMAT = "index.format";
+    public static final Setting<Integer> INDEX_FORMAT_SETTING =
+            Setting.intSetting(INDEX_FORMAT, 0, Setting.Property.IndexScope, Setting.Property.Final);
+
     public static final String KEY_IN_SYNC_ALLOCATIONS = "in_sync_allocations";
     static final String KEY_VERSION = "version";
     static final String KEY_ROUTING_NUM_SHARDS = "routing_num_shards";
@@ -1051,6 +1057,7 @@ public IndexMetaData build() {
             }
 
             final String uuid = settings.get(SETTING_INDEX_UUID, INDEX_UUID_NA_VALUE);
+
             return new IndexMetaData(new Index(index, uuid), version, primaryTerms, state, numberOfShards, numberOfReplicas, tmpSettings, mappings.build(),
                 tmpAliases.build(), customs.build(), filledInSyncAllocationIds.build(), requireFilters, initialRecoveryFilters, includeFilters, excludeFilters,
                 indexCreatedVersion, indexUpgradedVersion, getRoutingNumShards(), routingPartitionSize, waitForActiveShards);

core/src/main/java/org/elasticsearch/common/settings/IndexScopedSettings.java

@@ -77,6 +77,7 @@ public final class IndexScopedSettings extends AbstractScopedSettings {
         IndexMetaData.INDEX_BLOCKS_READ_ONLY_ALLOW_DELETE_SETTING,
         IndexMetaData.INDEX_PRIORITY_SETTING,
         IndexMetaData.INDEX_DATA_PATH_SETTING,
+        IndexMetaData.INDEX_FORMAT_SETTING,
         SearchSlowLog.INDEX_SEARCH_SLOWLOG_THRESHOLD_FETCH_DEBUG_SETTING,
         SearchSlowLog.INDEX_SEARCH_SLOWLOG_THRESHOLD_FETCH_WARN_SETTING,
         SearchSlowLog.INDEX_SEARCH_SLOWLOG_THRESHOLD_FETCH_INFO_SETTING,

core/src/test/java/org/elasticsearch/cluster/metadata/IndexMetaDataTests.java

@@ -32,6 +32,9 @@
 import java.io.IOException;
 import java.util.Set;
 
+import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.nullValue;
+
 public class IndexMetaDataTests extends ESTestCase {
 
     public void testIndexMetaDataSerialization() throws IOException {
@@ -121,4 +124,36 @@ public void testSelectShrinkShards() {
         assertEquals("the number of target shards (8) must be greater than the shard id: 8",
             expectThrows(IllegalArgumentException.class, () -> IndexMetaData.selectShrinkShards(8, metaData, 8)).getMessage());
     }
+
+    public void testIndexFormat() {
+        Settings defaultSettings = Settings.builder()
+                .put("index.version.created", 1)
+                .put("index.number_of_shards", 1)
+                .put("index.number_of_replicas", 1)
+                .build();
+
+        // matching version
+        {
+            IndexMetaData metaData = IndexMetaData.builder("foo")
+                    .settings(Settings.builder()
+                            .put(defaultSettings)
+                            // intentionally not using the constant, so upgrading requires you to look at this test
+                            // where you have to update this part and the next one
+                            .put("index.format", 6)
+                            .build())
+                    .build();
+
+            assertThat(metaData.getSettings().getAsInt(IndexMetaData.INDEX_FORMAT_SETTING.getKey(), 0), is(6));
+        }
+
+        // no setting configured
+        {
+            IndexMetaData metaData = IndexMetaData.builder("foo")
+                    .settings(Settings.builder()
+                            .put(defaultSettings)
+                            .build())
+                    .build();
+            assertThat(metaData.getSettings().getAsInt(IndexMetaData.INDEX_FORMAT_SETTING.getKey(), 0), is(0));
+        }
+    }
 }