Add user-defined cluster metadata (#33325)

Adds a place for users to store cluster-wide data they wish to associate
with the cluster via the Cluster Settings API. This is strictly for
user-defined data, Elasticsearch makes no other other use of these
settings.

docs/reference/modules/cluster/misc.asciidoc

@@ -22,6 +22,27 @@ user with access to the <<cluster-update-settings,cluster-update-settings>>
 API can make the cluster read-write again.
 
 
+[[user-defined-data]]
+==== User Defined Cluster Metadata
+
+User-defined metadata can be stored and retrieved using the Cluster Settings API.
+This can be used to store arbitrary, infrequently-changing data about the cluster
+without the need to create an index to store it. This data may be stored using
+any key prefixed with `cluster.metadata.`.  For example, to store the email
+address of the administrator of a cluster under the key `cluster.metadata.administrator`,
+issue this request:
+
+[source,js]
+-------------------------------
+PUT /_cluster/settings
+{
+  "persistent": {
+    "cluster.metadata.administrator": "[email protected]"
+  }
+}
+-------------------------------
+// CONSOLE
+
 [[cluster-max-tombstones]]
 ==== Index Tombstones
 

server/src/main/java/org/elasticsearch/cluster/service/ClusterService.java

@@ -52,6 +52,9 @@ public class ClusterService extends AbstractLifecycleComponent {
             Setting.positiveTimeSetting("cluster.service.slow_task_logging_threshold", TimeValue.timeValueSeconds(30),
                     Property.Dynamic, Property.NodeScope);
 
+    public static final org.elasticsearch.common.settings.Setting.AffixSetting<String> USER_DEFINED_META_DATA =
+        Setting.prefixKeySetting("cluster.metadata.", (key) -> Setting.simpleString(key, Property.Dynamic, Property.NodeScope));
+
     private final ClusterName clusterName;
 
     private final OperationRouting operationRouting;
@@ -68,6 +71,8 @@ public ClusterService(Settings settings, ClusterSettings clusterSettings, Thread
         this.clusterName = ClusterName.CLUSTER_NAME_SETTING.get(settings);
         this.clusterSettings.addSettingsUpdateConsumer(CLUSTER_SERVICE_SLOW_TASK_LOGGING_THRESHOLD_SETTING,
             this::setSlowTaskLoggingThreshold);
+        // Add a no-op update consumer so changes are logged
+        this.clusterSettings.addAffixUpdateConsumer(USER_DEFINED_META_DATA, (first, second) -> {}, (first, second) -> {});
         this.initialClusterStateCustoms = initialClusterStateCustoms;
         this.clusterApplierService = new ClusterApplierService(settings, clusterSettings, threadPool, this::newClusterStateBuilder);
     }

server/src/main/java/org/elasticsearch/common/settings/ClusterSettings.java

@@ -269,6 +269,7 @@ public void apply(Settings value, Settings current, Settings previous) {
                     HierarchyCircuitBreakerService.ACCOUNTING_CIRCUIT_BREAKER_OVERHEAD_SETTING,
                     IndexModule.NODE_STORE_ALLOW_MMAPFS,
                     ClusterService.CLUSTER_SERVICE_SLOW_TASK_LOGGING_THRESHOLD_SETTING,
+                    ClusterService.USER_DEFINED_META_DATA,
                     SearchService.DEFAULT_SEARCH_TIMEOUT_SETTING,
                     SearchService.DEFAULT_ALLOW_PARTIAL_SEARCH_RESULTS,
                     ElectMasterService.DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING,

server/src/test/java/org/elasticsearch/cluster/settings/ClusterSettingsIT.java

@@ -22,6 +22,7 @@
 import org.apache.logging.log4j.Level;
 import org.elasticsearch.action.admin.cluster.settings.ClusterUpdateSettingsRequestBuilder;
 import org.elasticsearch.action.admin.cluster.settings.ClusterUpdateSettingsResponse;
+import org.elasticsearch.action.admin.cluster.state.ClusterStateResponse;
 import org.elasticsearch.cluster.metadata.MetaData;
 import org.elasticsearch.cluster.routing.allocation.decider.EnableAllocationDecider;
 import org.elasticsearch.common.logging.ESLoggerFactory;
@@ -380,4 +381,34 @@ public void testLoggerLevelUpdate() {
         }
     }
 
+    public void testUserMetadata() {
+        String key = "cluster.metadata." + randomAlphaOfLengthBetween(5, 20);
+        String value = randomRealisticUnicodeOfCodepointLengthBetween(5, 50);
+        String updatedValue = randomRealisticUnicodeOfCodepointLengthBetween(5, 50);
+        logger.info("Attempting to store [{}]: [{}], then update to [{}]", key, value, updatedValue);
+
+        final Settings settings = Settings.builder().put(key, value).build();
+        final Settings updatedSettings = Settings.builder().put(key, updatedValue).build();
+        if (randomBoolean()) {
+            logger.info("Using persistent settings");
+
+            client().admin().cluster().prepareUpdateSettings().setPersistentSettings(settings).execute().actionGet();
+            ClusterStateResponse state = client().admin().cluster().prepareState().execute().actionGet();
+            assertEquals(value, state.getState().getMetaData().persistentSettings().get(key));
+
+            client().admin().cluster().prepareUpdateSettings().setPersistentSettings(updatedSettings).execute().actionGet();
+            ClusterStateResponse updatedState = client().admin().cluster().prepareState().execute().actionGet();
+            assertEquals(updatedValue, updatedState.getState().getMetaData().persistentSettings().get(key));
+        } else {
+            logger.info("Using transient settings");
+            client().admin().cluster().prepareUpdateSettings().setTransientSettings(settings).execute().actionGet();
+            ClusterStateResponse state = client().admin().cluster().prepareState().execute().actionGet();
+            assertEquals(value, state.getState().getMetaData().transientSettings().get(key));
+
+            client().admin().cluster().prepareUpdateSettings().setTransientSettings(updatedSettings).execute().actionGet();
+            ClusterStateResponse updatedState = client().admin().cluster().prepareState().execute().actionGet();
+            assertEquals(updatedValue, updatedState.getState().getMetaData().transientSettings().get(key));
+        }
+    }
+
 }