Add link to warning re. single-node clusters

server/src/main/java/org/elasticsearch/cluster/coordination/Coordinator.java

@@ -41,6 +41,7 @@
 import org.elasticsearch.cluster.service.MasterServiceTaskQueue;
 import org.elasticsearch.cluster.version.CompatibilityVersions;
 import org.elasticsearch.common.Priority;
+import org.elasticsearch.common.ReferenceDocs;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.component.AbstractLifecycleComponent;
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
@@ -831,10 +832,12 @@ public void run() {
                                     discover other nodes and form a multi-node cluster via the [{}={}] setting. Fully-formed clusters do \
                                     not attempt to discover other nodes, and nodes with different cluster UUIDs cannot belong to the same \
                                     cluster. The cluster UUID persists across restarts and can only be changed by deleting the contents of \
-                                    the node's data path(s). Remove the discovery configuration to suppress this message.""",
+                                    the node's data path(s). Remove the discovery configuration to suppress this message. See [{}] for \
+                                    more information.""",
                                 applierState.metadata().clusterUUID(),
                                 DISCOVERY_SEED_HOSTS_SETTING.getKey(),
-                                DISCOVERY_SEED_HOSTS_SETTING.get(settings)
+                                DISCOVERY_SEED_HOSTS_SETTING.get(settings),
+                                ReferenceDocs.FORMING_SINGLE_NODE_CLUSTERS
                             );
                         }
                     }

server/src/main/java/org/elasticsearch/common/ReferenceDocs.java

@@ -81,6 +81,7 @@ public enum ReferenceDocs {
     MAX_SHARDS_PER_NODE,
     FLOOD_STAGE_WATERMARK,
     X_OPAQUE_ID,
+    FORMING_SINGLE_NODE_CLUSTERS,
     // this comment keeps the ';' on the next line so every entry above has a trailing ',' which makes the diff for adding new links cleaner
     ;
 

server/src/main/resources/org/elasticsearch/common/reference-docs-links.json

@@ -41,5 +41,6 @@
   "LUCENE_MAX_DOCS_LIMIT": "size-your-shards.html#troubleshooting-max-docs-limit",
   "MAX_SHARDS_PER_NODE": "size-your-shards.html#troubleshooting-max-shards-open",
   "FLOOD_STAGE_WATERMARK": "fix-watermark-errors.html",
-  "X_OPAQUE_ID": "api-conventions.html#x-opaque-id"
+  "X_OPAQUE_ID": "api-conventions.html#x-opaque-id",
+  "FORMING_SINGLE_NODE_CLUSTERS": "modules-discovery-bootstrap-cluster.html#modules-discovery-bootstrap-cluster-joining"
 }

server/src/test/java/org/elasticsearch/cluster/coordination/CoordinatorTests.java

@@ -25,6 +25,7 @@
 import org.elasticsearch.cluster.node.DiscoveryNodeUtils;
 import org.elasticsearch.cluster.node.DiscoveryNodes;
 import org.elasticsearch.cluster.service.ClusterStateUpdateStats;
+import org.elasticsearch.common.ReferenceDocs;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
 import org.elasticsearch.common.io.stream.StreamOutput;
@@ -79,6 +80,8 @@
 import static org.elasticsearch.discovery.SettingsBasedSeedHostsProvider.DISCOVERY_SEED_HOSTS_SETTING;
 import static org.elasticsearch.monitor.StatusInfo.Status.HEALTHY;
 import static org.elasticsearch.monitor.StatusInfo.Status.UNHEALTHY;
+import static org.hamcrest.Matchers.allOf;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.greaterThanOrEqualTo;
@@ -1762,7 +1765,13 @@ public void testLogsWarningPeriodicallyIfSingleNodeClusterHasSeedHosts() {
                         @Override
                         public void match(LogEvent event) {
                             final String message = event.getMessage().getFormattedMessage();
-                            assertThat(message, startsWith("This node is a fully-formed single-node cluster with cluster UUID"));
+                            assertThat(
+                                message,
+                                allOf(
+                                    startsWith("This node is a fully-formed single-node cluster with cluster UUID"),
+                                    containsString(ReferenceDocs.FORMING_SINGLE_NODE_CLUSTERS.toString())
+                                )
+                            );
                             loggedClusterUuid = (String) event.getMessage().getParameters()[0];
                         }