Remove unsupported LEGACY_MULTI_NODE_DISCOVERY_TYPE (#112903)

This was never documented as supported anyway and has been deprecated
for several years so can now be removed.

Author: DaveCTurner

docs/changelog/112903.yaml

@@ -0,0 +1,17 @@
+pr: 112903
+summary: Remove unsupported legacy value for `discovery.type`
+area: Cluster Coordination
+type: breaking
+issues: []
+breaking:
+  title: Remove unsupported legacy value for `discovery.type`
+  area: Cluster and node setting
+  details: >-
+    Earlier versions of {es} had a `discovery.type` setting which permitted
+    values that referred to legacy discovery types. From v9.0.0 onwards, the
+    only supported values for this setting are `multi-node` (the default) and
+    `single-node`.
+  impact: >-
+    Remove any value for `discovery.type` from your `elasticsearch.yml`
+    configuration file.
+  notable: false

server/src/main/java/org/elasticsearch/discovery/DiscoveryModule.java

@@ -27,15 +27,12 @@
 import org.elasticsearch.cluster.version.CompatibilityVersions;
 import org.elasticsearch.common.Randomness;
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
-import org.elasticsearch.common.logging.DeprecationCategory;
-import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.network.NetworkService;
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Setting.Property;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.transport.TransportAddress;
-import org.elasticsearch.core.UpdateForV9;
 import org.elasticsearch.features.FeatureService;
 import org.elasticsearch.gateway.GatewayMetaState;
 import org.elasticsearch.indices.breaker.CircuitBreakerService;
@@ -70,8 +67,6 @@ public class DiscoveryModule extends AbstractModule {
 
     public static final String MULTI_NODE_DISCOVERY_TYPE = "multi-node";
     public static final String SINGLE_NODE_DISCOVERY_TYPE = "single-node";
-    @Deprecated
-    public static final String LEGACY_MULTI_NODE_DISCOVERY_TYPE = "zen";
 
     public static final Setting<String> DISCOVERY_TYPE_SETTING = new Setting<>(
         "discovery.type",
@@ -175,15 +170,11 @@ public DiscoveryModule(
             throw new IllegalArgumentException("Unknown election strategy " + ELECTION_STRATEGY_SETTING.get(settings));
         }
 
-        checkLegacyMultiNodeDiscoveryType(discoveryType);
-
         this.reconfigurator = getReconfigurator(settings, clusterSettings, clusterCoordinationPlugins);
         var preVoteCollectorFactory = getPreVoteCollectorFactory(clusterCoordinationPlugins);
         var leaderHeartbeatService = getLeaderHeartbeatService(settings, clusterCoordinationPlugins);
 
-        if (MULTI_NODE_DISCOVERY_TYPE.equals(discoveryType)
-            || LEGACY_MULTI_NODE_DISCOVERY_TYPE.equals(discoveryType)
-            || SINGLE_NODE_DISCOVERY_TYPE.equals(discoveryType)) {
+        if (MULTI_NODE_DISCOVERY_TYPE.equals(discoveryType) || SINGLE_NODE_DISCOVERY_TYPE.equals(discoveryType)) {
             coordinator = new Coordinator(
                 NODE_NAME_SETTING.get(settings),
                 settings,
@@ -215,22 +206,6 @@ public DiscoveryModule(
         logger.info("using discovery type [{}] and seed hosts providers {}", discoveryType, seedProviderNames);
     }
 
-    @UpdateForV9
-    private static void checkLegacyMultiNodeDiscoveryType(String discoveryType) {
-        if (LEGACY_MULTI_NODE_DISCOVERY_TYPE.equals(discoveryType)) {
-            DeprecationLogger.getLogger(DiscoveryModule.class)
-                .critical(
-                    DeprecationCategory.SETTINGS,
-                    "legacy-discovery-type",
-                    "Support for setting [{}] to [{}] is deprecated and will be removed in a future version. Set this setting to [{}] "
-                        + "instead.",
-                    DISCOVERY_TYPE_SETTING.getKey(),
-                    LEGACY_MULTI_NODE_DISCOVERY_TYPE,
-                    MULTI_NODE_DISCOVERY_TYPE
-                );
-        }
-    }
-
     // visible for testing
     static Reconfigurator getReconfigurator(
         Settings settings,

server/src/test/java/org/elasticsearch/discovery/DiscoveryModuleTests.java

@@ -219,20 +219,6 @@ public BiConsumer<DiscoveryNode, ClusterState> getJoinValidator() {
         assertTrue(onJoinValidators.contains(consumer));
     }
 
-    public void testLegacyDiscoveryType() {
-        newModule(
-            Settings.builder()
-                .put(DiscoveryModule.DISCOVERY_TYPE_SETTING.getKey(), DiscoveryModule.LEGACY_MULTI_NODE_DISCOVERY_TYPE)
-                .build(),
-            List.of(),
-            List.of()
-        );
-        assertCriticalWarnings(
-            "Support for setting [discovery.type] to [zen] is deprecated and will be removed in a future version. Set this setting to "
-                + "[multi-node] instead."
-        );
-    }
-
     public void testRejectsMultipleReconfigurators() {
         assertThat(
             expectThrows(