Deprecations for single data node setting

docs/reference/migration/migrate_7_14.asciidoc

@@ -55,4 +55,20 @@ cluster then you must upgrade the affected nodes. Once upgraded, they will join
 the cluster again.
 ====
 
+[discrete]
+[[breaking_714_core_deprecations]]
+=== Core deprecations
+
+[discrete]
+[[deprecate-single-data-node-watermark]]
+.Setting `cluster.routing.allocation.disk.watermark.enable_for_single_data_node=false` is deprecated.
+[%collapsible]
+====
+*Details* +
+The setting `cluster.routing.allocation.disk.watermark.enable_for_single_data_node`
+should never be explicitly set to false. In 8.0, the only legal value will be
+true. In a future release, the setting will be removed completely, with same
+behavior as if the setting was `true`.
+====
+
 // end::notable-breaking-changes[]

server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDecider.java

@@ -26,6 +26,8 @@
 import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.collect.ImmutableOpenMap;
+import org.elasticsearch.common.logging.DeprecationCategory;
+import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
@@ -35,6 +37,7 @@
 import org.elasticsearch.snapshots.SnapshotShardSizeInfo;
 
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
 import static org.elasticsearch.cluster.routing.allocation.DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_HIGH_DISK_WATERMARK_SETTING;
@@ -66,11 +69,28 @@
 public class DiskThresholdDecider extends AllocationDecider {
 
     private static final Logger logger = LogManager.getLogger(DiskThresholdDecider.class);
+    private static final DeprecationLogger deprecationLogger = DeprecationLogger.getLogger(DiskThresholdDecider.class);
 
     public static final String NAME = "disk_threshold";
 
     public static final Setting<Boolean> ENABLE_FOR_SINGLE_DATA_NODE =
-        Setting.boolSetting("cluster.routing.allocation.disk.watermark.enable_for_single_data_node", false, Setting.Property.NodeScope);
+        Setting.boolSetting("cluster.routing.allocation.disk.watermark.enable_for_single_data_node", false,
+            new Setting.Validator<Boolean>() {
+                @Override
+                public void validate(Boolean value) {
+                    // empty
+                }
+
+                @Override
+                public void validate(Boolean value, Map<Setting<?>, Object> settings, boolean isPresent) {
+                    if (value == Boolean.FALSE && isPresent) {
+                        deprecationLogger.deprecate(DeprecationCategory.SETTINGS, "watermark_enable_for_single_data_node",
+                            "setting [{}=false] is deprecated and will not be available in a future version",
+                            ENABLE_FOR_SINGLE_DATA_NODE.getKey());
+                    }
+                }
+            },
+            Setting.Property.NodeScope);
 
     public static final Setting<Boolean> SETTING_IGNORE_DISK_WATERMARKS =
         Setting.boolSetting("index.routing.allocation.disk.watermark.ignore", false,

server/src/test/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDeciderTests.java

@@ -1133,6 +1133,17 @@ Settings.EMPTY, new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLU
                 " actual free: [20.0%]"));
     }
 
+    public void testSingleDataNodeDeprecationWarning() {
+        Settings settings = Settings.builder()
+            .put(DiskThresholdDecider.ENABLE_FOR_SINGLE_DATA_NODE.getKey(), false)
+            .build();
+
+        new DiskThresholdDecider(settings, new ClusterSettings(settings, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS));
+
+        assertWarnings("setting [cluster.routing.allocation.disk.watermark.enable_for_single_data_node=false] is deprecated and" +
+            " will not be available in a future version");
+    }
+
     public void testDiskThresholdWithSnapshotShardSizes() {
         final long shardSizeInBytes = randomBoolean() ? 10L : 50L;
         logger.info("--> using shard size [{}]", shardSizeInBytes);

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/DeprecationChecks.java

@@ -86,7 +86,8 @@ private DeprecationChecks() {
                     NodeDeprecationChecks::checkMultipleDataPaths,
                     NodeDeprecationChecks::checkDataPathsList,
                     NodeDeprecationChecks::checkBootstrapSystemCallFilterSetting,
-                    NodeDeprecationChecks::checkSharedDataPathSetting
+                    NodeDeprecationChecks::checkSharedDataPathSetting,
+                    NodeDeprecationChecks::checkSingleDataNodeWatermarkSetting
                 )
             ).collect(Collectors.toList());
         }

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/NodeDeprecationChecks.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.cluster.node.DiscoveryNodeRole;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.bootstrap.BootstrapSettings;
+import org.elasticsearch.cluster.routing.allocation.decider.DiskThresholdDecider;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Setting.Property;
 import org.elasticsearch.common.settings.Settings;
@@ -420,4 +421,19 @@ static DeprecationIssue checkSharedDataPathSetting(final Settings settings, fina
         }
         return null;
     }
+
+    static DeprecationIssue checkSingleDataNodeWatermarkSetting(final Settings settings, final PluginsAndModules pluginsAndModules) {
+        if (DiskThresholdDecider.ENABLE_FOR_SINGLE_DATA_NODE.get(settings) == false
+            && DiskThresholdDecider.ENABLE_FOR_SINGLE_DATA_NODE.exists(settings)) {
+            String key = DiskThresholdDecider.ENABLE_FOR_SINGLE_DATA_NODE.getKey();
+            return new DeprecationIssue(DeprecationIssue.Level.CRITICAL,
+                String.format(Locale.ROOT, "setting [%s=false] is deprecated and will not be available in a future version", key),
+                "https://www.elastic.co/guide/en/elasticsearch/reference/7.14/" +
+                    "breaking-changes-7.14.html#deprecate-single-data-node-watermark",
+                String.format(Locale.ROOT, "found [%s] configured to false. Discontinue use of this setting or set it to true.", key)
+            );
+        }
+
+        return null;
+    }
 }

x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/NodeDeprecationChecksTests.java

@@ -40,6 +40,9 @@
 import static org.hamcrest.Matchers.nullValue;
 import static org.hamcrest.Matchers.startsWith;
 
+
+import org.elasticsearch.cluster.routing.allocation.decider.DiskThresholdDecider;
+
 public class NodeDeprecationChecksTests extends ESTestCase {
 
     public void testCheckDefaults() {
@@ -590,4 +593,28 @@ public void testSharedDataPathSetting() {
                 "Found shared data path configured. Discontinue use of this setting."
             )));
     }
+
+    public void testSingleDataNodeWatermarkSetting() {
+        Settings settings = Settings.builder()
+            .put(DiskThresholdDecider.ENABLE_FOR_SINGLE_DATA_NODE.getKey(), false)
+            .build();
+
+        List<DeprecationIssue> issues = DeprecationChecks.filterChecks(DeprecationChecks.NODE_SETTINGS_CHECKS, c -> c.apply(settings,
+            null));
+
+        final String expectedUrl =
+            "https://www.elastic.co/guide/en/elasticsearch/reference/7.14/" +
+                "breaking-changes-7.14.html#deprecate-single-data-node-watermark";
+        assertThat(issues, hasItem(
+            new DeprecationIssue(DeprecationIssue.Level.CRITICAL,
+                "setting [cluster.routing.allocation.disk.watermark.enable_for_single_data_node=false] is deprecated and" +
+                    " will not be available in a future version",
+                expectedUrl,
+                "found [cluster.routing.allocation.disk.watermark.enable_for_single_data_node] configured to false." +
+                    " Discontinue use of this setting or set it to true."
+            )));
+
+        assertWarnings("setting [cluster.routing.allocation.disk.watermark.enable_for_single_data_node=false] is deprecated and" +
+            " will not be available in a future version");
+    }
 }