Allocate newly created indices on data_hot tier nodes (#61342)

This commit adds the functionality to allocate newly created indices on nodes in the "hot" tier by
default when they are created.

This does not break existing behavior, as nodes with the `data` role are considered to be part of
the hot tier. Users that separate their deployments by using the `data_hot` (and `data_warm`,
`data_cold`, `data_frozen`) roles will have their data allocated on the hot tier nodes now by
default.

This change is a little more complicated than changing the default value for
`index.routing.allocation.include._tier` from null to "data_hot". Instead, this adds the ability to
have a plugin inject a setting into the builder for a newly created index. This has the benefit of
allowing this setting to be visible as part of the settings when retrieving the index, for example:

```
// Create an index
PUT /eggplant

// Get an index
GET /eggplant?flat_settings
```

Returns the default settings now of:

```json
{
  "eggplant" : {
    "aliases" : { },
    "mappings" : { },
    "settings" : {
      "index.creation_date" : "1597855465598",
      "index.number_of_replicas" : "1",
      "index.number_of_shards" : "1",
      "index.provided_name" : "eggplant",
      "index.routing.allocation.include._tier" : "data_hot",
      "index.uuid" : "6ySG78s9RWGystRipoBFCA",
      "index.version.created" : "8000099"
    }
  }
}
```

After the initial setting of this setting, it can be treated like any other index level setting.

This new setting is *not* set on a new index if any of the following is true:

- The index is created with an `index.routing.allocation.include.<anything>` setting
- The index is created with an `index.routing.allocation.exclude.<anything>` setting
- The index is created with an `index.routing.allocation.require.<anything>` setting
- The index is created with a null `index.routing.allocation.include._tier` value
- The index was created from an existing source metadata (shrink, clone, split, etc)

Relates to #60848

docs/reference/api-conventions.asciidoc

@@ -391,6 +391,7 @@ Returns:
       "index.creation_date": "1474389951325",
       "index.uuid": "n6gzFZTgS664GUfx0Xrpjw",
       "index.version.created": ...,
+      "index.routing.allocation.include._tier" : "data_hot",
       "index.provided_name" : "my-index-000001"
     }
   }
@@ -424,6 +425,13 @@ Returns:
         "version": {
           "created": ...
         },
+        "routing": {
+          "allocation": {
+            "include": {
+              "_tier": "data_hot"
+            }
+          }
+        },
         "provided_name" : "my-index-000001"
       }
     }

docs/reference/cluster/allocation-explain.asciidoc

@@ -113,7 +113,12 @@ DELETE my-index-000001
 [source,console]
 --------------------------------------------------
 PUT /my-index-000001?master_timeout=1s&timeout=1s
-{"settings": {"index.routing.allocation.include._name": "non_existent_node"} }
+{
+  "settings": {
+    "index.routing.allocation.include._name": "non_existent_node",
+    "index.routing.allocation.include._tier": null
+  }
+}
 
 GET /_cluster/allocation/explain
 {

qa/rolling-upgrade/src/test/java/org/elasticsearch/upgrades/RecoveryIT.java

@@ -224,6 +224,7 @@ public void testRelocationWithConcurrentIndexing() throws Exception {
                     // but the recovering copy will be seen as invalid and the cluster health won't return to GREEN
                     // before timing out
                     .put(INDEX_DELAYED_NODE_LEFT_TIMEOUT_SETTING.getKey(), "100ms")
+                    .put("index.routing.allocation.include._tier", "")
                     .put(SETTING_ALLOCATION_MAX_RETRY.getKey(), "0"); // fail faster
                 createIndex(index, settings.build());
                 indexDocs(index, 0, 10);
@@ -240,6 +241,7 @@ public void testRelocationWithConcurrentIndexing() throws Exception {
                     .put(IndexMetadata.INDEX_NUMBER_OF_REPLICAS_SETTING.getKey(), 0)
                     .put(INDEX_ROUTING_ALLOCATION_ENABLE_SETTING.getKey(), (String)null)
                     .put("index.routing.allocation.include._id", oldNode)
+                    .putNull("index.routing.allocation.include._tier")
                 );
                 ensureGreen(index); // wait for the primary to be assigned
                 ensureNoInitializingShards(); // wait for all other shard activity to finish
@@ -262,6 +264,7 @@ public void testRelocationWithConcurrentIndexing() throws Exception {
                 updateIndexSettings(index, Settings.builder()
                     .put(IndexMetadata.INDEX_NUMBER_OF_REPLICAS_SETTING.getKey(), 2)
                     .put("index.routing.allocation.include._id", (String)null)
+                    .putNull("index.routing.allocation.include._tier")
                 );
                 asyncIndexDocs(index, 60, 45).get();
                 ensureGreen(index);

server/src/main/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexService.java

@@ -68,6 +68,7 @@
 import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.mapper.MapperService.MergeReason;
 import org.elasticsearch.index.query.QueryShardContext;
+import org.elasticsearch.index.shard.IndexSettingProvider;
 import org.elasticsearch.indices.IndexCreationException;
 import org.elasticsearch.indices.IndicesService;
 import org.elasticsearch.indices.InvalidIndexNameException;
@@ -83,6 +84,7 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
@@ -127,6 +129,7 @@ public class MetadataCreateIndexService {
     private final SystemIndices systemIndices;
     private final ShardLimitValidator shardLimitValidator;
     private final boolean forbidPrivateIndexSettings;
+    private final Set<IndexSettingProvider> indexSettingProviders = new HashSet<>();
 
     public MetadataCreateIndexService(
         final Settings settings,
@@ -155,6 +158,19 @@ public MetadataCreateIndexService(
         this.shardLimitValidator = shardLimitValidator;
     }
 
+    /**
+     * Add a provider to be invoked to get additional index settings prior to an index being created
+     */
+    public void addAdditionalIndexSettingProvider(IndexSettingProvider provider) {
+        if (provider == null) {
+            throw new IllegalArgumentException("provider must not be null");
+        }
+        if (indexSettingProviders.contains(provider)) {
+            throw new IllegalArgumentException("provider already added");
+        }
+        this.indexSettingProviders.add(provider);
+    }
+
     /**
      * Validate the name for an index against some static rules and a cluster state.
      */
@@ -465,7 +481,7 @@ private ClusterState applyCreateIndexRequestWithV1Templates(final ClusterState c
 
         final Settings aggregatedIndexSettings =
             aggregateIndexSettings(currentState, request, MetadataIndexTemplateService.resolveSettings(templates),
-                null, settings, indexScopedSettings, shardLimitValidator);
+                null, settings, indexScopedSettings, shardLimitValidator, indexSettingProviders);
         int routingNumShards = getIndexNumberOfRoutingShards(aggregatedIndexSettings, null);
         IndexMetadata tmpImd = buildAndValidateTemporaryIndexMetadata(currentState, aggregatedIndexSettings, request, routingNumShards);
 
@@ -491,7 +507,7 @@ private ClusterState applyCreateIndexRequestWithV2Template(final ClusterState cu
         final Settings aggregatedIndexSettings =
             aggregateIndexSettings(currentState, request,
                 MetadataIndexTemplateService.resolveSettings(currentState.metadata(), templateName),
-                null, settings, indexScopedSettings, shardLimitValidator);
+                null, settings, indexScopedSettings, shardLimitValidator, indexSettingProviders);
         int routingNumShards = getIndexNumberOfRoutingShards(aggregatedIndexSettings, null);
         IndexMetadata tmpImd = buildAndValidateTemporaryIndexMetadata(currentState, aggregatedIndexSettings, request, routingNumShards);
 
@@ -537,7 +553,7 @@ private ClusterState applyCreateIndexRequestWithExistingMetadata(final ClusterSt
         }
 
         final Settings aggregatedIndexSettings = aggregateIndexSettings(currentState, request, Settings.EMPTY,
-            sourceMetadata, settings, indexScopedSettings, shardLimitValidator);
+            sourceMetadata, settings, indexScopedSettings, shardLimitValidator, indexSettingProviders);
         final int routingNumShards = getIndexNumberOfRoutingShards(aggregatedIndexSettings, sourceMetadata);
         IndexMetadata tmpImd = buildAndValidateTemporaryIndexMetadata(currentState, aggregatedIndexSettings, request, routingNumShards);
 
@@ -596,14 +612,64 @@ static Map<String, Object> parseV1Mappings(String mappingsJson, List<CompressedX
      * @return the aggregated settings for the new index
      */
     static Settings aggregateIndexSettings(ClusterState currentState, CreateIndexClusterStateUpdateRequest request,
-                                           Settings templateSettings, @Nullable IndexMetadata sourceMetadata, Settings settings,
-                                           IndexScopedSettings indexScopedSettings, ShardLimitValidator shardLimitValidator) {
-        Settings.Builder indexSettingsBuilder = Settings.builder();
+                                           Settings combinedTemplateSettings, @Nullable IndexMetadata sourceMetadata, Settings settings,
+                                           IndexScopedSettings indexScopedSettings, ShardLimitValidator shardLimitValidator,
+                                           Set<IndexSettingProvider> indexSettingProviders) {
+        // Create builders for the template and request settings. We transform these into builders
+        // because we may want settings to be "removed" from these prior to being set on the new
+        // index (see more comments below)
+        final Settings.Builder templateSettings = Settings.builder().put(combinedTemplateSettings);
+        final Settings.Builder requestSettings = Settings.builder().put(request.settings());
+
+        final Settings.Builder indexSettingsBuilder = Settings.builder();
         if (sourceMetadata == null) {
-            indexSettingsBuilder.put(templateSettings);
+            final Settings.Builder additionalIndexSettings = Settings.builder();
+            final Settings templateAndRequestSettings = Settings.builder()
+                .put(combinedTemplateSettings)
+                .put(request.settings())
+                .build();
+
+            // Loop through all the explicit index setting providers, adding them to the
+            // additionalIndexSettings map
+            for (IndexSettingProvider provider : indexSettingProviders) {
+                additionalIndexSettings.put(provider.getAdditionalIndexSettings(request.index(), templateAndRequestSettings));
+            }
+
+            // For all the explicit settings, we go through the template and request level settings
+            // and see if either a template or the request has "cancelled out" an explicit default
+            // setting. For example, if a plugin had as an explicit setting:
+            // "index.mysetting": "blah
+            // And either a template or create index request had:
+            // "index.mysetting": null
+            // We want to remove the explicit setting not only from the explicitly set settings, but
+            // also from the template and request settings, so that from the newly create index's
+            // perspective it is as though the setting has not been set at all (using the default
+            // value).
+            for (String explicitSetting : additionalIndexSettings.keys()) {
+                if (templateSettings.keys().contains(explicitSetting) && templateSettings.get(explicitSetting) == null) {
+                    logger.debug("removing default [{}] setting as it in set to null in a template for [{}] creation",
+                        explicitSetting, request.index());
+                    additionalIndexSettings.remove(explicitSetting);
+                    templateSettings.remove(explicitSetting);
+                }
+                if (requestSettings.keys().contains(explicitSetting) && requestSettings.get(explicitSetting) == null) {
+                    logger.debug("removing default [{}] setting as it in set to null in the request for [{}] creation",
+                        explicitSetting, request.index());
+                    additionalIndexSettings.remove(explicitSetting);
+                    requestSettings.remove(explicitSetting);
+                }
+            }
+
+            // Finally, we actually add the explicit defaults prior to the template settings and the
+            // request settings, so that the precedence goes:
+            // Explicit Defaults -> Template -> Request -> Necessary Settings (# of shards, uuid, etc)
+            indexSettingsBuilder.put(additionalIndexSettings.build());
+            indexSettingsBuilder.put(templateSettings.build());
         }
+
         // now, put the request settings, so they override templates
-        indexSettingsBuilder.put(request.settings());
+        indexSettingsBuilder.put(requestSettings.build());
+
         if (indexSettingsBuilder.get(IndexMetadata.SETTING_INDEX_VERSION_CREATED.getKey()) == null) {
             final DiscoveryNodes nodes = currentState.nodes();
             final Version createdVersion = Version.min(Version.CURRENT, nodes.getSmallestNonClientNodeVersion());

server/src/main/java/org/elasticsearch/cluster/node/DiscoveryNodeFilters.java

@@ -181,6 +181,9 @@ public boolean match(DiscoveryNode node) {
                         }
                     }
                 }
+            } else if ("_tier".equals(attr)) {
+                // Always allow _tier as an attribute, will be handled elsewhere
+                return true;
             } else {
                 String nodeAttributeValue = node.getAttributes().get(attr);
                 if (nodeAttributeValue == null) {

server/src/main/java/org/elasticsearch/index/shard/IndexSettingProvider.java

@@ -0,0 +1,36 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.index.shard;
+
+import org.elasticsearch.common.settings.Settings;
+
+/**
+ * An {@link IndexSettingProvider} is a provider for index level settings that can be set
+ * explicitly as a default value (so they show up as "set" for newly created indices)
+ */
+public interface IndexSettingProvider {
+    /**
+     * Returns explicitly set default index {@link Settings} for the given index. This should not
+     * return null.
+     */
+    default Settings getAdditionalIndexSettings(String indexName, Settings templateAndRequestSettings) {
+        return Settings.EMPTY;
+    }
+}

server/src/main/java/org/elasticsearch/node/Node.java

@@ -503,6 +503,9 @@ protected Node(final Environment initialEnvironment,
                     systemIndices,
                     forbidPrivateIndexSettings
             );
+            pluginsService.filterPlugins(Plugin.class)
+                .forEach(p -> p.getAdditionalIndexSettingProviders()
+                    .forEach(metadataCreateIndexService::addAdditionalIndexSettingProvider));
 
             final MetadataCreateDataStreamService metadataCreateDataStreamService =
                 new MetadataCreateDataStreamService(threadPool, clusterService, metadataCreateIndexService);

server/src/main/java/org/elasticsearch/plugins/Plugin.java

@@ -36,6 +36,7 @@
 import org.elasticsearch.env.Environment;
 import org.elasticsearch.env.NodeEnvironment;
 import org.elasticsearch.index.IndexModule;
+import org.elasticsearch.index.shard.IndexSettingProvider;
 import org.elasticsearch.repositories.RepositoriesService;
 import org.elasticsearch.script.ScriptService;
 import org.elasticsearch.threadpool.ExecutorBuilder;
@@ -197,4 +198,14 @@ public Set<DiscoveryNodeRole> getRoles() {
     public void close() throws IOException {
 
     }
+
+    /**
+     * An {@link IndexSettingProvider} allows hooking in to parts of an index
+     * lifecycle to provide explicit default settings for newly created indices. Rather than changing
+     * the default values for an index-level setting, these act as though the setting has been set
+     * explicitly, but still allow the setting to be overridden by a template or creation request body.
+     */
+    public Collection<IndexSettingProvider> getAdditionalIndexSettingProviders() {
+        return Collections.emptyList();
+    }
 }