Elasticsearch: Base the default number of shards on ES version

Before Elasticsearch 7.0, the default number of shards was 5. Since
version 6, there is a warning about that default number changing to 1
starting from 7.0.0. The related Issue 9768 first reported about it.

Set the default number of shards to 1 starting with 7.0, to remove that
index creation warning with V7, thus fit with what Elasticsearch 7.0+
expects. Stick to the previous default for versions earlier than 7, to
not potentially jeopardize existing deployments based on ES 6 or 5.

Making this value non-default requires a corresponding configuration
change on the Elasticsearch server side. And changing the number of
shards for the latter is a non-trivial procedure. [1] introduces that
and is already part of the Gerrit documentation.

[1] https://www.elastic.co/guide/en/elasticsearch/reference/current/getting-started-concepts.html#getting-started-shards-and-replicas

Bug: Issue 10499
Change-Id: I9e91c471b48c4c4c893927c153a5661dfccc2bcb

Documentation/config-gerrit.txt

@@ -2930,7 +2930,7 @@ Sets the number of shards to use per index. Refer to the
 link:https://www.elastic.co/guide/en/elasticsearch/reference/current/getting-started-concepts.html#getting-started-shards-and-replicas[
 Elasticsearch documentation] for details.
 +
-Defaults to 5.
+Defaults to 5 for Elasticsearch versions 5 and 6, and to 1 starting with Elasticsearch 7.
 
 [[elasticsearch.numberOfReplicas]]elasticsearch.numberOfReplicas::
 +

gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/AbstractElasticIndex.java

@@ -162,7 +162,7 @@ abstract class AbstractElasticIndex<K, V> implements Index<K, V> {
     }
 
     // Recreate the index.
-    String indexCreationFields = concatJsonString(getSettings(), getMappings());
+    String indexCreationFields = concatJsonString(getSettings(client.adapter()), getMappings());
     response =
         performRequest(
             "PUT", indexName + client.adapter().includeTypeNameParam(), indexCreationFields);
@@ -177,8 +177,8 @@ abstract class AbstractElasticIndex<K, V> implements Index<K, V> {
 
   protected abstract String getMappings();
 
-  private String getSettings() {
-    return gson.toJson(ImmutableMap.of(SETTINGS, ElasticSetting.createSetting(config)));
+  private String getSettings(ElasticQueryAdapter adapter) {
+    return gson.toJson(ImmutableMap.of(SETTINGS, ElasticSetting.createSetting(config, adapter)));
   }
 
   protected abstract String getId(V v);

gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticConfiguration.java

@@ -43,7 +43,7 @@ class ElasticConfiguration {
   static final String KEY_NUMBER_OF_REPLICAS = "numberOfReplicas";
   static final String DEFAULT_PORT = "9200";
   static final String DEFAULT_USERNAME = "elastic";
-  static final int DEFAULT_NUMBER_OF_SHARDS = 5;
+  static final int DEFAULT_NUMBER_OF_SHARDS = 0;
   static final int DEFAULT_NUMBER_OF_REPLICAS = 1;
 
   private final Config cfg;
@@ -101,4 +101,11 @@ class ElasticConfiguration {
   String getIndexName(String name, int schemaVersion) {
     return String.format("%s%s_%04d", prefix, name, schemaVersion);
   }
+
+  int getNumberOfShards(ElasticQueryAdapter adapter) {
+    if (numberOfShards == DEFAULT_NUMBER_OF_SHARDS) {
+      return adapter.getDefaultNumberOfShards();
+    }
+    return numberOfShards;
+  }
 }

gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticQueryAdapter.java

@@ -27,6 +27,7 @@ public class ElasticQueryAdapter {
   private final boolean useV5Type;
   private final boolean useV6Type;
   private final boolean omitType;
+  private final int defaultNumberOfShards;
 
   private final String searchFilteringName;
   private final String indicesExistParams;
@@ -40,6 +41,7 @@ public class ElasticQueryAdapter {
     this.useV5Type = !version.isV6OrLater();
     this.useV6Type = version.isV6();
     this.omitType = version.isV7OrLater();
+    this.defaultNumberOfShards = version.isV7OrLater() ? 1 : 5;
     this.versionDiscoveryUrl = version.isV6OrLater() ? "/%s*" : "/%s*/_aliases";
     this.searchFilteringName = "_source";
     this.indicesExistParams =
@@ -92,6 +94,10 @@ public class ElasticQueryAdapter {
     return omitType;
   }
 
+  int getDefaultNumberOfShards() {
+    return defaultNumberOfShards;
+  }
+
   String getType() {
     return getType("");
   }

gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticSetting.java

@@ -22,18 +22,18 @@ class ElasticSetting {
   private static final ImmutableMap<String, String> CUSTOM_CHAR_MAPPING =
       ImmutableMap.of("\\u002E", "\\u0020", "\\u005F", "\\u0020");
 
-  static SettingProperties createSetting(ElasticConfiguration config) {
-    return new ElasticSetting.Builder().addCharFilter().addAnalyzer().build(config);
+  static SettingProperties createSetting(ElasticConfiguration config, ElasticQueryAdapter adapter) {
+    return new ElasticSetting.Builder().addCharFilter().addAnalyzer().build(config, adapter);
   }
 
   static class Builder {
     private final ImmutableMap.Builder<String, FieldProperties> fields =
         new ImmutableMap.Builder<>();
 
-    SettingProperties build(ElasticConfiguration config) {
+    SettingProperties build(ElasticConfiguration config, ElasticQueryAdapter adapter) {
       SettingProperties properties = new SettingProperties();
       properties.analysis = fields.build();
-      properties.numberOfShards = config.numberOfShards;
+      properties.numberOfShards = config.getNumberOfShards(adapter);
       properties.numberOfReplicas = config.numberOfReplicas;
       return properties;
     }