Import code from internal repository to git

java/com/google/domain/registry/model/common/EntityGroupRoot.java

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+// Copyright 2016 Google Inc. All Rights Reserved.
+//
+// Licensed 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 com.google.domain.registry.model.common;
+
+import com.google.domain.registry.model.BackupGroupRoot;
+
+import com.googlecode.objectify.Key;
+import com.googlecode.objectify.annotation.Entity;
+import com.googlecode.objectify.annotation.Id;
+
+/**
+ * The root key for the entity group which is known as the cross-tld entity group for historical
+ * reasons.
+ *
+ * <p>This exists as a storage place for common configuration options and global settings that
+ * aren't updated too frequently.  Entities in this entity group are usually cached upon load.  The
+ * reason this common entity group exists is because it enables strongly consistent queries and
+ * updates across this seldomly updated data.  This shared entity group also helps cut down on
+ * a potential ballooning in the number of entity groups enlisted in transactions.
+ *
+ * <p>Historically, each TLD used to have a separate namespace, and all entities for a TLD were in
+ * a single EntityGroupRoot for that TLD.  Hence why there was a "cross-tld" entity group -- it was
+ * the entity group for the single namespace where global data applicable for all TLDs lived.
+ */
+@Entity
+public class EntityGroupRoot extends BackupGroupRoot {
+
+  @Id
+  private String id;
+
+  /** The root key for cross-tld resources such as registrars. */
+  public static final Key<EntityGroupRoot> getCrossTldKey() {
+    return Key.create(EntityGroupRoot.class, "cross-tld");
+  }
+}