[GitHub] [lucene-solr] chatman commented on a change in pull request #1758: SOLR-14749: Provide a clean API for cluster-level event processing, Initial draft.

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chatman commented on a change in pull request #1758:
URL: https://github.com/apache/lucene-solr/pull/1758#discussion_r472263584



##########
File path: solr/core/src/java/org/apache/solr/cluster/events/ClusterSingleton.java
##########
@@ -0,0 +1,14 @@
+package org.apache.solr.cluster.events;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * Intended for {@link org.apache.solr.core.CoreContainer} plugins that should be
+ * enabled only one instance per cluster.
+ * <p>Implementation detail: currently these plugins are instantiated on the
+ * Overseer leader, and closed when the current node loses its leadership.</p>
+ */
+@Retention(RetentionPolicy.RUNTIME)
+public @interface ClusterSingleton {

Review comment:
       I'm not sure Solr core code should be responsible for coordinating plugins' coordination like this. I feel we should keep this coordination left to the plugins. I am NOT saying that the autoscaling plugin should do this on its own, but I am just saying that "some plugin" should do it for the autoscaling plugin. IOW, this coordination code shouldn't be in solr-core.
   As an example, we can have two plugins:
   "autoscaling-framework-plugin" depends on "lifecycle-coordination-plugin". The latter can have these concepts defined in them, including the actual implementation.

##########
File path: solr/core/src/java/org/apache/solr/cluster/events/ScheduledEvent.java
##########
@@ -0,0 +1,30 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF 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.apache.solr.cluster.events;
+
+/**
+ *
+ */
+public interface ScheduledEvent extends ClusterEvent {

Review comment:
       Can you please provide an example of a scheduled event? I am unable to see why a scheduled event is needed in Solr core.




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