sigram commented on a change in pull request #1684:
URL: https://github.com/apache/lucene-solr/pull/1684#discussion_r469366732



##########
File path: 
solr/core/src/java/org/apache/solr/cluster/placement/PlacementPlugin.java
##########
@@ -0,0 +1,41 @@
+/*
+ * 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.placement;
+
+/**
+ * Implemented by external plugins to control replica placement and movement 
on the search cluster (as well as other things
+ * such as cluster elasticity?) when cluster changes are required (initiated 
elsewhere, most likely following a Collection
+ * API call).
+ */
+public interface PlacementPlugin {

Review comment:
       I'm strongly against using static variables. 
   
   We can either assume that plugins are basically stateless and configured on 
the fly, or stateful - and in the latter case they could be re-configurable 
(which has its own problems) or not - that is, you have to create a new 
instance of the plugin if configuration has changed. In this case there's no 
problem with synchronization - the old instance of the plugin completes its 
work using the old config.




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