tkalkirill commented on code in PR #6199:
URL: https://github.com/apache/ignite-3/pull/6199#discussion_r2192028022


##########
modules/metastorage/src/main/java/org/apache/ignite/internal/metastorage/server/NotificationEnqueuedListener.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.ignite.internal.metastorage.server;
+
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import org.apache.ignite.internal.hlc.HybridTimestamp;
+import org.apache.ignite.internal.metastorage.Entry;
+
+/**
+ * Listener that gets invoked when {@link WatchProcessor}'s internal 
notification chain future is updated.
+ */
+@FunctionalInterface
+public interface NotificationEnqueuedListener {
+    /**
+     * Notifies this listener that {@link WatchProcessor}'s internal 
notification chain future is updated.
+     *
+     * <p>This must always be run under the same lock under which the 
notification future is updated.
+     *
+     * @param newNotificationFuture New notification future.
+     * @param entries Entries corresponding to the update (empty if the 
notification is not about a new revision,
+     *     but about Metastorage safe time advancement.
+     * @param timestamp Metastorage timestamp.
+     */
+    void onEnqueued(CompletableFuture<Void> newNotificationFuture, List<Entry> 
entries, HybridTimestamp timestamp);

Review Comment:
   > Why is it not safe? Are you talking about someone erroneously adding a 
callback that would behave badly in some way? 
   
   Yes.
   
   > We need to call it after we chain the current notification as for catalog 
updates, the action we add ('advance schema safe time') must be executed after 
the notification futures are completed. It might be that I didn't get your 
idea; in this case, please elaborate (maybe provide a sketch of how it could 
look)
   
   Okay, now I understand more.



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