[GitHub] [incubator-doris] vagetablechicken commented on a change in pull request #4771: [LoadBalance] make BeLoadRebalancer extends from base class Rebalancer

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vagetablechicken commented on a change in pull request #4771:
URL: https://github.com/apache/incubator-doris/pull/4771#discussion_r510254020



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File path: fe/fe-core/src/main/java/org/apache/doris/clone/Rebalancer.java
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@@ -0,0 +1,80 @@
+// 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.doris.clone;
+
+import org.apache.doris.catalog.TabletInvertedIndex;
+import org.apache.doris.clone.TabletScheduler.PathSlot;
+import org.apache.doris.system.SystemInfoService;
+import org.apache.doris.task.AgentBatchTask;
+import org.apache.doris.thrift.TStorageMedium;
+
+import com.google.common.collect.Lists;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+
+/*
+ * Rebalancer is responsible for
+ * 1. selectAlternativeTablets: selecting alternative tablets by one rebalance strategy,
+ * and return them to tablet scheduler(maybe contains the concrete moves, or maybe not).
+ * 2. createBalanceTask: given a tablet, try to create a clone task for this tablet.
+ * 3. getCachedSrcBackendId: if the rebalance strategy wants to delete the replica of a specified be,
+ * override this func.
+ * NOTICE:
+ * It may have a long interval between selectAlternativeTablets() & createBalanceTask(). So the concrete moves may be
+ * invalid when we createBalanceTask(), you should check the moves' validation.
+ */
+public abstract class Rebalancer {
+    // When Rebalancer init, the statisticMap is usually empty. So it's no need to be an arg.
+    // Only use updateLoadStatistic() to load stats.
+    protected Map<String, ClusterLoadStatistic> statisticMap = new HashMap<>();
+    protected TabletInvertedIndex invertedIndex;
+    protected SystemInfoService infoService;
+
+    public Rebalancer(SystemInfoService infoService, TabletInvertedIndex invertedIndex) {
+        this.infoService = infoService;
+        this.invertedIndex = invertedIndex;
+    }
+
+    public List<TabletSchedCtx> selectAlternativeTablets() {
+        List<TabletSchedCtx> alternativeTablets = Lists.newArrayList();
+        for (Map.Entry<String, ClusterLoadStatistic> entry : statisticMap.entrySet()) {
+            for (TStorageMedium medium : TStorageMedium.values()) {
+                alternativeTablets.addAll(selectAlternativeTabletsForCluster(entry.getKey(),
+                        entry.getValue(), medium));
+            }
+        }
+        return alternativeTablets;
+    }
+
+    protected abstract List<TabletSchedCtx> selectAlternativeTabletsForCluster(
+            String clusterName, ClusterLoadStatistic clusterStat, TStorageMedium medium);
+
+    public abstract void createBalanceTask(TabletSchedCtx tabletCtx, Map<Long, PathSlot> backendsWorkingSlots,
+                                           AgentBatchTask batchTask) throws SchedException;
+
+    public Long getCachedSrcBackendId(Long tabletId) {

Review comment:
       I prefer that cache should be managed by `Rebalancer`, no need to make `getCachedSrcBackendId` to delete. You can do whatever you want, e.g. only clear cache when stats update.
   
   About cancellations(scheduler canceled, task running cancelled, etc.):
   Before these cancellations, adding tablet to the scheduler may get failed. And after the clone task completed, the rest of a rebalance move may get failed too. So there're so many places that need to delete cache. I think we shouldn't 'delete the cache asap'. Maybe clearing cache when stats update is better.




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