Re: [PR] Extend strictReplicaGroup policy to pick a replica group even if there are unavaliable segments for more availablity [pinot]

["klsince (via GitHub)" <gi...@apache.org>]

klsince commented on code in PR #11847:
URL: https://github.com/apache/pinot/pull/11847#discussion_r1367558654


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pinot-broker/src/main/java/org/apache/pinot/broker/routing/instanceselector/FallbackReplicaGroupInstanceSelector.java:
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@@ -0,0 +1,428 @@
+/**
+ * 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.pinot.broker.routing.instanceselector;
+
+import java.time.Clock;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.TreeSet;
+import java.util.stream.Collectors;
+import javax.annotation.Nullable;
+import org.apache.commons.lang3.tuple.Pair;
+import org.apache.helix.model.ExternalView;
+import org.apache.helix.model.IdealState;
+import org.apache.helix.store.zk.ZkHelixPropertyStore;
+import org.apache.helix.zookeeper.datamodel.ZNRecord;
+import org.apache.pinot.broker.routing.adaptiveserverselector.AdaptiveServerSelector;
+import org.apache.pinot.common.metrics.BrokerMetrics;
+import org.apache.pinot.common.request.BrokerRequest;
+import org.apache.pinot.common.utils.HashUtil;
+import org.apache.pinot.common.utils.config.QueryOptionsUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+
+/**
+ * This extends the default strictReplicaGroup policy to allow fallback. When using default strictReplicaGroup policy,
+ * if none of replica groups have all segments available in it, all segments are marked as unavailable, which is very
+ * strict and makes queries less available. In this new policy, we allow to fall back to a replica group in that
+ * case, even though the selected replica group might have some segments unavailable. As strictReplicaGroup, all the
+ * remaining available segments are processed in the same replica group.
+ */
+public class FallbackReplicaGroupInstanceSelector extends BaseInstanceSelector {
+  private static final Logger LOGGER = LoggerFactory.getLogger(FallbackReplicaGroupInstanceSelector.class);
+  private final Map<Set<String>, InstanceGroup> _instanceGroups = new HashMap<>();

Review Comment:
   InstanceGroup is a key concept here. It is a set of the replica instances hosting a group of segments, e.g. segment[0-10] are kept on instance[0-2] (i.e. 3 replicas for those segments), then the InstanceGroup is (instance0/1/2) with each instance hosting all segment[0-10].
   
   The major change is to keep available/unavailable segments for every instance in a InstanceGroup, to allow us to dynamically select an instance from the InstanceGroup for its available segments, and report the unavailable segments from the selected instance.



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