[GitHub] [pinot] jasperjiaguo commented on a diff in pull request #9311: Adaptive Server Selection

[GitBox <gi...@apache.org>]

jasperjiaguo commented on code in PR #9311:
URL: https://github.com/apache/pinot/pull/9311#discussion_r975798700


##########
pinot-broker/src/main/java/org/apache/pinot/broker/routing/adaptiveserverselector/AdaptiveServerSelector.java:
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@@ -0,0 +1,55 @@
+/**
+ * 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.adaptiveserverselector;
+
+import java.util.List;
+import org.apache.commons.lang3.tuple.Pair;
+
+
+/**
+ * The {@code AdaptiveServerSelector} intelligently selects the best available server for a segment during query
+ * processing. The decision is made based on stats recorded for each server during query processing.
+ */
+public interface AdaptiveServerSelector {
+
+  /**
+   * Picks the best server to route a query from the list of candidate servers.
+   *
+   * @param serverCandidates Candidate servers from which the best server should be chosen.
+   * @return server identifier
+   */
+  String select(List<String> serverCandidates);
+
+  /**
+   * Returns the ranking of servers ordered from best to worst.
+   *
+   * @return List of servers ranked from best to worst.
+   */
+  List<String> fetchServerRanking();

Review Comment:
   Should we rename it to `fetchFullServerRanking` for more clarity?



##########
pinot-broker/src/main/java/org/apache/pinot/broker/routing/adaptiveserverselector/HybridSelector.java:
##########
@@ -0,0 +1,99 @@
+/**
+ * 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.adaptiveserverselector;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Random;
+import org.apache.commons.lang3.tuple.Pair;
+import org.apache.pinot.core.transport.server.routing.stats.ServerRoutingStatsManager;
+
+
+/**
+ * The {@code HybridSelector} is an AdaptiveServerSelector implementation that picks the best server based on the
+ * following parameters:
+ * 1. Num of in-flight requests (A)
+ * 2. EMA of in-flight requests (B)
+ * 3. EMA of latencies (C)
+ * This selector implementation is based on the ideas suggested in the paper -
+ * https://www.usenix.org/system/files/conference/nsdi15/nsdi15-paper-suresh.pdf
+ *
+ * The C3 server score for each server is calculated as follows. The server with the lowest C3 score is picked.
+ *       C3serverScore = Math.pow(A+B, N) * C
+ * N -> Configurable exponent with default value of 3.
+ */
+public class HybridSelector implements AdaptiveServerSelector {
+  private final ServerRoutingStatsManager _serverRoutingStatsManager;
+  private final Random _random;
+
+  public HybridSelector(ServerRoutingStatsManager serverRoutingStatsManager) {
+    _serverRoutingStatsManager = serverRoutingStatsManager;
+    _random = new Random();
+  }
+
+  @Override
+  public String select(List<String> serverCandidates) {
+    String selectedServer = null;
+    Double minValue = Double.MAX_VALUE;
+
+    // TODO: If two or more servers have the same score, break the tie intelligently.
+    for (String server : serverCandidates) {
+      Double score = _serverRoutingStatsManager.fetchC3ScoreForServer(server);
+
+      // No stats for this server. That means this server hasn't received any queries yet.
+      if (score == null) {
+        int randIdx = _random.nextInt(serverCandidates.size());

Review Comment:
   (nit) How about if we simply pick the server that has `score == null`? This gives more determinism and is less likely to create thundering herd problem, because the next call will also pick a null (if any). The current solution is fine for not too large `serverCandidates.size()`. The expectation of initializing all servers is `size*\sum_{j=1...size} 1/j` (see [this](https://math.stackexchange.com/questions/379525/probability-distribution-in-the-coupon-collectors-problem)). My only concern is that if the serverCandidates.size() could get large for balanced assignment, and we become unfortunate with random numbers.



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