[GitHub] [incubator-druid] surekhasaharan commented on a change in pull request #6901: Introduce published segment cache in broker

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

surekhasaharan commented on a change in pull request #6901: Introduce published segment cache in broker
URL: https://github.com/apache/incubator-druid/pull/6901#discussion_r271479675
 
 

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 File path: sql/src/main/java/org/apache/druid/sql/calcite/schema/MetadataSegmentView.java
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+/*
+ * 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.druid.sql.calcite.schema;
+
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.JavaType;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.base.Preconditions;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.google.inject.Inject;
+import org.apache.druid.client.BrokerSegmentWatcherConfig;
+import org.apache.druid.client.DataSegmentInterner;
+import org.apache.druid.client.JsonParserIterator;
+import org.apache.druid.client.coordinator.Coordinator;
+import org.apache.druid.concurrent.LifecycleLock;
+import org.apache.druid.discovery.DruidLeaderClient;
+import org.apache.druid.guice.ManageLifecycle;
+import org.apache.druid.java.util.common.DateTimes;
+import org.apache.druid.java.util.common.ISE;
+import org.apache.druid.java.util.common.StringUtils;
+import org.apache.druid.java.util.common.concurrent.Execs;
+import org.apache.druid.java.util.common.lifecycle.LifecycleStart;
+import org.apache.druid.java.util.common.lifecycle.LifecycleStop;
+import org.apache.druid.java.util.emitter.EmittingLogger;
+import org.apache.druid.java.util.http.client.Request;
+import org.apache.druid.server.coordinator.BytesAccumulatingResponseHandler;
+import org.apache.druid.sql.calcite.planner.PlannerConfig;
+import org.apache.druid.timeline.DataSegment;
+import org.jboss.netty.handler.codec.http.HttpMethod;
+import org.joda.time.DateTime;
+
+import javax.annotation.Nullable;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Iterator;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+/**
+ * This class polls the coordinator in background to keep the latest published segments.
+ * Provides {@link #getPublishedSegments()} for others to get segments in metadata store.
+ */
+@ManageLifecycle
+public class MetadataSegmentView
+{
+
+  private static final EmittingLogger log = new EmittingLogger(MetadataSegmentView.class);
+
+  private final DruidLeaderClient coordinatorDruidLeaderClient;
+  private final ObjectMapper jsonMapper;
+  private final BytesAccumulatingResponseHandler responseHandler;
+  private final BrokerSegmentWatcherConfig segmentWatcherConfig;
+
+  private final boolean isCacheEnabled;
+  @Nullable
+  private final ConcurrentMap<DataSegment, DateTime> publishedSegments;
+  private final ScheduledExecutorService scheduledExec;
+  private final long pollPeriodInMS;
+  private final LifecycleLock lifecycleLock = new LifecycleLock();
+  private final AtomicBoolean cachePopulated = new AtomicBoolean(false);
+
+  @Inject
+  public MetadataSegmentView(
+      final @Coordinator DruidLeaderClient druidLeaderClient,
+      final ObjectMapper jsonMapper,
+      final BytesAccumulatingResponseHandler responseHandler,
+      final BrokerSegmentWatcherConfig segmentWatcherConfig,
+      final PlannerConfig plannerConfig
+  )
+  {
+    Preconditions.checkNotNull(plannerConfig, "plannerConfig");
+    this.coordinatorDruidLeaderClient = druidLeaderClient;
+    this.jsonMapper = jsonMapper;
+    this.responseHandler = responseHandler;
+    this.segmentWatcherConfig = segmentWatcherConfig;
+    this.isCacheEnabled = plannerConfig.isMetadataSegmentCacheEnable();
+    this.pollPeriodInMS = plannerConfig.getMetadataSegmentPollPeriod();
+    this.publishedSegments = isCacheEnabled ? new ConcurrentHashMap<>(1000) : null;
+    this.scheduledExec = Execs.scheduledSingleThreaded("MetadataSegmentView-Cache--%d");
+  }
+
+  @LifecycleStart
+  public void start()
+  {
+    if (!lifecycleLock.canStart()) {
+      throw new ISE("can't start.");
+    }
+    try {
+      if (isCacheEnabled) {
+        scheduledExec.schedule(new PollTask(), pollPeriodInMS, TimeUnit.MILLISECONDS);
+      }
+      lifecycleLock.started();
+      log.info("MetadataSegmentView Started.");
+    }
+    finally {
+      lifecycleLock.exitStart();
+    }
+  }
+
+  @LifecycleStop
+  public void stop()
+  {
+    if (!lifecycleLock.canStop()) {
+      throw new ISE("can't stop.");
+    }
+    log.info("MetadataSegmentView is stopping.");
+    if (isCacheEnabled) {
+      scheduledExec.shutdown();
+    }
+    log.info("MetadataSegmentView Stopped.");
+  }
+
+  private void poll()
+  {
+    log.info("polling published segments from coordinator");
+    final JsonParserIterator<DataSegment> metadataSegments = getMetadataSegments(
+        coordinatorDruidLeaderClient,
+        jsonMapper,
+        responseHandler,
+        segmentWatcherConfig.getWatchedDataSources()
+    );
+
+    final DateTime timestamp = DateTimes.nowUtc();
+    while (metadataSegments.hasNext()) {
+      final DataSegment interned = DataSegmentInterner.intern(metadataSegments.next());
+      // timestamp is used to filter deleted segments
+      publishedSegments.put(interned, timestamp);
+    }
+    // filter the segments from cache whose timestamp is not equal to latest timestamp stored,
+    // since the presence of a segment with an earlier timestamp indicates that
+    // "that" segment is not returned by coordinator in latest poll, so it's
+    // likely deleted and therefore we remove it from publishedSegments
+    // Since segments are not atomically replaced because it can cause high
+    // memory footprint due to large number of published segments, so
+    // we are incrementally removing deleted segments from the map
+    // This means publishedSegments will be eventually consistent with
+    // the segments in coordinator
+    publishedSegments.entrySet().removeIf(e -> e.getValue() != timestamp);
+    cachePopulated.set(true);
+  }
+
+  public Iterator<DataSegment> getPublishedSegments()
+  {
+    if (isCacheEnabled) {
+      Preconditions.checkState(
+          lifecycleLock.awaitStarted(1, TimeUnit.MILLISECONDS) && cachePopulated.get(),
 
 Review comment:
   yeah, client would get `IllegalStateException` with the error message `hold on, still syncing published segments`, and they'll have to retry.

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