fix: Create a pool of Channels for each target.

google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/internal/ChannelCache.java

@@ -17,17 +17,27 @@
 package com.google.cloud.pubsublite.internal;
 
 import com.google.common.annotations.VisibleForTesting;
+import com.google.common.flogger.GoogleLogger;
 import io.grpc.Channel;
 import io.grpc.ManagedChannel;
 import io.grpc.ManagedChannelBuilder;
+import java.util.Deque;
+import java.util.LinkedList;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
 import java.util.function.Function;
 
 /** A ChannelCache creates and stores default channels for use with api methods. */
 public class ChannelCache {
+  private static final GoogleLogger log = GoogleLogger.forEnclosingClass();
+
   private final Function<String, ManagedChannel> channelFactory;
-  private final ConcurrentHashMap<String, ManagedChannel> channels = new ConcurrentHashMap<>();
+  private final ConcurrentHashMap<String, Deque<ManagedChannel>> channels =
+      new ConcurrentHashMap<>();
+
+  private static final int NUMBER_OF_CHANNELS_PER_TARGET = 10;
+  private static final String NUMBER_OF_CHANNELS_PER_TARGET_VM_OVERRIDE =
+      "google.cloud.pubsublite.channelCacheSize";
 
   public ChannelCache() {
     this(ChannelCache::newChannel);
@@ -40,20 +50,45 @@ public ChannelCache() {
   }
 
   @VisibleForTesting
-  void onShutdown() {
+  synchronized void onShutdown() {
     channels.forEachValue(
         channels.size(),
-        channel -> {
+        channels -> {
           try {
-            channel.shutdownNow().awaitTermination(60, TimeUnit.SECONDS);
+            for (ManagedChannel channel : channels) {
+              channel.shutdownNow().awaitTermination(60, TimeUnit.SECONDS);
+            }
           } catch (InterruptedException e) {
             e.printStackTrace();
           }
         });
   }
 
-  public Channel get(String target) {
-    return channels.computeIfAbsent(target, channelFactory);
+  public synchronized Channel get(String target) {
+    Deque<ManagedChannel> channelQueue = channels.computeIfAbsent(target, this::newChannels);
+    ManagedChannel channel = channelQueue.removeFirst();
+    channelQueue.addLast(channel);
+    return channel;
+  }
+
+  private Deque<ManagedChannel> newChannels(String target) {
+    int numberOfChannels = NUMBER_OF_CHANNELS_PER_TARGET;
+    String numberOfChannelsOverride = System.getProperty(NUMBER_OF_CHANNELS_PER_TARGET_VM_OVERRIDE);
+    if (numberOfChannelsOverride != null && !numberOfChannelsOverride.isEmpty()) {
+      try {
+        numberOfChannels = Integer.parseInt((numberOfChannelsOverride));
+      } catch (NumberFormatException e) {
+        log.atSevere().log(
+            "Unable to parse override for number of channels per target: %s",
+            numberOfChannelsOverride);
+      }
+    }
+
+    Deque<ManagedChannel> channels = new LinkedList<>();
+    for (int i = 0; i < numberOfChannels; i++) {
+      channels.add(channelFactory.apply(target));
+    }
+    return channels;
   }
 
   private static ManagedChannel newChannel(String target) {

google-cloud-pubsublite/src/test/java/com/google/cloud/pubsublite/internal/ChannelCacheTest.java

@@ -17,14 +17,14 @@
 package com.google.cloud.pubsublite.internal;
 
 import static com.google.common.truth.Truth.assertThat;
-import static org.mockito.ArgumentMatchers.any;
-import static org.mockito.Mockito.times;
-import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 import static org.mockito.MockitoAnnotations.initMocks;
 
 import io.grpc.Channel;
 import io.grpc.ManagedChannel;
+import java.util.HashSet;
+import java.util.Set;
 import java.util.function.Function;
 import org.junit.Before;
 import org.junit.Test;
@@ -34,7 +34,6 @@
 
 @RunWith(JUnit4.class)
 public class ChannelCacheTest {
-  @Mock ManagedChannel mockChannel;
   @Mock Function<String, ManagedChannel> channelFactory;
 
   @Before
@@ -44,14 +43,22 @@ public void setUp() {
 
   @Test
   public void reusesChannels() {
-    when(channelFactory.apply(any())).thenReturn(mockChannel);
+    when(channelFactory.apply("abc"))
+        .thenAnswer(
+            (target) -> {
+              ManagedChannel channel = mock(ManagedChannel.class);
+              when(channel.shutdownNow()).thenReturn(channel);
+              return channel;
+            });
     ChannelCache cache = new ChannelCache(channelFactory);
-    Channel chan1 = cache.get("abc");
-    Channel chan2 = cache.get("abc");
-    assertThat(chan1).isEqualTo(chan2);
-    verify(channelFactory, times(1)).apply("abc");
-    when(mockChannel.shutdownNow()).thenReturn(mockChannel);
+
+    // Only 10 Channels should be created.
+    Set<Channel> channels = new HashSet<>();
+    for (int i = 0; i < 20; i++) {
+      channels.add(cache.get("abc"));
+    }
+
+    assertThat(channels.size()).isEqualTo(10);
     cache.onShutdown();
-    verify(mockChannel, times(1)).shutdownNow();
   }
 }