feat: increase default number of channels when gRPC-GCP channel pool is enabled

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java

@@ -89,6 +89,12 @@ public class SpannerOptions extends ServiceOptions<Spanner, SpannerOptions> {
           "https://www.googleapis.com/auth/spanner.admin",
           "https://www.googleapis.com/auth/spanner.data");
   private static final int MAX_CHANNELS = 256;
+  private static final int DEFAULT_CHANNELS = 4;
+  // Set the default number of channels to GRPC_GCP_ENABLED_DEFAULT_CHANNELS when gRPC-GCP extension
+  // is enabled,
+  // to make sure there are sufficient channels available to move the sessions to a different
+  // channel if a network connection in a particular channel fails.
+  @VisibleForTesting static final int GRPC_GCP_ENABLED_DEFAULT_CHANNELS = 8;
   private final TransportChannelProvider channelProvider;
 
   @SuppressWarnings("rawtypes")
@@ -669,8 +675,7 @@ public static class Builder
 
     private GrpcInterceptorProvider interceptorProvider;
 
-    /** By default, we create 4 channels per {@link SpannerOptions} */
-    private int numChannels = 4;
+    private Integer numChannels;
 
     private String transportChannelExecutorThreadNameFormat = "Cloud-Spanner-TransportChannel-%d";
 
@@ -1122,8 +1127,7 @@ public Builder setHost(String host) {
 
     /** Enables gRPC-GCP extension with the default settings. */
     public Builder enableGrpcGcpExtension() {
-      this.grpcGcpExtensionEnabled = true;
-      return this;
+      return this.enableGrpcGcpExtension(null);
     }
 
     /**
@@ -1133,6 +1137,9 @@ public Builder enableGrpcGcpExtension() {
     public Builder enableGrpcGcpExtension(GcpManagedChannelOptions options) {
       this.grpcGcpExtensionEnabled = true;
       this.grpcGcpOptions = options;
+      if (this.numChannels == null) {
+        this.numChannels = GRPC_GCP_ENABLED_DEFAULT_CHANNELS;
+      }
       return this;
     }
 
@@ -1166,6 +1173,10 @@ public SpannerOptions build() {
         // As we are using plain text, we should never send any credentials.
         this.setCredentials(NoCredentials.getInstance());
       }
+      if (this.numChannels == null) {
+        this.numChannels = DEFAULT_CHANNELS;
+      }
+
       return new SpannerOptions(this);
     }
   }

google-cloud-spanner/src/test/java/com/google/cloud/spanner/SpannerOptionsTest.java

@@ -918,4 +918,38 @@ public void testCustomAsyncExecutorProvider() {
             .build();
     assertSame(service, options.getAsyncExecutorProvider().getExecutor());
   }
+
+  @Test
+  public void testDefaultNumChannelsWithGrpcGcpExtensionEnabled() {
+    SpannerOptions options =
+        SpannerOptions.newBuilder().setProjectId("test-project").enableGrpcGcpExtension().build();
+
+    assertEquals(options.getNumChannels(), SpannerOptions.GRPC_GCP_ENABLED_DEFAULT_CHANNELS);
+  }
+
+  @Test
+  public void testNumChannelsWithGrpcGcpExtensionEnabled() {
+    // Set number of channels explicitly, before enabling gRPC-GCP channel pool in SpannerOptions
+    // builder.
+    int numChannels = 5;
+    SpannerOptions options1 =
+        SpannerOptions.newBuilder()
+            .setProjectId("test-project")
+            .setNumChannels(numChannels)
+            .enableGrpcGcpExtension()
+            .build();
+
+    assertEquals(options1.getNumChannels(), numChannels);
+
+    // Set number of channels explicitly, after enabling gRPC-GCP channel pool in SpannerOptions
+    // builder.
+    SpannerOptions options2 =
+        SpannerOptions.newBuilder()
+            .setProjectId("test-project")
+            .enableGrpcGcpExtension()
+            .setNumChannels(numChannels)
+            .build();
+
+    assertEquals(options2.getNumChannels(), numChannels);
+  }
 }