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

@@ -696,6 +696,7 @@ public static class Builder
     private CloseableExecutorProvider asyncExecutorProvider;
     private String compressorName;
     private String emulatorHost = System.getenv("SPANNER_EMULATOR_HOST");
+    private boolean userSpecifiedNumChannels = false;
 
     private Builder() {
       // Manually set retry and polling settings that work.
@@ -815,6 +816,7 @@ public Builder setInterceptorProvider(GrpcInterceptorProvider interceptorProvide
      */
     public Builder setNumChannels(int numChannels) {
       this.numChannels = numChannels;
+      this.userSpecifiedNumChannels = true;
       return this;
     }
 
@@ -1123,6 +1125,9 @@ public Builder setHost(String host) {
     /** Enables gRPC-GCP extension with the default settings. */
     public Builder enableGrpcGcpExtension() {
       this.grpcGcpExtensionEnabled = true;
+      if (!userSpecifiedNumChannels) { // If numChannels is not set through options
+        this.numChannels = 8; // default when grpc-gcp channel pool is enabled
+      }
       return this;
     }
 

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

@@ -0,0 +1,64 @@
+package com.google.cloud.spanner;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class DefaultNumChannelsTest {
+
+  // With GRPC GCP channel pool enabled
+  @Test
+  public void testDefaultNumChannelsWithGrpcGcpEnabled() {
+    SpannerOptions.Builder builder = SpannerOptions.newBuilder()
+        .setProjectId("test-project")
+        .enableGrpcGcpExtension();
+
+    SpannerOptions options = builder.build();
+    assertThat(options.getNumChannels()).isEqualTo(8);
+  }
+
+  @Test
+  public void testNumChannelsWithGrpcGcpEnabledLater() {
+    SpannerOptions.Builder builder = SpannerOptions.newBuilder()
+        .setProjectId("test-project")
+        .setNumChannels(5)
+        .enableGrpcGcpExtension();
+
+    SpannerOptions options = builder.build();
+    assertThat(options.getNumChannels()).isEqualTo(5);
+  }
+
+  @Test
+  public void testNumChannelsWithGrpcGcpEnabled() {
+    SpannerOptions.Builder builder = SpannerOptions.newBuilder()
+        .setProjectId("test-project")
+        .enableGrpcGcpExtension()
+        .setNumChannels(5);
+
+    SpannerOptions options = builder.build();
+    assertThat(options.getNumChannels()).isEqualTo(5);
+  }
+
+  // Without enabling GRPC GCP channel pool
+  @Test
+  public void testDefaultNumChannelsWithOutGrpcGcpEnabled() {
+    SpannerOptions.Builder builder = SpannerOptions.newBuilder()
+        .setProjectId("test-project");
+
+    SpannerOptions options = builder.build();
+    assertThat(options.getNumChannels()).isEqualTo(4);
+  }
+
+  @Test
+  public void testNumChannelsWithOutGrpcGcpEnabled() {
+    SpannerOptions.Builder builder = SpannerOptions.newBuilder()
+        .setProjectId("test-project")
+        .setNumChannels(7);
+
+    SpannerOptions options = builder.build();
+    assertThat(options.getNumChannels()).isEqualTo(7);
+  }
+}