feat: use LazySpannerInitializer instead of DCL (GoogleCloudPlatform#4197)

olavloite · web-flow · commit 71b05255d669 · 2020-11-09T09:14:57.000-08:00
The Spanner client library now includes a lazy initializer for Spanner instances that can be used with Google Cloud Functions and other libraries that want to create an instance the first time one is needed and reuse this instance for subsequent requests. Fixes GoogleCloudPlatform#2862
diff --git a/functions/spanner/src/main/java/functions/HelloSpanner.java b/functions/spanner/src/main/java/functions/HelloSpanner.java
@@ -23,8 +23,8 @@
 import com.google.cloud.functions.HttpResponse;
 import com.google.cloud.spanner.DatabaseClient;
 import com.google.cloud.spanner.DatabaseId;
+import com.google.cloud.spanner.LazySpannerInitializer;
 import com.google.cloud.spanner.ResultSet;
-import com.google.cloud.spanner.Spanner;
 import com.google.cloud.spanner.SpannerException;
 import com.google.cloud.spanner.SpannerOptions;
 import com.google.cloud.spanner.Statement;
@@ -38,63 +38,28 @@
 public class HelloSpanner implements HttpFunction {
   private static final Logger logger = Logger.getLogger(HelloSpanner.class.getName());
 
-  @VisibleForTesting
-  static Spanner createSpanner() {
-    return SpannerOptions.newBuilder().build().getService();
-  }
-
-  // SpannerHolder is a holder class for a Spanner instance that is initialized lazily.
-  private static final class SpannerHolder {
-    private final Object lock = new Object();
-    private volatile boolean initialized;
-    private volatile DatabaseClient client;
-    private volatile Throwable error;
-
-    private SpannerHolder() {}
+  // TODO<developer>: Set these environment variables.
+  private static final String SPANNER_INSTANCE_ID =
+      MoreObjects.firstNonNull(System.getenv("SPANNER_INSTANCE"), "my-instance");
+  private static final String SPANNER_DATABASE_ID =
+      MoreObjects.firstNonNull(System.getenv("SPANNER_DATABASE"), "example-db");
 
-    // Initialize the {@link Spanner} instance in a method and not as a static variable, as it
-    // might throw an error, and we want to catch and log that specific error. An administrator must
-    // take action to mitigate the reason for the initialization failure, for example ensuring that
-    // the service account being used to access Cloud Spanner has permission to do so.
-    DatabaseClient get() throws Throwable {
-      if (!initialized) {
-        synchronized (lock) {
-          if (!initialized) {
-            try {
-              DatabaseId db =
-                  DatabaseId.of(
-                      SpannerOptions.getDefaultProjectId(),
-                      SPANNER_INSTANCE_ID,
-                      SPANNER_DATABASE_ID);
-              client = createSpanner().getDatabaseClient(db);
-            } catch (Throwable t) {
-              error = t;
-            }
-            initialized = true;
-          }
-        }
-      }
-      if (error != null) {
-        throw error;
-      }
-      return client;
-    }
-  }
+  private static final DatabaseId databaseId =
+      DatabaseId.of(
+          SpannerOptions.getDefaultProjectId(),
+          SPANNER_INSTANCE_ID,
+          SPANNER_DATABASE_ID);
 
-  // The SpannerHolder instance is shared across all instances of the HelloSpanner class.
-  private static final SpannerHolder SPANNER_HOLDER = new SpannerHolder();
+  // The LazySpannerInitializer instance is shared across all instances of the HelloSpanner class.
+  // It will create a Spanner instance the first time one is requested, and continue to return that
+  // instance for all subsequent requests.
+  private static final LazySpannerInitializer SPANNER_INITIALIZER = new LazySpannerInitializer();
 
   @VisibleForTesting
   DatabaseClient getClient() throws Throwable {
-    return SPANNER_HOLDER.get();
+    return SPANNER_INITIALIZER.get().getDatabaseClient(databaseId);
   }
 
-  // TODO<developer>: Set these environment variables.
-  private static final String SPANNER_INSTANCE_ID =
-      MoreObjects.firstNonNull(System.getenv("SPANNER_INSTANCE"), "my-instance");
-  private static final String SPANNER_DATABASE_ID =
-      MoreObjects.firstNonNull(System.getenv("SPANNER_DATABASE"), "example-db");
-
   @Override
   public void service(HttpRequest request, HttpResponse response) throws Exception {
     var writer = new PrintWriter(response.getWriter());
