[RRIO] [Throttle] Stub Enqueue PTransform (#29053)

sdks/java/io/rrio/src/main/java/org/apache/beam/io/requestresponse/ThrottleEnqueue.java

@@ -0,0 +1,61 @@
+/*
+ * 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.beam.io.requestresponse;
+
+import com.google.auto.value.AutoValue;
+import org.apache.beam.sdk.transforms.Create;
+import org.apache.beam.sdk.transforms.PTransform;
+import org.apache.beam.sdk.values.PCollection;
+import org.apache.beam.sdk.values.TypeDescriptor;
+
+/**
+ * {@link ThrottleEnqueue} enqueues {@link RequestT} elements yielding an {@link ApiIOError} {@link
+ * PCollection} of any enqueue errors.
+ */
+class ThrottleEnqueue<RequestT> extends PTransform<PCollection<RequestT>, PCollection<ApiIOError>> {
+
+  @SuppressWarnings({"unused"})
+  private final Configuration<RequestT> configuration;
+
+  private ThrottleEnqueue(Configuration<RequestT> configuration) {
+    this.configuration = configuration;
+  }
+
+  /** Configuration details for {@link ThrottleEnqueue}. */
+  @AutoValue
+  abstract static class Configuration<RequestT> {
+
+    static <RequestT> Builder<RequestT> builder() {
+      return new AutoValue_ThrottleEnqueue_Configuration.Builder<>();
+    }
+
+    abstract Builder<RequestT> toBuilder();
+
+    @AutoValue.Builder
+    abstract static class Builder<RequestT> {
+
+      abstract Configuration<RequestT> build();
+    }
+  }
+
+  @Override
+  public PCollection<ApiIOError> expand(PCollection<RequestT> input) {
+    // TODO(damondouglas): expand in a future PR.
+    return input.getPipeline().apply(Create.empty(TypeDescriptor.of(ApiIOError.class)));
+  }
+}