[RRIO] [Throttle] stub the dequeue transform (#29056)

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

@@ -0,0 +1,101 @@
+/*
+ * 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 java.util.Map;
+import org.apache.beam.io.requestresponse.ThrottleDequeue.Result;
+import org.apache.beam.sdk.Pipeline;
+import org.apache.beam.sdk.transforms.PTransform;
+import org.apache.beam.sdk.values.PCollection;
+import org.apache.beam.sdk.values.PCollectionTuple;
+import org.apache.beam.sdk.values.PInput;
+import org.apache.beam.sdk.values.POutput;
+import org.apache.beam.sdk.values.PValue;
+import org.apache.beam.sdk.values.TupleTag;
+import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.ImmutableMap;
+import org.joda.time.Instant;
+
+/**
+ * {@link ThrottleDequeue} dequeues {@link RequestT} elements at a fixed rate yielding a {@link
+ * Result} containing the dequeued {@link RequestT} {@link PCollection} and a {@link ApiIOError}
+ * {@link PCollection} of any errors.
+ */
+class ThrottleDequeue<RequestT> extends PTransform<PCollection<Instant>, Result<RequestT>> {
+
+  private static final TupleTag<ApiIOError> FAILURE_TAG = new TupleTag<ApiIOError>() {};
+
+  // TODO(damondouglas): remove suppress warnings after instance utilized.
+  @SuppressWarnings({"unused"})
+  private final Configuration<RequestT> configuration;
+
+  private ThrottleDequeue(Configuration<RequestT> configuration) {
+    this.configuration = configuration;
+  }
+
+  @Override
+  public Result<RequestT> expand(PCollection<Instant> input) {
+    // TODO(damondouglas): expand in a future PR.
+    return new Result<>(new TupleTag<RequestT>() {}, PCollectionTuple.empty(input.getPipeline()));
+  }
+
+  @AutoValue
+  abstract static class Configuration<RequestT> {
+
+    @AutoValue.Builder
+    abstract static class Builder<RequestT> {
+      abstract Configuration<RequestT> build();
+    }
+  }
+
+  /** The {@link Result} of dequeuing {@link RequestT}s. */
+  static class Result<RequestT> implements POutput {
+
+    static <RequestT> Result<RequestT> of(TupleTag<RequestT> requestsTag, PCollectionTuple pct) {
+      return new Result<>(requestsTag, pct);
+    }
+
+    private final Pipeline pipeline;
+    private final TupleTag<RequestT> requestsTag;
+    private final PCollection<RequestT> requests;
+    private final PCollection<ApiIOError> failures;
+
+    private Result(TupleTag<RequestT> requestsTag, PCollectionTuple pct) {
+      this.pipeline = pct.getPipeline();
+      this.requestsTag = requestsTag;
+      this.requests = pct.get(requestsTag);
+      this.failures = pct.get(FAILURE_TAG);
+    }
+
+    @Override
+    public Pipeline getPipeline() {
+      return pipeline;
+    }
+
+    @Override
+    public Map<TupleTag<?>, PValue> expand() {
+      return ImmutableMap.of(
+          requestsTag, requests,
+          FAILURE_TAG, failures);
+    }
+
+    @Override
+    public void finishSpecifyingOutput(
+        String transformName, PInput input, PTransform<?, ?> transform) {}
+  }
+}