feat: run a node once per item of a list input

core/src/main/java/com/google/adk/workflow/ParallelWorkers.java

@@ -0,0 +1,228 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed 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 com.google.adk.workflow;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectReader;
+import com.google.adk.JsonBaseModel;
+import com.google.adk.agents.Context;
+import com.google.adk.annotations.Experimental;
+import com.google.common.collect.ImmutableList;
+import com.google.genai.types.Content;
+import com.google.genai.types.Part;
+import com.google.genai.types.Schema;
+import com.google.genai.types.Type;
+import io.reactivex.rxjava3.core.Flowable;
+import io.reactivex.rxjava3.core.Single;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReferenceArray;
+import org.jspecify.annotations.Nullable;
+
+/**
+ * Static factories for workflow nodes that run a node once per item of a list input. Each item runs
+ * as a child of the returned node on its own branch, and the node outputs the items' results as a
+ * list in input order. A failed item cancels the others and fails the node, while an item that asks
+ * for input stops queued items from starting.
+ */
+@Experimental
+public final class ParallelWorkers {
+
+  /**
+   * Returns a node that runs {@code node} on every item at once. Otherwise it behaves as {@link
+   * #parallelNode(Node, int)} describes.
+   *
+   * @throws WorkflowConfigurationException if {@code node} is {@link Start}
+   */
+  public static Node parallelNode(Node node) {
+    return parallelNode(node, Integer.MAX_VALUE);
+  }
+
+  /**
+   * Returns a node that runs {@code node} on each item of a list input, at most {@code
+   * maxParallelWorkers} at a time and starting them in input order; the returned node takes {@code
+   * node}'s name, description, state schema and {@link Node#requiresAllPredecessors()}, and leaves
+   * its config and input and output schemas to each item's run. A text-only {@link Content}, such
+   * as a user message, is read as JSON, and a JSON list fans out. Its text reaches the items as a
+   * {@code String} if it is not JSON, or if it is a non-list value and the item's input schema
+   * takes a string; any other non-list input runs as one item.
+   *
+   * @throws WorkflowConfigurationException if {@code node} is {@link Start} or {@code
+   *     maxParallelWorkers} is less than 1
+   */
+  public static Node parallelNode(Node node, int maxParallelWorkers) {
+    if (node instanceof Start) {
+      throw new WorkflowConfigurationException("parallelNode cannot wrap Start.");
+    }
+    if (maxParallelWorkers < 1) {
+      throw new WorkflowConfigurationException(
+          String.format("maxParallelWorkers must be at least 1, but was %s.", maxParallelWorkers));
+    }
+    return new ParallelWorkerNode(node, maxParallelWorkers);
+  }
+
+  private static final class ParallelWorkerNode extends BaseNode {
+
+    private static final ObjectReader JSON_READER =
+        JsonBaseModel.getMapper()
+            .readerFor(Object.class)
+            .with(DeserializationFeature.FAIL_ON_TRAILING_TOKENS);
+    // No run ID: items subscribe in input order, so each gets the same generated ID on resume.
+    private static final RunNodeConfig ITEM_CONFIG =
+        RunNodeConfig.builder().useSubBranch(true).build();
+
+    private final Node worker;
+    private final int maxParallelWorkers;
+
+    ParallelWorkerNode(Node worker, int maxParallelWorkers) {
+      super(
+          worker.name(),
+          worker.description(),
+          /* rerunOnResume= */ true,
+          /* waitForOutput= */ false,
+          NodeConfig.builder().build(),
+          /* inputSchema= */ null,
+          /* outputSchema= */ null,
+          worker.stateSchema().orElse(null));
+      this.worker = worker;
+      this.maxParallelWorkers = maxParallelWorkers;
+    }
+
+    @Override
+    public boolean requiresAllPredecessors() {
+      return worker.requiresAllPredecessors();
+    }
+
+    @Override
+    public Flowable<?> runNode(Context context, @Nullable Object nodeInput) {
+      return Flowable.defer(
+          () -> {
+            Object input = nodeInput instanceof Content content ? unwrap(content) : nodeInput;
+            List<?> items = input instanceof List<?> list ? list : Collections.singletonList(input);
+            return new FanOut(context, items).run();
+          });
+    }
+
+    /**
+     * Returns the JSON value of text-only {@code content}, or its text if that is not JSON or is a
+     * non-list value for an item that takes a string; any other content is returned as it is.
+     */
+    private @Nullable Object unwrap(Content content) {
+      List<Part> parts = content.parts().orElse(ImmutableList.of());
+      if (parts.isEmpty() || parts.stream().anyMatch(part -> part.text().isEmpty())) {
+        return content;
+      }
+      String text = content.text();
+      Object json;
+      try {
+        json = JSON_READER.readValue(text);
+      } catch (JsonProcessingException e) {
+        return text;
+      }
+      if (json instanceof List<?> list) {
+        return list;
+      }
+      return worker.inputSchema().map(ParallelWorkerNode::acceptsString).orElse(false)
+          ? text
+          : json;
+    }
+
+    private static boolean acceptsString(Schema schema) {
+      return schema.type().map(type -> type.knownEnum() == Type.Known.STRING).orElse(false)
+          || schema.anyOf().orElse(ImmutableList.of()).stream()
+              .anyMatch(ParallelWorkerNode::acceptsString);
+    }
+
+    private final class FanOut {
+      private final Context context;
+      private final List<?> items;
+      // Written by the item runs, which may finish on different threads.
+      private final AtomicReferenceArray<Object> outputs;
+      private final AtomicReferenceArray<DynamicNodeFailedException> failures;
+      private final AtomicBoolean interrupted = new AtomicBoolean();
+
+      FanOut(Context context, List<?> items) {
+        this.context = context;
+        this.items = items;
+        this.outputs = new AtomicReferenceArray<>(items.size());
+        this.failures = new AtomicReferenceArray<>(items.size());
+      }
+
+      Flowable<?> run() {
+        return Flowable.range(0, items.size())
+            // An interrupt stops only the items that had to wait for a slot.
+            .takeWhile(index -> index < maxParallelWorkers || !interrupted.get())
+            .flatMapSingle(this::runItem, /* delayErrors= */ false, maxParallelWorkers)
+            // A failed item stops the rest; the typed parameter rules out takeUntil(Publisher).
+            .takeUntil((Boolean failed) -> failed)
+            .ignoreElements()
+            .andThen(Flowable.defer(this::finish));
+      }
+
+      /** Runs one item, or replays its result on resume, and emits whether it failed. */
+      private Single<Boolean> runItem(int index) {
+        return context
+            .runNode(worker, items.get(index), ITEM_CONFIG)
+            .doOnSuccess(output -> outputs.set(index, output))
+            .ignoreElement()
+            .toSingleDefault(false)
+            .onErrorResumeNext(error -> settle(index, error));
+      }
+
+      /**
+       * Records a failure so the lowest index wins a race, and keeps an interrupt from ending the
+       * rest.
+       */
+      private Single<Boolean> settle(int index, Throwable error) {
+        if (error instanceof DynamicNodeFailedException failure) {
+          failures.set(index, failure);
+          return Single.just(true);
+        }
+        if (error instanceof NodeInterruptedException) {
+          // The runtime has already added the item's interrupt IDs to this node.
+          interrupted.set(true);
+          return Single.just(false);
+        }
+        return Single.error(error);
+      }
+
+      /**
+       * Fails with the lowest-index failure, or else waits with the items that asked for input, or
+       * else outputs the items' results. Each failure's error event stays at its item.
+       */
+      private Flowable<?> finish() {
+        List<@Nullable Object> results = new ArrayList<>(items.size());
+        for (int i = 0; i < items.size(); i++) {
+          DynamicNodeFailedException failure = failures.get(i);
+          if (failure != null) {
+            return Flowable.error(failure);
+          }
+          results.add(outputs.get(i));
+        }
+        if (interrupted.get()) {
+          return Flowable.error(new NodeInterruptedException());
+        }
+        return Flowable.just(Collections.unmodifiableList(results));
+      }
+    }
+  }
+
+  private ParallelWorkers() {}
+}