[GSoC 2026] Kafka Streams runner — ExecutableStage (stateless ParDo) translator

runners/kafka-streams/build.gradle

@@ -61,6 +61,7 @@ dependencies {
   permitUnusedDeclared "org.apache.kafka:kafka-clients:$kafka_version"
 
   testImplementation project(path: ":sdks:java:core", configuration: "shadowTest")
+  testImplementation project(":sdks:java:harness")
   testImplementation library.java.hamcrest
   testImplementation library.java.junit
   testImplementation library.java.mockito_core

runners/kafka-streams/src/main/java/org/apache/beam/runners/kafka/streams/translation/ExecutableStageProcessor.java

@@ -0,0 +1,211 @@
+/*
+ * 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.runners.kafka.streams.translation;
+
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import org.apache.beam.model.pipeline.v1.RunnerApi;
+import org.apache.beam.runners.fnexecution.control.BundleProgressHandler;
+import org.apache.beam.runners.fnexecution.control.ExecutableStageContext;
+import org.apache.beam.runners.fnexecution.control.OutputReceiverFactory;
+import org.apache.beam.runners.fnexecution.control.RemoteBundle;
+import org.apache.beam.runners.fnexecution.control.StageBundleFactory;
+import org.apache.beam.runners.fnexecution.provisioning.JobInfo;
+import org.apache.beam.runners.fnexecution.state.StateRequestHandler;
+import org.apache.beam.sdk.fn.data.FnDataReceiver;
+import org.apache.beam.sdk.util.construction.graph.ExecutableStage;
+import org.apache.beam.sdk.values.WindowedValue;
+import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Iterables;
+import org.apache.kafka.streams.processor.api.Processor;
+import org.apache.kafka.streams.processor.api.ProcessorContext;
+import org.apache.kafka.streams.processor.api.Record;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Kafka Streams {@link Processor} that executes a fused {@link ExecutableStage} (stateless user
+ * code such as ParDo) in the Beam SDK harness over the Fn API.
+ *
+ * <p>For each {@link KStreamsPayload#isData() data} payload it unwraps the {@link WindowedValue}
+ * and feeds it to the harness through the stage's main input {@link FnDataReceiver}. Harness
+ * outputs are collected on the harness threads into {@link #pendingOutputs} and then flushed
+ * downstream on the Kafka Streams processing thread when the bundle closes — Kafka Streams' {@link
+ * ProcessorContext#forward} must only be called from the processing thread, so outputs are never
+ * forwarded directly from a harness callback.
+ *
+ * <p>A {@link KStreamsPayload#isWatermark() watermark} payload marks a bundle boundary: the open
+ * bundle (if any) is closed (flushing outputs), and the watermark is then forwarded downstream so
+ * that subsequent stages observe it after all data of the bundle.
+ *
+ * <p>This is the Kafka Streams analogue of Flink's {@code ExecutableStageDoFnOperator} and Spark's
+ * {@code SparkExecutableStageFunction}. State, timers, and side inputs are out of scope for this
+ * first version: the stage is executed with {@link StateRequestHandler#unsupported()} and no timer
+ * receivers.
+ */
+class ExecutableStageProcessor
+    implements Processor<byte[], KStreamsPayload<byte[]>, byte[], KStreamsPayload<byte[]>> {
+
+  private static final Logger LOG = LoggerFactory.getLogger(ExecutableStageProcessor.class);
+
+  private final RunnerApi.ExecutableStagePayload stagePayload;
+  private final JobInfo jobInfo;
+
+  // pendingOutputs is enqueued by SDK harness threads (inside the OutputReceiverFactory callback)
+  // and drained by the Kafka Streams processing thread on bundle close; needs to be thread-safe.
+  private final Queue<WindowedValue<byte[]>> pendingOutputs = new ConcurrentLinkedQueue<>();
+
+  private @Nullable ProcessorContext<byte[], KStreamsPayload<byte[]>> context;
+  private @Nullable ExecutableStageContext stageContext;
+  private @Nullable StageBundleFactory stageBundleFactory;
+  private @Nullable RemoteBundle currentBundle;
+
+  ExecutableStageProcessor(RunnerApi.ExecutableStagePayload stagePayload, JobInfo jobInfo) {
+    this.stagePayload = stagePayload;
+    this.jobInfo = jobInfo;
+  }
+
+  @Override
+  public void init(ProcessorContext<byte[], KStreamsPayload<byte[]>> context) {
+    this.context = context;
+    ExecutableStage executableStage = ExecutableStage.fromPayload(stagePayload);
+    this.stageContext = KafkaStreamsExecutableStageContextFactory.getInstance().get(jobInfo);
+    this.stageBundleFactory = stageContext.getStageBundleFactory(executableStage);
+  }
+
+  @Override
+  public void process(Record<byte[], KStreamsPayload<byte[]>> record) {
+    KStreamsPayload<byte[]> payload = record.value();
+    if (payload.isWatermark()) {
+      // NOTE: flushing the bundle on every received watermark is provisional. Once the
+      // WatermarkManager lands, a stage will receive watermarks from multiple parent instances and
+      // the output watermark becomes min() across them — the bundle should flush / the output
+      // watermark advance only when that minimum actually moves forward, not on every received
+      // watermark. Tracked in #38743.
+      closeBundleAndFlush(record);
+      forwardWatermark(record, payload.getWatermarkMillis());
+      return;
+    }
+    try {
+      ensureBundleOpen();
+      mainInputReceiver().accept(payload.getData());
+    } catch (Exception e) {
+      throw new RuntimeException("Failed to process element through SDK harness", e);
+    }
+  }
+
+  private void ensureBundleOpen() throws Exception {
+    if (currentBundle != null) {
+      return;
+    }
+    StageBundleFactory factory = checkInitialized(stageBundleFactory);
+    OutputReceiverFactory outputReceiverFactory =
+        new OutputReceiverFactory() {
+          @Override
+          public <OutputT> FnDataReceiver<OutputT> create(String pCollectionId) {
+            // Outputs are queued here on harness threads and drained on the processing thread
+            // after the bundle closes.
+            return receivedElement -> {
+              if (receivedElement != null) {
+                pendingOutputs.add((WindowedValue<byte[]>) receivedElement);
+              }
+            };
+          }
+        };
+    currentBundle =
+        factory.getBundle(
+            outputReceiverFactory,
+            StateRequestHandler.unsupported(),
+            BundleProgressHandler.ignored());
+  }
+
+  private FnDataReceiver<WindowedValue<?>> mainInputReceiver() {
+    RemoteBundle bundle = checkInitialized(currentBundle);
+    @SuppressWarnings("unchecked")
+    FnDataReceiver<WindowedValue<?>> receiver =
+        (FnDataReceiver<WindowedValue<?>>)
+            (FnDataReceiver<?>) Iterables.getOnlyElement(bundle.getInputReceivers().values());
+    return receiver;
+  }
+
+  private void closeBundleAndFlush(Record<byte[], KStreamsPayload<byte[]>> record) {
+    RemoteBundle bundle = currentBundle;
+    if (bundle == null) {
+      return;
+    }
+    try {
+      // close() blocks until the harness finishes the bundle and all outputs have been delivered
+      // to the output receiver (and hence enqueued in pendingOutputs).
+      bundle.close();
+    } catch (Exception e) {
+      throw new RuntimeException("Failed to close SDK harness bundle", e);
+    } finally {
+      currentBundle = null;
+    }
+    ProcessorContext<byte[], KStreamsPayload<byte[]>> ctx = checkInitialized(context);
+    WindowedValue<byte[]> output;
+    while ((output = pendingOutputs.poll()) != null) {
+      ctx.forward(
+          new Record<byte[], KStreamsPayload<byte[]>>(
+              record.key(), KStreamsPayload.data(output), record.timestamp()));
+    }
+  }
+
+  private void forwardWatermark(
+      Record<byte[], KStreamsPayload<byte[]>> record, long watermarkMillis) {
+    ProcessorContext<byte[], KStreamsPayload<byte[]>> ctx = checkInitialized(context);
+    ctx.forward(
+        new Record<byte[], KStreamsPayload<byte[]>>(
+            record.key(), KStreamsPayload.watermark(watermarkMillis), record.timestamp()));
+  }
+
+  @Override
+  public void close() {
+    try {
+      if (currentBundle != null) {
+        currentBundle.close();
+        currentBundle = null;
+      }
+    } catch (Exception e) {
+      LOG.warn("Error closing in-flight SDK harness bundle", e);
+    }
+    try {
+      if (stageBundleFactory != null) {
+        stageBundleFactory.close();
+        stageBundleFactory = null;
+      }
+    } catch (Exception e) {
+      LOG.warn("Error closing stage bundle factory", e);
+    }
+    try {
+      if (stageContext != null) {
+        stageContext.close();
+        stageContext = null;
+      }
+    } catch (Exception e) {
+      LOG.warn("Error closing executable stage context", e);
+    }
+  }
+
+  private static <T> T checkInitialized(@Nullable T value) {
+    if (value == null) {
+      throw new IllegalStateException("ExecutableStageProcessor used before init()");
+    }
+    return value;
+  }
+}

runners/kafka-streams/src/main/java/org/apache/beam/runners/kafka/streams/translation/ExecutableStageTranslator.java

@@ -0,0 +1,93 @@
+/*
+ * 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.runners.kafka.streams.translation;
+
+import java.io.IOException;
+import org.apache.beam.model.pipeline.v1.RunnerApi;
+import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Iterables;
+import org.apache.kafka.streams.Topology;
+
+/**
+ * Translates the {@code beam:runner:executable_stage:v1} URN.
+ *
+ * <p>Adds an {@link ExecutableStageProcessor} node to the topology, wired to the processor that
+ * produces the stage's input PCollection (resolved through {@link
+ * KafkaStreamsTranslationContext#getProcessorNameForPCollection}). The processor runs the fused
+ * user code in the SDK harness; its single output PCollection is registered so downstream
+ * translators can attach to this node.
+ *
+ * <p>Multi-output stages (additional outputs / side inputs / state / timers) are out of scope for
+ * this first version and are rejected so the limitation fails fast rather than silently dropping
+ * outputs.
+ */
+class ExecutableStageTranslator implements PTransformTranslator {
+
+  @Override
+  public void translate(
+      String transformId, RunnerApi.Pipeline pipeline, KafkaStreamsTranslationContext context) {
+    RunnerApi.PTransform transform = pipeline.getComponents().getTransformsOrThrow(transformId);
+
+    RunnerApi.ExecutableStagePayload stagePayload;
+    try {
+      stagePayload = RunnerApi.ExecutableStagePayload.parseFrom(transform.getSpec().getPayload());
+    } catch (IOException e) {
+      throw new IllegalArgumentException(
+          "Failed to parse ExecutableStagePayload for transform " + transformId, e);
+    }
+
+    // Fail fast on stage features that are not yet supported, so users get a clear message rather
+    // than a silent miss further down the harness/topology path.
+    if (stagePayload.getSideInputsCount() > 0) {
+      throw new UnsupportedOperationException(
+          "ExecutableStage "
+              + transformId
+              + " has side inputs; side inputs are not yet supported by the Kafka Streams runner.");
+    }
+    if (stagePayload.getUserStatesCount() > 0 || stagePayload.getTimersCount() > 0) {
+      throw new UnsupportedOperationException(
+          "ExecutableStage "
+              + transformId
+              + " uses user state or timers; stateful ParDo is not yet supported by the Kafka"
+              + " Streams runner.");
+    }
+    if (transform.getOutputsMap().size() > 1) {
+      throw new UnsupportedOperationException(
+          "ExecutableStage "
+              + transformId
+              + " has "
+              + transform.getOutputsMap().size()
+              + " outputs; multi-output stages are not yet supported by the Kafka Streams runner.");
+    }
+
+    // The payload distinguishes the main input from side inputs, so reading it from the payload
+    // is unambiguous even before we add side-input support.
+    String inputPCollectionId = stagePayload.getInput();
+    String parentProcessor = context.getProcessorNameForPCollection(inputPCollectionId);
+
+    Topology topology = context.getTopology();
+    topology.addProcessor(
+        transformId,
+        () -> new ExecutableStageProcessor(stagePayload, context.getJobInfo()),
+        parentProcessor);
+
+    if (!transform.getOutputsMap().isEmpty()) {
+      String outputPCollectionId = Iterables.getOnlyElement(transform.getOutputsMap().values());
+      context.registerPCollectionProducer(outputPCollectionId, transformId);
+    }
+  }
+}

runners/kafka-streams/src/main/java/org/apache/beam/runners/kafka/streams/translation/KStreamsPayload.java

@@ -19,6 +19,7 @@
 
 import java.util.Objects;
 import org.apache.beam.sdk.values.WindowedValue;
+import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.MoreObjects;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -121,6 +122,12 @@ public int hashCode() {
 
   @Override
   public String toString() {
-    return kind == Kind.DATA ? "Data{" + data + "}" : "Watermark{" + watermarkMillis + "}";
+    MoreObjects.ToStringHelper helper = MoreObjects.toStringHelper(this).add("kind", kind);
+    if (kind == Kind.DATA) {
+      helper.add("data", data);
+    } else {
+      helper.add("watermarkMillis", watermarkMillis);
+    }
+    return helper.toString();
   }
 }