apache · slachiewicz · Sep 10, 2026
diff --git a/flink-runtime/src/main/java/org/apache/flink/streaming/api/operators/SourceOperator.java b/flink-runtime/src/main/java/org/apache/flink/streaming/api/operators/SourceOperator.java
@@ -431,9 +431,7 @@ public void open() throws Exception {
                             sourceMetricGroup,
                             getProcessingTimeService(),
                             getExecutionConfig().getAutoWatermarkInterval(),
-                            mainInputActivityClock,
-                            getProcessingTimeService().getClock(),
-                            taskIOMetricGroup);
+                            mainInputActivityClock);
         } else {
             eventTimeLogic =
                     TimestampsAndWatermarks.createNoOpEventTimeLogic(

diff --git a/.../java/org/apache/flink/streaming/api/operators/source/ActivityClockPausingDataOutput.java b/.../java/org/apache/flink/streaming/api/operators/source/ActivityClockPausingDataOutput.java
@@ -0,0 +1,122 @@
+/*
+ * 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.flink.streaming.api.operators.source;
+
+import org.apache.flink.annotation.Internal;
+import org.apache.flink.runtime.event.WatermarkEvent;
+import org.apache.flink.streaming.api.operators.util.PausableRelativeClock;
+import org.apache.flink.streaming.api.watermark.Watermark;
+import org.apache.flink.streaming.runtime.io.PushingAsyncDataInput;
+import org.apache.flink.streaming.runtime.streamrecord.LatencyMarker;
+import org.apache.flink.streaming.runtime.streamrecord.RecordAttributes;
+import org.apache.flink.streaming.runtime.streamrecord.StreamRecord;
+import org.apache.flink.streaming.runtime.watermarkstatus.WatermarkStatus;
+
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * A {@link PushingAsyncDataInput.DataOutput} decorator that pauses the source's input activity
+ * clock while the downstream operators process what was emitted.
+ *
+ * <p>Everything emitted by a source is processed synchronously by the chained operators on the task
+ * thread before control returns to the source reader. During that time no split can be polled, so
+ * the elapsed time says nothing about whether a split has records. Without this decorator a slow
+ * chained operator, or a window firing triggered by a watermark, can exceed the idleness timeout
+ * and get a split with pending records marked idle, after which those records arrive behind an
+ * already advanced watermark and are dropped as late.
+ *
+ * <p>This is the same idea as pausing the clock during backpressure (FLIP-471): the idleness
+ * timeout only counts time during which the source was actually able to make progress on its input.
+ *
+ * @param <T> The type of the emitted records.
+ */
+@Internal
+public final class ActivityClockPausingDataOutput<T>
+        implements PushingAsyncDataInput.DataOutput<T> {
+
+    private final PushingAsyncDataInput.DataOutput<T> delegate;
+    private final PausableRelativeClock inputActivityClock;
+
+    public ActivityClockPausingDataOutput(
+            PushingAsyncDataInput.DataOutput<T> delegate,
+            PausableRelativeClock inputActivityClock) {
+        this.delegate = checkNotNull(delegate);
+        this.inputActivityClock = checkNotNull(inputActivityClock);
+    }
+
+    @Override
+    public void emitRecord(StreamRecord<T> streamRecord) throws Exception {
+        inputActivityClock.pause();
+        try {
+            delegate.emitRecord(streamRecord);
+        } finally {
+            inputActivityClock.unPause();
+        }
+    }
+
+    @Override
+    public void emitWatermark(Watermark watermark) throws Exception {
+        inputActivityClock.pause();
+        try {
+            delegate.emitWatermark(watermark);
+        } finally {
+            inputActivityClock.unPause();
+        }
+    }
+
+    @Override
+    public void emitWatermarkStatus(WatermarkStatus watermarkStatus) throws Exception {
+        inputActivityClock.pause();
+        try {
+            delegate.emitWatermarkStatus(watermarkStatus);
+        } finally {
+            inputActivityClock.unPause();
+        }
+    }
+
+    @Override
+    public void emitLatencyMarker(LatencyMarker latencyMarker) throws Exception {
+        inputActivityClock.pause();
+        try {
+            delegate.emitLatencyMarker(latencyMarker);
+        } finally {
+            inputActivityClock.unPause();
+        }
+    }
+
+    @Override
+    public void emitRecordAttributes(RecordAttributes recordAttributes) throws Exception {
+        inputActivityClock.pause();
+        try {
+            delegate.emitRecordAttributes(recordAttributes);
+        } finally {
+            inputActivityClock.unPause();
+        }
+    }
+
+    @Override
+    public void emitWatermark(WatermarkEvent watermark) throws Exception {
+        inputActivityClock.pause();
+        try {
+            delegate.emitWatermark(watermark);
+        } finally {
+            inputActivityClock.unPause();
+        }
+    }
+}
diff --git a/...a/org/apache/flink/streaming/api/operators/source/ProgressiveTimestampsAndWatermarks.java b/...a/org/apache/flink/streaming/api/operators/source/ProgressiveTimestampsAndWatermarks.java
@@ -27,12 +27,9 @@
 import org.apache.flink.api.common.eventtime.WatermarkOutputMultiplexer;
 import org.apache.flink.api.connector.source.ReaderOutput;
 import org.apache.flink.api.connector.source.SourceOutput;
-import org.apache.flink.runtime.metrics.groups.TaskIOMetricGroup;
 import org.apache.flink.streaming.api.operators.util.PausableRelativeClock;
 import org.apache.flink.streaming.runtime.io.PushingAsyncDataInput;
 import org.apache.flink.streaming.runtime.tasks.ProcessingTimeService;
-import org.apache.flink.util.clock.Clock;
-import org.apache.flink.util.clock.RelativeClock;
 
 import javax.annotation.Nullable;
 
@@ -66,11 +63,7 @@ public class ProgressiveTimestampsAndWatermarks<T> implements TimestampsAndWater
 
     private final long periodicWatermarkInterval;
 
-    private final RelativeClock mainInputActivityClock;
-
-    private final Clock clock;
-
-    private final TaskIOMetricGroup taskIOMetricGroup;
+    private final PausableRelativeClock mainInputActivityClock;
 
     @Nullable private SplitLocalOutputs<T> currentPerSplitOutputs;
 
@@ -84,17 +77,13 @@ public ProgressiveTimestampsAndWatermarks(
             TimestampsAndWatermarksContextProvider watermarksContextProvider,
             ProcessingTimeService timeService,
             Duration periodicWatermarkInterval,
-            RelativeClock mainInputActivityClock,
-            Clock clock,
-            TaskIOMetricGroup taskIOMetricGroup) {
+            PausableRelativeClock mainInputActivityClock) {
 
         this.timestampAssigner = timestampAssigner;
         this.watermarksFactory = watermarksFactory;
         this.watermarksContextProvider = watermarksContextProvider;
         this.timeService = timeService;
         this.mainInputActivityClock = mainInputActivityClock;
-        this.clock = clock;
-        this.taskIOMetricGroup = taskIOMetricGroup;
 
         long periodicWatermarkIntervalMillis;
         try {
@@ -120,28 +109,36 @@ public ReaderOutput<T> createMainOutput(
                 currentMainOutput == null && currentPerSplitOutputs == null,
                 "already created a main output");
 
-        final WatermarkOutput watermarkOutput =
-                new WatermarkToDataOutput(output, watermarkUpdateListener);
-        IdlenessManager idlenessManager = new IdlenessManager(watermarkOutput);
-
         final WatermarkGenerator<T> watermarkGenerator =
                 watermarksFactory.createWatermarkGenerator(
                         watermarksContextProvider.create(mainInputActivityClock));
 
+        // Downstream operators process every emitted element synchronously on the task thread,
+        // during which no split can be polled. When something measures input activity time (i.e.
+        // idleness detection is configured), hide that time from the activity clocks; otherwise
+        // keep the plain output to leave the hot path untouched.
+        final PushingAsyncDataInput.DataOutput<T> recordOutput =
+                watermarksContextProvider.isInputActivityClockRequested()
+                        ? new ActivityClockPausingDataOutput<>(output, mainInputActivityClock)
+                        : output;
+
+        final WatermarkOutput watermarkOutput =
+                new WatermarkToDataOutput(recordOutput, watermarkUpdateListener);
+        IdlenessManager idlenessManager = new IdlenessManager(watermarkOutput);
+
         currentPerSplitOutputs =
                 new SplitLocalOutputs<>(
-                        output,
+                        recordOutput,
                         idlenessManager.getSplitLocalOutput(),
                         watermarkUpdateListener,
                         timestampAssigner,
                         watermarksFactory,
                         watermarksContextProvider,
-                        clock,
-                        taskIOMetricGroup);
+                        mainInputActivityClock);
 
         currentMainOutput =
                 new StreamingReaderOutput<>(
-                        output,
+                        recordOutput,
                         idlenessManager.getMainOutput(),
                         timestampAssigner,
                         watermarkGenerator,
@@ -237,8 +234,7 @@ private static final class SplitLocalOutputs<T> {
         private final WatermarkGeneratorSupplier<T> watermarksFactory;
         private final TimestampsAndWatermarksContextProvider watermarksContextProvider;
         private final WatermarkUpdateListener watermarkUpdateListener;
-        private final Clock clock;
-        private final TaskIOMetricGroup taskIOMetricGroup;
+        private final PausableRelativeClock mainInputActivityClock;
 
         private SplitLocalOutputs(
                 PushingAsyncDataInput.DataOutput<T> recordOutput,
@@ -247,16 +243,14 @@ private SplitLocalOutputs(
                 TimestampAssigner<T> timestampAssigner,
                 WatermarkGeneratorSupplier<T> watermarksFactory,
                 TimestampsAndWatermarksContextProvider watermarksContextProvider,
-                Clock clock,
-                TaskIOMetricGroup taskIOMetricGroup) {
+                PausableRelativeClock mainInputActivityClock) {
 
             this.recordOutput = recordOutput;
             this.timestampAssigner = timestampAssigner;
             this.watermarksFactory = watermarksFactory;
             this.watermarksContextProvider = watermarksContextProvider;
             this.watermarkUpdateListener = watermarkUpdateListener;
-            this.clock = clock;
-            this.taskIOMetricGroup = taskIOMetricGroup;
+            this.mainInputActivityClock = mainInputActivityClock;
 
             this.watermarkMultiplexer = new WatermarkOutputMultiplexer(watermarkOutput);
             this.localOutputs =
@@ -303,21 +297,20 @@ public void onIdleUpdate(boolean idle) {
         }
 
         private PausableRelativeClock createInputActivityClock(String splitId) {
-            // Dedicated inputActivityClock for a particular split. It will be paused both in case
-            // of back pressure and when split is paused due to watermark alignment.
-            PausableRelativeClock inputActivityClock = new PausableRelativeClock(clock);
+            // Dedicated inputActivityClock for a particular split, layered on the main input
+            // activity clock: it is paused whenever the main clock is (backpressure, downstream
+            // processing) and additionally when this split is paused due to watermark alignment.
+            PausableRelativeClock inputActivityClock =
+                    new PausableRelativeClock(mainInputActivityClock);
             inputActivityClocks.put(splitId, inputActivityClock);
-            taskIOMetricGroup.registerBackPressureListener(inputActivityClock);
             return inputActivityClock;
         }
 
         void releaseOutputForSplit(String splitId) {
             watermarkUpdateListener.splitFinished(splitId);
             localOutputs.remove(splitId);
             watermarkMultiplexer.unregisterOutput(splitId);
-            PausableRelativeClock inputActivityClock =
-                    requireNonNull(inputActivityClocks.remove(splitId));
-            taskIOMetricGroup.unregisterBackPressureListener(inputActivityClock);
+            requireNonNull(inputActivityClocks.remove(splitId));
         }
 
         void emitPeriodicWatermark() {

diff --git a/...rc/main/java/org/apache/flink/streaming/api/operators/source/TimestampsAndWatermarks.java b/...rc/main/java/org/apache/flink/streaming/api/operators/source/TimestampsAndWatermarks.java
@@ -24,10 +24,9 @@
 import org.apache.flink.api.common.eventtime.WatermarkStrategy;
 import org.apache.flink.api.connector.source.ReaderOutput;
 import org.apache.flink.metrics.MetricGroup;
-import org.apache.flink.runtime.metrics.groups.TaskIOMetricGroup;
+import org.apache.flink.streaming.api.operators.util.PausableRelativeClock;
 import org.apache.flink.streaming.runtime.io.PushingAsyncDataInput;
 import org.apache.flink.streaming.runtime.tasks.ProcessingTimeService;
-import org.apache.flink.util.clock.Clock;
 import org.apache.flink.util.clock.RelativeClock;
 
 import java.time.Duration;
@@ -104,9 +103,7 @@ static <E> TimestampsAndWatermarks<E> createProgressiveEventTimeLogic(
             MetricGroup metrics,
             ProcessingTimeService timeService,
             long periodicWatermarkIntervalMillis,
-            RelativeClock mainInputActivityClock,
-            Clock clock,
-            TaskIOMetricGroup taskIOMetricGroup) {
+            PausableRelativeClock mainInputActivityClock) {
 
         TimestampsAndWatermarksContextProvider contextProvider =
                 new TimestampsAndWatermarksContextProvider(metrics);
@@ -120,9 +117,7 @@ static <E> TimestampsAndWatermarks<E> createProgressiveEventTimeLogic(
                 contextProvider,
                 timeService,
                 Duration.ofMillis(periodicWatermarkIntervalMillis),
-                mainInputActivityClock,
-                clock,
-                taskIOMetricGroup);
+                mainInputActivityClock);
     }
 
     static <E> TimestampsAndWatermarks<E> createNoOpEventTimeLogic(
@@ -142,12 +137,30 @@ static <E> TimestampsAndWatermarks<E> createNoOpEventTimeLogic(
     class TimestampsAndWatermarksContextProvider {
         private final MetricGroup metrics;
 
+        /** Whether any created context handed out the input activity clock. */
+        private boolean inputActivityClockRequested;
+
         public TimestampsAndWatermarksContextProvider(MetricGroup metrics) {
             this.metrics = metrics;
         }
 
         public TimestampsAndWatermarksContext create(RelativeClock inputActivityClock) {
-            return new TimestampsAndWatermarksContext(metrics, inputActivityClock);
+            return new TimestampsAndWatermarksContext(
+                    metrics, inputActivityClock, () -> inputActivityClockRequested = true);
+        }
+
+        /**
+         * Returns true if a timestamp assigner or watermark generator created through this provider
+         * asked for the input activity clock, which is the case with {@link
+         * org.apache.flink.api.common.eventtime.WatermarkStrategy#withIdleness(java.time.Duration)}.
+         * Only then is it worth paying for keeping that clock accurate.
+         *
+         * <p>The decision to hide downstream processing time from the clock is taken once, when the
+         * main output is created. A supplier that asks for the clock lazily, after its first {@code
+         * createWatermarkGenerator} call returned, therefore keeps the previous behaviour.
+         */
+        public boolean isInputActivityClockRequested() {
+            return inputActivityClockRequested;
         }
     }
 }
diff --git a/.../java/org/apache/flink/streaming/api/operators/source/TimestampsAndWatermarksContext.java b/.../java/org/apache/flink/streaming/api/operators/source/TimestampsAndWatermarksContext.java
@@ -35,12 +35,27 @@ public final class TimestampsAndWatermarksContext
         implements TimestampAssignerSupplier.Context, WatermarkGeneratorSupplier.Context {
 
     private final MetricGroup metricGroup;
+
     private final RelativeClock inputActivityClock;
 
+    private final Runnable onInputActivityClockRequested;
+
     public TimestampsAndWatermarksContext(
             MetricGroup metricGroup, RelativeClock inputActivityClock) {
+        this(metricGroup, inputActivityClock, () -> {});
+    }
+
+    /**
+     * @param onInputActivityClockRequested invoked every time {@link #getInputActivityClock()} is
+     *     called, so that the owner can tell whether anything measures input activity time.
+     */
+    public TimestampsAndWatermarksContext(
+            MetricGroup metricGroup,
+            RelativeClock inputActivityClock,
+            Runnable onInputActivityClockRequested) {
         this.metricGroup = checkNotNull(metricGroup);
         this.inputActivityClock = inputActivityClock;
+        this.onInputActivityClockRequested = checkNotNull(onInputActivityClockRequested);
     }
 
     @Override
@@ -50,6 +65,7 @@ public MetricGroup getMetricGroup() {
 
     @Override
     public RelativeClock getInputActivityClock() {
+        onInputActivityClockRequested.run();
         return inputActivityClock;
     }
 }