Pausable channels

api/revapi.json

@@ -27,7 +27,18 @@
     "criticality" : "highlight",
     "minSeverity" : "POTENTIALLY_BREAKING",
     "minCriticality" : "documented",
-    "differences" : [ ]
+    "differences" : [
+        {
+            "code": "java.method.addedToInterface",
+            "new": "method io.smallrye.reactive.messaging.PausableChannel io.smallrye.reactive.messaging.ChannelRegistry::getPausable(java.lang.String)",
+            "justification": "Added PausableChannel to the ChannelRegistry interface to allow pausing and resuming channels."
+        },
+        {
+            "code": "java.method.addedToInterface",
+            "new": "method void io.smallrye.reactive.messaging.ChannelRegistry::register(java.lang.String, io.smallrye.reactive.messaging.PausableChannel)",
+            "justification": "Added PausableChannel to the ChannelRegistry interface to allow pausing and resuming channels."
+        }
+    ]
   }
 }, {
   "extension" : "revapi.reporter.json",
@@ -46,4 +57,4 @@
     "minCriticality" : "documented",
     "output" : "out"
   }
-} ]
+} ]

api/src/main/java/io/smallrye/reactive/messaging/ChannelRegistry.java

@@ -43,4 +43,8 @@ Subscriber<? extends Message<?>> register(String name,
 
     Map<String, Boolean> getOutgoingChannels();
 
+    void register(String name, PausableChannel pausable);
+
+    PausableChannel getPausable(String name);
+
 }

api/src/main/java/io/smallrye/reactive/messaging/PausableChannel.java

@@ -0,0 +1,24 @@
+package io.smallrye.reactive.messaging;
+
+/**
+ * A channel that can be paused and resumed.
+ */
+public interface PausableChannel {
+
+    /**
+     * Checks whether the channel is paused.
+     *
+     * @return {@code true} if the channel is paused, {@code false} otherwise
+     */
+    boolean isPaused();
+
+    /**
+     * Pauses the channel.
+     */
+    void pause();
+
+    /**
+     * Resumes the channel.
+     */
+    void resume();
+}

api/src/main/java/io/smallrye/reactive/messaging/PausableChannelConfiguration.java

@@ -0,0 +1,28 @@
+package io.smallrye.reactive.messaging;
+
+/**
+ * A channel that can be paused and resumed.
+ */
+public interface PausableChannelConfiguration {
+
+    /**
+     * The name of the property to configure whether the channel is pausable.
+     */
+    String PAUSABLE_PROPERTY = "pausable";
+
+    /**
+     * The name of the property to configure whether the channel is initially paused.
+     */
+    String PAUSED_PROPERTY = "initially-paused";
+
+    /**
+     * The name of the channel.
+     */
+    String name();
+
+    /**
+     * Whether the channel is paused at subscribe time.
+     */
+    boolean initiallyPaused();
+
+}

documentation/mkdocs.yml

@@ -34,6 +34,7 @@ nav:
             - 'Message Context' : concepts/message-context.md
             - 'Metadata Injection': concepts/incoming-metadata-injection.md
             - 'Generic Payloads': concepts/generic-payloads.md
+            - 'Pausable Channels': concepts/pausable-channels.md
 
   - Kafka:
         - kafka/kafka.md

documentation/src/main/docs/concepts/pausable-channels.md

@@ -0,0 +1,34 @@
+# Pausable Channels
+
+Based on reactive streams, Smallrye Reactive Messaging ensures that channels are back-pressured.
+This means that the flow of messages is controlled by the downstream consumer,
+whether it is a processing method or outgoing channel.
+Sometimes you may want to pause the flow of messages, for example, when the consumer is not ready to process them.
+
+Injected [`@Channel`](emitter.md#retrieving-channels) streams are not subscribed to by default, so the flow of messages is controlled by the application.
+But for [`@Incoming`](model.md#incoming-and-outgoing) methods, the flow of messages is controlled by the runtime.
+
+Pausable channels are useful when you want to control the flow of messages within your application.
+
+## Creating a Pausable Channel
+
+To use pausable channels, you need to activate it with the configuration property `pausable` set to `true`.
+
+```properties
+mp.messaging.incoming.my-channel.pausable=true
+# optional, by default the channel is NOT paused initially
+mp.messaging.outgoing.my-channel.initially-paused=true
+```
+
+## Controlling the flow of messages
+
+If a channel is configured to be pausable,
+you can get the `PausableChannel` by channel name from the `ChannelRegistry` programmatically,
+and pause or resume the channel as needed:
+
+``` java
+{{ insert('pausable/PausableController.java') }}
+```
+
+!!!warning
+    Pausable channels only work with back-pressure aware subscribers, with bounded downstream requests.

documentation/src/main/java/pausable/PausableController.java

@@ -0,0 +1,39 @@
+package pausable;
+
+import jakarta.annotation.PostConstruct;
+import jakarta.enterprise.context.ApplicationScoped;
+import jakarta.inject.Inject;
+
+import org.eclipse.microprofile.reactive.messaging.Incoming;
+
+import io.smallrye.reactive.messaging.ChannelRegistry;
+import io.smallrye.reactive.messaging.PausableChannel;
+
+@ApplicationScoped
+public class PausableController {
+
+    @Inject
+    ChannelRegistry registry;
+
+    @PostConstruct
+    public void resume() {
+        // Wait for the application to be ready
+        // Retrieve the pausable channel
+        PausableChannel pausable = registry.getPausable("my-channel");
+        // Pause the processing of the messages
+        pausable.resume();
+    }
+
+    public void pause() {
+        // Retrieve the pausable channel
+        PausableChannel pausable = registry.getPausable("my-channel");
+        // Pause the processing of the messages
+        pausable.pause();
+    }
+
+    @Incoming("my-channel")
+    void process(String message) {
+        // Process the message
+    }
+
+}

smallrye-reactive-messaging-kafka/src/test/java/io/smallrye/reactive/messaging/kafka/PausableChannelTest.java

@@ -0,0 +1,191 @@
+package io.smallrye.reactive.messaging.kafka;
+
+import static java.util.concurrent.TimeUnit.SECONDS;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.await;
+
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.atomic.LongAdder;
+
+import jakarta.enterprise.context.ApplicationScoped;
+import jakarta.inject.Inject;
+
+import org.apache.kafka.common.serialization.IntegerDeserializer;
+import org.apache.kafka.common.serialization.IntegerSerializer;
+import org.apache.kafka.common.serialization.StringDeserializer;
+import org.apache.kafka.common.serialization.StringSerializer;
+import org.eclipse.microprofile.reactive.messaging.Incoming;
+import org.eclipse.microprofile.reactive.messaging.Message;
+import org.eclipse.microprofile.reactive.messaging.Outgoing;
+import org.junit.jupiter.api.Test;
+
+import io.smallrye.mutiny.Multi;
+import io.smallrye.reactive.messaging.ChannelRegistry;
+import io.smallrye.reactive.messaging.PausableChannel;
+import io.smallrye.reactive.messaging.annotations.Blocking;
+import io.smallrye.reactive.messaging.kafka.base.KafkaCompanionTestBase;
+import io.smallrye.reactive.messaging.kafka.base.KafkaMapBasedConfig;
+
+public class PausableChannelTest extends KafkaCompanionTestBase {
+
+    public static final int COUNT = 100;
+
+    private KafkaMapBasedConfig commonConfig() {
+        return kafkaConfig("mp.messaging.outgoing.out")
+                .with("topic", topic)
+                .put("key.serializer", StringSerializer.class.getName())
+                .put("value.serializer", IntegerSerializer.class.getName())
+                .withPrefix("mp.messaging.incoming.data")
+                .put("topic", topic)
+                .put("cloud-events", false)
+                .put("commit-strategy", "throttled")
+                .put("auto.offset.reset", "earliest")
+                .put("key.deserializer", StringDeserializer.class.getName())
+                .put("value.deserializer", IntegerDeserializer.class.getName());
+    }
+
+    @Test
+    public void testPausableChannelInitiallyPaused() {
+        addBeans(MyMessageProducer.class);
+        ConsumerApp application = runApplication(commonConfig()
+                .with("pausable", true)
+                .with("initially-paused", true), ConsumerApp.class);
+        ChannelRegistry pausableChannels = get(ChannelRegistry.class);
+        PausableChannel pauser = pausableChannels.getPausable("data");
+
+        long firstStep = COUNT / 10;
+        long secondStep = COUNT / 5;
+        long finalStep = COUNT;
+        await().pollDelay(1, SECONDS).untilAsserted(() -> assertThat(application.getCount()).isEqualTo(0L));
+        assertThat(pauser.isPaused()).isTrue();
+        pauser.resume();
+        await().untilAsserted(() -> assertThat(application.getCount()).isGreaterThan(firstStep));
+        pauser.pause();
+        await().untilAsserted(() -> assertThat(application.getCount()).isBetween(firstStep, finalStep));
+        assertThat(pauser.isPaused()).isTrue();
+        pauser.resume();
+        await().untilAsserted(() -> assertThat(application.getCount()).isGreaterThan(secondStep));
+        pauser.pause();
+        await().untilAsserted(() -> assertThat(application.getCount()).isBetween(secondStep, finalStep));
+        assertThat(pauser.isPaused()).isTrue();
+        pauser.resume();
+        await().untilAsserted(() -> assertThat(application.getCount()).isEqualTo(COUNT));
+    }
+
+    @Test
+    public void testPausableChannel() {
+        addBeans(MyMessageProducer.class);
+        ConsumerApp application = runApplication(commonConfig()
+                .with("pausable", true), ConsumerApp.class);
+        ChannelRegistry pausableChannels = get(ChannelRegistry.class);
+        PausableChannel pauser = pausableChannels.getPausable("data");
+
+        long firstStep = COUNT / 10;
+        long secondStep = COUNT / 5;
+        long finalStep = COUNT;
+        await().untilAsserted(() -> assertThat(application.getCount()).isGreaterThan(firstStep));
+        assertThat(pauser.isPaused()).isFalse();
+        pauser.pause();
+        await().untilAsserted(() -> assertThat(application.getCount()).isBetween(firstStep, finalStep));
+        pauser.resume();
+        await().untilAsserted(() -> assertThat(application.getCount()).isGreaterThan(secondStep));
+        pauser.pause();
+        await().untilAsserted(() -> assertThat(application.getCount()).isBetween(secondStep, finalStep));
+        pauser.resume();
+        await().untilAsserted(() -> assertThat(application.getCount()).isEqualTo(COUNT));
+    }
+
+    @Test
+    public void testPausableChannelWithPauser() {
+        addBeans(MyMessageProducer.class);
+        ConsumerAppWithPauser application = runApplication(commonConfig()
+                .with("pausable", true), ConsumerAppWithPauser.class);
+        ChannelRegistry pausableChannels = get(ChannelRegistry.class);
+        PausableChannel pauser = pausableChannels.getPausable("data");
+
+        assertThat(pauser.isPaused()).isFalse();
+        await().untilAsserted(() -> {
+            if (pauser.isPaused()) {
+                pauser.resume();
+            }
+            assertThat(application.getCount()).isEqualTo(COUNT);
+        });
+        assertThat(application.getPaused()).isEqualTo(5);
+    }
+
+    @ApplicationScoped
+    public static class ConsumerApp {
+
+        LongAdder count = new LongAdder();
+        List<Integer> list = new CopyOnWriteArrayList<>();
+
+        @Incoming("data")
+        @Blocking
+        public void consume(Integer message) throws InterruptedException {
+            list.add(message);
+            count.increment();
+            Thread.sleep(50);
+        }
+
+        public List<Integer> get() {
+            return list;
+        }
+
+        public long getCount() {
+            return count.longValue();
+        }
+    }
+
+    @ApplicationScoped
+    public static class ConsumerAppWithPauser {
+
+        @Inject
+        ChannelRegistry registry;
+
+        LongAdder count = new LongAdder();
+        LongAdder paused = new LongAdder();
+        List<Integer> list = new CopyOnWriteArrayList<>();
+
+        @Incoming("data")
+        @Blocking
+        public void consume(Integer message) throws InterruptedException {
+            list.add(message);
+            count.increment();
+            if (count.longValue() % 20 == 0) {
+                PausableChannel data = registry.getPausable("data");
+                data.pause();
+                paused.increment();
+            }
+        }
+
+        public List<Integer> get() {
+            return list;
+        }
+
+        public long getCount() {
+            return count.longValue();
+        }
+
+        public long getPaused() {
+            return paused.longValue();
+        }
+    }
+
+    @ApplicationScoped
+    public static class MyMessageProducer {
+
+        List<Integer> produced = new CopyOnWriteArrayList<>();
+
+        @Outgoing("out")
+        public Multi<Message<Integer>> generate() {
+            return Multi.createFrom().range(0, COUNT)
+                    .map(i -> Message.of(i, () -> {
+                        produced.add(i);
+                        return CompletableFuture.completedFuture(null);
+                    }));
+        }
+    }
+
+}

smallrye-reactive-messaging-kafka/src/test/java/io/smallrye/reactive/messaging/kafka/base/WeldTestBase.java

@@ -41,6 +41,7 @@
 import io.smallrye.reactive.messaging.providers.extension.MutinyEmitterFactoryImpl;
 import io.smallrye.reactive.messaging.providers.extension.ObservationDecorator;
 import io.smallrye.reactive.messaging.providers.extension.OutgoingObservationDecorator;
+import io.smallrye.reactive.messaging.providers.extension.PausableChannelDecorator;
 import io.smallrye.reactive.messaging.providers.extension.ReactiveMessagingExtension;
 import io.smallrye.reactive.messaging.providers.impl.ConfiguredChannelFactory;
 import io.smallrye.reactive.messaging.providers.impl.ConnectorFactories;
@@ -112,6 +113,7 @@ public void initWeld() {
         weld.addBeanClass(ContextDecorator.class);
         weld.addBeanClass(ObservationDecorator.class);
         weld.addBeanClass(OutgoingObservationDecorator.class);
+        weld.addBeanClass(PausableChannelDecorator.class);
         weld.disableDiscovery();
     }