[Service Bus] Allow 0 prefetch and dynamically use batch size to request link credits

sdk/core/azure-core-amqp/src/main/java/com/azure/core/amqp/implementation/AmqpReceiveLink.java

@@ -25,7 +25,7 @@ public interface AmqpReceiveLink extends AmqpLink {
     Flux<Message> receive();
 
     /**
-     * Adds the specified number of credits to the link.
+     * Schedule to adds the specified number of credits to the link.
      *
      * The number of link credits initialises to zero. It is the application's responsibility to call this method to
      * allow the receiver to receive {@code credits} more deliveries.
@@ -34,6 +34,21 @@ public interface AmqpReceiveLink extends AmqpLink {
      */
     void addCredits(int credits);
 
+    /**
+     * Adds the specified number of credits to the link.
+     *
+     * The number of link credits initialises to zero. It is the application's responsibility to call this method to
+     * allow the receiver to receive {@code credits} more deliveries.
+     *
+     * It will update the credits in local memory instantly so {@link #getCredits()} will get
+     * the updated credits immediately. But the service side may get the credits added with a latency.
+     * As a contrast, {@link #getCredits()} may return an unchanged value for a short while after
+     * {@link #addCredits(int)} is called to schedule the credit addition and before the job dispatcher executes it.
+     *
+     * @param credits Number of credits to add to the receive link.
+     */
+    void addCreditsInstantly(int credits);
+
     /**
      * Gets the current number of credits this link has.
      *

sdk/core/azure-core-amqp/src/main/java/com/azure/core/amqp/implementation/ReactorReceiver.java

@@ -107,6 +107,11 @@ public void addCredits(int credits) {
         }
     }
 
+    @Override
+    public void addCreditsInstantly(int credits) {
+        receiver.flow(credits);
+    }
+
     @Override
     public int getCredits() {
         return receiver.getRemoteCredit();

sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/ServiceBusAsyncConsumer.java

@@ -34,9 +34,7 @@ class ServiceBusAsyncConsumer implements AutoCloseable {
         this.linkProcessor = linkProcessor;
         this.messageSerializer = messageSerializer;
         this.processor = linkProcessor
-            .map(message -> this.messageSerializer.deserialize(message, ServiceBusReceivedMessage.class))
-            .publish(receiverOptions.getPrefetchCount())
-            .autoConnect(1);
+            .map(message -> this.messageSerializer.deserialize(message, ServiceBusReceivedMessage.class));
     }
 
     /**

sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/ServiceBusClientBuilder.java

@@ -94,7 +94,7 @@ public final class ServiceBusClientBuilder {
 
     // Using 0 pre-fetch count for both receive modes, to avoid message lock lost exceptions in application
     // receiving messages at a slow rate. Applications can set it to a higher value if they need better performance.
-    private static final int DEFAULT_PREFETCH_COUNT = 1;
+    private static final int DEFAULT_PREFETCH_COUNT = 0;
     private static final String NAME_KEY = "name";
     private static final String VERSION_KEY = "version";
     private static final String UNKNOWN = "UNKNOWN";
@@ -670,11 +670,13 @@ public ServiceBusSessionProcessorClientBuilder maxConcurrentSessions(int maxConc
 
         /**
          * Sets the prefetch count of the processor. For both {@link ServiceBusReceiveMode#PEEK_LOCK PEEK_LOCK} and
-         * {@link ServiceBusReceiveMode#RECEIVE_AND_DELETE RECEIVE_AND_DELETE} modes the default value is 1.
+         * {@link ServiceBusReceiveMode#RECEIVE_AND_DELETE RECEIVE_AND_DELETE} modes the default value is 0.
          *
          * Prefetch speeds up the message flow by aiming to have a message readily available for local retrieval when
          * and before the application starts the processor.
          * Setting a non-zero value will prefetch that number of messages. Setting the value to zero turns prefetch off.
+         * Using a non-zero prefetch risks of losing messages even though it has better performance.
+         * @see <a href="https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-prefetch">Service Bus Prefetch</a>
          *
          * @param prefetchCount The prefetch count.
          *
@@ -1442,9 +1444,9 @@ ServiceBusReceiverAsyncClient buildAsyncClient(boolean isAutoCompleteAllowed) {
     }
 
     private void validateAndThrow(int prefetchCount) {
-        if (prefetchCount < 1) {
+        if (prefetchCount < 0) {
             throw logger.logExceptionAsError(new IllegalArgumentException(String.format(
-                "prefetchCount (%s) cannot be less than 1.", prefetchCount)));
+                "prefetchCount (%s) cannot be less than 0.", prefetchCount)));
         }
     }
 

sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/ServiceBusReceiverAsyncClient.java

@@ -573,7 +573,18 @@ Flux<ServiceBusReceivedMessage> peekMessagesAt(int maxMessages, long sequenceNum
      * @return An <b>infinite</b> stream of messages from the Service Bus entity.
      */
     public Flux<ServiceBusReceivedMessage> receiveMessages() {
-        return receiveMessagesWithContext()
+        // Without limitRate(), if the user calls receiveMessages().subscribe(), it will call
+        // ServiceBusReceiveLinkProcessor.request(long request) where request = Long.MAX_VALUE.
+        // We turn this one-time non-backpressure request to continuous requests with backpressure.
+        // If receiverOptions.prefetchCount is set to non-zero, it will be passed to ServiceBusReceiveLinkProcessor
+        // to auto-refill the prefetch buffer. A request will retrieve one message from this buffer.
+        // If receiverOptions.prefetchCount is 0 (default value),
+        // the request will add a link credit so one message is retrieved from the service.
+        return receiveMessagesNoBackPressure().limitRate(1, 0);
+    }
+
+    Flux<ServiceBusReceivedMessage> receiveMessagesNoBackPressure() {
+        return receiveMessagesWithContext(0)
             .handle((serviceBusMessageContext, sink) -> {
                 if (serviceBusMessageContext.hasError()) {
                     sink.error(serviceBusMessageContext.getThrowable());
@@ -598,6 +609,10 @@ public Flux<ServiceBusReceivedMessage> receiveMessages() {
      * @return An <b>infinite</b> stream of messages from the Service Bus entity.
      */
     Flux<ServiceBusMessageContext> receiveMessagesWithContext() {
+        return receiveMessagesWithContext(1);
+    }
+
+    Flux<ServiceBusMessageContext> receiveMessagesWithContext(int highTide) {
         final Flux<ServiceBusMessageContext> messageFlux = sessionManager != null
             ? sessionManager.receive()
             : getOrCreateConsumer().receive().map(ServiceBusMessageContext::new);
@@ -610,16 +625,19 @@ Flux<ServiceBusMessageContext> receiveMessagesWithContext() {
             withAutoLockRenewal = messageFlux;
         }
 
-        final Flux<ServiceBusMessageContext> withAutoComplete;
+        Flux<ServiceBusMessageContext> result;
         if (receiverOptions.isEnableAutoComplete()) {
-            withAutoComplete = new FluxAutoComplete(withAutoLockRenewal, completionLock,
+            result = new FluxAutoComplete(withAutoLockRenewal, completionLock,
                 context -> context.getMessage() != null ? complete(context.getMessage()) : Mono.empty(),
                 context -> context.getMessage() != null ? abandon(context.getMessage()) : Mono.empty());
         } else {
-            withAutoComplete = withAutoLockRenewal;
+            result = withAutoLockRenewal;
         }
 
-        return withAutoComplete
+        if (highTide > 0) {
+            result = result.limitRate(highTide, 0);
+        }
+        return result
             .onErrorMap(throwable -> mapError(throwable, ServiceBusErrorSource.RECEIVE));
     }
 

sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/ServiceBusReceiverClient.java

@@ -575,12 +575,12 @@ public void rollbackTransaction(ServiceBusTransactionContext transactionContext)
      */
     @Override
     public void close() {
-        asyncClient.close();
-
         SynchronousMessageSubscriber messageSubscriber = synchronousMessageSubscriber.getAndSet(null);
         if (messageSubscriber != null && !messageSubscriber.isDisposed()) {
             messageSubscriber.dispose();
         }
+
+        asyncClient.close();
     }
 
     /**
@@ -590,19 +590,20 @@ public void close() {
     private void queueWork(int maximumMessageCount, Duration maxWaitTime,
                            FluxSink<ServiceBusReceivedMessage> emitter) {
         final long id = idGenerator.getAndIncrement();
-        final SynchronousReceiveWork work = new SynchronousReceiveWork(id, maximumMessageCount, maxWaitTime, emitter);
-
+        final int prefetch = asyncClient.getReceiverOptions().getPrefetchCount();
+        final int toRequest = prefetch != 0 ? Math.min(maximumMessageCount, prefetch) : maximumMessageCount;
+        final SynchronousReceiveWork work = new SynchronousReceiveWork(id,
+            toRequest,
+            maxWaitTime, emitter);
         SynchronousMessageSubscriber messageSubscriber = synchronousMessageSubscriber.get();
         if (messageSubscriber == null) {
-            long prefetch = asyncClient.getReceiverOptions().getPrefetchCount();
-            SynchronousMessageSubscriber newSubscriber = new SynchronousMessageSubscriber(prefetch, work);
-
+            SynchronousMessageSubscriber newSubscriber = new SynchronousMessageSubscriber(toRequest, work);
             if (!synchronousMessageSubscriber.compareAndSet(null, newSubscriber)) {
                 newSubscriber.dispose();
                 SynchronousMessageSubscriber existing = synchronousMessageSubscriber.get();
                 existing.queueWork(work);
             } else {
-                asyncClient.receiveMessages().subscribeWith(newSubscriber);
+                asyncClient.receiveMessagesNoBackPressure().subscribeWith(newSubscriber);
             }
         } else {
             messageSubscriber.queueWork(work);

sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/ServiceBusSessionManager.java

@@ -309,7 +309,7 @@ private Flux<ServiceBusMessageContext> getSession(Scheduler scheduler, boolean d
                     onSessionRequest(1L);
                 }
             }))
-            .publishOn(scheduler);
+            .publishOn(scheduler, 1);
     }
 
     private Mono<ServiceBusManagementNode> getManagementNode() {

sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/ServiceBusSessionReceiver.java

@@ -63,14 +63,23 @@ class ServiceBusSessionReceiver implements AutoCloseable {
         this.receiveLink = receiveLink;
         this.lockContainer = new LockContainer<>(ServiceBusConstants.OPERATION_TIMEOUT);
 
-        receiveLink.setEmptyCreditListener(() -> 1);
+        receiveLink.setEmptyCreditListener(() -> 0);
 
         final Flux<ServiceBusMessageContext> receivedMessagesFlux = receiveLink
             .receive()
             .publishOn(scheduler)
             .doOnSubscribe(subscription -> {
                 logger.verbose("Adding prefetch to receive link.");
-                receiveLink.addCredits(prefetch);
+                if (prefetch > 0) {
+                    receiveLink.addCredits(prefetch);
+                }
+            })
+            .doOnRequest(request -> {  // request is of type long.
+                if (prefetch == 0) {  //  add "request" number of credits
+                    receiveLink.addCredits((int) request);
+                } else {  // keep total credits "prefetch" if prefetch is not 0.
+                    receiveLink.addCredits(Math.max(0, prefetch - receiveLink.getCredits()));
+                }
             })
             .takeUntilOther(cancelReceiveProcessor)
             .map(message -> {

sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/SynchronousMessageSubscriber.java

@@ -57,8 +57,6 @@ protected void hookOnSubscribe(Subscription subscription) {
 
         if (Operators.setOnce(UPSTREAM, this, subscription)) {
             this.subscription = subscription;
-            remaining.addAndGet(requested);
-            subscription.request(requested);
             subscriberInitialized = true;
             drain();
         } else {
@@ -140,7 +138,6 @@ private void drainQueue() {
 
             while ((currentWork = workQueue.peek()) != null
                 && (!currentWork.isProcessingStarted() || bufferMessages.size() > 0)) {
-
                 // Additional check for safety, but normally this work should never be terminal
                 if (currentWork.isTerminal()) {
                     // This work already finished by either timeout or no more messages to send, process next work.
@@ -155,6 +152,9 @@ private void drainQueue() {
                     // timer to complete the currentWork in case of timeout trigger
                     currentTimeoutOperation = getTimeoutOperation(currentWork);
                     currentWork.startedProcessing();
+                    final long calculatedRequest = currentWork.getNumberOfEvents() - remaining.get();
+                    remaining.addAndGet(calculatedRequest);
+                    subscription.request(calculatedRequest);
                 }
 
                 // Send messages to currentWork from buffer
@@ -174,15 +174,6 @@ private void drainQueue() {
                         currentTimeoutOperation.dispose();
                     }
                     logger.verbose("The work [{}] is complete.", currentWork.getId());
-                } else {
-                    // Since this work is not complete, find out how much we should request from upstream
-                    long creditToAdd = currentWork.getRemaining() - (remaining.get() + bufferMessages.size());
-                    if (creditToAdd > 0) {
-                        remaining.addAndGet(creditToAdd);
-                        subscription.request(creditToAdd);
-                        logger.verbose("Requesting [{}] from upstream for work [{}].", creditToAdd,
-                            currentWork.getId());
-                    }
                 }
             }
         }

sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/implementation/ServiceBusReceiveLinkProcessor.java

@@ -45,7 +45,6 @@ public class ServiceBusReceiveLinkProcessor extends FluxProcessor<ServiceBusRece
     private final Object queueLock = new Object();
     private final AtomicBoolean isTerminated = new AtomicBoolean();
     private final AtomicInteger retryAttempts = new AtomicInteger();
-    private final AtomicBoolean linkCreditsAdded = new AtomicBoolean();
     private final AtomicReference<String> linkName = new AtomicReference<>();
 
     // Queue containing all the prefetched messages.
@@ -200,12 +199,7 @@ public void onNext(ServiceBusReceiveLink next) {
             oldSubscription = currentLinkSubscriptions;
 
             currentLink = next;
-            next.setEmptyCreditListener(() -> {
-                final int creditsToAdd = getCreditsToAdd(0);
-                linkCreditsAdded.set(creditsToAdd > 0);
-
-                return creditsToAdd;
-            });
+            next.setEmptyCreditListener(() -> 0);
 
             currentLinkSubscriptions = Disposables.composite(
                 next.receive().publishOn(Schedulers.boundedElastic()).subscribe(message -> {
@@ -499,6 +493,9 @@ private void drainQueue() {
                     if (receiveMode != ServiceBusReceiveMode.PEEK_LOCK) {
                         pendingMessages.decrementAndGet();
                     }
+                    if (prefetch > 0) { // re-fill messageQueue if there is prefetch configured.
+                        checkAndAddCredits(currentLink);
+                    }
                 } catch (Exception e) {
                     logger.error("Exception occurred while handling downstream onNext operation.", e);
                     throw logger.logExceptionAsError(Exceptions.propagate(
@@ -545,18 +542,14 @@ private void checkAndAddCredits(AmqpReceiveLink link) {
             return;
         }
 
-        // Credits have already been added to the link. We won't try again.
-        if (linkCreditsAdded.getAndSet(true)) {
-            return;
-        }
-
-        final int credits = getCreditsToAdd(link.getCredits());
-        linkCreditsAdded.set(credits > 0);
-
-        logger.info("Link credits to add. Credits: '{}'", credits);
+        synchronized (lock) {
+            final int linkCredits = link.getCredits();
+            final int credits = getCreditsToAdd(linkCredits);
+            logger.info("Link credits='{}', Link credits to add: '{}'", linkCredits, credits);
 
-        if (credits > 0) {
-            link.addCredits(credits);
+            if (credits > 0) {
+                link.addCredits(credits);
+            }
         }
     }
 
@@ -571,22 +564,40 @@ private int getCreditsToAdd(int linkCredits) {
         }
 
         final int creditsToAdd;
-        if (messageQueue.isEmpty() && !hasBackpressure) {
-            creditsToAdd = prefetch;
+        final int expectedTotalCredit;
+        if (prefetch == 0) {
+            if (r <= Integer.MAX_VALUE) {
+                expectedTotalCredit = (int) r;
+            } else {
+                //This won't really happen in reality.
+                //For async client, receiveMessages() calls "return receiveMessagesNoBackPressure().limitRate(1, 0);".
+                //So it will request one by one from this link processor, even though the user's request has no
+                //back pressure.
+                //For sync client, the sync subscriber has back pressure.
+                //The request count uses the the argument of method receiveMessages(int maxMessages).
+                //It's at most Integer.MAX_VALUE.
+                expectedTotalCredit = Integer.MAX_VALUE;
+            }
         } else {
-            synchronized (queueLock) {
-                final int queuedMessages = pendingMessages.get();
-                final int pending = queuedMessages + linkCredits;
-
-                if (hasBackpressure) {
-                    creditsToAdd = Math.max(Long.valueOf(r).intValue() - pending, 0);
-                } else {
-                    // If the queue has less than 1/3 of the prefetch, then add the difference to keep the queue full.
-                    creditsToAdd = minimumNumberOfMessages >= queuedMessages
-                        ? Math.max(prefetch - pending, 1)
-                        : 0;
-                }
+            expectedTotalCredit = prefetch;
+        }
+        logger.info("linkCredits: '{}', expectedTotalCredit: '{}'", linkCredits, expectedTotalCredit);
+
+        synchronized (queueLock) {
+            final int queuedMessages = pendingMessages.get();
+            final int pending = queuedMessages + linkCredits;
+
+            if (hasBackpressure) {
+                creditsToAdd = Math.max(expectedTotalCredit - pending, 0);
+            } else {
+                // If the queue has less than 1/3 of the prefetch, then add the difference to keep the queue full.
+                creditsToAdd = minimumNumberOfMessages >= queuedMessages
+                    ? Math.max(expectedTotalCredit - pending, 0)
+                    : 0;
             }
+            logger.info("prefetch: '{}', requested: '{}', linkCredits: '{}', expectedTotalCredit: '{}', queuedMessages:"
+                    + "'{}', creditsToAdd: '{}', messageQueue.size(): '{}'", getPrefetch(), r, linkCredits,
+                expectedTotalCredit, queuedMessages, creditsToAdd, messageQueue.size());
         }
 
         return creditsToAdd;

sdk/servicebus/azure-messaging-servicebus/src/test/java/com/azure/messaging/servicebus/ServiceBusClientBuilderTest.java

@@ -202,7 +202,7 @@ void invalidPrefetch() {
             .receiveMode(ServiceBusReceiveMode.PEEK_LOCK);
 
         // Act & Assert
-        assertThrows(IllegalArgumentException.class, () -> receiverBuilder.prefetchCount(0));
+        assertThrows(IllegalArgumentException.class, () -> receiverBuilder.prefetchCount(-1));
     }
 
     @MethodSource("getProxyConfigurations")