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Channel Decorators

SmallRye Reactive Messaging supports decorating reactive streams of incoming and outgoing channels for implementing cross-cutting concerns such as monitoring, tracing or message interception.

Two symmetrical APIs are proposed for decorating publisher and subscriber channels, PublisherDecorator and SubscriberDecorator respectively.

Important

@Incoming channels and channels bound to an outbound connector are both Subscribers. Conversely @Outgoing channels and channels bound to an inbound connector are Publishers.

For example, to provide a decorator which counts consumed messages from incoming connector, implement a bean exposing the interface PublisherDecorator:

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@ApplicationScoped
public class ConsumedMessageDecorator implements PublisherDecorator {

    private final Map<String, AtomicLong> counters = new HashMap<>();

    @Override
    public Multi<? extends Message<?>> decorate(Multi<? extends Message<?>> publisher, String channelName,
            boolean isConnector) {
        if (isConnector) {
            AtomicLong counter = new AtomicLong();
            counters.put(channelName, counter);
            return publisher.onItem().invoke(counter::incrementAndGet);
        } else {
            return publisher;
        }
    }

    @Override
    public int getPriority() {
        return 10;
    }

    public long getMessageCount(String channel) {
        return counters.get(channel).get();
    }
}

Decorators' decorate method is called only once per channel at application deployment when graph wiring is taking place. Decorators are very powerful because they receive the stream of messages (Mutiny Multi<Message<?>>) and potentially return a new stream of messages.

Note that if a decorator is available it will be called on every channel. The decorate method receives the channel name and whether the channel is a connector or not as parameters. Decorators are called ordered from highest to lowest priority (from the least value to the greatest), obtained using the jakarta.enterprise.inject.spi.Prioritized#getPriority method.

Note

The SubscriberDecorator receive a list of channel names because @Incoming annotation is repeatable and consuming methods can be linked to multiple channels.

Intercepting Outgoing Messages

Decorators can be used to intercept and alter messages, both on incoming and outgoing channels. Smallrye Reactive Messaging provides a SubscriberDecorator by default to allow intercepting outgoing messages for a specific channel.

To provide an outgoing interceptor implement a bean exposing the interface OutgoingInterceptor, qualified with a @Identifier with the channel name to intercept. Only one interceptor is allowed to be bound for interception per outgoing channel. If no interceptors are found with a @Identifier but a @Default one is available, it is used. When multiple interceptors are available, the bean with the highest priority is used.

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package interceptors;

import jakarta.enterprise.context.ApplicationScoped;

import org.eclipse.microprofile.reactive.messaging.Message;

import io.smallrye.common.annotation.Identifier;
import io.smallrye.reactive.messaging.OutgoingInterceptor;
import io.smallrye.reactive.messaging.OutgoingMessageMetadata;

@Identifier("channel-a")
@ApplicationScoped
public class MyInterceptor implements OutgoingInterceptor {

    @Override
    public Message<?> onMessage(Message<?> message) {
        return message.withPayload("changed " + message.getPayload());
    }

    @Override
    public void onMessageAck(Message<?> message) {
        message.getMetadata(OutgoingMessageMetadata.class)
                .ifPresent(m -> m.getResult());
    }

    @Override
    public void onMessageNack(Message<?> message, Throwable failure) {

    }
}

An OutgoingInterceptor can implement these three methods:

  • Message<?> onMessage(Message<?> message) : Called before passing the message to the outgoing connector for transmission. The message can be altered by returning a new message from this method.
  • void onMessageAck(Message<?> message) : Called after message acknowledgment. This callback can access OutgoingMessageMetadata which will hold the result of the message transmission to the broker, if supported by the connector. This is only supported by MQTT and Kafka connectors.
  • void onMessageNack(Message<?> message, Throwable failure) : Called before message negative-acknowledgment.

Note

If you are willing to adapt an incoming message payload to fit a consuming method receiving type, you can use MessageConverters.