Reactor Gateways

The "Reactive Gateways" offer a reactive API wrapper around the command, query and event bus. Most of the operations are similar to those from non-reactive gateways, simply replacing the CompletableFuture with either a Mono or Flux. In some cases, the API is expended to ease use of common reactive patterns.

Reactor doesn’t allow null values in streams. Any null value returned from the handler will be mapped to Mono#empty().
Retrying operations

All operations support Reactor’s retry mechanism:

reactiveQueryGateway.query(query, ResponseType.class).retry(5);

This call will retry sending the query a maximum of five times when it fails.

Configuration in SpringBoot

This extension can be added as a Spring Boot starter dependency to your project using group id org.axonframework.extensions.reactor and artifact id axon-reactor-spring-boot-starter. The implementation of the extension can be found here.

Reactor command gateway

This section describes the methods on the ReactorCommandGateway.

send

Sends the given command once the caller subscribes to the command result. Returns immediately.

A common pattern is using the REST API to send a command. In this case it is recommend to for example use WebFlux, and return the command result Mono directly to the controller:

Sending a command from a Spring WebFlux Controller.
class SpringCommandController {

    private final ReactorCommandGateway reactiveCommandGateway;

    @PostMapping
    public Mono<CommandHandlerResponseBody> sendCommand(@RequestBody CommandBody command) {
        return reactiveCommandGateway.send(command);
    }
}
If the command handling function returns type void, Mono<CommandHandlerResponseBody> should be replaced with Mono<Void>

Another common pattern is "send and forget":

Function that sends a command and returns immediately without waiting for the result.
class CommandDispatcher {

    private final ReactorCommandGateway reactiveCommandGateway;

    public void sendAndForget(MyCommand command) {
         reactiveCommandGateway.send(command)
                               .subscribe();
    }
}

sendAll

This method uses the given Publisher of commands to dispatch incoming commands.

This operation is available only in the Reactor extension. Use it to connect third-party streams that delivers commands.
Connects an external input stream directly to the Reactor Command Gateway.
class CommandPublisher {

    private final ReactorCommandGateway reactiveCommandGateway;

    @PostConstruct
    public void startReceivingCommands(Flux<CommandBody> inputStream) {
        reactiveCommandGateway.sendAll(inputStream)
                              .subscribe();
    }
}
The sendAll operation will keep sending commands until the input stream is canceled.
send and sendAll do not offer any backpressure, yet. The only backpressure mechanism in place is that commands will be sent sequentially; thus once the result of a previous command arrives. The number of commands is prefetched from an incoming stream and stored in a buffer for sending (see Flux#concatMap). This slows down sending, but does not guarantee that the Subscriber will not be overwhelmed with commands if they are sent too fast.

Reactor query gateway

query

Sends the given query, expecting a response in the form of responseType from a single source.

Recommended way of using the Reactor query gateway within a Spring REST controller.
class SpringQueryController {

    private final ReactorQueryGateway reactiveQueryGateway;

    // The query's Mono is returned to the Spring controller. Subscribe control is given to Spring Framework.
    @GetMapping
    public Mono<SomeResponseType> findAll(FindAllQuery query, Class<SomeResponseType> responseType) {
        return reactiveQueryGateway.query(query, responseType);
    }
}

scatterGather

Sends the given query, expecting a response in the form of responseType from several sources within a specified duration.

Sends a given query that stops after receiving three results, or after 5 seconds.
class SpringQueryController {

    private final ReactorQueryGateway reactiveQueryGateway;

    @GetMapping
    public Flux<SomeResponseType> findMany(FindManyQuery query) {
        return reactiveQueryGateway.scatterGather(query, SomeResponseType.class, Duration.ofSeconds(5)).take(3);
    }
}

Subscription queries

Firstly, the Reactor API for subscription queries in Axon is not new.

However, we noticed several patterns which are often used, such as:

  • Concatenating initial results with query updates in a single stream, or

  • skipping the initial result all together.

As such, the Reactor Extension provides several methods to ease usage of these common patterns.

subscriptionQuery

Sends the given query, returns the initial result and keeps streaming incremental updates until a subscriber unsubscribes from the Flux.

Note that this method should be used when the response type of the initial result and incremental update match.

Flux<ResultType> resultFlux = reactiveQueryGateway.subscriptionQuery("criteriaQuery", ResultType.class);

The above invocation through the ReactorQueryGateway is equivalent to:

class SubscriptionQuerySender {

    private final ReactorQueryGateway reactiveQueryGateway;

    public Flux<SomeResponseType> sendSubscriptionQuery(SomeQuery query, Class<SomeResponseType> responseType) {
        return reactiveQueryGateway.subscriptionQuery(query, responseType, responseType)
                                   .flatMapMany(result -> result.initialResult()
                                                                .concatWith(result.updates())
                                                                .doFinally(signal -> result.close()));
    }
}

subscriptionQueryMany

Sends the given query, returns the initial result and keeps streaming incremental updates until a subscriber unsubscribes from the Flux.

This operation should be used when the initial result contains multiple instances of the response type which needs to be flattened. Additionally, the response type of the initial response and incremental updates need to match.

Flux<ResultType> resultFlux = reactiveQueryGateway.subscriptionQueryMany("criteriaQuery", ResultType.class);

The above invocation through the ReactorQueryGateway is equivalent to:

class SubscriptionQuerySender {

    private final ReactorQueryGateway reactiveQueryGateway;

    public Flux<SomeResponseType> sendSubscriptionQuery(SomeQuery query, Class<SomeResponseType> responseType) {
        return reactiveQueryGateway.subscriptionQuery(query,
                                                      ResponseTypes.multipleInstancesOf(responseType),
                                                      ResponseTypes.instanceOf(responseType))
                                   .flatMapMany(result -> result.initialResult()
                                                                .flatMapMany(Flux::fromIterable)
                                                                .concatWith(result.updates())
                                                                .doFinally(signal -> result.close()));
    }
}

queryUpdates

Sends the given query and streams incremental updates until a subscriber unsubscribes from the Flux.

This method could be used when subscriber is only interested in updates.

Flux<ResultType> updatesOnly = reactiveQueryGateway.queryUpdates("criteriaQuery", ResultType.class);

The above invocation through the ReactorQueryGateway is equivalent to:

class SubscriptionQuerySender {

    private final ReactorQueryGateway reactiveQueryGateway;

    public Flux<SomeResponseType> sendSubscriptionQuery(SomeQuery query, Class<SomeResponseType> responseType) {
        return reactiveQueryGateway.subscriptionQuery(query, ResponseTypes.instanceOf(Void.class), responseType)
                                   .flatMapMany(result -> result.updates()
                                                                .doFinally(signal -> result.close()));
    }
}
In the above shown methods, the subscription query is closed automatically after a subscriber has unsubscribed from the Flux. When using the regular QueryGateway, the subscription query needs to be closed manually however.

Reactor event gateway

Reactive variation of the EventGateway. Provides support for reactive return types such as Flux.

publish

Publishes the given events once the caller subscribes to the resulting Flux.

This method returns events that were published. Note that the returned events may be different from those the user has published, granted an interceptor has been registered which modifies events.

Example of dispatcher modified events, returned to user as the result Flux.
class EventPublisher {

    private final ReactorEventGateway reactiveEventGateway;

    // Register a dispatch interceptor to modify the event messages
    public EventPublisher() {
        reactiveEventGateway.registerDispatchInterceptor(
            eventMono -> eventMono.map(event -> GenericEventMessage.asEventMessage("intercepted" + event.getPayload()))
        );
    }

    public void publishEvent() {
        Flux<Object> result = reactiveEventGateway.publish("event");
    }
}

Interceptors

Axon provides a notion of interceptors. The Reactor gateways allow for similar interceptor logic, namely the ReactorMessageDispatchInterceptor and ReactorResultHandlerInterceptor.

These interceptors allow us to centrally define rules and filters that will be applied to a message stream.

Interceptors will be applied in order they have been registered to the given component.

React dispatch interceptors

The ReactorMessageDispatchInterceptor should be used to centrally apply rules and validations for outgoing messages. Note that a ReactorMessageDispatchInterceptor is an implementation of the default MessageDispatchInterceptor interface used throughout the framework. The implementation of this interface is described as follows:

@FunctionalInterface
public interface ReactorMessageDispatchInterceptor<M extends Message<?>> extends MessageDispatchInterceptor<M> {

    Mono<M> intercept(Mono<M> message);

    @Override
    default BiFunction<Integer, M, M> handle(List<? extends M> messages) {
        return (position, message) -> intercept(Mono.just(message)).block();
    }
}

It thus defaults the MessageDispatchInterceptor#handle(List<? extends M> method to utilize the ReactorMessageDispatchInterceptor#intercept(Mono<M>) method. As such, a ReactorMessageDispatchInterceptor could thus be configured on a plain Axon gateway too. Here are a couple of examples how a message dispatch interceptor could be used:

Dispatch interceptor that adds key-value pairs to the message’s MetaData.
class ReactorConfiguration {

    public void registerDispatchInterceptor(ReactorCommandGateway reactiveGateway) {
        reactiveGateway.registerDispatchInterceptor(
            msgMono -> msgMono.map(msg -> msg.andMetaData(Collections.singletonMap("key1", "value1")))
        );
    }
}
Dispatch interceptor that discards the message, based on a security flag in the Reactor Context.
class ReactorConfiguration {

    public void registerDispatchInterceptor(ReactorEventGateway reactiveGateway) {
        reactiveGateway.registerDispatchInterceptor(
            msgMono -> msgMono.filterWhen(v -> Mono.subscriberContext()
                              .filter(ctx-> ctx.hasKey("security"))
                              .map(ctx->ctx.get("security")))
        );
    }
}

Reactor result handler interceptors

The ReactorResultHandlerInterceptor should be used to centrally apply rules and validations for incoming messages, a.k.a. results. The implementation of this interface is described as follows:

@FunctionalInterface
public interface ReactorResultHandlerInterceptor<M extends Message<?>, R extends ResultMessage<?>> {

    Flux<R> intercept(M message, Flux<R> results);
}

The parameters are the message that has been sent, and a Flux of results from that message, which is going to be intercepted. The message parameter can be useful if you want to apply a given result rule only for specific messages. Here are a couple of examples how a message result interceptor could be used:

This type of interceptor is available only in the Reactor Extension.
Result interceptor which discards all results that have a payload matching blockedPayload
class ReactorConfiguration {

    public void registerResultInterceptor(ReactorQueryGateway reactiveGateway) {
        reactiveGateway.registerResultHandlerInterceptor(
            (msg, results) -> results.filter(r -> !r.getPayload().equals("blockedPayload"))
        );
    }
}
Result interceptor which validates that the query result does not contain an empty String.
class ReactorConfiguration {

    public void registerResultInterceptor(ReactorQueryGateway reactiveGateway) {
        reactiveQueryGateway.registerResultHandlerInterceptor(
            (query, results) -> results.flatMap(r -> {
                if (r.getPayload().equals("")) {
                    return Flux.<ResultMessage<?>>error(new RuntimeException("no empty strings allowed"));
                } else {
                    return Flux.just(r);
                }
            })
        );
    }
}
Result interceptor which discards all results where the queryName matches myBlockedQuery.
class ReactorConfiguration {

    public void registerResultInterceptor(ReactorQueryGateway reactiveGateway) {
        reactiveQueryGateway.registerResultHandlerInterceptor(
            (q, results) -> results.filter(it -> !((boolean) q.getQueryName().equals("myBlockedQuery")))
        );
    }
}
Result interceptor which limits the result waiting time to thirty seconds per message.
class ReactorConfiguration {

    public void registerResultInterceptor(ReactorCommandGateway reactiveGateway) {
        reactiveGateway.registerResultHandlerInterceptor(
            (msg,results) -> results.timeout(Duration.ofSeconds(30))
        );
    }
}
Result interceptor which limits the number of results to five entries, and logs all results.
class ReactorConfiguration {

    public void registerResultInterceptor(ReactorCommandGateway reactiveGateway) {
        reactiveGateway.registerResultHandlerInterceptor(
            (msg,results) -> results.log().take(5)
        );
    }
}