EventMaid

EventMaid is a library for building messaging architectures.

It provides components to integrate parts of your business logic in a loosely coupled fashion. This allows for applications to be highly extensible and easily tested.

Maven Dependency

<dependency>
    <groupId>de.quantummaid.eventmaid</groupId>
    <artifactId>core</artifactId>
    <version>...</version>
</dependency>

Motivation

Messaging is a form of communication, that exchanges encapsulated messages between parts of an application or different applications. In contrast to other types of integration it models both the carrier and the send messages as distinct concepts of the application. This can provide several benefits if used correctly. But it can also generate overhead and complexity. This library focuses on a lightweight implementation of messaging patterns to be wire use cases and objects within one application. For integrating different applications (over network, shared memory, etc.) other libraries exist.

This library provides implementations for typical forms of message carriers like Channels or MesssageBus. Explicitly modelling these transport mechanisms as distinct objects have an beneficiary influence on the coupling of independent parts of the application. When parts of the application want to communicate with each other, they send messages. The sender puts its message on the MessageBus. The MessageBus then ensures, that the message is delivered to all subscribers. The sender does not need to know the number or type of the subscribers. The subscribers have no knowledge about the sender. This leads to a very loosely coupled integration. As sender and subscriber can be added or removed dynamically during runtime, the application becomes very flexible.

Both, MessageBus and Channel, can be configured to provide asynchronous aspects using Threads. This simplifies code using these objects, as a lot of asynchronous and synchronization problems are already solved. Dynamically scaling out Threads only required to change the configuration Channels and MessageBus. The rest of the application can remain mostly agnostic to it.

These messaging patterns ease the integration of Frameworks with the application's use cases. But also the communication between use cases is greatly simplified with a MessageBus. Even at the scope of Domain Objects messaging patterns can provide loosely coupling and dynamism.

Installation

To use EventMaid add the following Maven depedency to your 'pom.xml`:

<dependency>
    <groupId>de.quantummaid.eventmaid</groupId>
    <artifactId>core</artifactId>
    <version>0.9.14</version>
</dependency>

Basic Concepts

Channel

A common task in message driven architectures is sending messages from a bunch of producers to an arbitrary amount of consumers, handling errors and allowing to add Filters dynamically. In EventMaid Channels provide these kind of properties:

• add or remove sender and receiver dynamically
• define the type of send messages, when sender and receiver have agreed on the format of the send messages
• change the messages during the transportation via Filter: changing the contents of the message, blocking invalid messages,...
• abstract configuration: once the Channel is created, the participants should be agnostic about the used configuration, e.g. whether the Channel is asynchronous
• dynamic extension points: add Filter, add logging or even replace the subscriber during tests
• monitoring: get information about the number of messages that got delivered successful, blocked or failed with exceptions

Creating a Channel

Channels can be created using the ChannelBuilder class:

Channel<TestMessage> channel = ChannelBuilder.aChannel(TestMessage.class)
    .forType(ChannelType.SYNCHRONOUS)
    .withDefaultAction(Consume.consumeMessage(m -> {
        System.out.println(m);
    }))
    .build();

channel.send(new TestMessage());

Channels can be of type SYNCHRONOUS (which is the default) or of type ASYNCHRONOUS. Synchronous Channels will execute the delivery on the Thread calling send. Asynchronous Channels bring their own Threadpool with them. For more details on how to create and configure asynchronous Channels see Configuring the Channel

At the end of a Channel an Action is executed for each message. Actions abstract the consumer part of the messaging. The simplest Action is the Consume Action which executes the given logic for each message that reached the end of the Channel. Other Actions allow dynamic subscriptions or jumps to other Channels.

Actions

Each message reaching the end of the Channel will be consumed by an Action. This can be the default Action defined during creation time or a dynamically changed by Filter (explained below). Several default Actions exist:

Consume

A Consume Action calls the given Consumer function for every message that reached the end of the Channel:

Action<T> action = Consume.consumeMessage(processingContext -> {
    T payload = processingContext.getPayload();
    System.out.println(payload);
});

A shortcut exists, if only the payload is needed:

Action<T> action = Consume.consumePayload(payload -> {
    System.out.println(payload);
});

Subscription

The Subscription Action extends the Consume Action with the ability of having several consumers, called Subscriber. The Subscription Action allows adding and removing Subscriber dynamically:

Subscription<Object> subscription = Subscription.subscription();

Consumer<Object> consumer = message -> System.out.println(message);

SubscriptionId subscriptionId = subscription.addSubscriber(consumer);

subscription.removeSubscriber(subscriptionId);

Subscriber<Object> subscriber = ConsumerSubscriber.consumerSubscriber(consumer);
SubscriptionId subscriptionId = subscription.addSubscriber(subscriber);
subscription.removeSubscriber(subscriber);

boolean notEmpty = subsription.hasSubscribers();

The addSubscriber method is overloaded to accept either a java Consumer or a Subscriber object. Classes implementing this interface get more control over the management of the SubscriptionId or the acceptance of messages, e.g. they can preempt the delivery, so that other subscriber do receive the message. See Subscriber for more details. The addSubscriber methods returns a SubscriptionId, which is an unique UUID generated for each Subscriber. It can be used to uniquely identify a Subscriber. The removeSubscriber method makes use of this to remove subscriptions.

Jump

In more complex messaging architectures larger processing logics are often split into smaller, logical pieces by chaining Channels. Each Channel is then responsible for a smaller part in this flow. These Channels can be connected via Jump Actions. A Jump Action takes a message and sends it on the next Channel:

Channel<T> nextChannel = ...;
Jump<T> jump = Jump.jumpTo(nextChannel);

The reason to use Jumps and not a Consume calling send on the next Channel is the control structure used in Channels. Messages send over Channels are enveloped in ProcessingContext objects. These context objects contain history information over past Channels useful for debugging. The Jump Action handles these context information during the change of Channels (For more information about the ProcessingContext object see Processing Context).

Adding Filter to Channel

Channels provide an extensible mechanism for processing messages: Filter. A send message traverses all Filter before being consumed by the final Action. Each Filter has two options: It can allow the message to pass or it can block the message. A blocked message will stop its propagation through the remaining Filter and will never reach the final Action:

channel.addProcessFilter((processingContext, filterActions) -> {
    TestMessage message = processingContext.getPayload();
    if (isValid()) {
        message.validated = true;
        filterActions.pass(processingContext);
    } else {
        filterActions.block(processingContext);
    }
});

Calling filterActions.pass will propagate the message to the next Filter. filterActions.block will stop the propagation. If none of these methods are called, the message is also blocked. But not calling the block method should be avoided as Filter should be written as explicit as possible (Also the message is marked as forgotten and not as blocked in the ChannelStatistics).

As mentioned earlier, each message is always enveloped in a ProcessingContext control structure. To get access to the original message use getPayload. But the pass and block methods again expect the ProcessingContext object. Filter can freely access the ProcessingContext object. The most common usage would be to replace the Action, that is executed at the Channel's end:

channel.addPostFilter((processingContext, filterActions) -> {
    if(!processingContext.actionWasChanged()) {
        processingContext.changeAction(Consume.consumeMessage(m -> {}));
    }
});

Filter can be added at three different stages: Pre, Process, Post. These three different extension points serve as coarse-grained ordering. All Filter in the Pre Stage are always executed before the Filter in the Process stage, which themselves execute before the Post Filter. Within these stages the order of Filter follows the contract of java's concurrent list.

For each of three stages methods exists to query registered Filter and to remove Filter:

List<Filter<ProcessingContext<T>>> preFilter = channel.getPreFilter();
List<Filter<ProcessingContext<T>>> processFilter = channel.getProcessFilter();
List<Filter<ProcessingContext<T>>> postFilter = channel.getPostFilter();
        
channel.removePreFilter(filter);
channel.removeProcessFilter(filter);
channel.removePostFilter(filter);        

Call and Return

A special Action that can only be used inside a Filter is the Call Action. It is used to perform an immediate jump to a different Channel. The transport of the message is resumed the moment the other Channel executes a Return as it's final Action. This Call/Return combination allows Filter to add arbitrarily complex logic dynamically to a Channel.

Channel<TestMessage> differentChannel = ChannelBuilder.aChannel(TestMessage.class)
    .withDefaultAction(Return.aReturn())
    .build();

channel.addPostFilter((processingContext, filterActions) -> {
    Call.callTo(differentChannel, processingContext);
    System.out.println("Returned from other Channel.");
    filterActions.pass(processingContext);
});

Calls can be nested arbitrarily and don't need to return. But executing a Return without a previous Call will result in an error.

The factory method Call.callTo executes the Call directly. If access to the Call object is needed, a two step alternative exists:

channel.addPostFilter((processingContext, filterActions) -> {
    final Call<TestMessage> call = Call.prepareACall(differentChannel);
    doSomethingWith(call);
    call.execute(processingContext);
});

Once a Call and its matching Return object was executed, both objects are linked to each other:

//suppose these are the two related actions
Return<Object> returnAction = Return.aReturn();
Call<Object> callAction = Call.prepareACall(otherChannel);


ChannelProcessingFrame<Object> returnFrame = callAction.getReturnFrame();
assertThat(returnFrame.getAction(), equalTo(returnAction));

ChannelProcessingFrame<Object> callFrame = returnAction.getRelatedCallFrame();
assertThat(callFrame.getAction(), equalTo(callAction));

Channel<Object> callActionTargetChannel = callAction.getTargetChannel();

Channel Statistics

Each Channel provides basic logging in form of statistics itself: It logs the number of messages, that were

• accepted: message was received by Channel and transport was started. A message is always accepted or an exception is thrown
• queued: asynchronous Channel can queue messages, if no Threads are available. This statistic resembles the number of currently waiting messages
• blocked: number of messages that were blocked by Filter
• forgotten: number of messages that were neither passed nor blocked by Filter
• successful: number of messages that passed all Filter and executed the final Action without error
• failed: if an exception is thrown during a Filter or the final Action, the message is marked as failed

ChannelStatusInformation statusInformation = channel.getStatusInformation();
ChannelStatistics statistics = statusInformation.getChannelStatistics();
BigInteger acceptedMessages = statistics.getAcceptedMessages();
BigInteger queuedMessages = statistics.getQueuedMessages();
BigInteger blockedMessages = statistics.getBlockedMessages();
BigInteger forgottenMessages = statistics.getForgottenMessages();
BigInteger successfulMessages = statistics.getSuccessfulMessages();
BigInteger failedMessages = statistics.getFailedMessages();
Date timestamp = statistics.getTimestamp();

Each statistic contains a timestamp indicating the date, when the given numbers were approximately valid.

Closing the Channel

Each Channel can be closed to free resources in case the Channel was stateful (Asynchronous Channels are stateful). The following methods exists:

boolean finishRemainingTasks = true;
channel.close(finishRemainingTasks);

boolean closed = channel.isClosed();

try {
    boolean terminationSucceeded = channel.awaitTermination(5, MILLISECONDS);
} catch (InterruptedException e) {
    
}

These methods follow the contract, that classes from the standard java library with these sort of methods abide to. Channel (as all closable Classes in EventMaid) also implement the AutoClosable interface and can be used in a try-with-resources statement.

Configuring the Channel

Configuring a Channel is done using the respective ChannelBuilder class' methods.

ChannelBuilder.aChannel()
.forType(ChannelType.ASYNCHRONOUS)
.withAsynchronousConfiguration(asyncConfig)
.withDefaultAction(Subscription.subscription())
.withChannelExceptionHandler(customExceptionHandler)
.withActionHandlerSet(customActionHandlerSet)
.build();

The available Actions were discussed in Actions

Type

There exists two types of Channels: ChannelType.SYNCHRONOUSand ChannelType.ASYNCHRONOUS. Sending on synchronous Channels is executed on the Thread calling send. Asynchronous Channels provide their own Threads using a Threadpool. Asynchronous Channels require an additional AsynchronousConfiguration.

There exists two convenience methods to ease the creation of a fitting asynchronous configuration:

int numberOfThreads = 5;
AsynchronousConfiguration.constantPoolSizeAsynchronousPipeConfiguration(numberOfThreads);
int maximumBoundOfQueuedMessages = 100;
AsynchronousConfiguration.constantPoolSizeAsynchronousPipeConfiguration(numberOfThreads, maximumBoundOfQueuedMessages);

In case a more fine-tuned configuration is needed, two constructor and getter are provided:

AsynchronousConfiguration configuration = new AsynchronousConfiguration();
configuration.setCorePoolSize(5);

int corePoolSize = 5;
int maximumPoolSize = 10;
int maximumTimeout = 15;
TimeUnit timeUnit = MILLISECONDS;
LinkedBlockingQueue<Runnable> threadPoolWorkingQueue = new LinkedBlockingQueue<>();
new AsynchronousConfiguration(corePoolSize, maximumPoolSize, maximumTimeout, timeUnit, threadPoolWorkingQueue);

These configuration properties are identically to those available for the java ThreadPoolExecutor class, as the asynchronous Channel uses such underneath. For a comprehensive documentation please consult the java doc of the ThreadPoolExecutor class.

ChannelExceptionHandler

The default exception behaviour is to throw each exception on the Thread it occurs on. This might not be sufficient for a multi-threaded configuration. Therefore a custom exception handler can be set, that gets access to all internal exceptions.

ChannelExceptionHandler<T> channelExceptionHandler = new ChannelExceptionHandler<T>() {
    @Override
    public boolean shouldSubscriberErrorBeHandledAndDeliveryAborted(ProcessingContext<T> message, Exception e) {
        boolean abortDeliveryAndHandleError = true;
        return abortDeliveryAndHandleError;
    }

    @Override
    public void handleSubscriberException(ProcessingContext<T> message, Exception e) {

    }

    @Override
    public void handleFilterException(ProcessingContext<T> message, Exception e) {

    }
};

When an exception occurs during the accept method of a subscriber, first the shouldSubscriberErrorBeHandledAndDeliveryAborted method is called. This method can decide whether the exception should count as such and the delivery should be aborted. A true results in the message being marked as failed in the statistics. No further subscriber gets the message delivered and the handleSubscriberException method is called in the end. Given a false the exception is ignored and the delivery continues normally.

In case of a Filter throwing an exception, the handleFilterException method is called. An exception inside a Filter always counts as failed and aborts the propagation to subsequent Filter or any final Action.

ActionHandlerSet

Actions serve only as representative container for the information necessary to execute them. Any logic regarding their execution is handled by the ActionHandlers. This allows exchanging logic without changing Actions and makes debuging easier. The ActionHandlerSet contains one handler for each Action.

When a message reaches the end of a Channel, the ActionHandlerSet serves as a lookup object for an ActionHandler matching the Channel's final Action. When a suitable handler is found, its handle method is called. When no handler is registered an exception is thrown.

ActionHandlerSet<Object> defaultActionHandlerSet = DefaultActionHandlerSet.defaultActionHandlerSet();
defaultActionHandlerSet.registerActionHandler(CustomAction.class, new CustomActionHandler());
        
ChannelBuilder.aChannel()
.withActionHandlerSet(actionHandlerSet);

A more in depth explanation about writing custom Actions and ActionHandlerSets is given in Custom Actions.

Processing Context

Channels can be chained into arbitrary complex structures. The Channels are connected via Actions (and Calls inside Filter). Filters within these Channels might share data or the history might be of interest for debugging purpose. Since these type of information should not be stored inside the payload itself, a wrapping context object is needed, the ProcessingContext.

Each message contains its own ProcessingContext object. It wraps the message's payload and optional error payload. Each ProcessingContext creates a new unique MessageId for each message. Additionally, an optional CorrelationId can be set or derived from the MessageId. CorrelationIds are used heavily in combination with a MessageBus to link related messages to each other. Also more used in the context of a MessageBus is the ProcessingContext's EventType. The MessageBus explained later uses these EventTypes to decide, to which subscribers the current message should be routed. Each ProcessingContext also brings a meta data map from type Map<Object, Object> to store additional data about the message, which does not belong in the payload.

T payload = processingContext.getPayload();
Object errorPayload = processingContext.getErrorPayload();
EventType eventType = processingContext.getEventType();
Map<Object, Object> metaData = processingContext.getContextMetaData();

MessageId messageId = processingContext.getMessageId();
CorrelationId correlationId = processingContext.getCorrelationId();
CorrelationId correlationIdForMessage = processingContext.generateCorrelationIdForAnswer();
assertTrue(correlationIdForMessage.matches(messageId));

The history of Channels is represented as a linked list of ChannelProcessingFrames. This list includes a frame for each traversed Channel. Each frame contains a reference to its previous and next frame and to its respective Channel. When a Channel is traversed to its end, the actual final Action is also stored in the frame. The ProcessingContext object serves as root object referencing the first, initial frame and the frame of the currently traversed Channel.

ChannelProcessingFrame<T> initialProcessingFrame = processingContext.getInitialProcessingFrame();
Channel<T> channel = initialProcessingFrame.getChannel();
ChannelProcessingFrame<T> previousFrame = initialProcessingFrame.getPreviousFrame();
ChannelProcessingFrame<T> nextFrame = initialProcessingFrame.getNextFrame();
Action<T> executedAction = initialProcessingFrame.getAction();

Calls are also included in the linked ChannelProcessingFrames list, although stored a little bit differently. Let's suppose we have 4 Channels:

• Channel A contains a Filter executing a Call to Channel B. The default Action of Channel A is a Jump to Channel D
• Channel B is the target of the Call within Channel A. As default Action a Jump to Channel C is executed.
• Channel C just executes a Return as Action returning the control to Channel A
• Channel D is the last Channel with Consume as Action.

The linked list of ChannelProcessingFrames would consist of the following 5 entries:

1. a frame for Channel A with a Action Call as soon as the Call is executed
2. a frame for Channel B with the default Action Jump to Channel C
3. a frame for Channel C with the Action Return back to Channel A
4. a frame for Channel A with the default Action Jump to Channel D
5. a frame for Channel D with Consume as final Action

So in general one frame is added per Channel, except for a Call. In this case an extra ChannelProcessingFrames is added to indicate the branching of the flow.

Additionally the ProcessingContext object provides a MetaDataMap for sharing data between Channels or Filter.

ProcessingContext<Object> processingContext = ProcessingContext.processingContext(message);

Map<Object, Object> metaData = processingContext.getContextMetaData();

MessageBus

Channels are restricted to a specific type. This can be a benefit as the format of the communication between producer and consumer is defined by the Channel itself. But this solution comes short when several formats or communications are to be supported by the same transport object.

The solution is a MessageBus. Any type of message can be send over a MessageBus. Subscribers are then able to pick the type of messages they are interested in via type-based subscription. This makes integrating distinct parts of an application possible.

A MessageBus is structured as follows:

Every message is accepted by the AcceptingChannel. The AcceptingChannel is responsible for the configuration (synchronous or asynchronous) and can also contain Filter that need access to all messages. Messages, that passed the AcceptingChannel, are distributed into subscriber-specific Channels. Every EventType, which has at least on subscriber, corresponds to a Channel, that delivers all message of its type to its subscribers. On this Channel Filter can be added, that are specific for all messages of this EventType.

Using the MessageBus

MessageBus messageBus = MessageBusBuilder.aMessageBus()
    .forType(MessageBusType.SYNCHRONOUS)
    .build();


final EventType eventType = eventTypeFromString("requestX");
 SubscriptionId subscriptionId = messageBus.subscribe(eventType, o -> {
     System.out.println(o);
 });
        
 messageBus.send(eventType, new TestMessage());
        
messageBus.unsubcribe(subscriptionId);

The MessageBusBuilder is used to configure and create a MessageBus. The subscribe method is again overloaded to either aaccept a Subscriber or a java consumer. The first parameter defines the EventType of the subscription. All messages of this type are delivered to all subscribers for this type. The returned subscriptionId is used in case the subscriber should should be removed to not received any more messages.

As discussed in ProcesscingContext earlier, messages can contain a CorrelationId to express, that a set of messages are related. The typical use case is sending a response/answer to a previous request. The MessageBus allows sending and subscribing messages based on CorrelationIds.

CorrelationId correlationId = CorrelationId.newUniqueCorrelationId();
messageBus.subscribe(correlationId, processingContext -> {
    Object payload = processingContext.getPayload();
    System.out.println(payload);
});

EventType eventType = EventType.eventTypeFromString("answerForX");
messageBus.send(eventType, new TestMessage(), correlationId);

The CorrelationId based subscriber gets access to the complete ProcessingContext object, as it holds the MessageId and the CorrelationId. For the EventType based subscribe function, a subscribeRaw version exists, in case the normal subscriber needs access to the ProcessingContext object:

messageBus.subscribeRaw(eventType, processingContext -> {
    Object payload = processingContext.getPayload();
    System.out.println(payload);
});

Adding Filter to the MessageBus

The MessageBus can add Filters, that get access to all messages:

final Filter<Object> filter = new Filter<Object>() {
    @Override
    public void apply(Object message, FilterActions<Object> filterActions) {
        //filter logic
    }
};
messageBus.add(filter);

List<Filter<Object>> allFilter = messageBus.getFilter();

messageBus.remove(filter);

In case a more fine-grained filtering is needed, the MessageBus allows to query for the specific Channel for a given tyoe. On this Channel Filter can be added as already described in Filter

MessageBusStatusInformation statusInformation = messageBus.getStatusInformation();
Channel<TestMessage> channel = statusInformation.getChannelFor(eventType);
channel.addPreFilter(filter);
channel.addProcessFilter(filter);
channel.addPostFilter(filter);

In case the underlying ProcessingContext object is needed, an addRaw method is provided:

messageBus.addRaw(new Filter<ProcessingContext<Object>>() {
    @Override
    public void apply(ProcessingContext<Object> processingContext, 
                      FilterActions<ProcessingContext<Object>> filterActions) {
        //filterLogic
    }
});

MessageBus Statistics

Similar to the Channel the MessageBus collects statistics about all messages:

MessageBusStatusInformation statusInformation = messageBus.getStatusInformation();
MessageBusStatistics statistics = statusInformation.getCurrentMessageStatistics();
BigInteger acceptedMessages = statistics.getAcceptedMessages();
BigInteger queuedMessages = statistics.getQueuedMessages();
BigInteger blockedMessages = statistics.getBlockedMessages();
BigInteger forgottenMessages = statistics.getForgottenMessages();
BigInteger successfulMessages = statistics.getSuccessfulMessages();
BigInteger failedMessages = statistics.getFailedMessages();
Date timestamp = statistics.getTimestamp();

It's important to note, that these statistics count only for the accepting channel. Once, the message is given to the type specific channel and the CorrelationId based subscribers, the result is no longer contained in the above statistics. For instance, when a message has been successfully traversed the accepting channel and was delivered to the type specific channel, it is marked als successful. Errors in the type specific channel or the correlation based subscriber are not included. The type specific statistic are collected as usual by the channel itself.

MessageBus Debug Information

the MessageBusStatusInformationinterface provides useful debug information. It allows to query the currently registered subscriber and error listener.

MessageBusStatusInformation statusInformation = messageBus.getStatusInformation();
List<Subscriber<?>> allSubscribers = statusInformation.getAllSubscribers();
Map<Class<?>, List<Subscriber<?>>> subscribersPerType = statusInformation.getSubscribersPerType();
List<MessageBusExceptionListener<?>> exListener = statusInformation.getAllExceptionListener();

Closing the MessageBus

The methods to close the MessageBus are similar to those described for Channels in Closing the Channel:

boolean finishRemainingTasks = true;
messageBus.close(finishRemainingTasks);

boolean closed = messageBus.isClosed();

try {
    boolean awaitSucceeded = messageBus.awaitTermination(5, SECONDS);
} catch (InterruptedException e) {

}

Configuring the MessageBus

All configuration is done using the MessageBusBuilder class. All configurable properties have default values. This creates a synchronous MessageBus. The default MessageBusExceptionHandler throws all exceptions on the calling Thread. Whenever a class specific Channel has to be created, a synchronous one is created by the default MessageBusChannelFactory.

MessageBusBuilder.aMessageBus()
.forType(MessageBusType.SYNCHRONOUS)
.withAsynchronousConfiguration(asynchronousConfiguration)
.withExceptionHandler(new MessageBusExceptionHandler() {
    @Override
    public boolean shouldDeliveryChannelErrorBeHandledAndDeliveryAborted(ProcessingContext<?> message, Exception e, Channel<?> channel) {
        final boolean abortDeliveryAndHandleException = false;
        return abortDeliveryAndHandleException;
    }

    @Override
    public void handleDeliveryChannelException(ProcessingContext<?> message, Exception e, Channel<?> channel) {

    }

    @Override
    public void handleFilterException(ProcessingContext<?> message, Exception e, Channel<?> channel) {

    }
})
.withAChannelFactory(new MessageBusChannelFactory() {
    @Override
    public <T> Channel<?> createChannel(Class<T> tClass, Subscriber<T> subscriber, MessageBusExceptionHandler messageBusExceptionHandler) {
        ChannelExceptionHandler<T> channelExceptionHandler = delegatingExceptionHandlerTo(messageBusExceptionHandler);
        return ChannelBuilder.aChannel()
            .withDefaultAction(Subscription.subscription())
            .withChannelExceptionHandler(channelExceptionHandler)
            .build();
    }
})
.build();

The type and the AsynchronousConfiguration are similar to those used for Channels described in Configuring the Channel.

The default MessageBusExceptionHandler throws all exceptions. It can be replaced using withExceptionHandler method. When an exception is thrown in one of the subscriber the shouldDeliveryChannelErrorBeHandledAndDeliveryAborted is called to decide, whether the exception should be handled and the delivery is aborted or whether the exception should be ignored. In case the exception should be handled, the message is marked as failed in the statistics and the handleDeliveryChannelException method is called. When an exception is raised in any Filter (general or class specific Channel), the delivery is aborted, the message is marked as failed and the control is given to handleFilterException. After each of the handle_Exception methods all suitable exception listener are called.

The MessageBusChannelFactory is used to create the class-specific Channel, that delivers the messages to the Subscribers for this class. The default implementation creates a synchronous Channel, that redirect errors to the MessageBusErrorHandler. But in case more control over the configuration of these Channels is needed, a custom implementation can be given here. The creation of a new happen Channel can be requested in two cases: First a subscriber is added for a not yet known class. Second, an unknown message was sent. Then for the class of the message and all newly discovered parent classes, a new Channel is created. * discovered parent classes, a new {@code Channel} is created. Care has to be taken to handle or redirect the errors correctly. Also important to note is, that the close call to the MessageBus will not be propagated to the MessageBusChannelFactory. If a custom MessageBusChannelFactory contains state that requires a teardown, the synchronisation with the close call has to be enforced manually.

Dynamically adding exception listener

Once the MessageBus is created, the given MessageBusExceptionHandler can not be changed. But since subscribers are added or removed to or from a MessageBus in a highly dynamical way, a static exception handler becomes a problem. Therefore the MessageBus provides a way to register exception listener for specific EventType or CorrelationId on the fly. These listener will always be called after the MessageBusExceptionHandler received the exception.

messageBus.onException(correlationId, new MessageBusExceptionListener<Object>() {
    @Override
    public void accept(ProcessingContext<Object> processingContext, Exception e) {
                
    }
});

SubscriptionId subscriptionId = messageBus.onException(eventType, new MessageBusExceptionListener<Object>() {
    @Override
    public void accept(ProcessingContext<Object> processingContext, Exception e) {

    }
});

messageBus.unregisterExceptionListener(subscriptionId);

Advance Concepts

QCEC

A MessageBus allows for a loosely coupled form of communication, where Sender and Subscriber do not need to know from each other. They don't even know the number of the others as members of both sides can join or leave dynamically. Configuring the MessageBus in an asynchronous way allows for independently integrated parts of the application.

The integration points between the different use cases and Frameworks are a fitting example for the beneficiary use of an asynchronous MessageBus. But aspects like loose coupling and dynamic extensibility are of great benefit even in more coupled parts of the application, like within a use cases. Use cases execute their logic by assembling different parts of the application. A MessageBus can be of great help here. QCEC defines concepts and practices how to use a MessageBus inside the context of use cases or components with similar requirements of loosely coupling, extensibility and testability while having more shared context than integration between use cases and Frameworks.

QCEC (qcc) stands for Query, Constraint, Event and Command. These four concepts when combined with a synchronous MessageBus ease the assembling of logic into a use case. Queries are responsible to retrieve information out of other objects. Constraints inform others about a requirement, that, if violated, should rise an exception. The purpose of Events is to inform others or to share information with them. Commands perform updates on Domain Objects or Repositories.

Queries

Use cases need to retrieve information from the objects they interact with. Having a list of all objects of interest results in high coupling. By using a MessageBus, a message can be distributed to these objects without the use case having too much knowledge about them. The message is written as Query. This means, that subscriber upon receiving the Query object can use Query specific methods to store their data into the object. Let's take an example, in which we want to query all of our apple trees about the number of apples they currently hold. We define a custom Query, that is responsible to query how many apples all of our apple trees have:

class NumberOfApplesQuery implements Query<Integer>{
    private int sumOfApples;
        
    public void reportPartialResult(int numberOfApples){
        this.sumOfApples+=numberOfApples;
    }
        
    @Override
    public Integer result() {
        return sumOfApples;
    }
}

The AppleTree class can subscribe itself on the NumberOfApplesQuery. Each AppleTree reports its number of apples, whenever someone wants to know how many apples there are:

class AppleTree {
    public AppleTree(int numberOfApples, QueryResolver queryResolver) {
        queryResolver.answer(NumberOfApplesQuery.class, numberOfApplesQuery -> {
            numberOfApplesQuery.reportPartialResult(numberOfApples);
        });
    }
}

Now the use case does not not how to interact with AppleTree objects. He just needs to send the query:

MessageBus messageBus = MessageBusBuilder.aMessageBus()
    .forType(MessageBusType.SYNCHRONOUS)
    .build();

QueryResolver queryResolver = QueryResolverFactory.aQueryResolver(messageBus);

new AppleTree(1, queryResolver);
new AppleTree(3, queryResolver);

int numberOfApples = queryResolver.queryRequired(new NumberOfApplesQuery());
assertEquals(4, numberOfApples);

Executing a Query on the QueryResolver allows querying all AppleTrees about their stock. Although in this example the querying code already knows how many apples there are, it should be obvious, that the AppleTrees could be created somewhere else without compromising the validity of the code. The querying code does not even know about the existence of the AppleTrees. There could be different kinds of AppleTrees and the querying code would still be the same.

There exists two different methods for querying query and queryRequired:

Optional<Integer> optional = queryResolver.query(new NumberOfApplesQuery());
int numberOfApples = optional.orElseThrow(() -> new UnsupportedOperationException("Expected a query result."));
        
int numberOfApples = queryResolver.queryRequired(new NumberOfApplesQuery());

The query method allows queries not having a result and therefore returning an optional. The queryRequired method throws an UnsupportedOperationException when there is no result.

The answer method returns an SubscriptionId object. This can be used for the unsubscribe method to stop answering methods. The answer can also be used on super classes or interfaces. In this case all subclasses will result in the answer method to be called with the respective instance.

SubscriptionId subscriptionId = queryResolver.answer(Query.class, q -> handle(q));
queryResolver.unsubscribe(subscriptionId);

Per default queries are delivered to all Subscribers and the result is returned afterwards. But queries can be stopped early, when it's apparent, that further Subscribers won't add value to the result. To stop a query early, override the finished method. Once it returns true, the query is stopped and the result is returned immediately.

class PreemptiveQuery implements Query<Object> {
     private Object result;

     public void setResult(Object result) {
        this.result = result; 
     }

     @Override
     public Object result() {
        return result; 
     }

     @Override
     public boolean finished() {
         return result != null; 
     }
}

When subscribing for queries, superclasses can be used. The underlying MessageBus ensures, that all subclasses of the class used in answer will also call the consumer.

Constraints

Queries are used to retrieve data from others. They should not throw an exception, because it would mix up the partially retrieved data with the exception. But it's often necessary to ensure, that a specific constraint holds and if it does not, to raise an exception. This differs from queries in that way as a Constraint either holds or an exception is thrown. But a constraint will never return data.

Let's suppose we want to ensure, that the usernames of users are unique. We use a Constraint:

class UniqueUsernameConstraint {
     public String usernameToCheck;

     public UniqueUsernameConstraint(String usernameToCheck) {
        this.usernameToCheck = usernameToCheck;
     }
}

The User class is responsible to protect the uniqueness of its username:

class User {
    private String username;

    public User(String username, ConstraintEnforcer constraintEnforcer) {
        this.username = username;
        constraintEnforcer.respondTo(UniqueUsernameConstraint.class, uniqueUsernameConstraint -> {
        if(uniqueUsernameConstraint.usernameToCheck.equals(username)){
            throw new UsernameAlreadyInUseException(username);
            }
        });
    }
}

Now any code can send Constraints on the ConstraintEnforcer object to ensure, that the unique username constraint holds.

MessageBus messageBus = MessageBusBuilder.aMessageBus()
    .forType(MessageBusType.SYNCHRONOUS)
    .build();

ConstraintEnforcer constraintEnforcer = ConstraintEnforcerFactory.aConstraintEnforcer(messageBus);
        
new User("Tim", constraintEnforcer);
        
constraintEnforcer.enforce(new UniqueUsernameConstraint("Tim"));

Similar to the QueryResolver, the respondTo method allows for inheritance and interfaces. It also returns a SubscriptionId that can be used as parameter for the unsubscribe method to stop responding to constraints.

When subscribing for constraints, superclasses can be used. The underlying MessageBus ensures, that all subclasses of the class used in respondTo will also call the consumer.

Events

Queries retrieve information, constraints enforce rules and events are used to forward information. Events never return information and should not throw an exception. They are just used to indicate, that something happened.

Let's suppose a very basic login use case: Given a username and password, a login is tried. If it succeeded, an event is published to inform others, that the user went online.

MessageBus messageBus = MessageBusBuilder.aMessageBus()
    .forType(MessageBusType.SYNCHRONOUS)
    .build();

EventBus eventBus = EventBusFactory.aEventBus(messageBus);
        
boolean loginSuccessful = login(this.username, this.password);
if(loginSuccessful){
    eventBus.publish(new UserOnlineEvent(this.username));
}else{
    goBackToLoginForm();
}

class UserOnlineEvent {
    public String username;

    public UserOnlineEvent(String username) {
        this.username = username;
    }
}

The code publishing the event does not care, what others do with the information or even if there are others. It is of no concern for its functionality that other receive the event.

But other components might be interested, when a user goes online:

class UserOnlineView {
    private final List<String> userOnline = new ArrayList<>();

    public UserOnlineView(EventBus eventBus) {
        eventBus.reactTo(UserOnlineEvent.class, userOnlineEvent -> {
            final String username = userOnlineEvent.username;
            userOnline.add(username);
        });
    }
}

The UserOnlineView is dependent on the event and its information. But it doesn't care, who sent it. It just needs the information.

The EventBus has three methods: reactTo to add a subscriber for a class and all its subclasses. publish sends the Event on the underlying synchronous MessageBus. And unsubscribe removes the subscription for the given SubscriptionId.

When subscribing for events, superclasses can be used. The underlying MessageBus ensures, that all subclasses of the class used in reactTo will also call the consumer.

Commands

Aside from querying and aggregating data, use cases are responsible for a safe and secure update to the applications data. A common pattern is to model the update in form of Commands. A Command is a reusable abstraction over the the update. It gets the required parameter during its creation by the use case. During its invocation by the consuming counterpart it gets all information to execute its task. By moving the update logic out of the use case into a distinct object, the update process becomes decoupled from the use case and therefore reusable and testable.

DocumentBus

It's rarely the case that an application uses only Queries, Constraints or Events. Most of the time it's a mixture of these three. Therefore it becomes a burden to drag along a QueryResolver, a ConstraintEnforcer and an EventBus. It also becomes difficult to remember which SubscriptionId was used for which of these objects. Therefore the DocumentBus was created to combine these concepts and provide an easier to use interface.

It provides three entry methods: answer for Queries, ensure for Constraints and reactTo for Events, which represent the three respective methods of the QueryResolver, ConstraintEnforcer and EventBus. But the DocumentBus allows to enhance the subscription with conditionals and an automatic unsubscription.

Let's extend the AppleTree example with an DocumentBus. An AppleTree still reports his stock of apples to the NumberOfApplesQuery. But only if the query is from the owner of the tree. And the tree can only report as long as it is not cut down. Then it should stop its reporting and unsubscribe from the NumberOfApplesQuery.

DocumentBus documentBus = DocumentBusBuilder.aDefaultDocumentBus();
 
SubscriptionId subscriptionId = documentBus.answer(NumberOfAppleQuery.class)
    .onlyIf(numberOfAppleQuery -> numberOfAppleQuery.getOwner().equals(this.owner))
    .until(AppleTreeCutDownEvent.class, appleTreeCutDownEvent -> appleTreeCutDownEvent.getTree().equals(this))   
    .using(numberOfAppleQuery -> numberOfAppleQuery.reportPartial(this.numberOfApples))

The answer method takes the Query, for which itself or its subclasses the Consumer given in using should be called. The onlyIf method can add arbitrary many conditions. Only if all of them return true the Consumer given in using is called. The until method allows for one or several automatic unsubscriptions. Whenever one of these conditions return true, the subscription is removed and the AppleTree stops responding to the NumberOfAppleQuery. The returned SubscriptionId identified subscription for the query. It can be used to manually unsubscribe as long as an the until condition has not been met yet.

The same convenience interface exists for the Constraint's ensure and the Event's reactTo method:

documentBus.reactTo(AppleTreeCutDownEvent.class)
    .until(AppleTreeCutDownEvent.class, appleTreeCutDownEvent -> appleTreeCutDownEvent.getTree().equals(this))
    .using(appleTreeCutDownEvent -> releaseResources());
        
documentBus.ensure(TreeSpotFreeConstraint.class)
    .until(AppleTreeCutDownEvent.class, appleTreeCutDownEvent -> appleTreeCutDownEvent.getTree().equals(this))
    .using(treeSpotFreeConstraint -> {
        if(treeSpotFreeConstraint.getSpot().equals(this.spot)){
            throw new TreeSpotAlreadyOccupiedException(this.spot);
        }
    });

The onlyIf methods also exists for reactTo and ensure.

Sending objects is similar to the distinct single objects:

Optional<Integer> optional = documentBus.query(new NumberOfAppleQuery());
int numberOfApples = documentBus.queryRequired(new NumberOfAppleQuery());

documentBus.enforce(new TreeSpotFreeConstraint());
        
documentBus.publish(new AppleTreeCutDownEvent());

Message Function

Implementing a Request-Reply communication over an asynchronous MessageBus can be error-prone. Once the request is send, numerous ways exist, how to respond to it. It could be answered by different types for replies. Some model success responses, others error responses. But also exceptions can occur and no regular response is ever sent. In case several requests are simultaneously active, responses and exceptions have to be checked, if they correspond to the correct request.

The MessageFunction class simplifies this Request-Reply communication. When sending a request over a MessageFunction, a ResponseFuture is returned. This will fulfill, once a message with a CorrelationId is received, that matches the requests MessageId`, or an exception occured for either the request, or a correlated reply.

The future provides the methods getand await, which block the caller until a result is received or the optional timeout expired. The future also allows for a non blocking processing. Via the then method follow up actions can be defined, that are executed once a the future is fulfilled.

The following example models the process of buying a number of apples from a farmer. The BuyAppleRequest starts the negotiation. The farmer can accept the offer with an AcceptOfferReply or decline with a DeclineOfferReply based on his stock:

class Farmer {
    private int stock;

    public Farmer(MessageBus messageBus, int stock) {
        this.stock = stock;
        messageBus.subscribeRaw(buyAppleEventType, processingContext -> {
            BuyAppleRequest buyAppleRequest = processingContext.getPayload();
            
            CorrelationId correlationId = processingContext.generateCorrelationIdForAnswer();
            
            boolean acceptOffer = stock >= buyAppleRequest.numberOfApples;
            BuyAppleReply reply = new BuyAppleReply(acceptOffer);
            messageBus.send(replyEventType, reply, correlationId);
        });
    }
}

class BuyAppleRequest {
    public int numberOfApples;

    public BuyAppleRequest(int numberOfApples) {
        this.numberOfApples = numberOfApples;
    }
}

    
class BuyAppleReply  {
    boolean accept;
    
    BuyAppleReply(boolean accept){
        this.accept = accept;
    }
}

The CorrelationId is necessary to match a reply to its corresponding request. Instead of implementing a lot of subscriber and error logic itself the client code makes use of a MessageFunction:

MessageBus messageBus = MessageBusBuilder.aMessageBus()
    .forType(MessageBusType.ASYNCHRONOUS)
    .withAsynchronousConfiguration(asyncConfig)
    .build();

MessageFunction messageFunction = MessageFunctionBuilder.aMessageFunction(messageBus);

new Farmer(messageBus, 11);

BuyAppleRequest request = new BuyAppleRequest(5);
ResponseFutureresponseFuture = messageFunction.request(buyAppleEventType, request);
responseFuture.then((response, errorResponse, exception) -> {
    if (exception != null) {
        System.out.println("Exception occured: " + exception);
    } else {
        if (errorResponse == null) {
            System.out.println("payload received normally: " + response);
        } else {
            System.out.println("error payload: " + errorResponse);
        }
    }
});

The then method is invoked once the future fulfils. Inside the FollowUpAction, the exception should be checked first. It is not null, if the request or the reply caused an exception. The wasSuccessful field is true, if no exception occurred and the ProcessingContext error payload was null. Otherwise it is false. The response contains the payload of the reply.

ResponseFuture

The ResponseFuture implements the java Future interface. Therefore it provides methods to query or wait on the result. But it also defines additional get and wait methods to access the received ProcessingContext or error payload.

try {
    Object response = responseFuture.get();
    Object errorResponse = responseFuture.getErrorResponse();
    ProcessingContext<Object> processingContext = responseFuture.getRaw();
} catch (InterruptedException | ExecutionException e) {
    
}
try {
Object response = responseFuture.get(10, MILLISECONDS);
Object errorResponse = responseFuture.getErrorResponse(10, MILLISECONDS);
ProcessingContext<Object> processingContext = responseFuture.getRaw(10, MILLISECONDS);
} catch (InterruptedException | ExecutionException | TimeoutException e) {
            
}

boolean noExceptionOrErrorResponse = responseFuture.wasSuccessful();
boolean responseExceptionOrCancellationOccurred = responseFuture.isDone();
responseFuture.cancel(true);
boolean cancelled = responseFuture.isCancelled();

The get methods suspend the caller until a result was receied or the optional timeout expired. Cancelling a the future, will block its fulfillment. The underlying request or anything except that, will not be cancelled.

A future fulfills only once. So an exception during the sending of a request will fulfill the future. Subsequent replies to the request will be ignored and won't trigger any FollowUpActions twice. At any time only one FollowUpAction is allowed. When the future is cancelled, no FollowUpActions will be executed.

Use case invocation

In qcec we have seen, how to use messaging in the scope of a single use case. But invoking use cases using messaging provides the same benefits: low coupling and high extensibility. EventMaid provides several concepts to ease the configuration of the application's use case invocation.

UseCaseBus

The UseCaseBus is the most convenient form of invoking use cases over an asynchronous MessageBus. It provides an expressive builder interface to configure the invoked use cases, their serializing and deserializing logic. The following example should make the different steps more clear:

We want to invoke the SampleUseCase class over a MessageBus, that is shared between all use cases and probably some external frameworks handling http, websockets or other messaging systems to foreign systems. The use case defines appropriate parameter objets:

class UseCaseParameter {
    private final int someNumber;

    UseCaseParameter(int someNumber) {
        this.someNumber = someNumber;
    }
}

class SampleUseCase {

    public UseCaseReturnValue doStuff(UseCaseParameter request) {
        String message = "Received: " + request.someNumber;
        return new UseCaseReturnValue(message);
    }
}

class UseCaseReturnValue {
    private final String message;

    UseCaseReturnValue(String message) {
        this.message = message;
    }
}

Without any helper classes, the use case could be invoked over the MessageBus as follows:

messageBus.subscribe(useCaseEventType, o -> {
    SampleUseCase useCase = new SampleUseCase();
    UseCaseReturnValue returnValue = useCase.doStuff((UseCaseParameter) o);
    messageBus.send(useCaseResponseEventType, returnValue);
});

messageBus.send(useCaseEventType, new UseCaseParameter(5));

This solution has several problems. First of all, it becomes a burden to register all of the application's use cases or adapt to changes in those use cases. Second, sending the parameter as it is over the message bus creates a higher coupling as necessary. Each subscriber, that wants to access the objects of type useCaseEventType has to depend on the UseCaseParameter.class. Since the use case cannot know of potential users of this class, it has to take great care when changing the class. Therefore it is better to send data to and from use cases in a serialized form: as a Map<String,Object>. Potential consumer can take these map and extract only required data from it in a form, that each consumer can decide upon its own. In case of the use case it would be to deserialize it into a UseCaseParameter object:

messageBus.subscribe(useCaseEventType, map -> {
    SampleUseCase useCase = new SampleUseCase();
    UseCaseParameter useCaseParameter = deserialize(map);
    UseCaseReturnValue returnValue = useCase.doStuff(useCaseParameter);
    Map<String, Object> serializedReturnValue = serialize(returnValue);
    messageBus.send(useCaseResponseEventType, returnValue);
});

UseCaseParameter parameter = new UseCaseParameter(5);
Map<String, Object> requestMap = serialize(parameter);
messageBus.send(useCaseEventType, parameter);

Allthough sending the data in a serialized form, the invocatio of the use case became tidious, as three (de-)serialization steps have to be done. If the returnValue would be of interest, a fourth and final deserialization step would have been added to deserialize the UseCaseReturnValue back to the correct class. To make it more complicated, we do not reuse the the UseCaseParameter and UseCaseReturnValue classes, as they should only be owned by the use case itself. We introduce two additional classes:

class UseCaseRequest {
    private final int someNumber;

    UseCaseRequest(int someNumber) {
        this.someNumber = someNumber;
    }
}
    
class UseCaseResponse {
    private final String message;
    
    UseCaseResponse(String message) {
        this.message = message;
    }
}

Code invoking the use case will use these classes, so that the parameter and return classes, that the use case uses, can be freely changed, as the use case requires. Invocing the use case requires the following 5 steps:

1. Create a UseCaseRequest object, serialize it and send it on the MessageBus
2. Take the serialized request and deserialize it to a UseCaseParameter.
3. Invoke the use case with its single public method.
4. Serialize the UseCaseReturnValue and send it back.
5. Deserialize the response into a UseCaseResponse object.

Maintaining these 5 steps becomes a burden for several use cases. Therefore the UseCaseBus class was introduced. It provides a builder, that eases all the serialization/deserialization definitions and also the use case invocation. Taking the example from before, we can rewrite the code as follows:

UseCaseBus useCaseBus = UseCaseInvocationBuilder.anUseCaseBus() 
    .invokingUseCase(SampleUseCase.class).forType("useCase1").callingTheSingleUseCaseMethod() 
    .obtainingUseCaseInstancesUsingTheZeroArgumentConstructor()
    .mappingRequestsToUseCaseParametersOfType(UseCaseParameter.class).using((targetType, map) -> new UseCaseParameter((Integer) map.get("SampleUseCase.intParam")))
    .deserializingUseCaseResponsesOfType(UseCaseResponse.class).using((targetType, map) -> new UseCaseResponse((String) map.get("SampleUseCase.returnValue")))
    .throwAnExceptionByDefaultIfNoParameterMappingCanBeApplied()
    .serializingUseCaseRequestOfType(UseCaseRequest.class).using(useCaseRequest -> Map.of("SampleUseCase.intParam", useCaseRequest.someNumber))
    .serializingResponseObjectsOfType(UseCaseReturnValue.class).using(caseReturnValue -> Map.of("SampleUseCase.returnValue", caseReturnValue.message))
    .throwingAnExceptionByDefaultIfNoResponseMappingCanBeApplied()
    .puttingExceptionObjectNamedAsExceptionIntoResponseMapByDefault()
    .build(messageBus);

Let's explain each line step by step:

1. Starting the configuration of the UseCaseBus
2. defining which class to invoke for which type. Als defines, that the only public method should be invoked. Alternatives to calling specific methods in case not only a single method is present will be discussed later.
3. obtainingUseCaseInstancesUsingTheZeroArgumentConstructor: For each request a new use case instance will be created by using the constructor without parameters. Alternatively an injector can be set, that allows for greater control.
4. Define the deserialization steps: mappingRequestsToUseCaseParametersOfType is responsible to create the use case method's parameter from the map send over the MessageBus.
5. deserializingUseCaseResponsesOfType: After the invocation and the sending of the serialized return value, the fourth and last deserializing step is used to create an object from the response map.
6. The deserialization definitions need a default mapping. This method throws an exception, whenever no matching deserialization is found.
7. serializingUseCaseRequestOfType takes the request object and serialized it on the MessageBus. It takes the UseCaseRequest's message property and stores it as SampleUseCase.intParam in the map. That's the reason, the mappingRequestsToUseCaseParametersOfType method took the SampleUseCase.intParam property.
8. In case the use case returns a value, the serializingResponseObjectsOfType method allows for defining, how to put the value in a map.
9. The serialization definitions also need a default value, which would be also throw a exception.
10. Use case methods can throw exceptions. These exceptions need also also to be serialized, before sending them back to the caller (but as error payload). The puttingExceptionObjectNamedAsExceptionIntoResponseMapByDefault will take each exception and put the exception object under Exception in the map.
11. Set the used MessageBus and create the useCaseBus.

Once the UseCaseBus has been defined, it allows for invoking use cases and waiting for their result with as follows:

EventType eventType = EventType.eventTypeFromString("useCase1");
UseCaseRequest useCaseRequest = new UseCaseRequest(5);
try {
    PayloadAndErrorPayload<UseCaseResponse, Void> result = useCaseBus.invokeAndWait(eventType, useCaseRequest, UseCaseResponse.class, Void.class);
    final UseCaseResponse response = result.getPayload();
    System.out.println(response);
} catch (InterruptedException | ExecutionException e) {
    e.printStackTrace();
}

The invokeAndWait method takes the EventType, the data and two Classes as parameters. The data will be serialized using the definitions from above and will be send to the use case assigned to the specific EventType. Once a response has been received, the two classes are used to deserialize the response back to real objects.

The invokeAndWait method can also take an additional timeout. Also in case no final deserialization is needed, invokeAndWaitNotDeserialized versions exist, that return the serialized Map<String, Object> payloads:

try {
    PayloadAndErrorPayload<UseCaseResponse, ErrorResponseClass> result = useCaseBus.invokeAndWait(eventType, request, UseCaseResponse.class, ErrorResponseClass.class, 10, MILLISECONDS);
    PayloadAndErrorPayload<Map<String, Object>, Map<String, Object>> result2 = useCaseBus.invokeAndWaitNotDeserialized(eventType, request);
    PayloadAndErrorPayload<Map<String, Object>, Map<String, Object>> result3 = useCaseBus.invokeAndWaitNotDeserialized(eventType, request, 10, MILLISECONDS);
} catch (InterruptedException | ExecutionException | TimeoutException e) {
    e.printStackTrace();
}

The example above demonstrated the easiest form of configuring the UseCaseBus. But in case a more complex setup is needed, most of the methods described above have a more customizable version.

Invoking the use case with callingTheSingleUseCaseMethod is the easiest way, as the method simplifies all of the (de)serialization and method calling. But there exists cases, where the method cannot be used, for instance, if a use case has more than one public method. Then the invocation has to be defined explicitely:

UseCaseInvocationBuilder.anUseCaseBus()
.invokingUseCase(SampleUseCase.class).forType("t1").calling((sampleUseCase, o) -> {
    UseCaseParameter parameter = deserialize(o);
    UseCaseReturnValue returnValue = sampleUseCase.doStuff(parameter);
    Map<String, Object> responseMap = serialize(returnValue);
    return responseMap;
})
.invokingUseCase(SampleUseCase.class).forType("t3").callingVoid((sampleUseCase, o) -> {
    UseCaseParameter parameter = serialize(o);
    sampleUseCase.doStuff(parameter);
})
.invokingUseCase(SampleUseCase.class).forType("t2").callingBy((useCase, event, requestDeserializer, responseSerializer) -> {
    Map<String, Object> requestMap = (Map<String, Object>) event;
    UseCaseParameter parameter = requestDeserializer.deserialize(UseCaseParameter.class, requestMap);
    UseCaseReturnValue returnValue = useCase.doStuff(parameter);
    return responseSerializer.serialize(returnValue);
})

The calling method takes java BiFunction<U, Object, Map<String, Object>>. This function gets the use case instance, the request map and should return the response map. All (de)serialization and method calling is defined by the user. The callingVoid method is similar, except, that it expects the use case to not return a value. This results in an empty response map. The callingBy method provides access to the use case instance, the request map, the configured serializer and deserializer. It expects a filled response map as return value.

In case a custom logic is needed, to instantiate the use cases, the obtainingUseCaseInstancesUsing method allows for defining a injector. For each request, the injector will be called.

.obtainingUseCaseInstancesUsing(new UseCaseInstantiator() {
    @Override
    public <T> T instantiate(Class<T> type) {
        return newInstance(type);
    }
})

The deserialization definitions are usually class based. But in case more fine grained control, the mappingRequestsToUseCaseParametersThat or deserializingUseCaseResponsesThat methods exist, that take an BiPredicate<Class<?>, Map<String, Object>>, which also gets access to the current map. In case a different default deserialization instead of throwing an exception is needed, use the deserializeObjectsPerDefault method.

For the serialization, similar functions exists: serializingUseCaseRequestThat and serializingResponseObjectsThat take a Predicate<Object>, with access to the current object, that should be mapped to a map. In case null values should be mapped, the serializinguseCaseRequestsOfTypeVoid and serializingResponseObjectsOfTypeVoid methods exists, as null.getClass() won't work. In case a different default serialization instead of throwing an exception is needed, use the serializingObjectsByDefaultUsing method.

For the exception serializing, the same overloaded methods exist. serializingExceptionsOfType takes a class for which to include the following mapping defined in the using method. serializingExceptionsThat takes a Predicate<Exception> to decide, when to trigger. The puttingExceptionObjectNamedAsExceptionIntoResponseMapByDefault can be replaced by serializingExceptionsByDefaultUsing or throwingAnExceptionIfNoExceptionMappingCanBeFound. Please note, that the later will throw an exception outside the exception mapping function. This later exception will not be mapped, but instead, will be thrown as exception on the MessageBus.

UseCaseAdapter

Once UseCaseBus is configured invoking a use case is reduced to calling invokeAndWait. Although, this is very convenient to use, there exist cases, where more control is needed. A good example, is the functionality of the ResponseFuture's then method, which allows non blocking handling of responses. The invokeAndWait method always blocks, so it is not a valid substitute. For these cases, where more control in the sending and waiting on messages is needed, the the UseCaseInvocationBuilder can output a UseCaseAdapter, which fulfills only the use case invocation part of the UseCaseBus: It subscribes the defined use cases for their specific EventTypes on the MessageBus. Whenever a fitting message is received, the target use case is invoked with the defined (de)serialization. But the sending of requests and waiting of responses is left for the user. This allows for a more controlled sending, for instance using a MessageFunction. The following example describes the differences, when using a UseCaseAdapter instead of a UseCaseBus. The methods of the UseCaseInvocationBuilder are the same as for the UseCaseBus.

UseCaseAdapter useCaseAdapter = UseCaseInvocationBuilder.anUseCaseAdapter()
    .invokingUseCase(SampleUseCase.class).forType("t1").callingTheSingleUseCaseMethod()
    .obtainingUseCaseInstancesUsingTheZeroArgumentConstructor()
    .mappingRequestsToUseCaseParametersOfType(UseCaseParameter.class).using((targetType, map) -> new UseCaseParameter((Integer) map.get("SampleUseCase.intParam")))
    .deserializingUseCaseResponsesOfType(UseCaseResponse.class).using((targetType, map) -> new UseCaseResponse((String) map.get("SampleUseCase.returnValue")))
    .throwAnExceptionByDefaultIfNoParameterMappingCanBeApplied()
    .serializingResponseObjectsOfType(UseCaseReturnValue.class).using(caseReturnValue -> Map.of("SampleUseCase.returnValue", caseReturnValue.message))
    .serializingUseCaseRequestOfType(UseCaseRequest.class).using(useCaseRequest -> Map.of("SampleUseCase.intParam", useCaseRequest.someNumber))
    .throwingAnExceptionByDefaultIfNoResponseMappingCanBeApplied()
    .puttingExceptionObjectNamedAsExceptionIntoResponseMapByDefault()
    .buildAsStandaloneAdapter();

useCaseAdapter.attachAndEnhance(messageBus);

EventType eventType = EventType.eventTypeFromString("t1");
MessageFunction messageFunction = MessageFunctionBuilder.aMessageFunction(messageBus);
ResponseFuture responseFuture = messageFunction.request(eventType, mapData);        

The attachAndEnhance function registers the use cases on the given MessageBus. It returns a SerializedMessageBus, which is described further below:

SerializedMessageBus serializedMessageBus = useCaseAdapter.attachAndEnhance(messageBus);

It can also directly take an SerializedMessageBus:

useCaseAdapter.attachTo(serializedMessageBus);

Serialized MessageBus

The UseCaseBus simplifies invoking use cases. In case more control is needed when sending requests and handling responses, the UseCaseAdapter can be used. This works for all use cases, that take parameter and return a single (or none) return value. But there exists use cases, that send extra messages on the MessageBus during their execution. These messages should also be send in their serialized form to not brake any code, that excepts only objects of type Map<String, Object> on the MessageBus. This requirement forces use cases to know, how to serialize objects. But this information is usually defined during the creation of the UseCaseBus/ UseCaseAdapter. To not duplicate these kind of information, the SerializedMessageBus was created. The SerializedMessageBus wraps a normal MessageBus and provides methods to send messages, that are automatically serialized using the serializers defined in the UseCaseInvocationBuilder, and receive responses deserialized like the UseCaseBus would do.

A SerializedMessageBus can be created using its factory method:

SerializedMessageBus serializedMessageBus = SerializedMessageBus.aSerializedMessageBus(messageBus, deserializer, serializer);

But since the serializer and deserializer are defined in the UseCaseAdapter, its normally better to let the UseCaseAdapter create the wrapping SerializedMessageBus with the correct deserializer and serializer:

SerializedMessageBus serializedMessageBus = useCaseAdapter.attachAndEnhance(messageBus);

The attachAndEnhance will add all necessary subscriber to the MessageBus. It will then wrap the bus and return it with the correct (de)serializers.

The resulting SerializedMessageBus can send both serialized and not yet serialized data:

//sending raw data
Map<String, Object> data = new HashMap<>();
serializedMessageBus.send(eventType, data);

CorrelationId correlationId = newUniqueCorrelationId();
serializedMessageBus.send(eventType, data, correlationId);

Map<String, Object> errorData = new HashMap<>();
serializedMessageBus.send(eventType, data, errorData);
serializedMessageBus.send(eventType, data, errorData, correlationId);


//sending data, that is serialized first
UseCaseRequest data = new UseCaseRequest(5);
serializedMessageBus.serializeAndSend(eventType, data);
serializedMessageBus.serializeAndSend(eventType, data, correlationId);

ErrorData errorData = new ErrorData();
serializedMessageBus.serializeAndSend(eventType, data, errorData);
serializedMessageBus.serializeAndSend(eventType, data, errorData, correlationId);

It also allows for sending requests and waiting for their response:

//not serialized
Map<String, Object> data = new HashMap<>();
PayloadAndErrorPayload<Map<String, Object>, Map<String, Object>> result = serializedMessageBus.invokeAndWait(eventType, data);
PayloadAndErrorPayload<Map<String, Object>, Map<String, Object>> result = serializedMessageBus.invokeAndWait(eventType, data, 10, MILLISECONDS);

//only serialize request data
UseCaseRequest data = new UseCaseRequest(5);
PayloadAndErrorPayload<Map<String, Object>, Map<String, Object>> result = serializedMessageBus.invokeAndWaitSerializedOnly(eventType, data);
PayloadAndErrorPayload<Map<String, Object>, Map<String, Object>> result = serializedMessageBus.invokeAndWaitSerializedOnly(eventType, data, 10, MILLISECONDS);

//serialize request and deserialize response
PayloadAndErrorPayload<UseCaseResponse, ErrorResponse> result = serializedMessageBus.invokeAndWaitDeserialized(eventType, data, UseCaseResponse.class, ErrorResponse.class);
PayloadAndErrorPayload<UseCaseResponse, ErrorResponse> result = serializedMessageBus.invokeAndWaitDeserialized(eventType, data, UseCaseResponse.class, ErrorResponse.class, 10, MILLISECONDS);

Similar to the usual MessageBus, subscribers can be added and removed:

Subscriber<PayloadAndErrorPayload<Map<String, Object>, Map<String, Object>>> subscriber = new Subscriber<PayloadAndErrorPayload<Map<String, Object>, Map<String, Object>>>() {
    @Override
    public AcceptingBehavior accept(PayloadAndErrorPayload<Map<String, Object>, Map<String, Object>> message) {
        Map<String, Object> payload = message.getPayload();
        Map<String, Object> errorPayload = message.getErrorPayload();
        return AcceptingBehavior.MESSAGE_ACCEPTED;
    }

    @Override
    public SubscriptionId getSubscriptionId() {
        return subscriptionId;
    }
};
serializedMessageBus.subscribe(eventType, subscriber);
serializedMessageBus.subscribe(correlationId, subscriber);

Subscriber<PayloadAndErrorPayload<UseCaseResponse, ErrorResponse>> subscriber = new Subscriber<PayloadAndErrorPayload<UseCaseResponse, ErrorResponse>>() {
    @Override
    public AcceptingBehavior accept(PayloadAndErrorPayload<UseCaseResponse, ErrorResponse> message) {
        return AcceptingBehavior.MESSAGE_ACCEPTED;
    }

    @Override
    public SubscriptionId getSubscriptionId() {
        return subscriptionId;
    }
};
serializedMessageBus.subscribeDeserialized(eventType, subscriber, UseCaseResponse.class, ErrorResponse.class);
serializedMessageBus.subscribeDeserialized(correlationId, subscriber, UseCaseResponse.class, ErrorResponse.class);

serializedMessageBus.unsubscribe(subscriptionId);

Subscriber

Most of the functions, that take an Subscriber object, are overloaded to take also a Consumer object. Internally the Consumer object is mapped to a Subscriber, but the user does not have to burden itself with the handling of SubscriptionIds. But implementing your own Subscriber allows for greater control over the accepting and subscription mechanisms. The Subscriber interface defines two methods:

public interface Subscriber<T> {

    AcceptingBehavior accept(T message);

    SubscriptionId getSubscriptionId();
}

The accept method is called, whenever an object of the type T is received. The message should return an AcceptingBehavior object. This object can control, if the delivery of the message should be continued:

public AcceptingBehavior accept(Object message) {
    boolean continueDelivery = handle(message);
    return AcceptingBehavior.acceptingBehavior(continueDelivery);
}

A false will stop the delivery of the message to subsequent subscriber. If the result is known statically, two convenience constants can be used:

AcceptingBehavior.MESSAGE_ACCEPTED;
AcceptingBehavior.MESSAGE_ACCEPTED_AND_STOP_DELIVERY;

The second method getSubscriptionId should return a SubscriptionId, that is constant and unique for the Subscriber. The identification of an subscriber should be dependent on the equality of the SubscriberId returned by this method. equals and hashCode should behave accordingly.

Two convenience implementations of the Subscriber interface exist: The ConsumerSubscriber which creates a Subscriber from java consume and the PreemptiveSubscriber, which takes a java predicate. The return value of the predicate is used to decide, if the delivery is continued (return true) or if it is preempted (return false):

ConsumerSubscriber<Object> consumerSubscriber = ConsumerSubscriber.consumerSubscriber(m -> {
    System.out.println(m);
});

PreemptiveSubscriber<Object> preemptiveSubscriber = PreemptiveSubscriber.preemptiveSubscriber(m -> {
    if (shouldDeliveryContinue(m)) {
        return true;
    } else {
        return false;
    }
});

Custom Actions

The built-in Actions for Channels should cover most use cases. In case customization is needed, the Action interface can be implemented:

public interface Action<T> {}

It does not define any methods. An Action is only a container for necessary data. All the logic about executing the Action is done by the respective ActionHandler. For every custom Action, there must be an ActionHandler specifically written for this Action:

public interface ActionHandler<T extends Action<R>, R> { 
    void handle(T action, ProcessingContext<R> processingContext);
}

The ActionHandler interface defines two generic parameter: R is the generic given by the Action. Normally it is inherited by the type of the Channel. T corresponds the Action for which the ActionHandler is written. The handle method is called whenever a message with the Action has reached the end of the Channel. The following example implements a logging Action. This should clarify the generic parameter:

We define a custom Log Action, which contains a PrintStream as target.

class Log<T> implements Action<T> {
    private final PrintStream stream = System.out;

    public PrintStream getStream() {
        return stream;
    }
}

Additionally an ActionHandler is needed, so that Channel can execute the Log Action:

class LogActionHandler<T> implements ActionHandler<Log<T>, T> {
     @Override
     public void handle(Log<T> action, ProcessingContext<T> processingContext) {
        final PrintStream stream = action.getStream();
        stream.println(processingContext);
     }
}

When we want to use our Log Action, we have to make it known to the Channel. Each Channel has an ActionHandlerSet set during creation. Only those code Actions can be used as final code Action of the code Channel, that have a matching ActionHandler registered in the set. If an unknown Action is encountered, an NoHandlerForUnknownActionException is thrown.

To add your custom Action, register it the your custom ActionHandlerSet:

ActionHandlerSet<Object> actionHandlerSet = DefaultActionHandlerSet.defaultActionHandlerSet();
actionHandlerSet.registerActionHandler(Log.class, new LogActionHandler<>());
Channel<Object> channel = ChannelBuilder.aChannel(Object.class)
    .withDefaultAction(new Log<>())
    .withActionHandlerSet(actionHandlerSet)
    .build();

We used the default ActionHandlerSet, so that we do not have to register the built-in Actions and their ActionHandlers ourselves. But a completely different set can be built from scratch anytime with:

ActionHandlerSet<T> actionHandlerSet = ActionHandlerSet.emptyActionHandlerSet();

//manually registering all built-in actions
actionHandlerSet.registerActionHandler(Consume.class, ConsumerActionHandler.consumerActionHandler());
actionHandlerSet.registerActionHandler(Subscription.class, SubscriptionActionHandler.subscriptionActionHandler());
actionHandlerSet.registerActionHandler(Jump.class, JumpActionHandler.jumpActionHandler());
actionHandlerSet.registerActionHandler(Return.class, ReturnActionHandler.returnActionHandler());
actionHandlerSet.registerActionHandler(Call.class, CallActionHandler.callActionHandler());