Query Processing

Query Dispatching
Since version 3.1 Axon Framework also offers components for the Query handling. Although creating such a layer is fairly straight-forward, using Axon Framework for this part of the application has a number of benefits, such as the reuse of features such as interceptors and message monitoring.
The next sections provide an overview of the tasks related to setting up a Query dispatching infrastructure with the Axon Framework.
Query Gateway
The Query Gateway is a convenient interface towards the Query dispatching mechanism. While you are not required to use a Gateway to dispatch Queries, it is generally the easiest option to do so. Axon provides a QueryGateway interface and the DefaultQueryGateway implementation. The query gateway provides a number of methods that allow you to send a query and wait for a single or multiple results either synchronously, with a timeout or asynchronously. The query gateway needs to be configured with access to the Query Bus and a (possibly empty) list of QueryDispatchInterceptors.
Query Bus
The Query Bus is the mechanism that dispatches queries to Query Handlers. Queries are registered using the combination of the query request name and query response type. It is possible to register multiple handlers for the same request-response combination. Axon supports 3 query types:
Direct query
Scatter-gather query
Subscription query
Direct query
The direct query represents a query request to a single query handler. If no handler is found for a given query, a NoHandlerForQueryException is thrown. In case multiple handlers are registered, it is up to the implementation of the Query Bus to decide which handler is actually invoked. In the listing below we have a simple query handler:
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@QueryHandler // (1)
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public List<String> query(String criteria) {
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    // return the query result based on given criteria
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}
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(1) By default the name of the query is fully qualified class name of query payload (java.lang.String in our case). However, this behavior can be overridden by stating the queryName attribute of the @QueryHandler annotation.
If we want to query our view model, the List<String>, we would do something like this:
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// (1) create a query message
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GenericQueryMessage<String, List<String>> query =
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        new GenericQueryMessage<>("criteria", ResponseTypes.multipleInstancesOf(String.class));
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// (2) send a query message and print query response
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queryBus.query(query).thenAccept(System.out::println);
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(1) It is also possible to state the query name when we are building the query message, by default this is the fully qualified class name of the query payload.
(2) The response of sending a query is a java CompletableFuture, which depending on the type of the query bus may be resolved immediately. However, if a @QueryHandler annotated function's return type is CompletableFuture, the result will be returned asynchronously regardless of the type of the query bus.
Scatter-gather query
When you want responses from all of the query handlers matching your query message, the scatter-gather query is the type to use. As a response to that query a stream of results is returned. This stream contains a result from each handler that successfully handled the query, in unspecified order. In case there are no handlers for the query, or all handlers threw an exception while handling the request, the stream is empty.
In the listing below we have two query handlers:
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@QueryHandler(queryName = "query")
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public List<String> query1(String criteria) {
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    // return the query result based on given criteria
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}
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@QueryHandler(queryName = "query")
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public List<String> query2(String criteria) {
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    // return the query result based on given criteria
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}
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Since they are query handlers that are possibly in different components we would like to get result from both of them. So, we will use a scatter-gather query, like so:
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// create a query message
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GenericQueryMessage<String, List<String>> query =
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        new GenericQueryMessage<>("criteria", "query", ResponseTypes.multipleInstancesOf(String.class));
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// send a query message and print query response
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queryBus.scatterGather(query, 10, TimeUnit.SECONDS)
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        .map(Message::getPayload)
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        .flatMap(Collection::stream)
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        .forEach(System.out::println);
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Subscription query
The subscription query allows a client to get the initial state of the model it wants to query, and to stay up-to-date as the queried view model changes. In short it is an invocation of the Direct Query with possibility to be updated when the initial state changes. To up date a subscription with changes to the model, we will use the QueryUpdateEmitter component provided by Axon.
Let's extend our CardSummaryProjection example in the Quick Start section with a query handler for a specific GiftCard:
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@QueryHandler
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public CardSummary fetch(String cardId) {
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    return cardSummaries.stream()
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                        .filter(cs -> cs.getId().equals(cardId))
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                        .findFirst()
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                        .orElse(null);
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}
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This query handler will provide us with the initial state of a GiftCard. Once our GiftCard gets redeemed we would like to update any component which is interested in the updated state of that GiftCard. We'll achieve this by emitting an update using the QueryUpdateEmitter component within the event handler function of the RedeemedEvt event:
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@EventHandler
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public void on(RedeemedEvt evt) {
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    // (1)
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    cardSummaries.stream()
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                 .filter(cs -> evt.getId().equals(cs.getId()))
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                 .findFirst()
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                 .ifPresent(cardSummary -> {
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                     CardSummary updatedCardSummary = cardSummary.deductAmount(evt.getAmount());
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                     cardSummaries.remove(cardSummary);
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                     cardSummaries.add(updatedCardSummary);
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                 });
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    // (2)
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    queryUpdateEmitter.emit(String.class, cardId -> cardId.equals(evt.getId()), evt.getAmount());
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}
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(1) First, we update our view model the same way it was already done.
(2) If there is a subscription query interested in updates about this specific GiftCard we emit an update. The first parameter of the emission is the type of the query (String in our case) which corresponds to the query type in previously defined query handler. The second parameter is a predicate which will select the subscription query to be updated. In our case we will update only subscription queries interested in the GiftCard which has been updated. The third parameter is the actual update, which in our case is the redeemed amount. There are several overloads of the emit method present, feel free to take a look at JavaDoc for more specifics on that. Important thing to underline here is that an update is a message and that some overloads take the update message as a parameter (in our case we just sent the payload which was wrapped in the message) which enables us to attach meta-data for example.
Once we have query handling and emitting side implemented, we can issue a subscription query to get initial state of the GiftCard and be updated once this GiftCard is redeemed:
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// (1)
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commandGateway.sendAndWait(new IssueCmd("gc1", 100)); 
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// (2)
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GenericSubscriptionQueryMessage<String, CardSummary, Integer> query =
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                new GenericSubscriptionQueryMessage<>("gc1",
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                                                      ResponseTypes.instanceOf(CardSummary.class),
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                                                      ResponseTypes.instanceOf(Integer.class));
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// (3)
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SubscriptionQueryResult<QueryResponseMessage<CardSummary>, SubscriptionQueryUpdateMessage<Integer>> queryResult =
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                queryBus.subscriptionQuery(query);
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// (4)
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queryResult.handle(System.out::println, System.out::println);
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// (5)    
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commandGateway.sendAndWait(new RedeemCmd("gc1", 10));
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(1) Issuing a GiftCard with "gc1" id and initial value of 100.
(2) Creating a subscription query message to get the initial state of "gc1" GiftCard (this initial state is of type CardSummary) and to be updated once the state of GiftCard with id "gc1" is changed (in our case update means the card is redeemed). The type of the update is an Integer. Do note that the type of the update must match the type of the emission side.
(3) Once the message is created, we are sending it via the QueryBus. We receive a query result which contains two components: one is initialResult and the other is updates. In order to achieve 'reactiveness' we use Project Reactor's Mono for initialResult and Flux for updates.
Note Once the subscription query is issued, all updates are queued until the subscription to the Flux of updates is done. This behavior prevents losing of updates.
Note The Framework prevents issuing more than one query message with the same id. If it is necessary to be updated in several different places, create a new query message.
Note reactor-core dependency is mandatory for usage of subscription queries. However, it is a compile time dependency and it is not required for other Axon features.
(4) The SubscriptionQueryResult#handle(Consumer<? super I>, Consumer<? super U>) method gives us the possibility to subscribe to the initialResult and the updates in one go. If we want more granular control over the results, we can use the initialResult() and updates() methods on the query result.
(5) When we issue a RedeemCmd, our event handler in the projection will eventually be triggered, which will result in the emission of an update. Since we subscribed with the println() method to updates, the update will be printed out once it is received.
When we run our example, this is the output we will receive:
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GenericQueryResponseMessage{payload={CardSummary{id='gc1', initialAmount=100, remainingAmount=100}}, metadata={}, messageIdentifier='c8c49834-55b8-40f9-b5ef-8268f9f335d0'}
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GenericSubscriptionQueryUpdateMessage{payload={10}, metadata={'traceId'->'00cef44d-d09a-4abb-b62e-89a56fdef1f7', 'correlationId'->'30185a39-1652-45c5-b1a6-01c200f5b038'}, messageIdentifier='b7d6d6b3-110d-4b5b-afa2-1da7cb6568bf'}
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SimpleQueryBus
The SimpleQueryBus is the only Query Bus implementation in Axon 3.1. It does straightforward processing of queries in the thread that dispatches them. The SimpleQueryBus allows interceptors to be configured.
Query Interceptors
One of the advantages of using a query bus is the ability to undertake action based on all incoming queries. Examples are logging or authentication, which you might want to do regardless of the type of query. This is done using Interceptors.
There are different types of interceptors: Dispatch Interceptors and Handler Interceptors. Dispatch Interceptors are invoked before a query is dispatched to a Query Handler. At that point, it may not even be sure that any handler exists for that query. Handler Interceptors are invoked just before a Query Handler is invoked.
Dispatch Interceptors
Message Dispatch Interceptors are invoked when a query is dispatched on a Query Bus. They have the ability to alter the Query Message, by adding Meta Data, for example, or block the query by throwing an Exception. These interceptors are always invoked on the thread that dispatches the Query. Message Dispatch Interceptors are invoked when a query is dispatched on a Query Bus. They have the ability to alter the Query Message, by adding Meta Data, for example, or block the query by throwing an Exception. These interceptors are always invoked on the thread that dispatches the Query.
Structural validation
There is no point in processing a query if it does not contain all required information in the correct format. In fact, a query that lacks information should be blocked as early as possible. Therefore, an interceptor should check all incoming queries for the availability of such information. This is called structural validation.
Axon Framework has support for JSR 303 Bean Validation based validation. This allows you to annotate the fields on queries with annotations like @NotEmpty and @Pattern. You need to include a JSR 303 implementation (such as Hibernate-Validator) on your classpath. Then, configure a BeanValidationInterceptor on your Query Bus, and it will automatically find and configure your validator implementation. While it uses sensible defaults, you can fine-tune it to your specific needs.
Tip
You want to spend as few resources on an invalid queries as possible. Therefore, this interceptor is generally placed in the very front of the interceptor chain. In some cases, a Logging or Auditing interceptor might need to be placed in front, with the validating interceptor immediately following it.
The BeanValidationInterceptor also implements MessageHandlerInterceptor, allowing you to configure it as a Handler Interceptor as well.
Handler Interceptors
Message Handler Interceptors can take action both before and after query processing. Interceptors can even block query processing altogether, for example for security reasons.
Interceptors must implement the MessageHandlerInterceptor interface. This interface declares one method, handle, that takes three parameters: the query message, the current UnitOfWork and an InterceptorChain. The InterceptorChain is used to continue the dispatching process, whereas the UnitOfWork gives you (1) the message being handled and (2) provides the possibility to tie in logic prior, during or after (query) message handling (see UnitOfWork for more information about the phases).
Unlike Dispatch Interceptors, Handler Interceptors are invoked in the context of the Query Handler. That means they can attach correlation data based on the Message being handled to the Unit of Work, for example. This correlation data will then be attached to messages being created in the context of that Unit of Work.
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Dispatch Interceptors
Handler Interceptors