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ErraiBus supports a high-level RPC layer to make typical client-server RPC communication easy on top of the bus. While it is possible to use ErraiBus without ever using this API, you may find it to be a more useful and concise approach for exposing services to the clients.

Please note that this API has changed since version 1.0. RPC services provide a way of creating type-safe mechanisms to make client-to-server calls. Currently, this mechanism only support client-to-server calls, and not vice-versa.

Creating a service is straight forward. It requires the definition of a remote interface, and a service class which implements it. See the following:

The @Remote annotation tells Errai that we'd like to use this interface as a remote interface. The remote interface must be part of of the GWT client code. It cannot be part of the server-side code, since the interface will need to be referenced from both the client and server side code. That said, the implementation of a service is relatively simple to the point:

That's all there is to it. You use the same @Service annotation as described in Section 2.4. The presence of the remote interface tips Errai off as to what you want to do with the class.

Beginning with Errai 2.0.CR1, the default for automatic service discovery has changed in favour of CDI based applications, meaning RPC service discovery must be explicitly turned on in case Errai CDI is not used (the weld-integration.jar is not on the classpath). This can be done using an init-param in the servlet config of your web.xml:

Making calls

Calling a remote service involves use of the MessageBuilder API. Since all messages are asynchronous, the actual code for calling the remote service involves the use of a callback, which we use to receive the response from the remote method. Let's see how it works:

In the above example, we declare a remote callback that receives a Boolean, to correspond to the return value of the method on the server. We also reference the remote interface we are calling, and directly call the method. However, don't be tempted to write code like this:

The above code will never return a valid result. In fact, it will always return null, false, or 0 depending on the type. This is due to the fact that the method is dispatched asynchronously, as in, it does not wait for a server response before returning control. The reason we chose to do this, as opposed to emulate the native GWT-approach, which requires the implementation of remote and async interfaces, was purely a function of a tradeoff for simplicity.

Proxy Injection

An alternative to using the MessageBuilder API is to have a proxy of the service injected.

For calling the remote service, the callback objects need to be provided to the call method before the corresponding interface method is invoked.

The Errai IOC GWT module needs to be inherited to make use of caller injection. To do this, the following line needs to be added to the application's *.gwt.xml file. It is important that this line comes after the Errai Bus module:

Handling exceptions

Handling remote exceptions can be done by providing an ErrorCallback on the client:

As remote exceptions need to be serialized to be sent to the client, the @Portable annotation needs to be present on the corresponding exception class (see Marshalling). Further the exception class needs to be part of the client-side code. For more details on ErrorCallbacks see Handling Errors.

Global RPC exception handler

In a scenario where many different remote calls potentially throw the same exception types (e.g. exceptions related to authentication or authorization) it can be easier to register a global exception handler instead of providing error callbacks at each RPC invocation. This global exception handler is called in case an exception occurs in the process of a remote call that has no error callback associated with it. So, it will handle an otherwise uncaught exception.

Client-side Interceptors

Client-side remote call interceptors provide the ability to manipulate or bypass the remote call before it's being sent. This is useful for implementing crosscutting concerns like caching, for example when the remote call should be avoided if the data is already cached locally.

To have a remote call intercepted, either an interface method or the remote interface type has to be annotated with @InterceptedCall. If the type is annotated, all interface methods will be intercepted.

Note that an ordered list of interceptors can be used for specifying an interceptor chain e.g.

Implementing an interceptor is easy:

The RemoteCallContext passed to the aroundInvoke method provides access to the intercepted method's name and read/write access to the parameter values provided at the call site.

Calling proceed executes the next interceptor in the chain or the actual remote call if all interceptors have already been executed. If access to the result of the (asynchronous) remote call is needed in the interceptor, one of the overloaded versions of proceed accepting a RemoteCallback has to be used instead.

The result of the remote call can be manipulated by calling RemoteCallContext.setResult().

Not calling proceed in the interceptor bypasses the actual remote call, passing RestCallContext.getResult() to the RemoteCallBack provided at the call site.

Session and request objects in RPC endpoints

Before invoking an endpoint method Errai sets up an RpcContext that provides access to message resources that are otherwise not visible to RPC endpoints.

Batching remote calls

Some use cases require multiple interactions with the server to complete. Errai's RPC mechanism allows for batched invocations of remote methods that will be executed using a single server round-trip. This is useful for reducing the number of simultaneous HTTP connections and at the same time allows for reusing and combining fine-grained remote services.

Injecting a BatchCaller instead of a Caller<T> is all it takes to make use of batched remote procedure calls.

The remote methods will get executed only after sendBatch() was called. The method sendBatch accepts an additional RemoteCallback instance as a parameter which will we invoked when all remote calls have completed in success. Consequently, an ErrorCallback can also be provided which will get executed for all remote calls that have completed in failure.

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