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This section covers the core messaging concepts of the ErraiBus messaging framework.
It's important to understand the concept of how messaging works in ErraiBus. Service endpoints are given string-based names that are referenced by message senders. There is no difference between sending a message to a client-based service, or sending a message to a server-based service. In fact, a service of the same name may co-exist on both the client and the server and both will receive all messages bound for that service name, whether they are sent from the client or from the server.
Services are lightweight in ErraiBus, and can be declared liberally and extensively within your application to provide a message-based infrastructure for your web application. It can be tempting to think of ErraiBus simply as a client-server communication platform, but there is a plethora of possibilities for using ErraiBus purely with the GWT client context, such as a way to advertise and expose components dynamically, to get around the lack of reflection in GWT.
In fact, ErraiBus was originally designed to run completely within the client but quickly evolved into having the capabilities it now has today. So keep that in mind when you run up against problems in the client space that could benefit from runtime federation.
The MessageBuilder is the heart of the messaging API in ErraiBus. It provides a fluent / builder API, that is used for constructing messages. All three major message patterns can be constructed from the
MessageBuilder
.
Components that want to receive messages need to implement the
MessageCallback
interface.
But before we dive into the details, let look at some use cases first.
Sending Messages with the Client BusIn order to send a message from a client you need to create a
Message
and send it through an instance of
MessageBus
. In this simple example we send it to the subject 'HelloWorldService'.
public class HelloWorld implements EntryPoint {
// Get an instance of the RequestDispatcher
private RequestDispatcher dispatcher = ErraiBus.getDispatcher();
public void onModuleLoad() {
Button button = new Button("Send message");
button.addClickHandler(new ClickHandler() {
public void onClick(ClickEvent event) {
// Send a message to the 'HelloWorldService'.
MessageBuilder.createMessage()
.toSubject("HelloWorldService") // (1)
.signalling() // (2)
.noErrorHandling() // (3)
.sendNowWith(dispatcher); // (4)
});
[...]
}
}
}In the above example we build and send a message every time the button is clicked. Here's an explanation of what's going on as annotated above:
We specify the subject we wish to send a message to. In this case, "
HelloWorldService
".
We indicate that we wish to only signal the service, meaning, that we're not sending a qualifying command to the service. For information on this, read the section on Protocols .
We indicate that we do not want to provide an
ErrorCallback
to deal with errors for this message.
We transmit the message by providing an instance to the
RequestDispatcher
An astute observer will note that access to the
RequestDispatcher
differs within client code and server code. Because the client code does not run within a container, access to the
RequestDispatcher
and
MessageBus
is statically accessed using the
ErraiBus.get()
and
ErraiBus.getDispatcher()
methods. The server-side code, conversely, runs inside a dependency container for managing components. It currently uses Guice as the default container, but will more generically support the JSR-330 specification in the future.
Recieving Messages on the Server Bus / Server ServicesEvery message has a sender and at least one receiver. A receiver is as it sounds--it receives the message and does something with it. Implementing a receiver (also referred to as a service) is as simple as implementing our standard MessageCallback interface, which is used pervasively across, both client and server code. Let's begin with server side component that receives messages:
@Service
public class HelloWorldService implements MessageCallback {
public void callback(Message message) {
System.out.println("Hello, World!");
}
}
He we declare an extremely simple service. The
@Service
annotation provides a convenient, meta-data based way of having the bus auto-discover and deploy the service.
Sending Messages with the Server BusIn the following example we extend our server side component to reply with a message when the callback method is invoked. It will create a message and address it to the subject '
HelloWorldClient
':
@Service
public class HelloWorldService implements MessageCallback {
private RequestDispatcher dispatcher;
@Inject
public HelloWorldService(RequestDispatcher disaptcher) {
dispatcher = dispatcher;
}
public void callback(CommandMessage message) {
// Send a message to the 'HelloWorldClient'.
MessageBuilder.createMessage()
.toSubject("HelloWorldClient") // (1)
.signalling() // (2)
.with("text", "Hi There") // (3)
.noErrorHandling() // (4)
.sendNowWith(dispatcher); // (5)
});
}
}The above example shows a service which sends a message in response to receiving a message. Here's what's going on:
We specify the subject we wish to send a message to. In this case, "
HelloWorldClient
". We are sending this message to all clients which are listening in on this subject. For information on how to communicate with a single client, see Section 2.6.
We indicate that we wish to only signal the service, meaning that we're not sending a qualifying command to the service. For information on this, read the section on Protocols.
We add a message part called "text" which contains the value "Hi there".
We indicate that we do not want to provide an
ErrorCallback
to deal with errors for this message.
We transmit the message by providing an instance of the
RequestDispatcher
.
Receiving Messages on the Client Bus/ Client ServicesMessages can be received asynchronously and arbitriraily by declaring callback services within the client bus. As ErraiBus maintains an open COMET channel at all times, these messages are delivered in real time to the client as they are sent. This provides built-in push messaging for all client services.
public class HelloWorld implements EntryPoint {
private MessageBus bus = ErraiBus.get();
public void onModuleLoad() {
[...]
/**
* Declare a local service to receive messages on the subject
* "BroadcastReceiver".
*/
bus.subscribe("BroadcastReceiver", new MessageCallback() {
public void callback(CommandMessage message) {
/**
* When a message arrives, extract the "text" field and
* do something with it
*/
String messageText = message.get(String.class, "text");
}
});
[...]
}
}
In the above example, we declare a new client service called
"BroadcastReceiver"
which can now accept both local messages and remote messages from the server bus. The service will be available in the client to receive messages as long the client bus is and the service is not explicitly de-registered.
ConversationsConversations are message exchanges which are between a single client and a service. They are a fundmentally important concept in ErraiBus, since by default, a message will be broadcast to all client services listening on a particular channel.
When you create a reply with an incoming message, you ensure that the message you are sending back is received by the same client which sent the incoming message. A simple example:
@Service
public class HelloWorldService implements MessageCallback {
public void callback(CommandMessage message) {
// Send a message to the 'HelloWorldClient' on the client that sent us the
// the message.
MessageBuilder.createConversation(message)
.toSubject("HelloWorldClient")
.signalling()
.with("text", "Hi There! We're having a reply!")
.noErrorHandling().reply();
});
}
}
Note that the only difference between the example in the previous section (2.4) and this is the use of the
createConversation()}}method with {{MessageBuilder
.
It is possible to contruct a message and a default response handler as part of the
MessageBuilder
API. It should be noted, that multiple replies will not be possible and will result an exception if attempted. Using this aspect of the API is very useful for doing simple psuedo-synchronous conversive things.
You can do this by specifying a
MessageCallback
using the
repliesTo()
method in the
MessageBuilder
API after specifying the error handling of the message.
MessageBuilder.createMessage()
.toSubject("ConversationalService").signalling()
.with("SomeField", someValue)
.noErrorHandling()
.repliesTo(new MessageCallback() {
public void callback(Message message) {
System.out.println("I received a response");
}
})See the next section on how to build conversational services that can respond to such messages.
It is possible for the sender to infer, to whatever conversational service it is calling, what subject it would like the reply to go to. This is accomplished by utilizing the standard
MessageParts.ReplyTo
message part. Using this methodology for building conversations is generally encouraged.
Consider the following client side code:
MessageBuilder.createMessage()
.toSubject("ObjectService").signalling()
.with(MessageParts.ReplyTo, "ClientEndpoint")
.noErrorHandling().sendNowWith(dispatcher);And the conversational code on the server (for service ObjectService ):
MessageBuilder.createConversation(message)
.subjectProvided().signalling()
.with("Records", records)
.noErrorHandling().reply();
In the above examples, assuming that the latter example is inside a service called "
ObjectService
" and is referencing the incoming message that was sent in the former example, the message created will automatically reference the
ReplyTo
subject that was provided by the sender, and send the message back to the subject desired by the client on the client that sent the message.
Broadcasting messages to all clients listening on a specific subject is quite simple and involves nothing more than forgoing use of the reply API. For instance:
MessageBuilder.createMessage().
.toSubject("MessageListener")
.with("Text", "Hello, from your overlords in the cloud")
.noErrorHandling().sendGlobalWith(dispatcher);
If sent from the server, all clients currently connected, who are listening to the subject
"MessageListener"
will receive the message. It's as simple as that.
Communication from one client to another client is not directly possible within the bus federation, by design. This isn't to say that it's not possible. But one client cannot see a service within the federation of another client. We institute this limitation as a matter of basic security. But many software engineers will likely find the prospects of such communication appealing, so this section will provide some basic pointers on how to go about accomplishing it.
Relay ServicesThe essential architectural thing you'll need to do is create a relay service that runs on the server. Since a service advertised on the server is visible to all clients and all clients are visible to the server, you might already see where we're going with this.
By creating a service on the server which accepts messages from clients, you can create a simple protocol on-top of the bus to enable quasi peer-to-peer communication. (We say quasi, because it still needs to be routed through the server)
While you can probably imagine simply creating a broadcast-like service which accepts a message from one client and broadcasts it to the rest of the world, it may be less clear how to go about routing from one particular client to another particular client, so we'll focus on that problem. This is covered in Section 19.14.13, “Message Routing Information”
Every message that is sent between a local and remote (or server and client) buses contain session routing information. This information is used by the bus to determine what outbound queues to use to deliver the message to, so they will reach their intended recipients. It is possible to manually specify this information to indicate to the bus, where you want a specific message to go.
The utility class
org.jboss.errai.bus.server.util.ServerBusUtils
contains a utility method for extracting the String-based SessionID which is used to identify the message queue associated with any particular client. You may use this method to extract the
SessionID
from a message so that you may use it for routing. For example:
...
public void callback(Message message) {
String sessionId = ServerBusUtils.getSessionId(message);
// Record this sessionId somewhere.
...
}
The
SessionID
can then be stored in a medium, say a Map, to cross-reference specific users or whatever identifier you wish to allow one client to obtain a reference to the specific
SessionID
of another client. In which case, you can then provide the
SessionID
as a MessagePart to indicate to the bus where you want the message to go.
MessageBuilder.createMessage()
.toSubject("ClientMessageListener")
.signalling()
.with(MessageParts.SessionID, sessionId)
.with("Message", "We're relaying a message!")
.noErrorHandling().sendNowWith(dispatcher);
By providing the
SessionID
part in the message, the bus will see this and use it for routing the message to the relevant queue.
Now you're routing from client-to-client!
It may be tempting however, to try and include destination
SessionIDs
at the client level, assuming that this will make the infrastructure simpler. But this will not achieve the desired results, as the bus treats
SessionIDs
as transient. Meaning, the
SessionID
information is not ever transmitted from bus-to-bus, and therefore is only directly relevant to the proximate bus.