Interface for message buses.
A message bus is the main logical component to facilitate communication between (possibly distributed) parts of an application using Message Passing as its underlying architectural pattern. It allows registered sender and recipient objects, subsequently referred to as Message Endpoints, to exchange structured data in the form of user defined messages[1].
Depending on their usage, messages are classified into a number of types, such as commands, events and documents. In Unreal Engine, all these messages are modeled with regular built-in or user defined UStructs that may either be empty or contain data in the form of FProperty fields[2]. Before being dispatched, messages are internally wrapped into so called Message Context objects (IMessageContext) that contain additional information about a message, such as when it was sent and the sender and recipients.
The sending and receiving of messages is not limited to message endpoints within the same thread or process, but may be extended to other applications running on the same computer or on other computers connected to a network with so called Message Transport Plug-ins, such as the UdpMessaging plug-in that ships with Unreal Engine. The main goal of a message bus is to hide the technical details of the underlying transport mechanisms, so that users can focus on implementing their distributed applications rather than worrying about how data gets from one endpoint to another. A message bus makes it look as if all senders and recipients are located within the same process, regardless of whether that is actually the case or not. It is possible to restrict the reach of messages using so called Message Scopes (EMessageScope).
All message recipients (objects implementing the see IMessageReceiver interface) must be registered with the message bus through the see IMessageBus.Register method. Before a recipient is destroyed it should unregister itself using the see IMessageBus.Unregister method. Message senders (objects implementing the see IMessageSender interface) do not register with the bus, but instead pass a reference to themselves each time they send a message. The IMessageReceiver and IMessageSender both are very low-level interfaces into the messaging system. Most users will prefer to use instances of the see FMessageEndpoint class instead, which provides a much more convenient way of sending and receiving messages.
Message buses in Unreal Engine support the following two common messaging patterns: Request-Reply and Publish-Subscribe. Please note that higher level concepts, such as Remote Procedure Call (RPC) are not part of the messaging system, but we may provide them as separate features in the future.
In the Request-Reply pattern a message is sent to one or more particular message recipients using the IMessageBus.Send method. Message recipients implement the see IMessageRecipient interface and are uniquely identified by their addresses (FMessageAddress). After a message is received, the recipients may then reply with another message using the same IMessageBus.Send method. Alternatively, a previously received message may be forwarded to another recipient using the see IMessageBus.Forward method. This pattern is useful when message recipients already know about each other and wish to communicate directly, i.e. to exchange commands or events.
In the Publish-Subscribe pattern a message is sent to all message recipients on the bus using the see IMessageBus.Publish method. Only recipients that previously subscribed to the type of the sent message using the IMessageBus.Subscribe method will actually receive the message. All other recipients will not receive the message. After a published message is received, the recipients may respond with another message, either directly to the message sender using IMessageBus.Send method, or by publishing another message using the the IMessageBus.Publish method. This pattern is useful for discovering recipients on the bus and to message recipients that are unknown.
Most applications will often use a combination of both Request-Reply and Publish-Subscribe. A typical implementation of distributed applications involves service providers and service consumers (a service can be any useful functionality provided by a system). The service consumers will often publish a special message in order to discover service providers. Service providers are subscribed to these special messages and will respond with another message that contains information about the provider and that is sent back directly to the consumer. The consumer is now aware of the service provider's existence and its message address, and it can then request services by directly sending all subsequent messages directly to it.
The dispatching of sent and published messages to the correct recipients is accomplished by the Message Router, which is an internal component of the message bus. The message router maintains an address book of known message endpoints that allows it to determine the destination of messages. If a message cannot be delivered, it is forwarded to a so called Dead Letter Channel, which is a queue of messages that can be inspected for debugging purposes[3].
It is possible to intercept messages before they are being routed to recipients by registering so called Message Interceptors (objects implementing the IMessageInterceptor interface) with the message bus. This feature enables advanced use cases that require inspection or manipulation of messages contents, such as message filtering and enriching, splitting and aggregating, re-sequencing or authentication[4].
The messaging system also provides a facility for debugging the system itself through the so called Message Tracer (an internal object implementing the see IMessageTracer interface) that can be accessed with the IMessageBus.GetTracer() method. The message tracer is currently used in the Messaging Debugger - a visual debugging tool for the Messaging System in Unreal Frontend and the Unreal Editor.
Notes:
[1] The messaging implementation in Unreal Engine is more or less closely following the terminology in the Enterprise Integration Patterns book at http://www.eaipatterns.com/eaipatterns.html. A good introduction to messaging concepts can also be found on MSDN at http://msdn.microsoft.com/en-us/library/ff647328.aspx
[2] It is currently also required for such structures to implement a specific TStructOpsTypeTraits type trait or otherwise they will not be recognized as legal message types. Search for 'WithMessagingHandling' in the code base to find existing examples. We hope to remove the need for these type traits soon.
[3] Dead letter channels are not implemented yet.
[4] There are currently no built-in mechanisms for authentication and authorization, but they are on the to-do list.
- See also
- FMessageAddress, FMessageEndpoint, IMessageReceiver, IMessageSender
Inheritance diagram for IMessageBus:
1.9.8 