The document provides an overview of Unreal Engine's message bus system. It discusses how the message bus allows for exchanging data between endpoints using messages. It describes creating message types, endpoints that send and receive messages, and how publish-subscribe and request-response patterns work. The document also discusses using message bridges to connect message buses across processes and machines.

1. UE4 Twitch Stream 5/5/2016
Unreal Message Bus Overview
Gerke Max Preussner
max.preussner@epicgames.com
@gmpreussner

2. Process
Message
Endpoints
Message Bus
Message Bus:
Framework for exchanging commands,
events, documents, or any other data
inside UStructs between endpoints
USTRUCT()
struct FHelloMessage
{
GENERATED_BODY()
UPROPERTY(EditAnywhere, Category=“Message”)
FString HelloText;
FHelloMessage(const FString& InText) : HelloText(InText) { }
};
Declare a message type
(you can also use any existing UStruct, such as FVector or FColor)

3. Process
SenderEndpoint = FMessageEndpoint::Builder(“FMySender”, MessageBus);
ReceiverEndpoint = FMessageEndpoint::Builder(“FMyReceiver”, MessageBus)
.Handling<FHelloMessage>(this, &FMyReceiver::HandleHelloMessage);
Create a sender endpoint
Create a receiver endpoint
Message Endpoints:
Helper classes for sending and receiving
messages on the message bus

4. Process
Subscribers
Publisher
SenderEndpoint = FMessageEndpoint::Builder(“FMySender”, MessageBus);
ReceiverEndpoint = FMessageEndpoint::Builder(“FMyReceiver”, MessageBus)
.Handling<FHelloMessage>(this, &FMyReceiver::HandleHelloMessage);
if (ReceiverEndpoint.IsValid())
{
ReceiverEndpoint->Subscribe<FHelloMessage>();
}
SenderEndpoint.Publish(new FHelloMessage(TEXT(“Hey there!”)));
void FMyReceiver::HandleHelloMessage(const FHelloMessage& Message,
const TSharedRef<IMessageContextRef>& Context)
{
GLog->Log(*Message.HelloText); // prints “Hey there!”
}
Subscribe receiver to FHelloMessage messages
Publish a message to all subscribers
Handle received FHelloMessage
Publish-Subscribe:
Messages are sent to all endpoints that
subscribed to that message type

5. Process
Responder
Requester
SenderEndpoint = FMessageEndpoint::Builder(“FMySender”, MessageBus);
ReceiverEndpoint = FMessageEndpoint::Builder(“FMyReceiver”, MessageBus)
.Handling<FHelloMessage>(this, &FMyReceiver::HandleHelloMessage);
if (ReceiverEndpoint.IsValid())
{
ReeiverEndpoint->Subscribe<FHelloMessage>();
}
SenderEndpoint.Publish(new FHelloMessage(TEXT(“Hey there!”)));
void FMyReceiver::HandleHelloMessage(const FHelloMessage& Message,
const TSharedRef<IMessageContextRef>& Context)
{
SenderEndpoint->Send(new FReplyMessage(TEXT(“O hai!”)), Context->GetSender());
}
Send back a response
Request-Response:
Messages are sent directly to one or more
recipients using their message address

6. Process Process
Message Bridge
Message Bridges:
Connect two message buses in separate
processes via some transport technology

7. Process Process
UDP
TCP
HTTP
Named Pipes
Shared Mem
ZeroMQ
RabbitMQ
etc.
Message Bridges:
Connect two message buses in separate
processes via some transport technology

8. Computer
Computer
Computer
Process Process Process
Process
Internet
(via Message Tunnel)
UE4 Instance
3DStudio Max
Maya
Build Farm
Hardware I/O
etc.

9. The following slides explain UE4’s layered architecture for device management
as a more complex example use case for how the Messaging system can be used
to simplify the process of writing distributed applications.
The diagrams show how the same code architecture is being used to support a
number of different hardware configurations.
The last slide shows how the function calls flow through the various layers. Note
that some of this code has changed over the course of the last year. The intent is
to demonstrate how interprocess communication can be implemented, and how
function calls change to messages and back to function calls.

10. UserPCHostPC
ITargetDevice
ITargetDeviceService
ITargetDeviceProxy
Device Application
Platform specific interfaces
Platform agnostic interfaces
Physical Tier
Legend:
Target Device Management
Uses a multi-layered multi-tier
architecture instead of having the
application talk directly to the devices
MessageBus
Platform SDK

11. Device
Daemon
UserPCHostPC
ITargetDevice
ITargetDeviceService
ITargetDeviceProxy
MessageBus
IPP
Application
Message Bus
UserPCHostPC
ITargetDevice
ITargetDeviceService
ITargetDeviceProxy
Device Application
Platform specific interfaces
Platform agnostic interfaces
Physical Tier
Legend:
• Target devices are usually connected
to some host computer and communicate
via some platform SDK
Example: XBoxOne, PS4
• Some target devices can be deployed to
without a host computer if they have a
network connection
Example: Windows, Mac, iPad, iPhone
MessageBus
Platform SDK

12. User/HostPC
ITargetDevice
ITargetDeviceService
ITargetDeviceProxy
Device MessageBus
Platform SDK
Application
UserPCHostPC/Device
ITargetDevice
ITargetDeviceService
ITargetDeviceProxy
MessageBus
Application
UserPC/HostPC/Device
ITargetDevice
ITargetDeviceService
ITargetDeviceProxy
MessageBus
Application
UserPCHostPC
ITargetDevice
ITargetDeviceService
ITargetDeviceProxy
Device
MessageBus
Platform SDK
Application
• Target device is another computer
Example: Deploying to another PC
• Target device is the local computer
Example: Deploying to your own PC
• Target device is another device
connected to the local PC
Example: iPhone, iPad on your
own computer
• Target device is another device
connected to another PC
Example: iPhone, iPad on someone
else’s computer

13. Application ILauncher
Launch()
FLauncherWorker
Run()
ITargetDeviceProxy
PerformTask()
FLauncherDeployToDeviceTask
DeployApp(TransactionId)
ITargetDeviceService
targetdevice.deploy.file
targetdevice.deploy.commit
ITargetDevice
Local
File
Cache
Deploy(Directory)
IPP iOS Device
Copy Files
Deploy Files
targetdevice.deploy.finished (AppId)
OnDeployCommitted(TransactionId, AppId)
Deploy
Launch
FLauncherLaunchTask
PerformTask()
LaunchApp(AppId)
targetdevice.launch
Launch(AppId)
Launch Daemon
iosdaemon.launch [Url]
iosdaemon.launch .success
targetdevice.launch.finished
return AppId Device
File
System
Message Bus
Function Call
Vendor Specific
Legend: