Documentation/Messaging

This document was written during the Georgetown Hackfest. In the meantime, a message broker solution has been implemented in the CTK Plugin Framework. A in-process implementation is available and an out-of-process implementation is available for experimentation.

Use cases

Event Management

Each component/application generates many different types of events. A centralized event manager (Hub and Spoke) can be used to aggregate/dispatch events

1. Synchronization: Window/Leveling events in one window should be synchronized across all viewer windows
2. Temporal calibration: Different components have different update frequency, a centralized manager can be used to filter UpdateEvents, so the whole system is updating at the same frequency

System Integration

Different system/application have different data format or are running on different physical devices. A event bus can be used for system integration

Solution

1. UseMessage Broker for event management

AMQP model
Source: http://upload.wikimedia.org/wikipedia/en/9/9d/The-amqp-model-for-wikipedia.svg

1. Use Message Bus and Publisher/Subscriber design pattern for system integration

Message Bus Source: http://i.msdn.microsoft.com/dynimg/IC97398.gif

Message Bus Source: http://i.msdn.microsoft.com/dynimg/IC17158.gif

Implementation

1. Message layer: OpenIGTLink
   • OpenIGTLink defines many common data format and message structure
   • It has plain vanilla socket support, might switch to ZeroMQ for transportation
2. Transportation layer abstraction: ZeroMQ
   • ZeroMQ is a small and fast implementation of the Advanced Message Queuing Protocol under the LGPL license
   • It supports both synchronous and asynchronous messaging mode
   • It supportsTCP, multicast/PGM, inter-process, and inter-thread transportation
   • CMake version of library can be found here http://github.com/PatrickCheng/zeromq2
   • API can be found here http://api.zeromq.org/zmq.html