HOLMS.Messaging

This module implements the messaging layer of HOLMS. It's basically syntactic sugar over the RabbitMQ library, with a few nice benefits:

- Users don't have to learn another complicated library (RabbitMQ) with its
  idiosyncratic arguments and options
- The RabbitMQ components are hidden from the interface, removing the need
  for direct inclusion of the RabbitMQ library
- The component enforces a constrained style, including setup and publication
  to an (internally) named exchange, queue setup/teardown for listeners,
  use of pub/sub style, and automatic message encoding/decoding using protobuf
- Simplified interfaces for testing

Use

Start by creating a connection factory, the source of connections to the messaging system:

var cf = new MessageConnectionFactory("localhost");

Next, open a connection:

var cn = cf.OpenConnection();

Connections are freely shareable between threads, and are appropriate objects for inclusion in static DI/dependency injection container. However, dependency management systems should still maintain a reference to the connection factory, to re-establish the connection in case it is interrupted.

Next, create a channel -- this must be used by only one thing at once, it is not "thread safe":

var ch = cn.GetChannel();

Channels are the object on which publication and subscription occurs.

To publish an event:

var dto = new DTOForPublication(); ch.Publish("context.subject", dto);

Be sure to include messages here: https://shortbar.atlassian.net/wiki/display/PMC/Messaging+Design

To subscribe to all messages on a topic:

void d(object msg) { if (d is OneTypeOfMessage) { ... } else if (d is SecondTypeOfMessage) { ... } }

var listener = ch.CreateListenerForTopics(d, new string[] {"operations.checkins", "operations.checkouts"});

listener.Start();

Listeners can be started and stopped at any time. Retain a reference to them in a component with an ongoing subscription.