Using NServiceBus.Router to send messages between distributed sites

Running the project

1. Start the projects in debug mode.
2. Press Enter a couple of times.
3. Observe the PingHandler logging processed message IDs in the Server window.
4. Observe the PongHandler logging processed message IDs in the Client window.

Code walk-through

The solution consists of five projects.

Shared

The Shared project contains the message contracts.

Client

The Client project contains an NServiceBus endpoint that sends the Ping messages and expects Pong responses. It is configured to send the Ping messages through a local router:

var transport = endpointConfiguration.UseTransport<LearningTransport>();
var routingSettings = transport.Routing().ConnectToRouter("SiteA");
routingSettings.RouteToEndpoint(typeof(Ping), "Samples.Router.Sites.Server");

RouterA

The RouterA project sets up the client-side router. This router is set up to forward messages sent to SiteB to the SiteB router via the Tunnel interface.

var routing = routerConfig.UseStaticRoutingProtocol();
routing.AddRoute(
    destinationFilter: (iface, destination) => destination.Site == "SiteB",
    destinationFilterDescription: "Forward to SiteB",
    gateway: "SiteB",
    iface: "Tunnel");

It does not contain any forwarding configuration because the Pong reply message is routed automatically.

RouterB

The RouterB project sets up the server-side router. This router is set up to forward messages coming out of the Tunnel interface to the local transport.

var routing = routerConfig.UseStaticRoutingProtocol();
routing.AddForwardRoute("Tunnel", "Local");

Server

The Server project contains an NServiceBus endpoint that processes the Ping messages and sends the Pong messages as a response.

How it works

The Client sends the Ping message to the router using local transport. The message carries a special header that denotes which site or sites the message should be delivered to.

When the router in the origin site (RouterA) picks up the message it looks at the header and dispatches the message to the router in the destination site(s) using the tunnel transport (MSMQ in this sample). Before dispatching, the router stamps the message with a header denoting the origin site.

When the router in the destination site (RouterB) receives the message it looks at the type of the message and calculates the receiving endpoint name based on the forwarding rules configured.

When the Server receives the Ping message it replies with a Pong message. The reply does not require any routing because the routers along the path of the the original Ping message leave breadcrumbs that can be used to route the reply along the same path in the opposite direction.