How MassTransit Publish and Subscribe works

This is a follow-on from my last post, A First Look at MassTransit. Here’s my take on how publish and subscribe works. It’s based on a very brief scan of the MT code, so there could well be misunderstandings and missing details.

The core component of MassTransit is the ServiceBus, it’s the primary API that services use to subscribe to and publish messages. The ServiceBus has an inbound and outbound pipeline. When publish is called, a message gets sent down the pipeline until a component that cares that message type dispatches it to an endpoint. Similarly, when a message is received it is passed down the input pipeline giving each component a chance to process it.

Understanding how the input and output pipelines are populated is the key to understanding how MassTransit works. It’s instructive to get a printout of your pipelines by inspecting them with the PipelineViewer. I’ve created a little class to help with this:

 

using System.IO;
using MassTransit.Pipeline;
using MassTransit.Pipeline.Inspectors;

namespace MassTransit.Play.Helpers
{
    public class PipelineWriter : IPipelineWriter
    {
        private readonly TextWriter writer;
        private readonly IServiceBus bus;

        public PipelineWriter(IServiceBus bus, TextWriter writer)
        {
            this.bus = bus;
            this.writer = writer;
        }

        public void Write()
        {
            writer.WriteLine("InboundPipeline:\r\n");
            WritePipeline(bus.InboundPipeline);
            writer.WriteLine("OutboundPipeline:\r\n");
            WritePipeline(bus.OutboundPipeline);
        }

        private void WritePipeline(IPipelineSink<object> pipeline)
        {
            var inspector = new PipelineViewer();
            pipeline.Inspect(inspector);
            writer.WriteLine(inspector.Text);
        }
    }
}

Let’s look at the sequence of events when RuntimeServices.exe, a subscriber service and a publishing service start up.

When RuntimeServices starts up the SubscriptionService creates a list of ‘SubscriptionClients’. Initially this is empty.

When our subscriber comes on line, it sends an AddSubscriptionClient message to the subscription service. The subscription service then adds our subscriber to its list of subscription clients.

Next our publisher comes on line. It also sends an AddSubscriptionClient message to the subscription service. It too gets added to the subscription clients list.

When the subscriber subscribes to a particular message type, ServiceBus sends an AddSubscription message to SubscriptionService which in turn scans its list of subscription clients and sends the AddSubscription message to each one.

SubscriptionService also adds the subscription to its list of subscriptions so that when any other services come on line it can update them with the list.

The publisher receives the AddSubscription message that was broadcast to all the subscription clients and adds the subscriber endpoint to its outbound pipeline. Note that the Subscriber also receives it’s own AddSubscription message back and adds itself to its outbound pipeline (not shown in the diagram).

The subscriber also adds a component to its inbound pipeline to listen for messages of the subscribed type. I haven’t show this in the diagram either.

When the publisher publishes a message, it sends the message down its outbound pipeline until it is intercepted by the subscriber’s endpoint and dispatched to the subscriber’s queue. The subscription service is not involved at this point.

I hope this is useful if you’re trying to get to grips with MassTransit. Thanks to Dru for clarifying some points for me.