Dev Blog - Johan Danforth - Contents tagged with CQRS

A One File .NET Micro Service Bus for MSMQ?

Friday, August 19, 2011

9 Comments

This last year our company has invested quite some time in looking at CQRS, which led to looking at great looking service-buses like nServiceBus, Rhino Service Bus and Mass Transit, which led me to do some bus-coding on my own, mostly for fun and for learning MSMQ.

Inspired by the service buses mentioned above, the result became a bare-bones-one-file “micro service bus” that I call MiniBuss which sits on top of MSMQ. The file itself is only some 400 lines of code and supports send, receive, reply, publish and subscribe/unsubscribe.

Setting up a sender may look something like this:

var bus = new MiniBuss.ServiceBus();
 
 bus.RegisterMessageEndpoint<HelloCommand>("minibuss_receiver1@johan-dell-ssd");
 
 bus.Start();
 
 bus.Send(new HelloCommand { Guid = Guid.NewGuid(), Message = "Hello" });

Create the bus, register a message and tell it where messages of this type should go, start the bus and send the message.

Setting up a receiver may look something like this:

var bus = new ServiceBus { LocalEndpoint = "minibuss_receiver1" };
 
 bus.RegisterMessageHandler<HelloCommand>(command => Console.WriteLine(command.Message + " Guid: " + command.Guid));
 
 bus.Start();

Create the bus and tell it which endpoint to listen to (which creates a local MSMQ queue if necessary) and tell it which message type to listen for and which delegate to kick off when such a message is received.

Similarly, when doing a receive/reply, you would have to create the bus on the sender side with a local endpoint and register a message-handler for replies, like this:

var bus = new MiniBuss.ServiceBus { LocalEndpoint = "minibuss_sender1" };
 
 bus.RegisterMessageEndpoint<HelloCommand>("minibuss_receiver1@johan-dell-ssd");
 
 bus.RegisterMessageHandler<HelloResponse>(reply => Console.WriteLine("Reply from receiver: " + reply.Message));
 
 bus.Start();
 
 Console.WriteLine("Sending command...");
 bus.Send(new HelloCommand { Guid = Guid.NewGuid(), Message = "Hello" });

The receiver would do a bus.reply() like this:

var bus = new ServiceBus { LocalEndpoint = "minibuss_receiver1" };
 
 bus.RegisterMessageHandler<HelloCommand>(command =>
 {
     Console.WriteLine(command.Message + " Guid: " + command.Guid);
 
     bus.Reply(new HelloResponse { Guid = Guid.NewGuid(), Message = "Hello back!" });
 });
 
 bus.Start();

The MiniBus also supports publish to multiple subscribers. A simple publisher would create a bus with a local endpoint (to receive subscribe/unsubscribe commands), tell it to handle subscriptions for a certain event, then start publishing something every 5 seconds (as an example):

var bus = new MiniBuss.ServiceBus { LocalEndpoint = "minibuss_publisher1" };
 
 bus.HandleSubscriptionsFor<SomethingHappenedEvent>();
 
 bus.Start();    
 
 Task.Factory.StartNew(() => PublishingThread(bus), TaskCreationOptions.LongRunning);
 
 Console.WriteLine("Done, press ENTER to exit");
 Console.ReadLine();

private static void PublishingThread(MiniBuss.ServiceBus bus)
 {
     while (true)
     {
         Thread.Sleep(5000);
         var guid = Guid.NewGuid();
         Console.WriteLine("Publishing event with guid " + guid);
         bus.Publish(new SomethingHappenedEvent() { Guid = guid, Sent = DateTime.Now });
     }
 }

Any clients interesting in subscribing to events from the publisher would create a bus with a local endpoint, start the bus and then send a subscribe command to the publisher, telling it you’re interested in subscribing to a certain type of event and which delegate to handle it:

var bus = new MiniBuss.ServiceBus {LocalEndpoint = "minibuss_subscriber1"};
 
 bus.Start();
 
 bus.Subscribe<SomethingHappenedEvent>("minibuss_publisher1@localhost", 
     @event => Console.WriteLine("something happened at {0}, event id {1}",
         @event.Sent, @event.Guid));
 
 Console.WriteLine("Waiting for events, press ENTER to exit");
 Console.ReadLine();
 
 bus.UnSubscribe<SomethingHappenedEvent>("minibuss_publisher1");

Now, the question is, what do you think? I know there are issues with the code, like how to make message-handlers multi-threaded/concurrent without messing things up (it’s single threaded right now), and how to best handle exceptions, rollbacks and re-tries . Right now handling exceptions in send and receive works pretty well within a TransactionScope() together with ADO.NET, and if there’s an exception, the message is moved the an error-queue. No re-try or anything, only rollback and move to xxx_error. Also, the publisher doesn’t persist subscriptions, so if it is restarted subscribers wouldn’t know. You know, things like that.

I’m a user of the micro-orm called Dapper and like it a lot, so I’m thinking that maybe I should release this micro-bus as open source and see where people may take it, if anywhere? Maybe just down the drain because they figure out this service bus is dangerous to use and risk loosing messages or something (which would be extremely good to know :)

Or maybe this code is useless because you already got nServiceBus and Rhino out there and coders don’t need another service bus?

What do you say?