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Just over a week ago, Dru Sellers and I recorded a podcast with Elegant Coders Jarod Ferguson and David Starr. We spent a couple of hours discussing event driven architecture (EDA), messaging, and MassTransit. Through the magic of editing, they managed to end up with a solid eighty minutes (most likely to achieve a straight up G rating).

Check it out on their web site (where you can listen, or subscribe via iTunes/PodCastRSS).

Last weekend, Dru Sellers and I (along with 100 or so others) attended the Continuous Improvement in Software Development (referred to as KaizenConf, likely due to the URL) conference in Austin, TX. We left Tulsa late Friday night after taking my girls trick-or-treating for Halloween and drove all night arriving in Austin around 5:30 AM. After a quick nap and a shower, we went to the first day of the conference.

The night before at the opening, those who were there put up a series of topics that they wanted to discuss. One of those talks was on Enterprise Service Bus (ESB) patterns and implementations using MassTransit. I think it was pretty obvious where Dru and I were going to spend our time right after lunch. There were a number of people that we have had conversations with that also attended and they were excited to learn more about distributed application designs and how to implement them using open-source tools like MassTransit.

The First Session: Alternative Architectures

After the morning announcements, Dru and I joined Ben Scheirman and Ayende Rahien in a conversations about alternatives to the RDBMS. The conversation started a bit rough, but quickly opened up into other ways to store data in our applications. Ayende brought up CouchDB and the things he learned about it. There was also some in depth discussions about proper use of databases and separating transactional data from reporting to avoid transaction blocks. Some concepts on how to achieve the appropriate separation, including asynchronous ETL (extract-transform-load) were discussed. The Map/Reduce algorithm was also covered and some examples were given in the discussion of how to map data into dimensions for reporting. The proceedings from this talk can be found on the wiki.

The Second Session: Lean Architecture

My second session of the day was on Lean Architecture. This discussion was forward looking and related to how development teams could branch out and work on features with the intent of releasing individual features as they are complete (instead of waiting for a big release with many unrelated features). There was a lot of talk about how the teams work, how version control and build processes would need to adapt to handle the complexity of dealing with multiple development versions of a single code base. I think one of my biggest take aways from this session was the need for an integration branch that is created from the trunk before each merge. It is then possible to integrate a branch into the trunk without immediately impacting the trunk. Once the simulation branch merge is complete and tested, the simulation is merged into the trunk (which should be easy, since it was created from the trunk originally). This alone would help to ensure a solid trunk that can be delivered on demand. The proceedings from this talk are also on the wiki.

Over lunch we enjoyed some amazing Austin weather while talking about projects and recent events. We then went to setup for the discussion on ESB/MassTransit. Upon arrival, we were surprised at the number of people in attendance. There was a crowd of at least 20 people and the video crew was setup with remote microphones and the works. Once we had gotten the projector setup and demos loaded, we started into the discussion.

The Third Session: ESB Patterns and MassTransit

The conversation started with a general discussion about messaging patterns. A reading list was presented, along with the major patterns that are used in a publish/subscribe system like an ESB. Applications for this type of system including everything from a command/query interactive system to migrating a batch processing application to a more real-time asynchronous process. We touched on where MassTransit was in the development lifecycle and some of the things we learned over the last year of development. We showed some of the sample applications, including the new web service bridge sample for connecting external customer web services to an internal domain.

Needless to say, we were amazed at the response we got related to MT and ESB patterns in general. It was great to have Ayende and Jeremy Miller bouncing ideas around regarding the project and integration with other existing systems. The notes from the discussion are located on the KaizenConf Wiki.

The Last Session: A Mixed Bag

My last session of the day was split between two completely different sessions. One was on Advanced IOC usage, and the other was on moving from Project to Product. I bounced between these two sessions, picking up some interesting bits from each one. Both were late in the day, so I was fading pretty fast considering I only had 90 minutes of sleep the previous night. Good concepts were captured in the notes, so be sure to check those out (I’ll have to just to remember them).

After the sessions for the day, we went to the Hyatt with Ben (B#) and provided input as he started writing a new sample for MassTransit. The context for this new sample was the Gregor Hohpe article about how Starbucks Does Not Use Two-Phase Commit. This was mostly a learning exercise for Ben, however, we gained some insight watching him build it out. I think Dru refactored the host quite a bit to remove some extraneous ceremony that was just unnecessary. Once we had the first bits working, it was time to find some dinner and the rest of the gang. We had some great Tex-Mex at Trudy’s and then went back to the hotel bar to watch Texas Tech put the smack down on U-Texas Longhorns.

The next morning we met up at Starbucks (go figure) to complete the new sample Ben was building (yes, the irony of working on a sample about Starbucks @ Starbucks didn’t get past us). We ended up converting from using regular message consumers to a saga-based approach given the requirements and it took another 30 minutes to get it all working. Oren had some great ideas on how we could tweak little things here and there to eliminate some friction as well. Once we were done, we headed on over to the conference to see what was in store for the morning sessions.

This year, the focus of the conference was on continuous improvement. Therefore, instead of just having additional sessions on Sunday morning the attendees voted on sessions that they wanted to help improve. At least, that’s how I understood it, we were working on the Starbucks sample, remember? Anyway, when we arrived we found that the ESB Patterns session was one of the topics. The room was full with something like 22 votes on the page. I was surprised to see the response of the people, truly surprised.

Continuous Improvement: ESB Patterns and MassTransit

The actions are summarized on the wiki, and included things like documentation, more sample applications, and help on how to add messaging to an existing application. We also dug into the things needed to help people build and understanding messaging components in their applications. Things like diagnostics, conventions to avoid common missteps, hands-on labs to walk through building a message-based component, and a few other getting started items were brought up. From a technical side, we talked about additional patterns such as an overall conversation id to correlate multiple sagas (like a saga of sagas), as well as built-in support for compensation.

A large part of the discussion was on how functional programming features could be used to enhance the system. An example in Erlang of a message exchange was drawn up, along with how it might be written in C#. Matthew Podwysocki was consulted on how this example might be written in F#, and there was discussion about how the threading model of F# doesn’t really support the style used by Erlang. Some work is still needed there I suppose but we’ll continue to be open about the possibilities offered by functional languages. Glenn Block also talked about getting in touch with the connected systems group to see what they could offer.

County Line BBQ

With that, the conference was closed. Once again, the open space technology used for the event was awesome. The rules of open space dictate that what happened is the only thing that could have happened, and I agree with that completely. It was great to be a part of yet another excellent event and I look forward to future installments in other locations. Be sure to keep up with the wiki, as videos from most of the sessions will be made available online soon. One of our improvements was to better document our proceedings, and I think we’ve managed to succeed on that one for sure.

One of the more common scenarios I hear about when talking about building a service-oriented architecture has to do with providing a web service to external customers. This is often in the context of providing information from an internal system. The problem comes with how many of these systems are built.

In many situations, a web service is created (perhaps with ASP.NET Web services, an .asmx) which accepts a request from a client, performs some query against the production database, and returns the results. This is a very tightly-coupled way of meeting the requirements. How does one go about reducing coupling? Well, all too often the answer is “just add another web service.” You can see where this is going. The control still follows a chain of web service requests up until the final connection to the database.

The new WebServiceBridge sample in MassTransit demonstrates how to break this coupling using the publish/subscribe architecture. When the external web service receives a request, a message is asynchronously published. A subscription to the requested information is added at the same time. A completely insulated service then handles the request, publishing the response to the bus. The web service request that was pending is completed and the results of the query are returned to the client.

In the sample, this all happens with the least amount of load on the web server by using the asynchronous nature of web methods in ASP.NET. This reduces the load on the web servers, making it easier to handle multiple requests without extensive hardware requirements.

Last night I made a screencast that goes through this new sample and shows how it works. You can download the screencast here (please Save As to conserve bandwidth). Check it out and let me know what you think. Questions and comments are always welcome!

This past Thursday and Friday, the Tulsa TechFest was held at OSU Tulsa in Tulsa (could I write Tulsa one more time, I knew I could). Attendance was high and most of the sessions I attended were in rooms full of people. The day started off early Thursday morning, but that’s not the start of the story.

The night before, Dru and Rob came by the house for a little pre-conference warmup (and by warmup, I don’t mean this). Rob went through his presentation on continuous integration one last time while Dru and I worked through our presentation on messaging (and how to do it with MassTransit, of course). The night ended early for me, but Rob and Dru met up with Ben at the hotel and closed the bar (and by closed, I mean walked in at last call and got one beer).

The next morning we all met up and caught up on things since the last gathering. Since the last time we saw Ben, he’d been through a hurricane and granted the MVP Award from Microsoft. We then planned out our day of sessions based on the information currently available to us.

The speaker for the CSS talk was unable to attend, so the four of us convened an open-space session on CSS. The discussion in the fishbowl was good with a lot of interesting topics. Ben gave an on-screen demonstration of CSS from the ground up for those in the room that were new to it, providing context for the audience. CSS is extremely important considering it is the best (only?) way to layout and style websites consistently across browsers. I think everyone brought up how much of a turd IE 6 is when it comes to CSS compatibility.

After lunch, it was time for Dru and I to present our session on message-driven architecture (using MassTransit). You can see the first hour of the session on video here. The crowd really got into it, asked a lot of questions, and hopefully came away with an understanding of asynchronous application design and messaging.

After that session, we sat down with a guy that works for Sun and talked about enterprise application architecture. It was interesting comparing the mature open-source nature of Java to the budding open-source landspace in .NET. After the closing session and prize giveaway, there was a speakers dinner (Rib Crib, good stuff). Once we had eaten, we went to the hotel and did a little code sharing and Dru and Ben went through ASP.NET MVC some more. Then we went over to Dirty’s Tavern for some post-day fun. I was worn out, so I went back to my car and called it a night.

The next day was full of interesting stuff. A nice introduction to ASP.NET MVC by Ben, some extensive coverage of log4net by Dru, and I gave a presentation on iPhone development. Outside of the actual sessions there were a lot of great conversations about development and tools in general. We also recorded Ray Lewallen’s session on Behavior Driven Development, which can be viewed here.

My iPhone development session was purely introductory to show the tools and how they are used to build and deliver applications for the iPhone. The room was absolutely packed and hopefully everyone walked away with some good information. I know at least one guy did, he left two seconds after I said that building iPhone applications requires a Mac!

To wrap it up, the event was a huge success. There were a ton of people there, the vendor room was always alive with activity (likely due to Chris Koenig and his Rock Band setup giving some much needed ADHD relief between sessions). Chris also had a couple of great sessions on Silverlight and the new features in 2.0 that should really improve the use of Silverlight for Rich Internet Applications (RIAs).

I’m happy to say that we’ve just tied a bow around the latest release of MassTransit. Release 0.4 includes a number of new features and some tweaks to the internals as well. I’m going to describe a few of those features below, but you can grab the latest from the trunk or download the 0.4 release.

Building MassTransit

Since Visual Studio 2008 has been out for almost a year, it is now required to open the updated solution for MassTransit. In the main folder, the MassTransit-2008.sln is the one to use to build and run the unit tests. Many of the samples solutions are also 2008 solutions. The assemblies, however, are still targeting the .NET 2.0 framework, making them usable on both 2005 and 2008 projects. With only the .NET 3.5 framework installed, you should be able to run the build.bat to build the project without Visual Studio (our CI server does this).

Timeout Service

To enable automated support for timeouts in sagas, a new timeout service is available. This is a general service that can be used to schedule timeouts for whatever purpose may be needed. To schedule a timeout, the application should publish a ScheduleTimeout message with the duration or time when a response should be sent. The application/service can then consume the TimeoutExpired message, which will be published by the timeout service when the timeout period expires.

Message Deferral Service

One of the scenarios I often find in our systems is the need to poll a remote resource to determine if an operation has completed. To support this behavior without custom code in each instance, a new message deferral service has been added. This can be used to defer the delivery of a message until a period of time expires. The deferral services leverages the timeout service for scheduling and we republish a message after that timeout expires.

For example, we have a CheckRemoteResponseStatus message in one of our systems. This is initially published after a request is submitted to a remote system and a remote transaction id is returned. The first time the consumer gets the message, it checks the remote system for a response. In most cases, the response is immediately available and the saga continues. However, sometimes the remote system is too busy to respond and returns a pending status. In this case, the same CheckRemoteResponseStatus message is published within a DeferMessage. The deferred message service handles that message and will republish the original CheckRemoteResponseStatus message when the timeout expires. The saga will then handle the message to see if a response is now available. The saga keeps track of how many times the remote status has been checked and uses a sliding interval that increases as the retry count increases. Eventually, the final retry results in a failed transaction and is handle appropriately.

The nice thing about this is there was no custom retry logic required, and a common timeout and message deferral service were used. There are likely other cases within the application that will benefit from this shared functionality.

Transactional Queue Support

With 0.4, the entire method of reading from the endpoints has been redesigned. Previously, a single receive thread was used to receive from the endpoint which then dispatched the message handling to the dispatcher inside the service bus. This has been redesigned to use the dispatcher threads to perform the actual receive from the endpoint, using a transaction (ala System.Transactions) to handle the message reception. This keeps the transaction to a single thread while at the same time allowing concurrent message reception.

The transaction carries over into actions that are part of the message consumer. If a database update is part of the consumer, that database update can cause the entire message to rollback if it fails. If any exception is thrown, the entire reception of the message, any additional database operations, new messages sent, etc. will all be rolled back with the transaction.

Performance Improvements

Dru spent some time in NYC with Ayende Rahien reviewing the MT source code and Oren recommended changing from using locks to ReaderWriterLocks to improve concurrency. The changes in the threading system, along with the elimination of a lot of locking in favor of reader/writer locks has nearly doubled the throughput of messages when using a multi-core system using MSMQ. There have been a number of other internal tweaks as well to improve the concurrency of the bus dispatcher.

Control Bus

To enable competing consumer in a publish/subscribe environment, the control messages need to be on a separate bus from the data messages. To allow multiple services to compete against a single data channel (single MSMQ) in order to load balance and handle failure scenarios, the services cannot compete on the control messages such as subscriptions. The subscription client has been tweaked to allow it to operate on a separate bus from the data bus, at the same time notifying the subscription service of messages handled by the local endpoints of the service.

It’s easy to setup a single service that consumes messages from multiple buses (which in turn each have a specific endpoint being serviced). When a component is created to consume a message, the specific bus/endpoint that received the message is injected into the component (via setter injection) so that any subsequent messages can be published to the appropriate bus.

Health Service

The health service has been added making it easy to monitor endpoints and identify when an endpoint goes down. Periodic heartbeats are sent to the service and when a heartbeat hasn’t been received in a while, it marks that endpoint as down and attempts to directly ping it to get a response. The heartbeats can be subscribed, so a monitoring tool can keep track of which endpoints are there and what they are handling.

Configuration Model

To make it easier to use the bus in different containers, a new configuration model has been added to build and configure a service bus instance. This will ultimately result in moving a lot of the code used by build a service bus out of the container-specific facilities (such as the Windsor container).

Host Improvements

The ability to create and deploy Windows services has gotten easier with the updates to the Host assembly in MassTransit. With only a few files to define the lifecycle of a service, it is easy to get the ability to run, test, install and deploy a service. This includes services that are using the service bus. There is little to no coupling between the host and service bus, making it usable for a variety of purposes.

Learning MassTransit

A lot of requests have come for information on how to learn to use MassTransit. During Tulsa TechFest this week, we’re going to record our presentation and make it available online within a few days. This should give at least some introduction on how to use MassTransit (the presentation is mainly on distributed architecture, but we’re using MT for the demo bits). We’re also talking about doing a couple of podcasts on how to use it as well. Depending upon how that goes, we’ll try to do a couple of screencasts on “creating your first project with MT.”

The best way to discover how to use the code is to review the samples. The WinFormSample gives an overall example of how a variety of features are used. The HeavyLoad shows how many of the pieces work as well. The samples folder has a few others that demonstrate how to use MT in other scenarios.

So, check out the new release and give us some feedback on how the new features are working. We’ve already got a few backlog items that we’re slating for 0.5 based on some other contexts that have come up in our applications. Feel free to post on the message group or send either Dru or I an e-mail or tweet if you have any questions.

VMware Fusion 2.0 was released yesterday and I was anxious to upgrade. Rather than feel the full pain myself, I let brewbie go first. He got it setup and installed and said all was good, so I figured I’d give it a shot.

I was in a VM working on some new code that I had neither saved to disk nor commited to Subversion. I figured I’d live on the edge and just suspend the VM so I could upgrade Fusion. Once Fusion had exited, I downloaded the update and fired up the installer. Less than a minute later version 2.0 was installed and ready to go (no reboot required). I clicked the icon for my 2003 Server VM and it fired up just a few seconds, picking up right where I left off. Needless to say, I was very impressed.

The release notes said that previous VMs should update the VMware Tools to take advantage of the new features available in 2.0 so I went ahead and restarted Windows 2003 (1st time). Once it was back up, the new tools package installed and prompted for another reboot (2nd time). The reboots were amazingly quick and I was back up and running in no time.

Since it was all up and running, I started to configure the new features in Fusion 2.0. There is a new keyboard mapping feature so you can map certain key combinations to work within your VM. It works pretty slick, making it possible finally map a key for INSERT. There is also a new feature to make it so that clicking mailto links in the VM opens the Compose Mail window in Mac Mail. VERY COOL! I was less impressed with the browser linking since it changed my default browser to EverNote (WTF?). I changed it back to Safari and then disabled the feature.

After the short interruption, I felt that it was time to create a new VM for Windows Server 2008. I was following the guide on the Windows 2008 Workstation site to build a workstation-grade installation of Server 2008. Since VMware 2.0 supports things like Shader Model 2.0 with 3D graphics, I wanted to see it work. It took a couple of hours to get everything setup and configured, but the end result was a clean install of 2008 server configured for a workstation environment.

I then installed Steam, and downloaded a fresh copy of TeamFortress 2. I fired it up and it failed. It turns out that the tools by default only enable a certain level of hardware acceleration. I went to the advanced troubleshooting tab and cranked up the acceleration. I could now run everything, including Microsoft PhotoSynth and TeamFortress 2. The frame rate was pretty amazing considering it was running in a VM — seriously impressive actually. I experienced a lot of sound breakup when IO was being performed, so there is still some tweaking to do there I suppose.

Once play time was over, I installed Visual Studio 2008, TortoiseSVN, VisualSVN, Resharper, and updated. I pulled down the latest trunk of MassTransit and built it. The build took 16 seconds, compared to 21 seconds on my 2003 VM. However, when I ran the load test I was surprised to see that MSMQ performance was about half of what it was with 2003 Server. I ran the test a couple of times with the same result. I’m not sure what is at work here, but it seems like MSMQ4 has some different performance numbers than MSMQ3. I’ll do some additional tests and post more on this later. Maybe MSMQ was slowed down to make WCF look better (joking, of course).

In the end, the upgrade to 2.0 was painless – completely painless infact. I highly recommend it to take advantage of all the new features.

Download MassTransit 0.3 Here

Last night, Dru and I released version 0.3 of MassTransit. This is a pretty hefty milestone release with a lot of new functionality. I’m going to run down a list of some of the new features that made it into this version.

• Fault Messages
  If a Consumer throws an exception, a Fault<T> message will be published. The Fault envelope will contain the exception that was thrown, along with the message that caused the exception.
• Loopback Endpoint
  To support in-process communication, a loopback endpoint was created. This is entirely in memory and does not have any persistence support. But it makes it easy to test features of the system without requiring MSMQ and also makes for some interesting application design choices for dispatching work.
• Thread Pool Configuration
  As part of the Windsor facility, the thread pool size for dispatching messages can be configured.
• Batch Message Threading
  Dispatching batch messages using Batch<T> now uses a separate thread for each batch being consumed instead of using one of the dispatcher threads. This should avoid thread starvation under heavy message loads.
• Message Groups
  To allow messages to be grouped and sent as a single message (a coarse-grained style of messaging outlined in Fowler’s PoEAA), a new container-style MessageGroup allows a set of messages to be combined into one message and delivered. The consumer can then choose to handle the messages as a group (for unit-of-work style operations) or to split the messages out and dispatch them individually.
• Saga Support
  Long-lived transactions (Saga) are now supported and can be built using MassTransit. You can read all about that on my previous blog entry.
• Grid Support
  New support for distributed tasks via MassTransit has been added. This makes it easy to create distributed applications that process large amounts of data. A good sample is in the works, but presently it uses some fairly proprietary and confidential data so I can’t just hand that out as a sample. The structure makes it fairly easy to write a MapReduce style log analyzer though, if that gives you any ideas.
• Internal Changes
  A number of internal, under-the-hood changes took place. The dispatchers were rewritten to be insulated from each bus instance to enable a shared type information cache. There were also some tweaks in how dispatchers are plugged into the pipeline to make it easier to open the extensibility model for new types of dispatchers (such as the saga dispatcher that was just added).
• WinForm Sample
  A new sample showing how the use the service bus in a WinForm application was built. This includes a nice visual SubscriptionManager GUI to help see the subscriptions as they are added and removed by an application.
• Host Changes
  To make it easier to build Windows services containing message handlers, the Host project was updated with a cleaner style. The PubSub and WinFormSample/SubscriptionManagerGUI have been converted to this new structure and it was pretty easy for me to add (having never seen it before).

Those are the big hits, I’m sure there are some minor things I’ve missed. We’ll try to update the road map this week to give a picture of where we are and what we have left to get done. It’s likely that 0.3 will get a few days of soak time before we gear up for the next release anyway.

It’s important to note that all development is now being done in VS2008 (using .NET 3.5 for the test projects) but the main assemblies are still .NET 2.0 targets. It’s like that this will change in the next version or two considering most of the projects are trending towards the new platform.

One of the first applications we built with MassTransit provides messaging for a long-running transaction started by an application submitting a request. The request is formatted into a X12 envelope and sent to a web service. An intermediate response is returned (a X12 997) with a correlation identifier for the request. Another web service is polled for the response, which can be the result or an indication that the request is still pending. When the response is received, the X12 document is translated and stored in the database. Finally, the user is notified that the transaction is complete and the result is displayed.

As work proceeded on this application, I started to recognize the need for something to coordinate the different steps in a transaction involving multiple loosely-coupled services. Due to the duration of these transactions (the example above can take anywhere from three to sixty seconds), it is unreasonable to keep a single System.Transactions style transaction open the entire time. I started researching how others approached the problem and found a couple of articles that helped. After reading Sagas by Hector Garcaa-Molrna and Kenneth Salem (© 1987 ACM, PDF) and the chapter on sagas in the upcoming book Practical SOA by Arnon Rotem-Gal-Oz (PDF), I started to think about how this could be implemented within MassTransit.

I should note that NServiceBus (another open source service bus) also supports sagas, but I purposely avoiding taking a look at how Udi Dahan implemented them. Once saga support in MassTransit is complete I plan to review the source for NServiceBus to see how the implementations differ. I spoke with Udi at ALT.NET Seattle and his writing has been both educational and inspirational. A lot of great discussions in the NServiceBus mailing list have been an excellent resource as well.

So after a few weeks of trying to flesh out the structure (using TDD, of course), I finally arrived at what I think will be a highly usable infrastructure for handling sagas. In the project MassTransit.Saga.Tests, I’ve created a test that simulates a user registering for a web site. The class for the registration is shown below.

public class RegisterUserSaga :
	InitiatedBy< RegisterUser >,
	Orchestrates< UserVerificationEmailSent >,
	Orchestrates< UserValidated >,
	ISaga< RegisterUserSaga >
{
	private string _displayName;
	private string _email;
	private string _password;
	private string _username;

	public RegisterUserSaga(Guid correlationId)
	{
		CorrelationId = correlationId;
	}

	public Guid CorrelationId { get; private set; }
	public IObjectBuilder Builder { get; set; }
	public IServiceBus Bus { get; set; }
	public Action< RegisterUserSaga > Save { get; set; }

	public void Consume(RegisterUser message)
	{
		_displayName = message.DisplayName;
		_username = message.Username;
		_password = message.Password;
		_email = message.Email;

		Save(this);
		Bus.Publish(new SendUserVerificationEmail(CorrelationId, _email));
	}

	public void Consume(UserVerificationEmailSent message)
	{
		// once the verification e-mail has been sent, we allow 24 hours to pass before we
		// remove this transaction from the registration queue
		Bus.Publish(new UserRegistrationPending(CorrelationId));
		Bus.Publish(new UpdateSagaTimeout(CorrelationId, TimeSpan.FromHours(24)));
	}

	public void Consume(UserValidated message)
	{
		// at this point, the user has clicked the link in the validation e-mail
		Bus.Publish(new UserRegistrationComplete(CorrelationId));
		Bus.Publish(new CompleteSaga(CorrelationId));
	}
}

At the top of the class, the messages consumed by the saga are specified. InitiatedBy indicates the message initiates a new instance of the saga. Orchestrates is for messages that are part of the saga once it has been initiated. All saga instances are identified by a Guid and all messages consumed by the saga should have the CorrelatedBy< Guid > interface. A saga must also implement the ISaga generic interface to allow certain properties to be set giving the saga instance access to the bus and the object builder.

Once the saga class is added to the bus (via the AddComponent method), any messages consumed by the saga will be dispatched to the saga instance. A generic ISagaRepository must also be registered in the container so that sagas can be persisted between messages. The saga dispatcher uses the repository to either load or create the instance of the saga. Since instances of the saga class are saved, the class can expect the members to also be persisted between messages allowing state to be retained.

There is still some work to be done, including a service to handle timeouts and retries. It will be up to the developer to handle any compensating actions that need to be taken in the case of a failure. Therefore, it is highly suggested that the saga also consume any Fault< T > messages that are published when a message consumer throws an exception — particularly if the consumer is not part of the saga (such as an application or domain service).

The code is currently in the trunk and slated to be part of the 0.3 release.