Udi Dahan   Udi Dahan – The Software Simplist
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Archive for the ‘Workflow’ Category



Watch out for superficial invariants

Sunday, November 16th, 2014

superficialAs I was reading a blog post on CQRS, Aggregate Roots, and Invariants here, I became aware of a mistake I’ve seen many developers make over the years and I thought I’d call it out real quick.

Superficial Invariants

Taken from the blog post mentioned above: “For example, an employee cannot take more annual leave than they have.”

This falls into the trap of applying mathematical thinking (which we developers possess in great quantities) to the business world. The business world isn’t that mathematical (in general), and tends to have many more shades of gray expressed as “business rules” which can, and do, change.

Rules – not invariants

Employees can’t take more annual leave than they have.
… unless their manager approves.
… but that’s only up to 2 days.
… unless their manager is a VP, and then it’s up to 5 days.
… and that negative balance will be deducted from next year’s leave.
… Oh, and if the employee leaves the company before then, then the value of those negative days will be deducted from their final paycheck.

Impact on your domain

First of all, I hope you see that this isn’t something that you would trivially implement on an Employee object.

If you read these rules more carefully, you’ll probably notice that they’re speaking about a long-running process.

First, there is a request for leave. Then there’s an approval (with certain rules) which may come sometime later. And the approval itself may not even end the process – if the balance becomes negative.

And, as you’ve probably heard me say before, you end up with sagas as your aggregate roots (see Race conditions don’t exist from 4 years ago).

And a word about Bounded Contexts

Notice that these rules don’t care very much about things like the employee’s name, phone number, email, etc. Similarly, logic that deals with that data probably doesn’t care about the number of days of leave an employee takes.

In other words, these sets of data and logic can be said to belong to different Sub-Domains (in DDD terminology).

As such, it can make sense to take the annual leave logic and put it in a bounded context separate from the one responsible for the contact info.

In closing

In many of the samples and blog posts I see online, an overly simplified problem domain is implemented showing how the given implementation technique would be applied.

The problem is that developers then use that implementation technique as a “cookie cutter”, trying to fit real-world requirements into it, and then end up making a pretty big mess.

The more you delve into real-world requirements of business domains, the less you’ll see of mathematical invariants (unless, maybe, you’re building a physics engine for a game or something) and the more you’ll see long-running processes unfolding in front of you.

Regardless of whether you use NServiceBus sagas or not, start looking at the world as dynamic long-running processes rather than static noun-centric entities.



NSBcon London 2014

Tuesday, April 22nd, 2014

NSBconSince my last post announcing NSBcon you probably haven’t gone to take a look at what’s happening.

The speaker lineup for NSBcon London 2014 is now complete and we’ve got a really great mix of talks, if I do say so myself.

I’ve already mentioned that Oren and Greg will be there, but I wanted to talk a bit about the rest of the roster:

First up – Wonga

If you’re living in the UK, you almost can’t avoid seeing these ads.

Wonga

What you probably didn’t know is that Wonga has been running on NServiceBus for years now.
(No – we had nothing to do with the ads, and there’s nothing we can do about them.)

Charlie Barker was there from the beginning and has lived to tell the tale:

We faced the problem of scaling our platform to meet rapid growth in customers whilst at the same time increasing the team from 15 to 200 people spread across five countries. We did this by transitioning from N-Tier to SOA even as we were delivering new features and keeping the platform stable. No small feat.

This is an interesting story to hear, not only from the point of view of NServiceBus, and I’d definitely recommend grabbing Charlie over lunch or over a beer. This is probably one of the higher profile startup successes in the UK you’ll find and, as always, the behind the scenes story is just fascinating.

And to the cloud!

No self-respecting technology conference these days can go without spending at least some time talking about the cloud – and we won’t be the ones to buck the trend, even though we have a healthy disrespect for all sorts of things, including the ourselves :-)

You’ll hear from one of the foremost Azure MVPs and all around cloudy Belgian Yves Goeleven (whose last name nobody is really sure how to pronounce). Yves has been the driving force behind getting all the various bits of Azure infrastructure integrated into NServiceBus and, if you get him to just the right level of inebriation, will spill all the dirty little secrets of the Azure platform that Microsoft doesn’t want anyone to know.

Dylan Beattie will then relate his tales of creating loosely-coupled encoding workflows for audio and video on the cloud at Spotlight – one of the world’s leading resources for professional actors, casting directors, and production professionals. It ain’t easy making ordinary people into stars.

And so much more

To see the complete lineup, go to NSBcon.com.

And me – what will I be talking about? That’s a good question.

Not to steal my own thunder (after stealing everybody else’s), but you’ll hear about the deeper integration we’ve got planned for SignalR, making your event-driven architecture extend from the back of your systems all the way to the browser, so much simpler and smoother than you ever thought possible.

I’ll also tell you about how we’re going to enable you to run on queues that don’t support distributed transactions (like Service Bus for Windows Server) without having to worry about making your logic idempotent. For some background, see my blog post on Life without distributed transactions.

In short – it’s going to be a kick-ass conference.

Check it out.

* and for those of you paying out of pocket, contact nsbcon@particular.net for a discount.



The Danger of Centralized Workflows

Wednesday, July 13th, 2011

It isn’t uncommon for me to have a client or student at one of my courses ask me about some kind of workflow tool. This could be Microsoft Workflow Foundation, BizTalk, K2, or some kind of BPEL/orchestration engine. The question usually revolves around using this tool for all workflows in the system as opposed to the SOA-EDA-style publish/subscribe approach I espouse.

The question

The main touted benefit of these workflow-centric architectures is that we don’t have to change the code of the system in order to change its behavior resulting in ultimate flexibility!

Some of you may have already gone down this path and are shaking your heads remembering how your particular road to hell was paved with the exact same good intentions.

Let me explain why these things tend to go horribly wrong.

What’s behind the curtain

It starts with the very nature of workflow – a flow chart, is procedural in nature. First do this, then that, if this, then that, etc. As we’ve experienced first hand in our industry, procedural programming is fine for smaller problems but isn’t powerful enough to handle larger problems. That’s why we’ve come up with object-oriented programming.

I have yet to see an object-oriented workflow drag-and-drop engine. Yes, it works great for simple demo-ware apps. But if you try to through your most complex and volatile business logic at it, it will become a big tangled ball of spaghetti – just like if you were using text rather than pictures to code it.

And that’s one of the fundamental fallacies about these tools – you are still writing code. The fact that it doesn’t look like the rest of your code doesn’t change that fact. Changing the definition of your workflow in the tool IS changing your code.

On productivity

Sometimes people mention how much more productive it would be to use these tools than to write the code “by hand”. Occasionally I hear about an attempt to have “the business” use these tools to change the workflows themselves – without the involvement of developers (“imagine how much faster we could go without those pesky developers!”).

For those of us who have experienced this first-hand, we know that’s all wrong.

If “the business” is changing the workflows without developer involvement, invariably something breaks, and then they don’t know what to do. They haven’t been trained to think the way that developers have – they don’t really know how to debug. So the developers are brought back in anyway and from that point on, the business is once again giving requirements and the devs are the one implementing it.

Now when it comes to developer productivity, I can tell you that the keyboard is at least 10x more productive than the mouse. I can bang out an if statement in code much faster than draggy-dropping a diamond on the canvas, and two other activities for each side of the clause.

On maintainability

Sometimes the visualization of the workflow is presented as being much more maintainable than “regular code”.

When these workflows get to be to big/nested/reused, it ends up looking like the wiring diagram of an Intel chip (or worse). Check out the following diagram taken from the DailyWTF on a customer friendly system:

stateModel

The bigger these get, the less maintainable they are.

Now, some would push back on this saying that a method with 10,000 lines of code in it may be just as bad, if not worse. The thing is that these workflow tools guide developers down a path where it is very likely to end up with big, monolithic, procedural, nested code. When working in real code, we know we need to take responsibility for the cleanliness of our code using object-orientation, patterns, etc and refactoring things when they get too messy.

Here is where I’d bring up the SOA/pub-sub approach as an alternative – there is no longer this idea of a centralized anything. You have small pieces of code, each encapsulating a single business responsibility, working in concert with each other – reacting to each others events.

Productivity take 2: testing and version control

If you’re going to take your most complex and volatile business logic and put it into these workflow tools, have you thought about how your going to test it? How do you know that it works correctly? It tends to be VERY difficult to unit-test these kinds of workflows.

When a developer is implementing a change request, how do they know what other workflows might have been broken? Do they have to manually go through each and every scenario in the system to find out? How’s that for productivity?

Assuming something did break and the developer wants to see a diff – what’s different in the new workflow from the old one, what would that look like? When working with a team, the ability to diff and merge code is at the base of the overall team productivity.

What would happen to your team if you couldn’t diff or merge code anymore?
In this day and age, it should be considered irresponsible to develop without these version control basics.

In closing

There are some cases where these tools might make sense, but those tend to be much more rare than you’d expect (and there are usually better alternatives anyway). Regardless, the architectural analysis should start without the assumption of centralized workflow, database, or centralized anything for that matter.

If someone tries to push one of these tools/architectures on you, don’t walk away – run!



Race Conditions Don’t Exist

Tuesday, August 31st, 2010

crossing-the-finish-lineNot in the business world anyway.

The problem is that, as software developers, we’re all too quick to accept them at face value. We don’t question the requirements – in all fairness, it was never our job to do so. We were the ones that implemented them, preferably quickly.

For example

Let’s say we get the requirement the following requirements:

1. If the order was already shipped, don’t let the user cancel the order.
2. If the order was already cancelled, don’t let the user ship the order.

The race condition here is when we have two users who are looking at the same order, which is neither cancelled nor shipped yet, and each submits a command – one to ship the order, the other to cancel it.

In these cases, the code is simple – just an if statement before performing the relevant command.

So what’s the problem

A microsecond difference in timing shouldn’t make a difference to core business behaviors. Which means that we’ve actually got here is a bug in the requirements. Users are actually dictating solutions here rather than requirements.

Let’s ask our stakeholders, “why shouldn’t we let users cancel a shipped order? I mean, the users don’t want the products.”

And the stakeholders would respond with something like, “well, we don’t want to refund the user’s money then. Or, at least, not all their money. Well, maybe if they return the products in their original packaging, *then* we could give a full refund.”

And as we drilled deeper, “when do refunds need to be given? Right away, in the same transaction?”

The stakeholders would explain, “no, refunds don’t need to be given right away.”

It turns out we were missing the concept of a refund, as well as assuming that all things needed to be processed and enforced immediately. Once we dug into the requirements, we found that there is actually plenty of time to allow both transactions to go through. We just need to add some checks during shipping’s long-running process to see if the order was cancelled, and then to cut the process short.

So is everything a long-running process then?

That’s actually a fair question – long-running processes are a lot more common than at first appears.

What we’re seeing is that cancellation is now a command that has no reason to fail – just like CQRS tells us. When this command is performed, it publishes the OrderCancelled event, which the billing service subscribes to.

Billing then starts a long-running process (a saga, in NServiceBus lingo), also listening to events from the shipping process, ultimately making a decision when a refund should be given, and for how much.

Deeper business analysis

As we discuss matters more with our business stakeholders, we hear that most orders are actually cancelled within an hour of being submitted. It is quite rare for orders to be cancelled days later.

In which case, we could look at modeling the acceptance of an order as a long-running process itself.

When a user places an order, we don’t immediately publish an event indicating the acceptance of an order, instead a saga is kicked off – which opens up a timeout for an hour later. If a cancellation command arrives during that period of time, the user gets a full refund (seeing as we didn’t charge anything since billing didn’t get the accepted event to begin with), and the saga just shuts itself down. If the timeout occurs an hour later, and the saga didn’t get a cancel command, then the order is actually accepted and the event is published.

Yes, sagas are everywhere, once you learn to see with business eyes, and no race conditions are left.

In closing

Any time you see requirements that indicate a race condition, dig deeper.

What you’re likely to find are some additional business concepts as well as the introduction of time and the creation of long-running business processes. The implementation at that point will pivot from being trivial if-statements to being richer sagas.

Keep an eye out.



Make WCF and WF as Scalable and Robust as NServiceBus

Monday, June 30th, 2008

This topic is getting more play as more people are using WCF and WF in real-world scenarios, so I thought I’d pull the things that I’ve been watching in this space together:

Reliabilitydoctor

Locking in SqlWorkflowPersistenceService (via Ron Jacobs) where, if you want predictable persistence (MS: ‘none of our customers asked for this to be easy’), you need to use a custom activity (which Ron was kind enough to supply).

“Given what I learned today I’d have to say that I’d be very careful about using workflows with an optimistic locking.  Detecting these types of situations is not that simple.”

Let’s think about that. If we’re doing pessimistic locking, we get into the problem of, if a host restarts (as the result of a critical windows patch or some other unexpected occurrence), that the workflow won’t be able to be handled by any other host in the meantime (you didn’t care so much about your SLA, did you?).

Luckily, someone’s come up with a hack that works around this robustness problem in Scalable Workflow Persistence and Ownership.

“So this code will attempt to load workflow instances with expired locks every second. Is it a hack? Yes. But without one of two things in the SqlWorkflowPersistenceService its the sort of code you have to write to pick up unlocked workflow instances robustly.”

This will seriously churn the table used to store your workflows, decreasing performance of workflows that haven’t timed out. Oh well.

Testability

Implementing WCF Services without Referencing WCF (via Mark Seemann):

“More than a year ago, I wrote my first post on unit testing WCF services. One of my points back then was that you have to be careful that the service implementation doesn’t use any of the services provided by the WCF runtime environment (if you want to keep the service testable). As soon as you invoke something like OperationContext.Current, your code is not going to work in a unit testing scenario, but only when hosted by WCF.”

After pointing out some of the more basic difficulties in testability a straightforward WCF implementation brings, Mark turns the heat up in his follow-up post, Modifying Behavior of WCF-Free Service Implementations:

“Perhaps you need to control the service’s ConcurrencyMode, or perhaps you need to set UseSynchronizationContext. These options are typically controlled by the ServiceBehaviorAttribute. You may also want to provide an IInstanceProvider via a custom attribute that implements IContractBehavior. However, you can’t set these attributes on the service implementation itself, since it mustn’t have a reference to System.ServiceModel.”

Wow – all the things required to make a WCF service scalable and thread-safe make it difficult to test. In the end, we’re beginning to see how many hoops we have to go through in order to get separation of concerns, but until we can take all this and get it out of our application code, it’s an untenable solution. I hope Mark will continue with this series, if only so I can take the framework that might grow out of it and use it as a generic WCF transport for NServiceBus.

Comparisonapples and oranges

After the Neuron-NServiceBus comparison that Sam and I had, we talked some more. After going through some of the rational and thinking, Sam even put nServiceBus into his WCF-Neuron comparison talk. Sam had this to say about nServiceBus:

“The bottom line is: I like what I see. Although it’s a framework, not an ESB product like Neuron, it’s a powerful framework that takes the right approach on SOA and enforces a paradigm of reliable one-way, *non-blocking* calls. That is the point of the talk tonight overall; we need to get away from the stack world of synchronous RPC calls to true asynchronous non-blocking message based SOA systems.”

The main concern I have with a WCF+WF based solution is that developers need to know a lot in order to make it testable, scalable, and robust. In nServiceBus, that’s baked into the design. It would be extremely difficult for a developer writing application logic to interfere with when persistence needs to happen, or the concurrency strategy of long-running workflows. The fact that message handlers in the service layer don’t need concurrency modes, instance providers, or any of that junk make them testable by default.



Sagas Solve Stupid Transaction Timeouts

Monday, June 23rd, 2008

It turns out that there was a subtle, yet dangerous problem in the use of System.Transactions – a transaction could timeout, rollback, and the connection bound to that transaction could still change data in the database. image

Think about that a second.

Scary, isn’t it?

At TechEd Israel I had a discussion with Manu on this very issue, just under a different hat:

What’s the difference between a short-running workflow and a long-running one?

Manu suggested that we look at the actual time that things ran to differentiate between them. I asserted that if any external communication was involved in some part of state-management logic, that logic should automatically be treated as long-running.

Manu’s reasoning was that the complexity involved in writing long-running workflows was not justified for things that ran quickly, even if there was communication involved. Many developers don’t think twice about synchronously calling some web services in the middle of their database transaction logic. In the many Microsoft presentations I’ve been at on WF, not once has it been mentioned that state machines should be used when external communication is involved.

The problem that I have with this guidance is how do you know how quickly a remote call will return?

Do you just run it all locally on your machine, measure, and if it doesn’t take more than a second or so, then you’re OK?

The fact of the matter is that we can never know what the response time of a remote call will be. Maybe the remote machine is down. Maybe the remote process is down. Maybe someone changed the firewall settings and now we’re doing 10KB/s instead of 10MB/s. Maybe the local service is down and we’re communicating with the backup on the other side of the Pacific Ocean.

But the thing is, Manu’s right.

Writing long-running workflows (with WF) is more complex than is justified. My guess is that since WF wasn’t specifically designed for long-running workflows only, that this complexity crept in.nservicebus_logo_small

Sagas in nServiceBus were specifically designed for long-running workflows only.

Maybe that’s what kept them simple.

Since all external communication is done via one-way, non-blocking messaging only, each step of a saga runs as quick as if no communication were done at all. This keeps the time the transaction in charge of handling a message is open as short as possible. That, in turn, leads to the database being able to support more concurrent users.

In short, sagas are both more scalable and more robust.

No need to worry about garbaging-up your database.



Prevent technology blow-ups from killing your project

Wednesday, February 27th, 2008

Why should you put a 10 foot pole between yourself and technology?

Well, because Microsoft (or insert vendor of your choice here – they’re all equally guilty of this) tend to deprecate (as in kill) the technology they evangelised just last year/month/week.

Microsoft Sql Server Notification Services are the latest victim.

I hope you don’t have any application code tied to that technology.

Not that it’s the only one.

Workflow Foundation’s warts have started coming out from behind the shiny veneer. It turns out that the threading model is… problematic and requires all sorts of workarounds. Hope those are stable. It’s not like they could have known that we need a high performance way to run our business logic out of the box. I hope you don’t have to change your application code (sorry, pictures diagrams) when you get blocked threads when trying to cancel irrelevant workflows (customer no longer does business with us – cancel order processing workflows).

I forgot to mention that the solution above is for single-box parallelism – if you want true scale-out, you need to go back to solution that “require talented software developer use of call-external-method and handle-external-event activities along with the CLR thread-pool“. That’s OK – I have yet to meet a team/company who attests that they have below average developers.

I apologize for the somewhat sarcastic tone of this post.

It’s just that I’m sick of Microsoft handing developers razor-sharp knives, pointy end forward, and after the developer loses a couple of fingers mentions “oh yah, watch out for these pointy, sharp bits”.

To the developers out there – maybe we need kevlar suits before handling these hazardous materials.

To Microsoft – you think that this doesn’t alienate your customers?

We’re all in the same boat together.

I’m hoping that ALT.NET can help.



NServiceBus on Virtual TechEd

Thursday, February 21st, 2008

Well, I had almost forgot about that interview.

When I was at TechEd Barcelona last November (07), the morning after I flew in I experienced “the fish bowl” and Virtual TechEd for the first time. Anyway, after a short chat – and quite to my surprise, my interviewer, Paul Foster, decided that we should talk about nServiceBus.

So here it is. The Microsoft/Marketing friendly description of what nServiceBus is and how nicely it plays with things like WCF and WF. Always be a gracious guest. Don’t bite the hand that feeds you. But a nibble here and there – well, that you can get away with :-)

Download:

Virtual TechEd site is gone and in it’s place is something else, not related to software.



Sagas and Unit Testing – Business Process Verification Made Easy

Monday, February 4th, 2008

Sagas have always been designed with unit testing in mind. By keeping them disconnected from any communications or persistence technology, it was my belief that it should be fairly easy to use mock objects to test them. I’ve heard back from projects using nServiceBus this way that they were pleased with their ability to test them, and thought all was well.

Not so.

The other day I sat down to implement and test a non-trivial business process, and the testing was far from easy. Now as developers go, I’m not great, or an expert on unit testing or TDD, but I’m above average. It should not have been this hard. And I tried doing it with Rhino.Mocks, TypeMock, and finally Moq. It seemed like I was in a no-mans-land, between trying to do state-based testing, and setting expectations on the messages being sent (as well as correct values in those messages), nothing flowed.

Until I finally stopped trying to figure out how to test, and focused on what needed to be tested. I mean, it’s not like I was trying to build a generic mocking framework like Daniel.

Here’s an example business process, or actually, part of one, and then we’ll see how that can be tested. By the way, there will be a post coming soon which describes how we go about analysing a system, coming up with these message types, and how these sagas come into being, so stay tuned. Either that, or just come to my tutorial at QCon.

On with the process:

1. When we receive a CreateOrderMessage, whose “Completed” flag is true, we’ll send 2 AuthorizationRequestMessages to internal systems (for managers to authorize the order), one OrderStatusUpdatedMessage to the caller with a status “Received”, and a TimeoutMessage to the TimeoutManager requesting to be notified – so that the process doesn’t get stuck if one or both messages don’t get a response.

2. When we receive the first AuthorizationResponseMessage, we notify the initiator of the Order by sending them a OrderStatusUpdatedMessage with a status “Authorized1”.

3. When we get “timed out” from the TimeoutManager, we check if at least one AuthorizationResponseMessage has arrived, and if so, publish an OrderAcceptedMessage, and notify the initator (again via the OrderStatusUpdatedMessage) this time with a status of “Accepted”.

And here’s the test:


    public class OrderSagaTests 
    { 
        private OrderSaga orderSaga = null; 
        private string timeoutAddress; 
        private Saga Saga;     

        [SetUp] 
        public void Setup() 
        { 
            timeoutAddress = "timeout"; 
            Saga = Saga.Test(out orderSaga, timeoutAddress); 
        }     

        [Test] 
        public void OrderProcessingShouldCompleteAfterOneAuthorizationAndOneTimeout() 
        { 
            Guid externalOrderId = Guid.NewGuid(); 
            Guid customerId = Guid.NewGuid(); 
            string clientAddress = "client";     

            CreateOrderMessage createOrderMsg = new CreateOrderMessage(); 
            createOrderMsg.OrderId = externalOrderId; 
            createOrderMsg.CustomerId = customerId; 
            createOrderMsg.Products = new List<Guid>(new Guid[] { Guid.NewGuid() }); 
            createOrderMsg.Amounts = new List<float>(new float[] { 10.0F }); 
            createOrderMsg.Completed = true;     

            TimeoutMessage timeoutMessage = null;     

            Saga.WhenReceivesMessageFrom(clientAddress) 
                .ExpectSend<AuthorizeOrderRequestMessage>( 
                    delegate(AuthorizeOrderRequestMessage m) 
                    { 
                        return m.SagaId == orderSaga.Id; 
                    }) 
                .ExpectSend<AuthorizeOrderRequestMessage>( 
                    delegate(AuthorizeOrderRequestMessage m) 
                    { 
                        return m.SagaId == orderSaga.Id; 
                    }) 
                .ExpectSendToDestination<OrderStatusUpdatedMessage>( 
                    delegate(string destination, OrderStatusUpdatedMessage m) 
                    { 
                        return m.OrderId == externalOrderId && destination == clientAddress; 
                    }) 
                .ExpectSendToDestination<TimeoutMessage>( 
                    delegate(string destination, TimeoutMessage m) 
                    { 
                        timeoutMessage = m; 
                        return m.SagaId == orderSaga.Id && destination == timeoutAddress; 
                    }) 
                .When(delegate { orderSaga.Handle(createOrderMsg); });     

            Assert.IsFalse(orderSaga.Completed);     

            AuthorizeOrderResponseMessage response = new AuthorizeOrderResponseMessage(); 
            response.ManagerId = Guid.NewGuid(); 
            response.Authorized = true; 
            response.SagaId = orderSaga.Id;     

            Saga.ExpectSendToDestination<OrderStatusUpdatedMessage>( 
                    delegate(string destination, OrderStatusUpdatedMessage m) 
                    { 
                        return (destination == clientAddress && 
                                m.OrderId == externalOrderId && 
                                m.Status == OrderStatus.Authorized1); 
                    }) 
                .When(delegate { orderSaga.Handle(response); });     

            Assert.IsFalse(orderSaga.Completed);     

            Saga.ExpectSendToDestination<OrderStatusUpdatedMessage>( 
                    delegate(string destination, OrderStatusUpdatedMessage m) 
                    { 
                        return (destination == clientAddress && 
                                m.OrderId == externalOrderId && 
                                m.Status == OrderStatus.Accepted); 
                    }) 
                .ExpectPublish<OrderAcceptedMessage>( 
                    delegate(OrderAcceptedMessage m) 
                    { 
                        return (m.CustomerId == customerId); 
                    }) 
                .When(delegate { orderSaga.Timeout(timeoutMessage.State); });     

            Assert.IsTrue(orderSaga.Completed); 
        } 
    }

You might notice that this style is a bit similar to the fluent testing found in Rhino Mocks. That’s not coincidence. It actually makes use of Rhino Mocks internally. The thing that I discovered was that in order to test these sagas, you don’t need to actually see a mocking framework. All you should have to do is express how messages get sent, and under what criteria those messages are valid.

If you’re wondering what the OrderSaga looks like, you can find the code right here. It’s not a complete business process implementation, but its enough to understand how one would look like:


using System; 
using System.Collections.Generic; 
using ExternalOrderMessages; 
using NServiceBus.Saga; 
using NServiceBus; 
using InternalOrderMessages;     

namespace ProcessingLogic 
{ 
    [Serializable] 
    public class OrderSaga : ISaga<CreateOrderMessage>, 
        ISaga<AuthorizeOrderResponseMessage>, 
        ISaga<CancelOrderMessage> 
    { 
        #region config info     

        [NonSerialized] 
        private IBus bus; 
        public IBus Bus 
        { 
            set { this.bus = value; } 
        }     

        [NonSerialized] 
        private Reminder reminder; 
        public Reminder Reminder 
        { 
            set { this.reminder = value; } 
        }     

        #endregion     

        private Guid id; 
        private bool completed; 
        public string clientAddress; 
        public Guid externalOrderId; 
        public int numberOfPendingAuthorizations = 2; 
        public List<CreateOrderMessage> orderItems = new List<CreateOrderMessage>();     

        public void Handle(CreateOrderMessage message) 
        { 
            this.clientAddress = this.bus.SourceOfMessageBeingHandled; 
            this.externalOrderId = message.OrderId;     

            this.orderItems.Add(message);     

            if (message.Completed) 
            { 
                for (int i = 0; i < this.numberOfPendingAuthorizations; i++) 
                { 
                    AuthorizeOrderRequestMessage req = new AuthorizeOrderRequestMessage(); 
                    req.SagaId = this.id; 
                    req.OrderData = orderItems;     

                    this.bus.Send(req); 
                } 
            }     

            this.SendUpdate(OrderStatus.Recieved);     

            this.reminder.ExpireIn(message.ProvideBy - DateTime.Now, this, null); 
        }     

        public void Timeout(object state) 
        { 
            if (this.numberOfPendingAuthorizations <= 1) 
                this.Complete(); 
        }     

        public Guid Id 
        { 
            get { return id; } 
            set { id = value; } 
        }     

        public bool Completed 
        { 
            get { return completed; } 
        }     

        public void Handle(AuthorizeOrderResponseMessage message) 
        { 
            if (message.Authorized) 
            { 
                this.numberOfPendingAuthorizations--;     

                if (this.numberOfPendingAuthorizations == 1) 
                    this.SendUpdate(OrderStatus.Authorized1); 
                else 
                { 
                    this.SendUpdate(OrderStatus.Authorized2); 
                    this.Complete(); 
                } 
            } 
            else 
            { 
                this.SendUpdate(OrderStatus.Rejected); 
                this.Complete(); 
            } 
        }     

        public void Handle(CancelOrderMessage message) 
        {     

        }     

        private void SendUpdate(OrderStatus status) 
        { 
            OrderStatusUpdatedMessage update = new OrderStatusUpdatedMessage(); 
            update.OrderId = this.externalOrderId; 
            update.Status = status;     

            this.bus.Send(this.clientAddress, update); 
        }     

        private void Complete() 
        { 
            this.completed = true;     

            this.SendUpdate(OrderStatus.Accepted);     

            OrderAcceptedMessage accepted = new OrderAcceptedMessage(); 
            accepted.Products = new List<Guid>(this.orderItems.Count); 
            accepted.Amounts = new List<float>(this.orderItems.Count);     

            this.orderItems.ForEach(delegate(CreateOrderMessage m) 
                                        { 
                                            accepted.Products.AddRange(m.Products); 
                                            accepted.Amounts.AddRange(m.Amounts); 
                                            accepted.CustomerId = m.CustomerId; 
                                        });     

            this.bus.Publish(accepted); 
        } 
    } 
}

All this code is online in the subversion repository under /Samples/Saga.

Questions, comments, and general thoughts are always appreciated.



[Presentation files] Asynchronous Systems Architecture for the Web

Monday, January 7th, 2008

We had a great turnout yesterday at the Web Developer Community (not user group <grin/>). I passed on the presentation files and code samples to Noam but figured that the rest of my readers might enjoy them as well.

The (pdf) presentation is here: Asynchronous Systems Architecture for the Web

The code sample is here: Asynchronous User Management Code Sample

In the sample, you can see the use of sagas to manage the user registration process; store user email and hashed password, send a confirmation “email”, when the user clicks the “link”, the web server will take the saga id found in the url, and send a message with that id. This will cause the saga to complete and the user to be written to the “database”.

Since I didn’t have an email component on my laptop, and I’m guessing you don’t either, the saga just writes the url to the console. Copy and paste it from there into the browser, and you’re good to go.

A Word on TimeoutExceptions

One other thing that I want to call to your attention. When stepping-through the code in the debugger, you’re liable to spend more time than the Transaction Coordinator likes, which will cause it to rollback and try the message again. This is supposed to happen and occurs by design.

When you’re actually working with a database in a high performance environment, there will be cases where one transaction locks a page of a table and may cause other transactions to either timeout or be chosen as victims and just tossed. The behavior that best handles this scenario is just to retry the transaction.

However, you don’t have to write ugly code that checks for the specific error codes of each specific database for your code to work properly. The infrastructure will automatically do that for you – just let the exception happen. No need to write any try-catch code.

The sample is built on the newly released version of nServiceBus (1.6.1) but already contains all the binaries so you don’t have to set anything up yourself.

What’s coming for nServiceBus

We’re working towards a 2.0 release in the June-July timeframe which, beyond having the necessary documentation, web site, samples and everything any self-respecting open-source project has, is going to have some amazing grid-style features that will make all the message-priority & dynamic-routing stuff look “so last year”. Stay tuned.



   


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“So far, I heard about Service Oriented architecture all over. Everyone mentions it – the big buzz word. But, when I actually asked someone for what does it really mean, no one managed to give me a complete satisfied answer. Finally in his excellent course “Advanced Distributed Systems”, I got the answers I was looking for. Udi went over the different motivations (principles) of Services Oriented, explained them well one by one, and showed how each one could be technically addressed using NService bus. In his course, Udi also explain the way of thinking when coming to design a Service Oriented system. What are the questions you need to ask yourself in order to shape your system, place the logic in the right places for best Service Oriented system.

I would recommend this course for any architect or developer who deals with distributed system, but not only. In my work we do not have a real distributed system, but one PC which host both the UI application and the different services inside, all communicating via WCF. I found that many of the architecture principles and motivations of SOA apply for our system as well. Enough that you have SW partitioned into components and most of the principles becomes relevant to you as well. Bottom line – an excellent course recommended to any SW Architect, or any developer dealing with distributed system.”

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