My Technology Stack

A bunch of people have been posting answers to David Burela’s ‘What is your preferred technology stack?’ question and I thought I’d join in. At work we’re just finishing up on a smallish project that uses this stack:

• ASP.NET MVC 2.0
  If you’re developing web applications on Microsoft’s stack, the Web Forms vs MVC decision will have the most significant effect on your day to day development experience. I find Web Forms to be a harmful, leaky abstraction and cannot get enough of MVC’s simplicity, especially when it comes to AJAX development.
• MVC Contrib
  Fills in some of ASP.NET MVC’s gaps.
• jQuery
  Just don’t do JavaScript without it.
• jQuery.UI, jQuery.Validate, jQuery.Form, jqGrid
  I use a few other jQuery plugins but these are the most useful ones.
• NHibernate + Fluent NHibernate + NHibernate.Validator
  This should hardly come as a surprise, given that I’m a member of the Fluent NHibernate team. We’re using a trunk build of NHibernate 3.0 which includes the work on the new LINQ provider. In terms of Fluent NHibernate usage, we go the automapping route as our domain models are pretty straightforward. We sacrifice domain “purity” and use NHibernate.Validator’s attributes because they are just so damn convenient. I’d be quite comfortable with making a move to the fluent interface though.
• Castle Windsor
  When it comes to IoC containers, I don’t have particularly strong feelings. I’ve used Structure Map a bit and I prefer it to Windsor but its easy to go with Windsor because Sharp Architecture includes it. My next home project that needs a container will use AutoFac as I’ve been meaning to try it.
• Sharp Architecture
  Glue for ASP.NET MVC, Fluent NHibernate, Castle.Windsor and other projects. Sharp Architecture gives you a bunch of handy stuff and from a rapid development perspective its quite nice to drop in. But I’m not entirely comfortable with some of the assumptions it makes. For larger projects, I’ll reuse a bunch of the code from Sharp Architecture, rather than referencing it as a binary.
• xVal
  Acts as a bridge between NHibernate.Validator and jQuery.Validate. This way we stick [NotEmpty] style attributes on our domain model and we get client side validation for free. I’m aware that ASP.NET MVC 2.0 includes some xVal style functionality but I wasn’t able to figure out if there was a better option so I just stuck with xVal since it worked so nice on my last MVC (1.0) project.
• Elmah
  Drop it into your solution in 10 minutes and then forget about exception logging.
• NUnit
  I’d like to try some other unit testing frameworks but its hard to find the time. NUnit works well enough for now.
• Microsoft Team Foundation Server
  Ugh. We use it for source control, builds, and work tracking – and its my fault! (a mistake from many years ago, sigh). I wish we were using git.

The only problem with a stack like this is that when you decide to use a newer NHibernate build, you’ve got a whole bunch of projects to recompile. I’m still waiting for some clever folks to sort this problem out :)

Talking Shop

Last week I was featured on the Talking Shop Down Under podcast. Fellow aussie Richard Banks was kind enough to extend me an invitation a few weeks back to join him on his show but I told him there were “… many more interesting aussies to interview before we get to me” and suggested he start with Joe Albahari (LINQPad rocks!!). Boy I knew I was in trouble when the very next podcast rolled through featuring Joe – Richard sure doesn’t waste any time!

In the podcast we discuss Fluent NHibernate, git, and my reasons for doing open source. For those of you that were hoping for some more Reactive Extensions content, I’m afraid I’ve been neglecting Rx lately so I didn’t feel qualified to talk much about it. Maybe next time?

Thanks to Richard for having me on the show, it was lots of fun. If you do have a listen, please feel free to leave a comment – this was my first podcast and I haven’t done a great deal of public speaking so I could benefit from some constructive criticism. Cheers!

Advanced Distributed Systems Design with Udi Dahan

I spent last week in Melbourne for a 5 day training course on distributed systems, run by Udi Dahan. I’ve been a fan of Udi’s work for a while now, so when I discovered that he was on his way to Australia I jumped at the chance to be immersed in his approach to designing scalable and reliable distributed systems. Let me tell you – I was not disappointed. I found Udi’s course to be very challenging – not in terms of difficulty, but rather that it was full of ideas that challenged my preconceptions. I was also very impressed with Udi’s presentation and communication skills. He really knows the material and has an amazing ability to give helpful, informative answers to really dumb questions :)

The course was focused on a few key ideas:

• Service oriented architecture
• Message based systems
• Command / query responsibility segregation
• Domain models

If you’d like a more detailed breakdown of the course content, Udi has provided an overview on his website.

I’ve tried to understand SOA before and struggled with the abstractness of the concept. I guess part of my difficulty has always come from the fact that I still approach the world with a “developer” mindset – I’ve never really bought into the concept of a non-coding architect and sometimes I struggle to understand the meaning and implications of an idea if I can’t see the actual effect on the code we write.

Thanks to Udi, I now have a basic understanding of what SOA means. I am comfortable with the idea of identifying business capabilities and describing them as services that consist of business components, which in turn consist of autonomous components that are individually deployable. I’m also comfortable with the idea of these autonomous components communicating with each other via a message bus using a combination of asynchronous request-reply and publish-subscribe. I am not yet sure how well my understanding will map to the existing literature on SOA, as I got the impression from some of the other students that Udi’s version of SOA was somewhat different to the SOA they had read about. My impression of SOA is that it is a slippery subject, where only the most abstract and vague definitions are widely accepted. That said, I can see a great deal of value in the analytical process and technical approach that Udi taught us.

Having skimmed Enterprise Integration Patterns, I was already familiar with the idea of message based systems and the advantages of temporal decoupling before I began the course. But Udi’s coverage of the topic helped fill in large gaps and also gave me a great deal of appreciation for the power and simplicity of his open source .NET based service bus, NServiceBus. My co-worker Malcolm nearly did a backflip when he saw how easy it is to begin using NServiceBus alongside WCF.

Command / query responsibility segregation (CQRS) is about recognising the difference between queries and commands as part of system architecture, and designing completely different paths for handling the two types of operations. Going into the course, I thought I was comfortable with the idea as I’d watched a number of videos on the topic from Greg Young, and had read a number of blog posts. It turned out that Udi had his own take on CQRS that I did not pick up on when reading his posts on the topic. Speaking of Udi’s posts on the topic of CQRS, I highly recommend this one if you’d like to learn more.

So how was Udi’s version of CQRS different to what I expected? One example immediately comes to mind. A necessary part of CQRS is some sort of persistent view model to facilitate the queries, and a transactional data model to facilitate the commands. So far, so good. Udi then introduced the idea that there may be some relationships that are only recorded in the persistent view model, and not in the transactional model – if later on you need that data in your transactional store, then backfill it with an ETL from your view model! Boy did that raise some incredulous looks, especially from yours truly! “What happened to my single source of truth?” I cried.

Udi also had some interesting ideas about domain models. In particular, we went into quite a bit of detail on the idea of aggregates, and how to ensure consistency. Has it ever occurred to you that perhaps there is something wrong with loading a Customer, and then lazily loading their Orders at some later point? What happens if one of the Orders are modified by another process, after the Customer is loaded but before the Orders are lazily loaded? Now the model loaded into memory is inconsistent! Even if your optimistic concurrency will protect you in most of these scenarios, its entirely possible that you’re updating some other object based on logic that has operated on your inconsistent snapshot. This problem seemed obvious once it was pointed out, but it had never occurred to me before.

There were so many interesting discussions and points of contention over the 5 days, this post has barely begun to skim the surface. It really was an eye opening experience for me, and some serious food for thought. If you ever get the opportunity to attend Udi’s course, don’t hesitate. Its worth it!

Now that I’ve finally managed to get something written down on the course, I’ll let myself read some of the summaries written by other attendees.

Reacting to the Reactive Framework: Part 8

Today I caught wind of some news that was bound to rouse me from my blogging slumber. The Reactive Framework, now known as Reactive Extensions (or Rx for short) has been released on DevLabs. The good news is that this release includes versions for .NET 3.5, .NET 4 Beta 2 and Silverlight 3, and documentation. The bad news is that at this stage, the documentation is pretty sparse - single sentence summaries and zero examples. Certainly, the summaries will help developer make sense of the impressive range of extension methods available on IObservable. For example, the comment on the rather cryptically named Amb - Returns the observable sequence that reacts first - makes its purpose quite clear, even if it doesn’t help explain the name. However the same cannot be said for all the cases, such as Dematerialize - Dematerializes the explicit notification values of an observable sequence as implicit notifications. Perhaps that will make sense once I understand the difference between explicit and implicit notifications, but its unlikely that the documentation (in its current form) will help me understand this. No matter - this intermediate release is certainly welcome and I now look forward to diving back in to Rx with at least some clue of what things are supposed to do :)

If you are interested in the Reactive Extensions I recommend you check out the team blog. The announcement post has plenty of links for you to click and learn more. I also recommend Matt Podwysocki’s series of Rx posts. Also, don’t forget that now Rx is on DevLabs, there’s a dedicated forum.

Reacting to the Reactive Framework: Part 7

The demo application for today allows the user to experiment with a couple of the mechanisms that I tried for part 6. As you may recall, I wanted an IObservable<int[]> that would return the three selections made from three groups of radio buttons, and only begin returning values once a selection had been made from all three. The demo app allows you to switch between using SelectMany, ForkJoin and CombineLatest and observe how the behavior changes:

The first approach I tried was SelectMany - I’ll use the query comprehension syntax as it is much more readable than calling the SelectMany extension method directly:

from s1 in choiceControl1.OptionSelections
from s2 in choiceControl2.OptionSelections
from s3 in choiceControl3.OptionSelections
select new[] { s1, s2, s3 }

This syntax might be readable, but its also a little misleading. It looks very uniform, like it wouldn’t matter which order you wrote those three ‘from’ statements in. But if you are familiar with LINQ statements like this one, you will know that the ordering IS important. To observe this, activate the SelectMany option in the app above, and then make your selections in reverse order i.e. select an option from group 3, then group 2, then group 1. Notice how you don’t get a selection readout? What’s actually happening here is that the selections made in groups 2 and 3 are ignored until a selection is made in group 1. Then the selections made in group 3 are ignored, until a selection is made in group 2. Then finally selections from group 3 will trigger the observable and raise a result. I could go into more detail on what’s happening here, but is there much point? Its obvious that this implementation is not at all close to the desired result. Lets move on.

When I came across the signature for ForkJoin, I thought I had found what I was looking for:

public static IObservable<TSource[]> ForkJoin<TSource>(params IObservable<TSource>[] sources); 

It converts many IObservable<T>’s into one IObservable<T[]>, which is exactly what I want. The bad news is that it only works once. Go ahead and try ForkJoin in the sample app – the status text will update once a selection has been made from all three groups, but it will not update again as the selection continues to change. It did occur to me that perhaps I could use ForkJoin and somehow re-subscribe each time it fires, but I was reluctant to go down that path because it doesn’t feel like a functional (as in functional programming) solution.

The demo app for today uses CombineLatest in the same way as in my previous post:

choiceControl1.OptionSelections
  .CombineLatest(choiceControl2.OptionSelections, (i, j) => new[] { i, j })
  .CombineLatest(choiceControl3.OptionSelections, (array, k) => new[] { array[0], array[1], k }))

This works fine, though its not pretty. I’ll keep my eye out for a better solution, but before I finish up today I want to look at the code behind today’s demo app:

<StackPanel Margin="20">
    <TextBlock Text="Combining Example" HorizontalAlignment="Center" Margin="10" />
    <StackPanel Orientation="Vertical" HorizontalAlignment="Left">
        <TextBlock Text="Select a means of combination:" />
        <RadioButton Name="rbSelectMany" Content="Use SelectMany" />
        <RadioButton Name="rbForkJoin" Content="Use ForkJoin" />
        <RadioButton Name="rbCombineLatest" Content="Use CombineLatest" />                               
        <TextBlock Text="Select an option from each group below, and then experiment with changing your selections." TextWrapping="Wrap" Margin="0,10,0,0"  />
    </StackPanel>            
    <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" >
        <SilverlightApp:ChoiceControl Name="choiceControl1" Heading="Group 1"/>
        <SilverlightApp:ChoiceControl Name="choiceControl2" Heading="Group 2"/>
        <SilverlightApp:ChoiceControl Name="choiceControl3" Heading="Group 3"/>
    </StackPanel>
    <TextBlock Name="statusText" Text="Status Text" TextAlignment="Center"></TextBlock>
</StackPanel>

public MainPage()
{
    InitializeComponent();

    IObservable<IObservable<int[]>> observerSelections = Observable.Merge(
        Observable.FromEvent<RoutedEventArgs>(rbSelectMany, "Checked")
            .Select(_ => from s1 in choiceControl1.OptionSelections
                         from s2 in choiceControl2.OptionSelections
                         from s3 in choiceControl3.OptionSelections
                         select new[] { s1, s2, s3 }),
        Observable.FromEvent<RoutedEventArgs>(rbForkJoin, "Checked")
            .Select(_ => Observable.ForkJoin(choiceControl1.OptionSelections, choiceControl2.OptionSelections, choiceControl3.OptionSelections)),
        Observable.FromEvent<RoutedEventArgs>(rbCombineLatest, "Checked")
            .Select(_ => choiceControl1.OptionSelections
                            .CombineLatest(choiceControl2.OptionSelections, (i, j) => new[] { i, j })
                            .CombineLatest(choiceControl3.OptionSelections, (array, k) => new[] { array[0], array[1], k }))
        );

    IDisposable subscription = null;
    observerSelections.Subscribe(observer =>
                                 {
                                     if (subscription != null)
                                         subscription.Dispose();

                                     choiceControl1.ClearAll();
                                     choiceControl2.ClearAll();
                                     choiceControl3.ClearAll();
                                     statusText.Text = string.Empty;

                                     subscription = observer.Subscribe(UpdateSelectedOptions);                                                                               
                                 });
}

private void UpdateSelectedOptions(int[] values)
{
    statusText.Text = string.Format("Option {0}, Option {1}, Option {2}",
                                    values[0], values[1], values[2]);
}

If you recall in the previous post, I was using the Observable.Merge method to glue three separate events together into one observable, and today I’m using the same technique on the radio buttons that let you swap between the different implementations. The difference in this case is that instead of simply returning an integer, I’m returning an IObservable<int[]>, so the result is an IObservable<IObservable<int[]>>. I then subscribe to this so that each time the selected implementation changes, the existing subscription is disposed, the selections are cleared and a new subscription is instated.

What do you think? Am I being too clever for my own good here? Is an IObservable<IObservable<T>> taking it too far? Let me know in the comments!

The code for this post has been tagged in my github repository.

Reacting to the Reactive Framework: Part 6

Before I dive into today’s code, there are two things worth mentioning:

1. If you haven’t already, make haste to Jafar Husain’s blog. Jafar is posting truly interesting examples of using the Reactive Framework - none of the ‘hello world’ fare I’m peddling here.
2. I mentioned in my previous post that I was having trouble using System.Reactive.dll with non-silverlight projects. Jb Evain has the goods on how you might get around this if you were so inclined. For now I’ll stick with Silverlight as its well suited for including interactive demonstrations in my blog posts.

Today I want to look at combining events using the Reactive Framework (Rx). Here is the demo app for today: (Once again I will remind readers using RSS readers to view this post in a proper browser window so you can see the embedded Silverlight app.)

The idea behind this example is that I am not interested in the selections until an option has been selected from all three groups. From then on, I’d like to be informed whenever the selection changes. This application consists of three instances of a user control, one for each group. Here is the relevant markup and code for the user control:

<StackPanel Margin="20">
    <TextBlock Text="{Binding Heading}" Margin="5" />
    <RadioButton Name="optionButton1" Content="Option 1" />
    <RadioButton Name="optionButton2" Content="Option 2" />
    <RadioButton Name="optionButton3" Content="Option 3" />
</StackPanel>

public ChoiceControl()
{
    InitializeComponent();

    OptionSelections = Observable.Merge(
        Observable.FromEvent<RoutedEventArgs>(optionButton1, "Checked").Select(_ => 1),
        Observable.FromEvent<RoutedEventArgs>(optionButton2, "Checked").Select(_ => 2),
        Observable.FromEvent<RoutedEventArgs>(optionButton3, "Checked").Select(_ => 3)
        );
}

public IObservable<int> OptionSelections { get; private set; }

As you can see, I am creating IObservables from the Checked events of the radio buttons. I’m using the Select method to make observables that simply return the index, and I’m merging those together into one IObservable<int>. So if the user checked option 1, then option 2, then option 3, the OptionSelections would call OnNext on its subscribed observers with values 1, 2 and 3 respectively.

As I mentioned before, this app has 3 instances of this user control:

<StackPanel x:Name="LayoutRoot" Margin="50" Background="Azure" Width="400">
    <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" >
        <SilverlightApp:ChoiceControl Name="choiceControl1" Heading="Choice 1"/>
        <SilverlightApp:ChoiceControl Name="choiceControl2" Heading="Choice 2"/>
        <SilverlightApp:ChoiceControl Name="choiceControl3" Heading="Choice 3"/>
    </StackPanel>
    <TextBlock Name="statusText" TextAlignment="Center"></TextBlock>
</StackPanel>

Now somehow I want to subscribe to the OptionSelections from each control, in such a way that I don’t see any results raised until an option from all three groups have been selected. After a few failed attempts, I stumbled upon this solution:

public MainPage()
{
    InitializeComponent();

    string formatString = "Option {0}, Option {1}, Option {2}";

    var selections = choiceControl1.OptionSelections
        .CombineLatest(choiceControl2.OptionSelections, (i, j) => new[] { i, j })
        .CombineLatest(choiceControl3.OptionSelections, (array, k) => new[] { array[0], array[1], k });

    selections.Subscribe(values => statusText.Text = string.Format(formatString, values[0], values[1], values[2]));            

}

I’m using the CombineLatest extension method, but it will only combine two IObservables so I have to use it twice. The first call returns an IObservable<int[]>, where the array contains two elements. I then combine that with the option selections from choiceControl3, again returning an IObservable<int[]>, this time with all 3 selections. The final step is simply to subscribe to the resulting observable and display the message.

This solution of combining once into an array of two elements and then combining again into an array of three doesn’t seem particularly elegant, but so far I haven’t managed to find anything better. Perhaps I could write a version of CombineLatest that takes more than two observables and hides this ugliness? Its probably not necessary – there’s a good chance that the perfect method already exists, and I’ve just missed it.

In my next post, I’d like to demonstrate some of the failed attempts I made before I found CombineLatest. This demo will give me a chance to create my first IObservable<IObservable<T>>, which I’ve been itching to do :)

Today’s code has been tagged in my github repository.

Reacting to the Reactive Framework: Part 5

It appears to have gone mostly under the radar, but the Reactive Framework is now out in the wild. The latest release of the Silverlight Toolkit includes System.Reactive.dll, mostly to facilitate the testing of the controls found within the toolkit. Jafar Husain broke the news earlier this week with this excellent post.

Today I want to do a demo that is very similar to the previous demos I’ve done, but of course this time I’ll be using the real Reactive Framework rather than my crappy attempt at implementing it that I’ve inflicted upon you in earlier posts. So far my experiments with using System.Reactive.dll with windows forms have not compiled properly so I’ll switch to using Silverlight for my examples. For now I will continue to focus on exposing button click events as an IObservable. Here is the code:

IObservable<Event<RoutedEventArgs>> clicks = Observable.FromEvent<RoutedEventArgs>(button1, "Click");

int count = 0;
clicks.Subscribe(() => count++);

IObservable<string> messages = from c in clicks
                               select string.Format("Clicked {0} time{1}", count, count > 1 ? "s" : "");
messages.Subscribe(s => button1.Content = s);

And here is the application:

If you are in a RSS reader, you probably won’t be able to see the embedded Silverlight app above. Pop this post out into its own browser tab so that you can revel in the glory of a button that tells you how many times its been clicked!

Lets take a look at what this code is doing. The first step is to convert from an Event to an IObservable:

IObservable<Event<RoutedEventArgs>> clicks = Observable.FromEvent<RoutedEventArgs>(button1, "Click");

The Observable class defines a large swath of extension methods for creating and manipulating IObservables. This overload of the FromEvent method is simple to use, but unfortunately it makes use of a magic string (“click”). There is another overload for FromEvent that allows us to do the same thing with compile-time safety, but it is more verbose. I’ve wrapped it in an extension method:

public static IObservable<Event<RoutedEventArgs>> GetClicks(this Button button)
{
    return Observable.FromEvent((EventHandler<RoutedEventArgs> genericHandler) => new RoutedEventHandler(genericHandler),
                                routedHandler => button.Click += routedHandler,
                                routedHandler => button.Click -= routedHandler);
}

This overload takes three functions as arguments. One to convert from a generic event handler (EventHandler<T>) to the specific event handler that the Click event uses (RoutedEventHandler). The conversion is straightforward, because the two event handlers have the same signature. The other two functions add and remove the handler. This extension method can be used like so:

IObservable<Event<RoutedEventArgs>> clicks = button1.GetClicks();

It would not surprise me if we eventually see extra libraries that define many of these extension methods for us. Ideally they would be unnecessary, and we could simply write:

IObservable<Event<RoutedEventArgs>> clicks = button1.GetObservableEvent(b => b.Click);

But unfortunately the dreaded “The event 'System.Windows.Controls.Primitives.ButtonBase.Click' can only appear on the left hand side of += or –=”  message rears its ugly head. Perhaps in C# 5 the story will be different.

Moving on, the code initializes a count variable and subscribes an increment function to the clicks:

int count = 0;
clicks.Subscribe(() => count++);

You’ll notice that I am simply passing an Action to the Subscribe method, rather than an IObserver. The Reactive Framework won’t force you to use an IObserver if all you want to do is call a function when a new event occurs.

Finally, the code converts the stream of click events into a stream of messages and subscribes to it:

IObservable<string> messages = from c in clicks
                               select string.Format("Clicked {0} time{1}", count, count > 1 ? "s" : "");
messages.Subscribe(s => button1.Content = s);

There are lots of other ways in which to rewrite today’s example code. This version is very short and still quite readable:

int count = 0;
button1.GetClicks().Select(x => ++count)
    .Subscribe(() => button1.Content = string.Format("Clicked {0} time{1}", count, count > 1 ? "s" : ""));

Or I could implement IObserver:

public class CountingButtonObserver : IObserver<Event<RoutedEventArgs>>
{
    private int _count = 0;
    public Button Button { get; set; }

    public void OnNext(Event<RoutedEventArgs> value)
    {
        _count++;
        Button.Content = string.Format("Clicked {0} time{1}", _count, _count > 1 ? "s" : "");
    }

    public void OnError(Exception exception) {}
    public void OnCompleted() {}
}

And use it like so:

button1.GetClicks().Subscribe(new CountingButtonObserver{ Button = button1});

Well, that’s probably enough for today. I’m going to continue to explore the Reactive Framework and see what interesting things I can find. Oh, and let me know if you come across any documentation for it – so far I haven’t found any and it can be quite a struggle to make sense of the various extension methods that are available.

I’ll be pushing my experiments with the Reactive Framework to a github repo.