Project Highlight: Diamond MVC Framework

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Anyone who has been around the D community for longer than an eye blink will have heard of vibe.d, undoubtedly the most widely-used web application framework written in the D programming language. Those same people could be excused if they haven’t yet heard of Diamond, announcements for which have only started showing up in the D forums relatively recently.

According to its website, Diamond is “a full-stack, cross-platform MVC/Template Framework” that’s “inspired by ASP.NET and uses vibe.d for its backend”. Jacob Jensen, the project’s author, explains.

I have always been interested in web development, so one of the few projects I started writing in D was a web server using Phobos’s std.socket. It wasn’t a notable project or anything, but more an experiment. Then I discovered vibe.d and toyed around with it. Unfortunately, coming from a background working with ASP.NET using Razor, I wasn’t a big fan of the Diet templates. So initially I started Diamond as an alternative template engine. However, I ended up just adding more and more to the project and it soon became more and more independent. At this point, you don’t really write a lot of vibe.d code when using it, because the most general vibe.d features have wrappers in Diamond to better interact with the rest of the project.

Development on Diamond began in early 2016, but he put it aside a few months later. Then in October of 2017, after picking up some contract web app work, Diamond was resurrected in its 2.0 form.

I decided I wanted to use D, so I simply did a complete revamp of the project and plan to keep maintaining it.

His biggest hurdle has been keeping the design of the framework user-friendly and minimizing complex interactions between controllers, views, and models.

It was a big challenge to ensure that everything worked together in a way that made it feel natural to work with. On top of that I had to make sure that Diamond worked under multiple build types. At first Diamond was written solely with the web in mind and thus only supported websites and web APIs, but I saw the potential to use the template engine for more than just the web, like email templates. Thus I introduced yet a third way of building Diamond applications, which had to be completely separated from the web part of Diamond without introducing complexity into user code or the build process. By introducing stand-alone support, Diamond is now able to be used with existing projects that aren’t already using it for the web, e.g. someone could use Diamond to extend their existing web pages without having to switch the whole project to Diamond, or simply use Diamond for only a small portion of it.

Aside from the challenge of maintaining user-friendliness, Jacob says he’s encountered only a few issues in developing the framework, most of which came when he was refactoring for the 2.0 release. One in particular is interesting, as his solution for it was a D language feature you don’t often hear much about.

When I was introducing attributes to controllers to avoid manual mapping of actions, it took a while to figure out the best approach to it without having to pass additional information about the controller to its base class.

To demonstrate, Controller subclasses could originally be declared like so:

class MyController(TView) : Controller!TView
{
    ...
}

After he initially added attributes, the refactoring required the base class template to know the derived type at compile time in order to reflect on its attributes. His initial solution required that subclasses specify their own types as an additional template parameter to the Controller template in the class declaration.

class MyController(TView) : Controller!(TView, MyController!TView)
{
    ...
}

He didn’t like it, but it was the only way he could see to make the base class aware of the derived type. Then he discovered D’s template this parameters.

Template this parameters allow any templated member function to know at compile time the static, i.e. declared, type of the instance on which the function is being called.

module base;
class Base 
{
    void printType(this T)() 
    {
        import std.stdio;
        writeln(typeid(T));
    }
}

class Derived : Base {}

void main()
{
    Derived d1 = new Derived;
    auto d2 = new Derived;
    Base b1 = new Derived;

    d1.printType();
    d2.printType();
    b1.printType();
}

And this prints (in modulename.TypeName format):

base.Derived
base.Derived
base.Base

In Diamond, this is used in the Controller constructor in order to parse the UDAs (User Defined Attributes) attached to the derived type at compile time:

class Controller(TView) : BaseController
{
    ...
    this(this TController)(TView view)
    {
        ...

        static if (hasUDA!(TController, HttpAuthentication))
        {
            ...
        }

        static if (hasUDA!(TController, HttpVersion))
        {
            ...
        }
        ...
    }
}

The caveat, and the price Jacob is willing to pay for the increased convenience to users, is that instances of derived types should never be declared to have the type of the base class. When working with templated types in D, it’s idiomatic to use type inference anyway:

// This won't pick up the MyController attributes, as the declared
// type is that of the base class
Controller!ViewImpl controller1 = new MyController!ViewImpl;

// But this will
MyController!ViewImpl controller3 = new MyController!ViewImpl;

// And so will this -- it's also more idiomatic
auto controller2 = new MyController!ViewImpl;

Overall, Jacob has found the transition from C# to D fairly painless.

Most code I was used to writing, coming from C#, is pretty straight-forward in D. One of the pros of D, however, is its compile-time functionality. I use it heavily in Diamond to parse templates, map routes and controller actions, etc. It’s a really powerful tool in development and probably the most powerful tool in D. I also really like templates in D. They’re implemented in a way that doesn’t make them seem complex, unlike in C++, where templates can often seem obscure and cryptic. D is probably the most natural programming language that I’ve used.

Diamond indirectly supports Mongo and Redis through vibe.d, and has its own MySQL ORM interface that uses the native MySQL library under the hood. He has some plans improve upon the database support, however.

I plan to rewrite the whole MySQL part, since it currently uses some deprecated features – it was based on some old code I had been using. Along with that, I plan on implementing some “generic services” that can be used to create internal services in the project, which will of course wrap database engines such as MySQL, Mongo, Redis, etc., creating a similar API between them all and exposing an easier way to implement sharding.

He also intends to add support for textual data formats other than JSON (such as XML) to make Diamond compatible with SOAP or WCF services, add improved support for components in the view, and provide better integration with JavaScript. He also would like to implement an app server for hosting Diamond applications.

Anyone intending to use D for web work today who, like Jacob, has experience using ASP.NET and Razor should feel right at home using Diamond. For the rest, it’s an alternative to using vibe.d directly that some may find more comfortable. You can find the Diamond source, the current documentation, and the in-development official website (for which Jacob is dog-fooding Diamond) all at GitHub.

The Making of ‘D Web Development’

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A long-time contributor to the D community, Kai Nacke is the author of ‘D Web Development‘ and the maintainer of LDC, the LLVM D Compiler. In this post, he tells the story of how his book came together. Currently, the eBook version is on sale for USD $10.00 as part of the publisher’s Back to School sale, as are ‘D Cookbook‘ by Adam Ruppe and ‘Learning D‘ by Michael Parker.


At the beginning of 2014, I was asked by Packt Publishing if I wanted to review the D Cookbook by Adam Ruppe. Of course I wanted to!

The review was stressful, but it was a lot of fun. At the end of the year came a surprising question for me: would I be willing to switch sides and write a book myself? Here, I hesitated. Sure, writing your own book is a dream, but is this at all possible on top of a regular job? The proposed topic, D Web Development, was interesting. Web technologies I knew, of course, but the vibe.d framework was for me only a large unit test for each LDC release.

My interest was awakened and I created a chapter overview, based solely on my experience as a developer and the online documentation of vibe.d. The result came out well and I was offered a contract. It came with an immediate challenge: I should set up a small project plan. How do you plan to write a book?!?

Without any experience in this area, I stuck to the following rules. For each chapter, I planned a little time frame. Each should include at least one weekend, for the larger chapters perhaps even two. I reserved some time for the Easter holiday, too. The first version of the book would therefore be ready at the beginning of July, when I started writing in mid-February.

Even the first chapter showed that this plan was much too optimistic. The writing went off quickly – as soon as I had something I could write about. But experimenting and testing took a lot of time. For one thing, I didn’t have much experience with vibe.d. There were sample programs that I wanted to develop Saturday to write about on Sunday. However, I was still searching for errors on Monday, without having written a single line!

On the other hand, there were still a few rough edges in vibe.d at the time, but I did not want to write that these would be changed or implemented in later versions of the library. So I developed a few patches for vibe.d, e.g. digest authentication. By the way, there were also new LDC releases to create. Fortunately, the LDC team had expanded, so I just took care of the release itself (thanks so much, folks!). The result was, of course, that I missed many of my milestones.

In May, the first chapters came back from the review process. Other content also had to be written, such as the text for the back of the book. In mid-December, the last chapter was finished and almost all review notes on the other chapters were incorporated. After a little Christmas break, the remaining notes were quickly incorporated and the pre-final version of each chapter was created in January. And then, on February 1, 2016, the news came that my book was now published. I’d done it! Almost exactly one year after I had started with the first chapter.

Was the work worth it? In any case, it was a very special experience. Would I do it again? Yes! Right now, I am playing with the idea of updating the book and expanding a chapter. Let’s see what happens…

Project Highlight: vibe.d

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Since the day Sönke Ludwig first announced vibe.d on the D Forums, the project has been a big hit in the D community. It’s the exclusive subject of one book, has a chapter of its own in another, and has been proven in production both commercially and otherwise. As so many projects do, it all started out of frustration.

I was dissatisfied with existing network web libraries (in particular with Node.js, which was the new big thing back then, because it was also built on an asynchronous I/O model). D 2.0 gained cross platform fiber support through the integration of DRuntime, which seemed like a perfect opportunity to avoid the shortcomings of Node.js’s programming model (“callback hell”). Together with D’s strong type checking and the high performance of natively compiled applications this had the ideal preconditions for creating a network framework.

From the initial release, work progressed on adding web and REST interface generators (vibe.web.web and vibe.web.rest, respectively).

This was made possible by D’s advanced meta programming facilities, string mixins and compile-time reflection in particular. The eventual addition of user-defined attributes to the language enabled some important advances later on, such as the recently added authorization framework.

Vibe.d is at its core an I/O and concurrency framework that makes heavy use of fibers which run in a quasi-parallel framework.

Every time an operation (e.g. reading from a socket) needs to wait, the fiber yields execution, so another fiber can run instead. Each fiber uses up very little memory compared to a full thread and switching between fibers is very cheap. This enables highly scalable applications that behave like normal multithreaded applications (save for the low-level issues associated with real multithreading).

At a higher level, it can serve as a web framework for backend development and provides functionality for protocols like HTTP and SMTP, database connectivity, and the parsing of data formats. A number of third-party packages that extend or complement vibe.d can be found in the DUB repository (Sönke is also the creator and maintainer of DUB, the D build tool and package manager).

Big changes are currently afoot with the project. Beginning with the release of vibe.d 0.7.27 in February 2016, work began on splitting the monolithic project into independent DUB packages. One goal is to make it possible to use one vibe.d component without pulling them all in, reducing build times in the process.

Another goal is to employ modern D idioms where possible and to improve memory usage and performance as far as possible. It is surprising how much D evolved in just the short amount of time that vibe.d has been alive!

Diet-NG, vibe.d’s template engine based on Jade, was the first to be granted independence. It went through a complete rewrite that adds a strong test suite, makes use of D’s ranges where possible, provides a more flexible API, and eliminates dependencies on other vibe.d packages. Now he’s working on the core package.

The vibe-core package encapsulates the whole event and fiber logic, including I/O, tasks, concurrency primitives and general operating system abstraction. The original design was based heavily on classes and interfaces and had a very high level operating system abstraction layer, resulting in several downsides. For example, there was a dependence on the GC and virtual function calls could be an issue on certain platforms. One of the main goals was to minimize performance overhead in the new implementation.

As part of his experimentation with different API idioms and slimming down the code base, he produced the eventcore library.

The API follows a proactor pattern, meaning that a callback gets invoked whenever a certain asynchronous operation finishes. This is in contrast to the reactor pattern that is exposed by the non-blocking Posix APIs. It was chosen mainly so that asynchronous I/O APIs, such as Windows overlapped I/O and POSIX AIO, could be supported.

On top of eventcore, the fiber logic was implemented in a completely general way, using a central fiber scheduler and a generic asyncAwait function. This means that lots of corner cases are handled in a much more robust way now and that improvements in all areas can be made much faster and with fewer chances of breaking anything.

Next on the list for independence is the HTTP package. Sönke plans to completely rebuild the package from the ground up, adding HTTP/2 support and making it possible to enable allocation-free request/response processing.

Other obvious candidates are MongoDB and Redis clients, JSON and BSON support, the serialization framework, and the Markdown parser. The goal for each of these packages is always to take a look at the code and employ modern D idioms during the process.

If you’ve never taken D for a spin, vibe.d is a fun playground in which to do so. It’s not difficult to get up and running, and easier still if you have experience with such frameworks in other languages. It’s also ready to put into production in its current state, despite the leading zero in the version number. As Sönke makes progress on breaking it up into separate packages, it will almost certainly become an even more integral part of the growing D community.

Project Highlight: libasync

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d6libasync is a cross-platform event loop library written completely in D.  It was created, and continues to be maintained, by Etienne Cimon, who started it as a native driver for vibe.d, a modular asynchronous I/O framework most often used for web app development in D.

In 2014 or so, I was looking for a framework to power my future web development projects. I wasn’t going to use an interpreted language, as binary executables were too attractive. I found vibe.d appealing because, coming from C++, it was relatively simple and featureful. So I studied it, along with the D programming language and the Phobos standard library.

vibe.d has always used libevent under the hood by default. This is where Etienne ran into a problem that bothered him.

I stumbled on some workflow issues when deploying vibe.d apps to other operating systems which may or may not have the right version of libevent in the package repository. I didn’t want to package a DLL with my server, or have to go through dependency hell with my software, and I wanted everything to be consistently written in D to reduce the mental complexity of switching programming languages or to debug other issues.

So he decided to study up on the system APIs across the platforms supported by DMD (Windows, Linux, *BSD and OS X) and create his own event loop library in D. Now he, and anyone using libasync, can issue a single command with DUB to compile and execute a web application without needing to worry about external event loop dependencies.

libasync takes advantage of D’s delegates to provide a very intuitive interface.

void testDNS() {
	auto dns = new shared AsyncDNS(g_evl);
	dns.handler((NetworkAddress addr) {
		writeln("Resolved to: ", addr.toString(), ", it took: ", g_swDns.peek().usecs, " usecs");
	}).resolveHost("127.0.0.1");
}

Etienne says of the code snippet above:

The D garbage collector will keep the AsyncDNS object in dns alive for as long as the delegate used in the parameter of dns.handler is alive in the heap, which is in this object. The delegate syntax is more simple to declare than Javascript, and it is also type-safe. This DNS resolver will work on any platform thrown at it, thanks to D’s compile-time version conditions.

libasync makes use of the asynchronous I/O facilities available on each supported platform and provides a number of event-handlers out of the box.

Cross-platform event handlers have been defined for DNS resolution, UDP Messages, (Buffered/Unbuffered) TCP Connections, TCP Listeners, File Operations, Thread-local (Notifiers) and Cross-thread Signals, Timers and File Watchers. The intrinsics involve EPoll for Linux, KQueue for OS X and BSD, and overlapped I/O for Windows. With all of these features thoroughly tested through a vibe.d driver, libasync has become a very fast and reliable library which I use in all of my projects. My benchmarks show it as being a little slower than the libevent driver in vibe.d, though its self-explanatory code base makes it seamless to understand, maintain, and deploy.

A libasync driver has been added to vibe.d and work is going on to improve the library’s performance.

The stability of the underlying OS features makes for very little need for changes, although there is a big improvement involving the proactor pattern in the works for libasync and a new architecture for vibe.d. Together, those two developments are likely to increase the library’s performance significantly.

If you find yourself needing an event loop in D and want to give libasync a spin, you can visit the library’s page at the DUB repository for information on how to add it as a dependency to your own DUB-managed projects. libasync, in turn, has only one dependency itself, another library maintained by Etienne that provides a set of allocators and allocator-friendly containers called memutils.

It wasn’t so long ago that anyone using D who wanted something like libasync or memutils would need to either roll their own or bind to a C library. The ever-expanding list of libraries in the DUB repository, created and made available by members of the D community like Etienne, make it much easier to jump into D today than ever before.