A simple Capstan example

A Simple Capstan Example

(Updated 14 April 2014: Add new URL for osv-base image.)

Capstan is a new tool for building OSv virtual machine images. If you have worked with other tools for making VMs, you’ll find that Capstan is really simple. It’s a lot like Docker actually—only you get a complete VM out of it and not just a container.

You’re probably used to blogs from sneaky tech evangelists who claim that something is simple and then post some complicated set of instructions. So just to keep your finger off the close button, here’s all you need to do.

  • Add a Make target to build your application as a shared object.

  • Write a short Capstanfile. (8 lines not counting comments).

  • Run Capstan.

That’s all there is to it. Finger off the close button now? Good. Ready?

Let’s make a VM that does something useful, say, serve this article to the entire Internet. Go ahead and git clone Capstan and follow along.

An easy example, plus Makefile work

Just to keep it simple, let’s borrow the short HTTP server example from libevent. The libevent project is a wrapper for convenient event-driven programming, and the library is used in high-profile projects such as Tor, the anonymous communications system, and Chromium, the basis for the Google Chrome web browser.

Best of all, libevent includes an easy-to-use HTTP implementation and sample code for using it. So I’ll copy their web server sample code, tweak it a little to make the web server I need, and set up a simple Makefile.

Those steps are all done in the code for this article, which is at dmarti/http-server.

You’ll need the development package for libevent installed. On my system, it’s called libevent-devel.

Here’s the target to pay attention to:

http-server.so : http-server.c
        $(CC) -o $@ -std=gnu99 -fPIC -shared -levent $``

Yes, that’s right, we’re using -fPIC (position independent code) and -shared (passed to the linker, make it build a shared library). And http-server.c has a function called main. What’s going on? This is because of the way OSv works. Your application on OSv isn’t a conventional ELF executable, but a .so file.

Besides building the actual HTTP server, I’ll also put in a Make target to create the HTML version of this article from the README, because I can. So I type make to build the web content and the web server.

Of course you can expand on this to build as complicated of an application and data set as you want. This is just an example to show you Capstan for now.

Step two: Add a Capstanfile

Now it’s time to tell Capstan how to create the virtual machine image. Building it is easy—just run make—so there’s the build section right there. Now we need to tell Capstan what files go into the image, so we populate the files section with the name of our web server (http-server.so) the libevent shared library, and some web content—just the HTML version of this article, plus a favicon.ico file. (For now I’m just copying my development systems’s copy of libevent into the image. For real use, I’ll come up with a more consistent way to keep track of build artifacts like this, probably borrowing them from some helpful Linux distribution. Yes, OSv can use libraries built on and for your 64-bit Linux box.)

Easy so far. Now for the cmdline option, which is like Docker’s CMD: the command that gets run when the image starts. The HTTP server just takes its DocumentRoot entry from the command line, so the command comes out as:

cmdline: /tools/http-server.so /www

There’s one more section in the Capstanfile: base. That’s a pre-built OSv image, which is available from Amazon S3. Capstan will automatically download this for you. It lives under .capstan in your home directory.

Putting it all together

Now, when we type capstan build, Capstan invokes make, then creates the VM image. It lives under .capstan in your home directory, at:


This is a QCOW2 image, ready to run under KVM or convert to your favorite format. That’s it. Told you it was simple. You can just do capstan run and point your browser to http://localhost:8080/ to see the site.

In an upcoming blog post, I’ll cover the recently added VirtualBox support in Capstan (hint: try -p vbox) and some other fun things you can do.

If you have any Capstan questions, please join the osv-dev mailing list on Google Groups. You can get updates on new OSv and Capstan progress by subscribing to this blog or folllowing @CloudiusSystems on Twitter.