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Signed-off-by: Waldemar Kozaczuk <>
12 contributors

Users who have contributed to this file

@penberg @miha-plesko @wkozaczuk @syuu1228 @gberginc @tzach @tomime @tgrabiec @satoru-takeuchi @humblec @dmarti @asias


Capstan is a command-line tool for rapidly running your application on OSv unikernel. It focuses on improving user experience when building the unikernel and attempts to support not only a variety of runtimes (C, C++, Java, Node.js etc.), but also a variety of ready-to-run applications (Hadoop HDFS, MySQL, SimpleFOAM etc.).


Building unikernels is generally a nightmare! It is a non-trivial task that requires deep knowledge of unikernel implementation. It depends on numerous installation tools and takes somewhat 10 minutes to prepare each unikernel once configured correctly. But an application-oriented developer is not willing to take a load of new knowledge about unikernel specifics, nor wait long minutes to compile! And that's where Capstan comes in.

Capstan tends to be a tool that one configures with application-oriented settings (Where is application entry point? What environment variables to pass? etc.) and then runs a command or two to quickly boot up a new unikernel with application. Measured in seconds.

Capstan has been designed to operate in two distinct modes:

  • a mode based on Capstanfile (conceptually similar to Dockerfile) where the Capstanfile specifies base OSv image, optional build command, list of files to append to the image and the boot command line (for details look at Capstanfile specification)
  • a mode based on packages (conceptually similar to Docker compose), where Capstan uses prebuilt artifacts: precompiled OSv kernel, Java runtime from the Linux host, precompiled MySQL, and many more.

Either way, all you have to do is to write a Capstanfile or identify what prebuilt packages you want to have available in your unikernel and that's it.


Capstan is designed to prepare and run OSv unikernel for you. With Capstan it is possible to:

  • prepare OSv unikernel without compiling anything but your application, in seconds
  • build OSv image using Capstanfile (ala Dockerfile) or compose it from pre-built packages
  • use any prebuilt package from the OSv github releases repository or S3 package repository, or a combination thereof
  • build your own packages or base images
  • set arbitrary size of the target unikernel filesystem
  • run OSv unikernel one one of the supported hypervisors

But Capstan is not a magic tool that could solve all the problems for you. Capstan does not:

  • compile your application. If you have Java application, you need to use javac compiler and compile the application yourself prior using Capstan tool!
  • inspect your application. Capstan does nothing with your application but copies it into the unikernel
  • overcome OSv unikernel limits. Consult OSv documentation about what these limits are since they all still apply. Most notably, you can only run single process inside unikernel (forks are forbidden).

Getting started

Capstan can be installed using precompiled binary or compiled from source. Step-by-step Capstan Installation Guide

Using Capstan is rather simple: open up your project directory and create Capstan configuration files there:

$ capstan package init --name {name} --title {title} --author {author}
$ capstan runtime init --runtime {runtime}
# edit meta/run.yaml to match your application structure

Add your application files to the project root directory and then use Capstan command to create unikernel image (consult CLI Reference for a list of available arguments):

$ capstan package compose {unikernel-image-name}

At this point, you have your unikernel image built. It contains all your project files plus all the precompiled artifacts that you asked for. In other words, the unikernel image contains everything and is ready to be started! As you might have expected, there is Capstan command to run unikernel for you (using KVM/QEMU hypervisor on Linux):

$ capstan run {unikernel-image-name}

Congratulations, your unikernel is up-and-running! Press CTRL + C to stop it.



Capstan is distributed under the 3-clause BSD license.


This code has been developed within the MIKELANGELO project (no. 645402), started in January 2015, and co-funded by the European Commission under the H2020-ICT-07-2014: Advanced Cloud Infrastructures and Services programme.