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This is an upgraded version of the original Capstan and is maintained by MIKELANGELO consortium. Although it is still possible, joining this repository with the original Capstan is not very likely. We're looking towards the first stable release at the moment.

Capstan is a command-line tool for rapidly running your application on OSv unikernel. It focuses on improving user experience during 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.).


Buildning 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 unikerel 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.

To achieve this, Capstan uses precompiled artefacts: precompiled OSv kernel, precompiled Java runtime, precompiled MySQL, and many more. All you have to do is to name what precompiled 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
  • use any precompiled package from the MIKELANGELO package repository, or a combination thereof
  • set arbitrary size of the target unikernel filesystem
  • run OSv unikernel using one of the supported providers

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

Being in project root directory, then use Capstan command to create unikernel (consult CLI Reference for a list of available arguments):

$ capstan package compose {unikernel-name}

At this point, you have your unikernel built. It contains all your project files plus all the precompiled artefacts that you asked for. In other words, the unikernel 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 hipervisor):

$ capstan run {unikernel-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.