About Hydra

Hydra is a tool for continuous integration testing and software release that uses a purely functional language to describe build jobs and their dependencies. Continuous integration is a simple technique to improve the quality of the software development process. An automated system continuously or periodically checks out the source code of a project, builds it, runs tests, and produces reports for the developers. Thus, various errors that might accidentally be committed into the code base are automatically caught. Such a system allows more in-depth testing than what developers could feasibly do manually:

  • Portability testing : The software may need to be built and tested on many different platforms. It is infeasible for each developer to do this before every commit.
  • Likewise, many projects have very large test sets (e.g., regression tests in a compiler, or stress tests in a DBMS) that can take hours or days to run to completion.
  • Many kinds of static and dynamic analyses can be performed as part of the tests, such as code coverage runs and static analyses.
  • It may also be necessary to build many different variants of the software. For instance, it may be necessary to verify that the component builds with various versions of a compiler.
  • Developers typically use incremental building to test their changes (since a full build may take too long), but this is unreliable with many build management tools (such as Make), i.e., the result of the incremental build might differ from a full build.
  • It ensures that the software can be built from the sources under revision control. Users of version management systems such as CVS and Subversion often forget to place source files under revision control.
  • The machines on which the continuous integration system runs ideally provides a clean, well-defined build environment. If this environment is administered through proper SCM techniques, then builds produced by the system can be reproduced. In contrast, developer work environments are typically not under any kind of SCM control.
  • In large projects, developers often work on a particular component of the project, and do not build and test the composition of those components (again since this is likely to take too long). To prevent the phenomenon of ``big bang integration'', where components are only tested together near the end of the development process, it is important to test components together as soon as possible (hence continuous integration ).
  • It allows software to be released by automatically creating packages that users can download and install. To do this manually represents an often prohibitive amount of work, as one may want to produce releases for many different platforms: e.g., installers for Windows and Mac OS X, RPM or Debian packages for certain Linux distributions, and so on.

In its simplest form, a continuous integration tool sits in a loop building and releasing software components from a version management system. For each component, it performs the following tasks:

  • It obtains the latest version of the component's source code from the version management system.
  • It runs the component's build process (which presumably includes the execution of the component's test set).
  • It presents the results of the build (such as error logs and releases) to the developers, e.g., by producing a web page.

Examples of continuous integration tools include Jenkins, CruiseControl Tinderbox, Sisyphus, Anthill and BuildBot. These tools have various limitations.

  • They do not manage the build environment . The build environment consists of the dependencies necessary to perform a build action, e.g., compilers, libraries, etc. Setting up the environment is typically done manually, and without proper SCM control (so it may be hard to reproduce a build at a later time). Manual management of the environment scales poorly in the number of configurations that must be supported. For instance, suppose that we want to build a component that requires a certain compiler X. We then have to go to each machine and install X. If we later need a newer version of X, the process must be repeated all over again. An ever worse problem occurs if there are conflicting, mutually exclusive versions of the dependencies. Thus, simply installing the latest version is not an option. Of course, we can install these components in different directories and manually pass the appropriate paths to the build processes of the various components. But this is a rather tiresome and error-prone process.
  • They do not easily support variability in software systems . A system may have a great deal of build-time variability: optional functionality, whether to build a debug or production version, different versions of dependencies, and so on. (For instance, the Linux kernel now has over 2,600 build-time configuration switches.) It is therefore important that a continuous integration tool can easily select and test different instances from the configuration space of the system to reveal problems, such as erroneous interactions between features. In a continuous integration setting, it is also useful to test different combinations of versions of subsystems, e.g., the head revision of a component against stable releases of its dependencies, and vice versa, as this can reveal various integration problems.

Hydra, is a continuous integration tool that solves these problems. It is built on top of the Nix package manager, which has a purely functional language for describing package build actions and their dependencies. This allows the build environment for projects to be produced automatically and deterministically, and variability in components to be expressed naturally using functions; and as such is an ideal fit for a continuous build system.

About Us

Hydra is the successor of the Nix Buildfarm, which was developed in tandem with the Nix software deployment system. Nix was originally developed at the Department of Information and Computing Sciences, Utrecht University by the TraCE project (2003-2008). The project was funded by the Software Engineering Research Program Jacquard to improve the support for variability in software systems. Funding for the development of Nix and Hydra is now provided by the NIRICT LaQuSo Build Farm project.

About this Manual

This manual tells you how to install the Hydra buildfarm software on your own server and how to operate that server using its web interface.


Hydra is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

Hydra is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

Hydra at

The installation of Hydra runs at That installation is used to build software components from the Nix, NixOS, GNU, Stratego/XT, and related projects.

If you are one of the developers on those projects, it is likely that you will be using the NixOS Hydra server in some way. If you need to administer automatic builds for your project, you should pull the right strings to get an account on the server. This manual will tell you how to set up new projects and build jobs within those projects and write a release.nix file to describe the build process of your project to Hydra. You can skip the next chapter.

If your project does not yet have automatic builds within the NixOS Hydra server, it may actually be eligible. We are in the process of setting up a large buildfarm that should be able to support open source and academic software projects. Get in touch.

Hydra on your own buildfarm

If you need to run your own Hydra installation, installation chapter explains how to download and install the system on your own server.