DOCBOOK_FILES = installation.xml introduction.xml manual.xml projects.xml hacking.xml
EXTRA_DIST = $(DOCBOOK_FILES)
xsltproc_opts =					\
 --param callout.graphics.extension \'.gif\'	\
 --param section.autolabel 1 			\
 --param section.label.includes.component.label 1

MD_FILES = src/*.md

# Include the manual in the tarball.
dist_html_DATA = manual.html

EXTRA_DIST = $(MD_FILES)

# Embed Docbook's callout images in the distribution.
EXTRA_DIST += images
manual.html: $(DOCBOOK_FILES)
	$(XSLTPROC) $(xsltproc_opts) --nonet --xinclude		\
	  --output manual.html					\
	  $(docbookxsl)/xhtml/docbook.xsl manual.xml
images:
	$(MKDIR_P) images/callouts
	cp $(docbookxsl)/images/callouts/*.gif images/callouts
	chmod +wx images images/callouts
install-data-hook: images
	$(INSTALL) -d $(DESTDIR)$(htmldir)/images/callouts
	$(INSTALL_DATA) images/callouts/* $(DESTDIR)$(htmldir)/images/callouts
	ln -sfn manual.html $(DESTDIR)$(htmldir)/index.html
distclean-hook:
	-rm -rf images

install: $(MD_FILES)
	mdbook build . -d $(docdir)

# Hydra User's Guide
- [Introduction](introduction.md)
- [Installation](installation.md)
- [Creating and Managing Projects](projects.md)
- [Using the external API](api.md)
-----------
[About](about.md)
[Hacking](hacking.md)

# Authors
* Eelco Dolstra, Delft University of Technology, Department of Software Technology
* Rob Vermaas, Delft University of Technology, Department of Software Technology
* Eelco Visser, Delft University of Technology, Department of Software Technology
* Ludovic Courtès

Using the external API
======================
To be able to create integrations with other services, Hydra exposes an
external API that you can manage projects with.
The API is accessed over HTTP(s) where all data is sent and received as
JSON.
Creating resources requires the caller to be authenticated, while
retrieving resources does not.
The API does not have a separate URL structure for it\'s endpoints.
Instead you request the pages of the web interface as `application/json`
to use the API.
List projects
-------------
To list all the `projects` of the Hydra install:
    GET /
    Accept: application/json
This will give you a list of `projects`, where each `project` contains
general information and a list of its `job sets`.
**Example**
    curl -i -H 'Accept: application/json' \
        https://hydra.nixos.org
**Note:** this response is truncated
    GET https://hydra.nixos.org/
    HTTP/1.1 200 OK
    Content-Type: application/json
    [
      {
        "displayname": "Acoda",
        "name": "acoda",
        "description": "Acoda is a tool set for automatic data migration along an evolving data model",
        "enabled": 0,
        "owner": "sander",
        "hidden": 1,
        "jobsets": [
          "trunk"
        ]
      },
      {
        "displayname": "cabal2nix",
        "name": "cabal2nix",
        "description": "Convert Cabal files into Nix build instructions",
        "enabled": 0,
        "owner": "simons@cryp.to",
        "hidden": 1,
        "jobsets": [
          "master"
        ]
      }
    ]
Get a single project
--------------------
To get a single `project` by identifier:
    GET /project/:project-identifier
    Accept: application/json
**Example**
    curl -i -H 'Accept: application/json' \
        https://hydra.nixos.org/project/hydra
    GET https://hydra.nixos.org/project/hydra
    HTTP/1.1 200 OK
    Content-Type: application/json
    {
      "description": "Hydra, the Nix-based continuous build system",
      "hidden": 0,
      "displayname": "Hydra",
      "jobsets": [
        "hydra-master",
        "hydra-ant-logger-trunk",
        "master",
        "build-ng"
      ],
      "name": "hydra",
      "enabled": 1,
      "owner": "eelco"
    }
Get a single job set
--------------------
To get a single `job set` by identifier:
    GET /jobset/:project-identifier/:jobset-identifier
    Content-Type: application/json
**Example**
    curl -i -H 'Accept: application/json' \
        https://hydra.nixos.org/jobset/hydra/build-ng
    GET https://hydra.nixos.org/jobset/hydra/build-ng
    HTTP/1.1 200 OK
    Content-Type: application/json
    {
      "errormsg": "evaluation failed due to signal 9 (Killed)",
      "fetcherrormsg": null,
      "nixexprpath": "release.nix",
      "nixexprinput": "hydraSrc",
      "emailoverride": "rob.vermaas@gmail.com, eelco.dolstra@logicblox.com",
      "jobsetinputs": {
        "officialRelease": {
          "jobsetinputalts": [
            "false"
          ]
        },
        "hydraSrc": {
          "jobsetinputalts": [
            "https://github.com/NixOS/hydra.git build-ng"
          ]
        },
        "nixpkgs": {
          "jobsetinputalts": [
            "https://github.com/NixOS/nixpkgs.git release-14.12"
          ]
        }
      },
      "enabled": 0
    }
List evaluations
----------------
To list the `evaluations` of a `job set` by identifier:
    GET /jobset/:project-identifier/:jobset-identifier/evals
    Content-Type: application/json
**Example**
    curl -i -H 'Accept: application/json' \
        https://hydra.nixos.org/jobset/hydra/build-ng/evals
**Note:** this response is truncated
    GET https://hydra.nixos.org/jobset/hydra/build-ng/evals
    HTTP/1.1 200 OK
    Content-Type: application/json
    {
      "evals": [
        {
          "jobsetevalinputs": {
            "nixpkgs": {
              "dependency": null,
              "type": "git",
              "value": null,
              "uri": "https://github.com/NixOS/nixpkgs.git",
              "revision": "f60e48ce81b6f428d072d3c148f6f2e59f1dfd7a"
            },
            "hydraSrc": {
              "dependency": null,
              "type": "git",
              "value": null,
              "uri": "https://github.com/NixOS/hydra.git",
              "revision": "48d6f0de2ab94f728d287b9c9670c4d237e7c0f6"
            },
            "officialRelease": {
              "dependency": null,
              "value": "false",
              "type": "boolean",
              "uri": null,
              "revision": null
            }
          },
          "hasnewbuilds": 1,
          "builds": [
            24670686,
            24670684,
            24670685,
            24670687
          ],
          "id": 1213758
        }
      ],
      "first": "?page=1",
      "last": "?page=1"
    }
Get a single build
------------------
To get a single `build` by its id:
    GET /build/:build-id
    Content-Type: application/json
**Example**
    curl -i -H 'Accept: application/json' \
        https://hydra.nixos.org/build/24670686
    GET /build/24670686
    HTTP/1.1 200 OK
    Content-Type: application/json
    {
      "job": "tests.api.x86_64-linux",
      "jobsetevals": [
        1213758
      ],
      "buildstatus": 0,
      "buildmetrics": null,
      "project": "hydra",
      "system": "x86_64-linux",
      "priority": 100,
      "releasename": null,
      "starttime": 1439402853,
      "nixname": "vm-test-run-unnamed",
      "timestamp": 1439388618,
      "id": 24670686,
      "stoptime": 1439403403,
      "jobset": "build-ng",
      "buildoutputs": {
        "out": {
          "path": "/nix/store/lzrxkjc35mhp8w7r8h82g0ljyizfchma-vm-test-run-unnamed"
        }
      },
      "buildproducts": {
        "1": {
          "path": "/nix/store/lzrxkjc35mhp8w7r8h82g0ljyizfchma-vm-test-run-unnamed",
          "defaultpath": "log.html",
          "type": "report",
          "sha256hash": null,
          "filesize": null,
          "name": "",
          "subtype": "testlog"
        }
      },
      "finished": 1
    }

Hacking
=======
This section provides some notes on how to hack on Hydra. To get the
latest version of Hydra from GitHub:
    $ git clone git://github.com/NixOS/hydra.git
    $ cd hydra
To build it and its dependencies:
    $ nix-build release.nix -A build.x86_64-linux
To build all dependencies and start a shell in which all environment
variables (such as PERL5LIB) are set up so that those dependencies can
be found:
    $ nix-shell
To build Hydra, you should then do:
    [nix-shell]$ ./bootstrap
    [nix-shell]$ configurePhase
    [nix-shell]$ make
You can run the Hydra web server in your source tree as follows:
    $ ./src/script/hydra-server

Installation
============
This chapter explains how to install Hydra on your own build farm
server.
Prerequisites
-------------
To install and use Hydra you need to have installed the following
dependencies:
-   Nix
-   PostgreSQL
-   many Perl packages, notably Catalyst, EmailSender, and NixPerl (see
    the [Hydra expression in
    Nixpkgs](https://github.com/NixOS/hydra/blob/master/release.nix) for
    the complete list)
At the moment, Hydra runs only on GNU/Linux (*i686-linux* and
*x86\_64\_linux*).
For small projects, Hydra can be run on any reasonably modern machine.
For individual projects you can even run Hydra on a laptop. However, the
charm of a buildfarm server is usually that it operates without
disturbing the developer\'s working environment and can serve releases
over the internet. In conjunction you should typically have your source
code administered in a version management system, such as subversion.
Therefore, you will probably want to install a server that is connected
to the internet. To scale up to large and/or many projects, you will
need at least a considerable amount of diskspace to store builds. Since
Hydra can schedule multiple simultaneous build jobs, it can be useful to
have a multi-core machine, and/or attach multiple build machines in a
network to the central Hydra server.
Of course we think it is a good idea to use the
[NixOS](http://nixos.org/nixos) GNU/Linux distribution for your
buildfarm server. But this is not a requirement. The Nix software
deployment system can be installed on any GNU/Linux distribution in
parallel to the regular package management system. Thus, you can use
Hydra on a Debian, Fedora, SuSE, or Ubuntu system.
Getting Nix
-----------
If your server runs NixOS you are all set to continue with installation
of Hydra. Otherwise you first need to install Nix. The latest stable
version can be found one [the Nix web
site](http://nixos.org/nix/download.html), along with a manual, which
includes installation instructions.
Installation
------------
The latest development snapshot of Hydra can be installed by visiting
the URL
[`http://hydra.nixos.org/view/hydra/unstable`](http://hydra.nixos.org/view/hydra/unstable)
and using the one-click install available at one of the build pages. You
can also install Hydra through the channel by performing the following
commands:
    nix-channel --add http://hydra.nixos.org/jobset/hydra/master/channel/latest
    nix-channel --update
    nix-env -i hydra
Command completion should reveal a number of command-line tools from
Hydra, such as `hydra-queue-runner`.
Creating the database
---------------------
Hydra stores its results in a PostgreSQL database.
To setup a PostgreSQL database with *hydra* as database name and user
name, issue the following commands on the PostgreSQL server:
    createuser -S -D -R -P hydra
    createdb -O hydra hydra
Note that *\$prefix* is the location of Hydra in the nix store.
Hydra uses an environment variable to know which database should be
used, and a variable which point to a location that holds some state. To
set these variables for a PostgreSQL database, add the following to the
file `~/.profile` of the user running the Hydra services.
    export HYDRA_DBI="dbi:Pg:dbname=hydra;host=dbserver.example.org;user=hydra;"
    export HYDRA_DATA=/var/lib/hydra
You can provide the username and password in the file `~/.pgpass`, e.g.
    dbserver.example.org:*:hydra:hydra:password
Make sure that the *HYDRA\_DATA* directory exists and is writable for
the user which will run the Hydra services.
Having set these environment variables, you can now initialise the
database by doing:
    hydra-init
To create projects, you need to create a user with *admin* privileges.
This can be done using the command `hydra-create-user`:
    $ hydra-create-user alice --full-name 'Alice Q. User' \
        --email-address 'alice@example.org' --password foobar --role admin
Additional users can be created through the web interface.
Upgrading
---------
If you\'re upgrading Hydra from a previous version, you should do the
following to perform any necessary database schema migrations:
    hydra-init
Getting Started
---------------
To start the Hydra web server, execute:
    hydra-server
When the server is started, you can browse to [http://localhost:3000/]()
to start configuring your Hydra instance.
The `hydra-server` command launches the web server. There are two other
processes that come into play:
-   The
    evaluator
    is responsible for periodically evaluating job sets, checking out
    their dependencies off their version control systems (VCS), and
    queueing new builds if the result of the evaluation changed. It is
    launched by the
    hydra-evaluator
    command.
-   The
    queue runner
    launches builds (using Nix) as they are queued by the evaluator,
    scheduling them onto the configured Nix hosts. It is launched using
    the
    hydra-queue-runner
    command.
All three processes must be running for Hydra to be fully functional,
though it\'s possible to temporarily stop any one of them for
maintenance purposes, for instance.
Serving behind reverse proxy
----------------------------
To serve hydra web server behind reverse proxy like *nginx* or *httpd*
some additional configuration must be made.
Edit your `hydra.conf` file in a similar way to this example:
    using_frontend_proxy 1
    base_uri example.com
`base_uri` should be your hydra servers proxied URL. If you are using
Hydra nixos module then setting `hydraURL` option should be enough.
If you want to serve Hydra with a prefix path, for example
[http://example.com/hydra]() then you need to configure your reverse
proxy to pass `X-Request-Base` to hydra, with prefix path as value. For
example if you are using nginx, then use configuration similar to
following:
    server {
        listen 433 ssl;
        server_name example.com;
        .. other configuration ..
        location /hydra/ {
            proxy_pass     http://127.0.0.1:3000;
            proxy_redirect http://127.0.0.1:3000 https://example.com/hydra;
            proxy_set_header  Host              $host;
            proxy_set_header  X-Real-IP         $remote_addr;
            proxy_set_header  X-Forwarded-For   $proxy_add_x_forwarded_for;
            proxy_set_header  X-Forwarded-Proto $scheme;
            proxy_set_header  X-Request-Base    /hydra;
        }
    }
Using LDAP as authentication backend (optional)
-----------------------------------------------
Instead of using Hydra\'s built-in user management you can optionally
use LDAP to manage roles and users.
The `hydra-server` accepts the environment variable
*HYDRA\_LDAP\_CONFIG*. The value of the variable should point to a valid
YAML file containing the Catalyst LDAP configuration. The format of the
configuration file is describe in the
[*Catalyst::Authentication::Store::LDAP*
documentation](https://metacpan.org/pod/Catalyst::Authentication::Store::LDAP#CONFIGURATION-OPTIONS).
An example is given below.
Roles can be assigned to users based on their LDAP group membership
(*use\_roles: 1* in the below example). For a user to have the role
*admin* assigned to them they should be in the group *hydra\_admin*. In
general any LDAP group of the form *hydra\_some\_role* (notice the
*hydra\_* prefix) will work.
    credential:
      class: Password
      password_field: password
      password_type: self_check
    store:
      class: LDAP
      ldap_server: localhost
      ldap_server_options.timeout: 30
      binddn: "cn=root,dc=example"
      bindpw: notapassword
      start_tls: 0
      start_tls_options
        verify:  none
      user_basedn: "ou=users,dc=example"
      user_filter: "(&(objectClass=inetOrgPerson)(cn=%s))"
      user_scope: one
      user_field: cn
      user_search_options:
        deref: always
      use_roles: 1
      role_basedn: "ou=groups,dc=example"
      role_filter: "(&(objectClass=groupOfNames)(member=%s))"
      role_scope: one
      role_field: cn
      role_value: dn
      role_search_options:
        deref: always
        

Introduction
============
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](http://nixos.org/nix/),
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.
License
-------
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](http://www.gnu.org/licenses/) for more details.
Hydra at `nixos.org`
--------------------
The `nixos.org` installation of Hydra runs at
[`http://hydra.nixos.org/`](http://hydra.nixos.org/). That installation
is used to build software components from the [Nix](http://nixos.org),
[NixOS](http://nixos.org/nixos), [GNU](http://www.gnu.org/),
[Stratego/XT](http://strategoxt.org), 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](installation.md) explains how to download and install the
system on your own server.

Creating and Managing Projects
==============================
Once Hydra is installed and running, the next step is to add projects to
the build farm. We follow the example of the [Patchelf
project](http://nixos.org/patchelf.html), a software tool written in C
and using the GNU Build System (GNU Autoconf and GNU Automake).
Log in to the web interface of your Hydra installation using the user
name and password you inserted in the database (by default, Hydra\'s web
server listens on [`localhost:3000`](http://localhost:3000/)). Then
follow the \"Create Project\" link to create a new project.
Project Information
-------------------
A project definition consists of some general information and a set of
job sets. The general information identifies a project, its owner, and
current state of activity. Here\'s what we fill in for the patchelf
project:
    Identifier: patchelf
The *identifier* is the identity of the project. It is used in URLs and
in the names of build results.
The identifier should be a unique name (it is the primary database key
for the project table in the database). If you try to create a project
with an already existing identifier you\'d get an error message from the
database. So try to create the project after entering just the general
information to figure out if you have chosen a unique name. Job sets can
be added once the project has been created.
    Display name: Patchelf
The *display name* is used in menus.
    Description: A tool for modifying ELF binaries
The *description* is used as short documentation of the nature of the
project.
    Owner: eelco
The *owner* of a project can create and edit job sets.
    Enabled: Yes
Only if the project is *enabled* are builds performed.
Once created there should be an entry for the project in the sidebar. Go
to the project page for the
[Patchelf](http://localhost:3000/project/patchelf) project.
Job Sets
--------
A project can consist of multiple *job sets* (hereafter *jobsets*),
separate tasks that can be built separately, but may depend on each
other (without cyclic dependencies, of course). Go to the
[Edit](http://localhost:3000/project/patchelf/edit) page of the Patchelf
project and \"Add a new jobset\" by providing the following
\"Information\":
    Identifier:     trunk
    Description:    Trunk
    Nix expression: release.nix in input patchelfSrc
This states that in order to build the `trunk` jobset, the Nix
expression in the file `release.nix`, which can be obtained from input
`patchelfSrc`, should be evaluated. (We\'ll have a look at `release.nix`
later.)
To realize a job we probably need a number of inputs, which can be
declared in the table below. As many inputs as required can be added.
For patchelf we declare the following inputs.
    patchelfSrc
    'Git checkout' https://github.com/NixOS/patchelf
    nixpkgs 'Git checkout' https://github.com/NixOS/nixpkgs
    officialRelease   Boolean false
    system   String value "i686-linux"
Building Jobs
-------------
Build Recipes
-------------
Build jobs and *build recipes* for a jobset are specified in a text file
written in the [Nix language](http://nixos.org/nix/). The recipe is
actually called a *Nix expression* in Nix parlance. By convention this
file is often called `release.nix`.
The `release.nix` file is typically kept under version control, and the
repository that contains it one of the build inputs of the
corresponding--often called `hydraConfig` by convention. The repository
for that file and the actual file name are specified on the web
interface of Hydra under the `Setup` tab of the jobset\'s overview page,
under the `Nix
      expression` heading. See, for example, the [jobset overview
page](http://hydra.nixos.org/jobset/patchelf/trunk) of the PatchELF
project, and [the corresponding Nix
file](https://github.com/NixOS/patchelf/blob/master/release.nix).
Knowledge of the Nix language is recommended, but the example below
should already give a good idea of how it works:
    let
      pkgs = import <nixpkgs> {}; ① 
      jobs = rec { ②
        tarball = ③
          pkgs.releaseTools.sourceTarball { ④
            name = "hello-tarball";
            src = <hello>; ⑤
            buildInputs = (with pkgs; [ gettext texLive texinfo ]);
          };
        build = ⑥
          { system ? builtins.currentSystem }:  ⑦
          let pkgs = import <nixpkgs> { inherit system; }; in
          pkgs.releaseTools.nixBuild { ⑧
            name = "hello";
            src = jobs.tarball;
            configureFlags = [ "--disable-silent-rules" ];
          };
      };
    in
      jobs ⑨
          
This file shows what a `release.nix` file for
[GNU Hello](http://www.gnu.org/software/hello/) would look like.
GNU Hello is representative of many GNU and non-GNU free software
projects:
-   it uses the GNU Build System, namely GNU Autoconf, and GNU Automake;
    for users, it means it can be installed using the
    usual
    ./configure && make install
    procedure
    ;
-   it uses Gettext for internationalization;
-   it has a Texinfo manual, which can be rendered as PDF with TeX.
The file defines a jobset consisting of two jobs: `tarball`, and
`build`. It contains the following elements (referenced from the figure
by numbers):
1.  This defines a variable `pkgs` holding the set of packages provided
    by [Nixpkgs](http://nixos.org/nixpkgs/).
    Since `nixpkgs` appears in angle brackets, there must be a build
    input of that name in the Nix search path. In this case, the web
    interface should show a `nixpkgs` build input, which is a checkout
    of the Nixpkgs source code repository; Hydra then adds this and
    other build inputs to the Nix search path when evaluating
    `release.nix`.
2.  This defines a variable holding the two Hydra jobs--an *attribute
    set* in Nix.
3.  This is the definition of the first job, named `tarball`. The
    purpose of this job is to produce a usable source code tarball.
4.  The `tarball` job calls the `sourceTarball` function, which
    (roughly) runs `autoreconf && ./configure &&
                make dist` on the checkout. The `buildInputs` attribute
    specifies additional software dependencies for the job.
    
    > The package names used in `buildInputs`--e.g., `texLive`--are the
    > names of the *attributes* corresponding to these packages in
    > Nixpkgs, specifically in the
    > [`all-packages.nix`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/all-packages.nix)
    > file. See the section entitled "Package Naming" in the Nixpkgs
    > manual for more information.
5.  The `tarball` jobs expects a `hello` build input to be available in
    the Nix search path. Again, this input is passed by Hydra and is
    meant to be a checkout of GNU Hello\'s source code repository.
6.  This is the definition of the `build` job, whose purpose is to build
    Hello from the tarball produced above.
7.  The `build` function takes one parameter, `system`, which should be
    a string defining the Nix system type--e.g., `"x86_64-linux"`.
    Additionally, it refers to `jobs.tarball`, seen above.
    Hydra inspects the formal argument list of the function (here, the
    `system` argument) and passes it the corresponding parameter
    specified as a build input on Hydra\'s web interface. Here, `system`
    is passed by Hydra when it calls `build`. Thus, it must be defined
    as a build input of type string in Hydra, which could take one of
    several values.
    The question mark after `system` defines the default value for this
    argument, and is only useful when debugging locally.
8.  The `build` job calls the `nixBuild` function, which unpacks the
    tarball, then runs `./configure && make
                && make check && make install`.
9.  Finally, the set of jobs is returned to Hydra, as a Nix attribute
    set.
Building from the Command Line
------------------------------
It is often useful to test a build recipe, for instance before it is
actually used by Hydra, when testing changes, or when debugging a build
issue. Since build recipes for Hydra jobsets are just plain Nix
expressions, they can be evaluated using the standard Nix tools.
To evaluate the `tarball` jobset of the above example, just
run:
    $ nix-build release.nix -A tarball
However, doing this with the example as is will probably
yield an error like this:
    error: user-thrown exception: file `hello' was not found in the Nix search path (add it using $NIX_PATH or -I)
The error is self-explanatory. Assuming `$HOME/src/hello` points to a
checkout of Hello, this can be fixed this way:
    $ nix-build -I ~/src release.nix -A tarball
Similarly, the `build` jobset can be evaluated:
    $ nix-build -I ~/src release.nix -A build
The `build` job reuses the result of the `tarball` job, rebuilding it
only if it needs to.
Adding More Jobs
----------------
The example illustrates how to write the most basic
jobs, `tarball` and `build`. In practice, much more can be done by using
features readily provided by Nixpkgs or by creating new jobs as
customizations of existing jobs.
For instance, test coverage report for projects compiled with GCC can be
automatically generated using the `coverageAnalysis` function provided
by Nixpkgs instead of `nixBuild`. Back to our GNU Hello example, we can
define a `coverage` job that produces an HTML code coverage report
directly readable from the corresponding Hydra build page:
    coverage =
      { system ? builtins.currentSystem }:
      let pkgs = import nixpkgs { inherit system; }; in
      pkgs.releaseTools.coverageAnalysis {
        name = "hello";
        src = jobs.tarball;
        configureFlags = [ "--disable-silent-rules" ];
      };
As can be seen, the only difference compared to `build` is the use of
`coverageAnalysis`.
Nixpkgs provides many more build tools, including the ability to run
build in virtual machines, which can themselves run another GNU/Linux
distribution, which allows for the creation of packages for these
distributions. Please see [the `pkgs/build-support/release`
directory](https://github.com/NixOS/nixpkgs/tree/master/pkgs/build-support/release)
of Nixpkgs for more. The NixOS manual also contains information about
whole-system testing in virtual machine.
Now, assume we want to build Hello with an old version of GCC, and with
different `configure` flags. A new `build_exotic` job can be written
that simply *overrides* the relevant arguments passed to `nixBuild`:
    build_exotic =
      { system ? builtins.currentSystem }:
      let
        pkgs = import nixpkgs { inherit system; };
        build = jobs.build { inherit system; };
      in
        pkgs.lib.overrideDerivation build (attrs: {
          buildInputs = [ pkgs.gcc33 ];
          preConfigure = "gcc --version";
          configureFlags =
            attrs.configureFlags ++ [ "--disable-nls" ];
        });
The `build_exotic` job reuses `build` and overrides some of its
arguments: it adds a dependency on GCC 3.3, a pre-configure phase that
runs `gcc --version`, and adds the `--disable-nls` configure flags.
This customization mechanism is very powerful. For instance, it can be
used to change the way Hello and *all* its dependencies--including the C
library and compiler used to build it--are built. See the Nixpkgs manual
for more.
Declarative projects
--------------------
Hydra supports declaratively configuring a project\'s jobsets. This
configuration can be done statically, or generated by a build job.
> **Note**
> 
> Hydra will treat the project\'s declarative input as a static definition
> if and only if the spec file contains a dictionary of dictionaries. If
> the value of any key in the spec is not a dictionary, it will treat the
> spec as a generated declarative spec.
### Static, Declarative Projects
Hydra supports declarative projects, where jobsets are configured from a
static JSON document in a repository.
To configure a static declarative project, take the following steps:
1.  Create a Hydra-fetchable source like a Git repository or local path.
2.  In that source, create a file called `spec.json`, and add the
    specification for all of the jobsets. Each key is jobset and each
    value is a jobset\'s specification. For example:
    ``` {.json}
    {
      "nixpkgs": {
        "enabled": 1,
        "hidden": false,
        "description": "Nixpkgs",
        "nixexprinput": "nixpkgs",
        "nixexprpath": "pkgs/top-level/release.nix",
        "checkinterval": 300,
        "schedulingshares": 100,
        "enableemail": false,
        "emailoverride": "",
        "keepnr": 3,
        "inputs": {
          "nixpkgs": {
              "type": "git",
              "value": "git://github.com/NixOS/nixpkgs.git master",
              "emailresponsible": false
          }
        }
      },
      "nixos": {
        "enabled": 1,
        "hidden": false,
        "description": "NixOS: Small Evaluation",
        "nixexprinput": "nixpkgs",
        "nixexprpath": "nixos/release-small.nix",
        "checkinterval": 300,
        "schedulingshares": 100,
        "enableemail": false,
        "emailoverride": "",
        "keepnr": 3,
        "inputs": {
          "nixpkgs": {
            "type": "git",
            "value": "git://github.com/NixOS/nixpkgs.git master",
            "emailresponsible": false
          }
        }
      }
    }
    ```
3.  Create a new project, and set the project\'s declarative input type,
    declarative input value, and declarative spec file to point to the
    source and JSON file you created in step 2.
Hydra will create a special jobset named `.jobsets`. When the `.jobsets`
jobset is evaluated, this static specification will be used for
configuring the rest of the project\'s jobsets.
### Generated, Declarative Projects
Hydra also supports generated declarative projects, where jobsets are
configured automatically from specification files instead of being
managed through the UI. A jobset specification is a JSON object
containing the configuration of the jobset, for example:
``` {.json}
    {
        "enabled": 1,
        "hidden": false,
        "description": "js",
        "nixexprinput": "src",
        "nixexprpath": "release.nix",
        "checkinterval": 300,
        "schedulingshares": 100,
        "enableemail": false,
        "emailoverride": "",
        "keepnr": 3,
        "inputs": {
            "src": { "type": "git", "value": "git://github.com/shlevy/declarative-hydra-example.git", "emailresponsible": false },
            "nixpkgs": { "type": "git", "value": "git://github.com/NixOS/nixpkgs.git release-16.03", "emailresponsible": false }
        }
    }
  
```
To configure a declarative project, take the following steps:
1.  Create a jobset repository in the normal way (e.g. a git repo with a
    `release.nix` file, any other needed helper files, and taking any
    kind of hydra input), but without adding it to the UI. The nix
    expression of this repository should contain a single job, named
    `jobsets`. The output of the `jobsets` job should be a JSON file
    containing an object of jobset specifications. Each member of the
    object will become a jobset of the project, configured by the
    corresponding jobset specification.
2.  In some hydra-fetchable source (potentially, but not necessarily,
    the same repo you created in step 1), create a JSON file containing
    a jobset specification that points to the jobset repository you
    created in the first step, specifying any needed inputs
    (e.g. nixpkgs) as necessary.
3.  In the project creation/edit page, set declarative input type,
    declarative input value, and declarative spec file to point to the
    source and JSON file you created in step 2.
Hydra will create a special jobset named `.jobsets`, which whenever
evaluated will go through the steps above in reverse order:
1.  Hydra will fetch the input specified by the declarative input type
    and value.
2.  Hydra will use the configuration given in the declarative spec file
    as the jobset configuration for this evaluation. In addition to any
    inputs specified in the spec file, hydra will also pass the
    `declInput` argument corresponding to the input fetched in step 1.
3.  As normal, hydra will build the jobs specified in the jobset
    repository, which in this case is the single `jobsets` job. When
    that job completes, hydra will read the created jobset
    specifications and create corresponding jobsets in the project,
    disabling any jobsets that used to exist but are not present in the
    current spec.
Email Notifications
-------------------
Hydra can send email notifications when the status of a build changes.
This provides immediate feedback to maintainers or committers when a
change causes build failures.
The simplest approach to enable Email Notifications is to use the ssmtp
package, which simply hands off the emails to another SMTP server. For
details on how to configure ssmtp, see the documentation for the
`networking.defaultMailServer` option. To use ssmtp for the Hydra email
notifications, add it to the path option of the Hydra services in your
`/etc/nixos/configuration.nix` file:
    systemd.services.hydra-queue-runner.path = [ pkgs.ssmtp ];
    systemd.services.hydra-server.path = [ pkgs.ssmtp ];

              final.nix perlDeps perl

              final.nix perlDeps perl mdbook

          configureFlags = [ "--with-docbook-xsl=${docbook_xsl}/xml/xsl/docbook" ];