# 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
- either PostgreSQL or SQLite
- many Perl packages, notably Catalyst, EmailSender, and NixPerl (see the Hydra expression in Nixpkgs 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 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, along with a manual, which includes installation instructions.

## Installation

Hydra can be installed using Nixpkgs:

    	    nix-env -Ai hydra -f /path/to/nixpkgs

This makes the tools available in your Nix user environment, `$HOME/.nix-profile` by default.

Alternatively, the latest development snapshot can be installed by visiting the URL `http://hydra.nixos.org/view/hydra/unstable` and use 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/trunk/channel/latest
    nix-channel --update
    nix-env -i hydra

Command completion should reveal a number of command-line tools from Hydra:

    hydra_build.pl            hydra_eval_jobs           hydra_server.pl
    hydra_control.sh          hydra_evaluator.pl        hydra_update_gc_roots.pl
    hydra_create.pl           hydra_queue_runner.pl

## Creating the database

Hydra stores its results in a database, which can be a PostgreSQL or SQLite database. The latter is easier to setup, but the former scales better.

To setup a PostgreSQL database with hydra as database name and user name, issue the following commands:

    createdb hydra
    echo "CREATE USER hydra WITH PASSWORD '<your-password>' ;" | psql hydra
    cat $prefix/share/hydra/sql/hydra-postgresql.sql | psql hydra
    echo "GRANT ALL ON DATABASE hydra TO hydra;" | psql hydra

Note that $prefix is the location of Hydra in the nix store.

For SQLite, the following command is all it takes to create the database:

    	      cat $prefix/share/hydra/sql/hydra-sqlite.sql | sqlite3 /path/to/hydra.sqlite

To add a user root with admin privileges, execute:

    echo "INSERT INTO Users(userName, emailAddress, password) VALUES ('root', 'some@email.adress.com', '$(echo -n foobar | sha1sum | cut -c1-40)');" | psql hydra
    echo "INSERT INTO UserRoles(userName, role) values('root', 'admin');" | psql hydra

For SQLite the same commands can be used, with `psql hydra` replaced by `sqlite3 /path/to/hydra.sqlite`.

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 `.profile` of the user running the Hydra services.

    export HYDRA_DBI="dbi:Pg:dbname=hydra;host=localhost;"
    export HYDRA_DATA=/var/lib/hydra

Make sure that the HYDRA_DATA directory exists and is writable for the user which will run the Hydra services. For a SQLite database, the `HYDRA_DBI` should be set to something like `dbi:SQLite:/path/to/hydra.sqlite`

## Getting Started

To start the Hydra web server, execute:

    hydra_server.pl

When the server is started, you can browse to http://localhost:3000/ to start configuring your Hydra instance.

The `hydra_server.pl` command launches the web server. There are two other processes that come into play:

- The evaluator is responsible for peridically 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.pl` 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.pl` 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.