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Note: Hydra 旨在供 Nix/NixOS 软件包开发者使用；若仅是普通使用 NixOS，则不需要它。

Hydra 是一个用于持续集成测试和软件发布的工具，它使用一种纯函数式语言来描述构建作业及其依赖关系。持续集成是一种提高软件开发过程质量的简单技术。一个自动化系统会持续或定期地检出项目的源代码，进行构建、运行测试，并为开发人员生成报告。因此，可能意外提交到代码库中的各种错误都会被自动发现。

Hydra 官方服务器提供了预构建的二进制包，以加快 Nixpkgs 的更新速度：用户无需在自己的计算机上进行编译。

Hydra 手册 提供了 Hydra 功能与特性的概述，以及最新的安装指南。

安装

完整部署的启用过程同样非常简单： 

  services.hydra = {
    enable = true;
    hydraURL = "http://localhost:3000"; # externally visible URL
    notificationSender = "hydra@localhost"; # e-mail of Hydra service
    # a standalone Hydra will require you to unset the buildMachinesFiles list to avoid using a nonexistant /etc/nix/machines
    buildMachinesFiles = [];
    # you will probably also want this, otherwise *everything* will be built from scratch
    useSubstitutes = true;
  };

The module will automatically enable postgresql if you do not change the services.hydra.dbi option. Database layout will be created automatically by the Hydra service, however keep in mind that some state will be stored in the database and a complete stateless configuration is currently not possible - do your backups.

Web Configuration

Hydra will provide the web interface at localhost port 3000. However you need to create a new admin user (as UNIX user hydra) before being able to perform any changes: 

# su - hydra
$ hydra-create-user alice --full-name 'Alice Q. User' \
    --email-address 'alice@example.org' --password-prompt --role admin

Virtual machine

If not configured explicitly to do otherwise, Hydra will specify localhost as the default build machine. By default, system features enabling builds to be performed in virtual machines like "kvm" or "nixos-test" are not enabled. Such jobs will be queued indefinitely. Those options can be activated as follows: 

{
  nix.buildMachines = [
    { hostName = "localhost";
      protocol = null;
      system = "x86_64-linux";
      supportedFeatures = ["kvm" "nixos-test" "big-parallel" "benchmark"];
      maxJobs = 8;
    }
  ];
}

This option leads to the file /etc/nix/machines being created. If the hydra service config is still set to buildMachinesFiles = [], then it will be ignored, so remove this option again or add /etc/nix/machines to it.

Flake jobset

Configure jobset to the following:

The Flake output should have the attribute hydraJobs containing an attribute set that may be nested and reference derivations.

A sample Flake output that makes Hydra build all packages could look like this: 

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  };
  outputs = { self, nixpkgs, ... }: {
    packages.x86_64-linux = {
      ...
    };

    hydraJobs = {
      inherit (self)
        packages;
    };
  };
}

Restricted Mode

Hydra evaluates flakes in restricted mode. This prevents access to files outside of the nix store, including those fetched as flake inputs. Update your nix.settings.allowed-uris to include URI prefixes from which you expect flake inputs to be fetched: 

nix.settings.allowed-uris = [
  "github:"
  "git+https://github.com/"
  "git+ssh://github.com/"
];

Build a single Package from nixpkgs

Right now it is not possible to build a single package from nixpkgs with just that input. You will need to provide a supplementary repository which defines what to build. For examples you can check the hydra-example by makefu and in the Hydra Manual.

Imperative Building

These steps are required to build the hello package.

  1. log into Hydra after creating a user with hydra-create-user
  1. create new project
  • identifier: example-hello
  • display name: example-hello
  1. Actions -> Create jobset
  • identifier: hello
  • Nix expression: release.nix in hydra-example -> will evaluate the file release.nix in the given input
  • check interval: 60
  • scheduling shares: 1
  • Inputs:
Input Name Type Value Note
nixpkgs git checkout https://github.com/nixos/nixpkgs nixos-21.11 will check out branch nixos-21.11, will be made available to the nix expression via <nixpkgs>.
hydra-example git checkout https://github.com/makefu/hydra-example hydra-example is used by the jobset as input, release.nix is in the root directory

After creation, the jobset should be in the evaluation phase where inputs will be fetched. This phase may take some time as the complete nixpkgs repository needs to be downloaded before continuing. The result of the evaluation should be a single job which will get built.

Declarative Building

Since 2016, Hydra supports declarative creation of jobsets. Check out the example repository and description by Shea Levy.

Hydra Internals

Definitions

This subsection provides an overview of the Hydra-specific definitions and how to configure them.

Project

A cluster of Jobs which are all coming from a single input (like a git checkout), the first thing you will need to create. Every Job should be able to be built independently from another. Most of the time the project maps to a single repository like nixpkgs. It is comparable to the project definition in Jenkins.

Jobsets

A Jobset is a list of jobs which will be run. Often a jobset fits to a certain branch (master, staging, stable). A Jobset is defined by its inputs and will trigger if these inputs change. For example when a new commit onto a branch is added. Jobsets may depend on each other.

Job

A closure which will be built as part of a job set (like a single package, iso image or tarball).

Release Set

Defines all the jobs which are described in your release. By convention a file calledrelease.nix is being used. See the Hydra manual for Build Recipes for a thorough description of the structure.

Evaluation

The process of interpreting nix code into a list of .drv files. These files are the build recipes for all related outputs. You can introspect these files by running nix show-derivation nixpkgs.hello.

Build

Instantiation of a Job which is being triggered by being part of the release set.

Known Issues

  • hydra-queue-runner sometimes gets stuck even with builds are in the queue, and the builds are not scheduled. The issue is being tracked here. In the meantime, a workaround is to add a cron job that regularly restarts the hydra-queue-runner systemd service. Possible fix: [1]
  • If you see error: unexpected end-of-file it can mean multiple things, some of them are:
  1. You have a miss-match between nix versions on the Hydra server and the builder
  1. It can also mean that hydra-queue-runner needs privileges on the build server. Reference: [2]
  • The default timeout for git operations is 600 seconds [3], which might cause fetches of large repositories like nixos/nixpkgs to fail: error fetching latest change from git repo at `https://github.com/nixos/nixpkgs': timeout. The timeout can be increased with the following configuration.nix snippet:
{
  services.hydra.extraConfig = ''
    <git-input>
      timeout = 3600
    </git-input>
  '';
}

Hydra for NixOS releases

Hydra is used for managing official Nix project releases. The project Hydra server: https://hydra.nixos.org/

Some Hydra trackers for Nix projects:

Resources

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