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Hydra

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What it is

Hydra is a continuous integration service for projects based on Nix. It checks out project source periodically or continuously, builds it, runs tests, and produces reports. Work is organized into jobsets whose inputs can trigger builds when they change; jobsets can also depend on one another. Flake jobsets are supported when their outputs expose a hydraJobs attribute set. Jobs can produce packages, ISO images, or tarballs, and jobset inputs can use Git checkout sources. Hydra also provides a JSON API. It is aimed at Nix and NixOS package developers, not people who only need to use NixOS. Hydra is open source under GPL-3.0 and listed at free. It is self-hosted, with no hosted option listed, and running it is currently supported only on NixOS; the repository recommends the NixOS module for production. Flake evaluation is restricted from accessing files outside the Nix store. Some state is stored in the database, so backups are needed, and the wiki reports that hydra-queue-runner can sometimes leave queued builds unscheduled.

Spec sheet

What it does
Hydra is a continuous-integration service for projects based on Nix.Read 14 Oct 2026 · github.com
Purpose
Hydra is a continuous integration service for Nix based projects.Read 4 Oct 2026 · github.com
Intended users
Hydra is intended for Nix and NixOS package developers and is not needed simply to use NixOS.Read 4 Oct 2026 · wiki.nixos.org
Deployment
The repository says running Hydra is currently only supported on NixOS and recommends the NixOS module for production.Read 4 Oct 2026 · github.com
API
The repository includes a JSON API specification in hydra-api.yaml and points users to Swagger Editor for exploration.Read 14 Oct 2026 · github.com
Integrations
Hydra jobset inputs can use Git checkout sources, and the documented examples use GitHub repositories.Read 4 Oct 2026 · github.com
License
Hydra is distributed under the GNU General Public License version 3.Read 14 Oct 2026 · github.com
API surface
The hydra-api.yaml file contains 1,267 lines, including 1,220 lines of code.Read 14 Oct 2026 · github.com
Binary cache
Successful builds become substitutable store paths that can feed a binary cache.Read 14 Oct 2026 · zemdregon.github.io
Build model
Hydra evaluates Nix expressions into derivations and builds them on configured machines.Read 14 Oct 2026 · zemdregon.github.io
Build outputs
A Hydra build is an instantiated job such as a package, ISO image, or tarball.Read 14 Oct 2026 · wiki.nixos.org
Build products
Hydra can build jobs such as packages, ISO images, and tarballs as part of a jobset.Read 4 Oct 2026 · wiki.nixos.org
Builds and tests
Hydra periodically or continuously checks out project source, builds it, runs tests, and produces reports.Read 4 Oct 2026 · wiki.nixos.org
Checks distinction
Hydra does not use a flake's checks output as its job tree; checks are generally local or forge-CI gates.Read 14 Oct 2026 · zemdregon.github.io
Database dependency
A self-hosted deployment uses PostgreSQL for persistent Hydra state.Read 14 Oct 2026 · zemdregon.github.io
Declarative configuration
Hydra supports both imperative and declarative project and jobset configuration.Read 14 Oct 2026 · wiki.nixos.org
Documentation
The project links to a Hydra User's Guide, NixOS Wiki material, hydra-cli, and a build-farm setup presentation.Read 14 Oct 2026 · github.com
Evaluations
An evaluation interprets Nix code into derivation files that define the build jobs.Read 14 Oct 2026 · wiki.nixos.org
Flake support
For flakes, Hydra consumes the hydraJobs output attribute as its job tree.Read 14 Oct 2026 · zemdregon.github.io
Flakes
Hydra supports Flake jobsets whose outputs expose a hydraJobs attribute set.Read 4 Oct 2026 · wiki.nixos.org
Foundation operations
The NixOS Foundation says its Hydra build farm has hundreds of macOS, x86-64 Linux, and aarch64 Linux build cores and executes more than 350,000 builds weekly.Read 14 Oct 2026 · nixos.org
Independent review coverage
G2's Hydra page says it does not have enough reviews to provide buying insight; the listing is for a project-management product rather than NixOS Hydra.Read 14 Oct 2026 · g2.com
Independent technical assessment
An independent Nix documentation site describes Hydra as heavyweight for self-hosting and suggests lightweight forge CI for most application repositories.Read 14 Oct 2026 · zemdregon.github.io
Jobsets
Jobsets combine inputs with a Nix expression and can re-evaluate when inputs change.Read 14 Oct 2026 · wiki.nixos.org
Known issue
The wiki reports that hydra-queue-runner can sometimes get stuck while builds remain queued and unscheduled.Read 4 Oct 2026 · wiki.nixos.org
Limit: database contention
A Hydra issue reports that evaluating very large jobsets can lock the builds table and delay queue-runner updates for around 30 minutes.Read 14 Oct 2026 · github.com
Limit: disk usage
A Hydra issue documents recurring disk-space pressure involving build logs, PostgreSQL, the Nix store, and caches.Read 14 Oct 2026 · github.com
Limit: large Git inputs
The NixOS Wiki says the default 600-second Git timeout can make fetching large repositories fail.Read 14 Oct 2026 · wiki.nixos.org
Limit: package targeting
The NixOS Wiki says a single nixpkgs package cannot currently be built from only that input; a supplementary repository is required.Read 14 Oct 2026 · wiki.nixos.org
Limit: queue reliability
The NixOS Wiki records that hydra-queue-runner can become stuck while builds remain queued, with a cron restart suggested as a workaround.Read 14 Oct 2026 · wiki.nixos.org
Local web interface
The sample NixOS configuration exposes Hydra at a configurable URL, using http://localhost:3000 in the example.Read 14 Oct 2026 · github.com
Maker legal status
The NixOS Foundation is a registered Dutch nonprofit with Chamber of Commerce number 63520583.Read 14 Oct 2026 · linkedin.com
Maker size
LinkedIn lists the NixOS Foundation as having 2–10 employees.Read 14 Oct 2026 · linkedin.com
Operational caveat
The wiki says Hydra stores some state in its database and a complete stateless configuration is currently not possible, so backups are needed.Read 4 Oct 2026 · wiki.nixos.org
Production warning
The README says its simple local setup instructions are insufficient for a production or public server.Read 14 Oct 2026 · github.com
Projects
A Hydra project groups related jobsets, commonly corresponding to one source repository.Read 14 Oct 2026 · wiki.nixos.org
Rating on G2
0 out of 5 from 1 review on the unrelated project-management Hydra listing.Read 14 Oct 2026 · g2.com
Release sets
Release sets describe collections of jobs, conventionally in a release.nix file.Read 14 Oct 2026 · wiki.nixos.org
Restricted evaluation
Hydra evaluates flakes in restricted mode, requiring allowed URI prefixes for external inputs.Read 14 Oct 2026 · wiki.nixos.org
Security behavior
Hydra evaluates flakes in restricted mode, which prevents access to files outside the Nix store, including fetched flake inputs.Read 4 Oct 2026 · wiki.nixos.org
Self-hosting
Hydra can be self-hosted through the NixOS services.hydra module.Read 14 Oct 2026 · github.com
Source inputs
Jobset inputs can include Git checkouts such as nixpkgs and the Hydra repository.Read 14 Oct 2026 · github.com
Supported operating system
The project README says running Hydra is currently supported only on NixOS.Read 14 Oct 2026 · github.com

Plans and prices

Hydra Free GPL-3.0 licensed; running Hydra is currently only supported on NixOS Read 4 Oct 2026 · github.com

Every Hydra plan and price →

Strong points and limits

Strong points

  • Builds projects and runs tests on a schedule or continuously.
  • Jobsets can trigger on input changes and depend on one another.
  • Jobs can produce packages, ISO images, and tarballs.
  • Provides a JSON API.
  • Licensed under GPL-3.0.

Limits

  • Running Hydra is currently supported only on NixOS.
  • No hosted option is listed.
  • Database state means backups are needed.
  • The queue runner can leave builds unscheduled.

Who it suits

Hydra is for developers building Nix or NixOS packages who need continuous integration. It is not needed simply to use NixOS.

Verdict

Hydra is a Nix-focused CI service with jobsets, build reports, and a JSON API. Its NixOS-only deployment support and operational caveats matter for teams planning to run it.

Where Specifiction places it

Alternatives to HydraAll alternatives →