Hydra

Hydra is a continuous integration system for projects built with Nix, particularly Nix and NixOS package development. It is best suited to teams comfortable running their own NixOS infrastructure. Its jobset-based builds and reports come without a software charge, but operating and maintaining Hydra is part of the deal.

Overview

Hydra checks out source periodically or continuously, builds it, runs tests, and produces reports. Jobsets organize that work around inputs: changes can trigger builds, and one jobset can depend on another. This makes Hydra a natural fit for projects already structured around Nix, not a general-purpose requirement for people who simply use NixOS.

Jobsets can build packages, ISO images, and tarballs. Git checkout inputs are supported, with documented examples using GitHub repositories. Flake jobsets work when their outputs expose a hydraJobs attribute set. Flake evaluation runs in restricted mode, preventing access to files outside the Nix store, including fetched flake inputs; projects must account for that boundary in how they structure builds.

Key features

Build workflows and API

Hydra combines source checkouts, builds, tests, and reports in one Nix-oriented workflow. The JSON API, defined by the repository's hydra-api.yaml specification, gives developers an interface for working with Hydra programmatically. The feature set is focused: its appeal lies in serving Nix jobs, rather than breadth across unrelated build environments.

Self-hosting and operations

Hydra is open source under GPL-3.0, and supports self-hosted runners and artifact or dependency caching. Running it is currently supported only on NixOS; the project recommends its NixOS module for production. That keeps deployment aligned with the system Hydra is designed for, but rules it out for organizations that require a supported non-NixOS host or a hosted CI service.

Some state lives in Hydra's database, so a fully stateless configuration is not possible and backups are necessary. The wiki also reports that hydra-queue-runner can sometimes leave builds queued without scheduling them. Those are meaningful operational costs for teams that need reliable build throughput and should be included in deployment planning.

Pricing

Hydra is free: the Hydra plan costs free and is GPL-3.0 licensed. There is no hosted option, so the absence of a software fee does not remove the work of providing NixOS infrastructure, storing artifacts, maintaining the database, and backing up state. The plan suits teams willing to run and support their own installation; teams seeking a managed service should look elsewhere.

Platforms

Hydra is categorized for API, Linux, self-hosted, and web use. In practice, its deployment constraint is more specific than the Linux label: running Hydra is currently supported only on NixOS, with the NixOS module recommended for production.

Who it's for

Hydra makes sense for Nix and NixOS package developers who want jobset-driven builds, tests, reports, and artifact or dependency caching, and who can take responsibility for NixOS deployment and database backups. It is not needed merely to use NixOS. Teams needing hosted CI, a supported deployment outside NixOS, or less operational responsibility should choose another approach.

Pros and cons

Pros

• No software charge: The GPL-3.0-licensed plan costs nothing, making the software accessible to teams that can supply their own infrastructure.
• Nix-focused jobsets: Input-triggered jobs, dependencies between jobsets, Flake support, and multiple build product types suit Nix project workflows.
• Self-hosted capabilities: Self-hosted runners, caching, and a JSON API support teams operating their own CI environment.

Cons

• NixOS deployment requirement: Running Hydra is currently supported only on NixOS, limiting infrastructure choices.
• Ongoing operations: Database state requires backups, and there is no hosted option to take over maintenance.
• Queue-runner concern: A reported issue can leave builds queued but unscheduled, a risk for teams that depend on predictable queue processing.

Alternatives

For a broader set of CI choices, browse Continuous Integration Software. Hydra remains the focused option for Nix jobsets and NixOS self-hosting; the alternatives below serve different needs.

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Verdict

Choose Hydra if your team builds Nix projects, wants jobset-based CI without a software fee, and can run NixOS while managing backups and queue operations. Its strongest reason to choose it is the close fit between its build model and Nix workflows; its strongest reason to look elsewhere is the burden of self-hosting on NixOS, compounded by the reported queue-runner issue.