Hydra CI

The Nix build server - not to be confused with Ouroboros Hydra.

For general information about nix, see Nix.


The Hydra jobsets are defined by the hydraJobs, hydraJobsPr and hydraJobsBors flake attributes (cf. mkSystemHydraJobs function flake.nix). There are slight variations between them to avoid test results caching for bors builds and to disable integration tests for PRs.

Here are the corresponding links to the jobsets for the different branches:

Required job

Within the jobset, one job is special. This determines whether the build is reporting to GitHub as a success or failure.

It is an aggregate job containing the following constituents:


This job is set up with the mkRequiredJob function in flake.nix.

Build Products

Some build jobs have build products which can be downloaded from the Hydra web interface. Other build systems sometimes call these “artefacts”.

When you click the “latest successful build” link for a job, it redirects (HTTP 302 Moved Temporarily) to the location of the current latest successful build. To make a download link which is always the latest, click the Details button next a build product, and copy the links.

A nix derivation builder script can register build products by adding lines to the file $out/nix-support/hydra-build-products.


Hydra evaluates the hydraJobs attribute of flake.nix file on the Hydra master host (hydra.iohk.io). It then sends the jobs out to the build farm to be built.

Evaluation fails if there are syntax errors in the Nix files, type errors, etc. Sometimes evaluation can fail due to memory exhaustion on Hydra master.

Note: If evaluation fails, the Hydra eval runner will retry until it succeeds. The ci/hydra-eval status will change from pending to failed, until the evaluation succeeds.

Restarting builds


After signing in with the adrestia user, you should be able to see the Actions→Restart button on the build job page.

The password for this user is in Vaultwarden.

Note about bors

If the bors batch has already failed, restarting the failed build in Hydra will have no effect. You will need to submit a new merge job to bors.

Links to this page