## Declarative Projects

Hydra supports declaratively configuring a project's jobsets.
This configuration can be done statically, or generated by a build job.

> **Note**
>
> Hydra will treat the project's declarative input as a static definition if and only if the spec file contains a dictionary of dictionaries.
> If the value of any key in the spec is not a dictionary, it will treat the spec as a generated declarative spec.

### Static, Declarative Projects

Hydra supports declarative projects, where jobsets are configured from a static JSON document in a repository.

To configure a static declarative project, take the following steps:

1.  Create a Hydra-fetchable source like a Git repository or local path.

2.  In that source, create a file called `spec.json`, and add the specification for all of the jobsets.
    Each key is jobset and each value is a jobset's specification.
    For example:

    ```json
    {
      "nixpkgs": {
        "enabled": 1,
        "hidden": false,
        "description": "Nixpkgs",
        "nixexprinput": "nixpkgs",
        "nixexprpath": "pkgs/top-level/release.nix",
        "checkinterval": 300,
        "schedulingshares": 100,
        "enableemail": false,
        "enable_dynamic_run_command": false,
        "emailoverride": "",
        "keepnr": 3,
        "inputs": {
          "nixpkgs": {
              "type": "git",
              "value": "git://github.com/NixOS/nixpkgs.git master",
              "emailresponsible": false
          }
        }
      },
      "nixos": {
        "enabled": 1,
        "hidden": false,
        "description": "NixOS: Small Evaluation",
        "nixexprinput": "nixpkgs",
        "nixexprpath": "nixos/release-small.nix",
        "checkinterval": 300,
        "schedulingshares": 100,
        "enableemail": false,
        "enable_dynamic_run_command": false,
        "emailoverride": "",
        "keepnr": 3,
        "inputs": {
          "nixpkgs": {
            "type": "git",
            "value": "git://github.com/NixOS/nixpkgs.git master",
            "emailresponsible": false
          }
        }
      }
    }
    ```

3.  Create a new project, and set the project's declarative input type, declarative input value, and declarative spec file to point to the source and JSON file you created in step 2.

Hydra will create a special jobset named `.jobsets`.
When the `.jobsets` jobset is evaluated, this static specification will be used for configuring the rest of the project's jobsets.


### Generated, Declarative Projects

Hydra also supports generated declarative projects, where jobsets are configured automatically from specification files instead of being managed through the UI.
A jobset specification is a JSON object containing the configuration of the jobset, for example:

```json
{
    "enabled": 1,
    "hidden": false,
    "description": "js",
    "nixexprinput": "src",
    "nixexprpath": "release.nix",
    "checkinterval": 300,
    "schedulingshares": 100,
    "enableemail": false,
    "enable_dynamic_run_command": false,
    "emailoverride": "",
    "keepnr": 3,
    "inputs": {
        "src": { "type": "git", "value": "git://github.com/shlevy/declarative-hydra-example.git", "emailresponsible": false },
        "nixpkgs": { "type": "git", "value": "git://github.com/NixOS/nixpkgs.git release-16.03", "emailresponsible": false }
    }
}
```

To configure a declarative project, take the following steps:

1.  Create a jobset repository in the normal way (e.g. a git repo with a `release.nix` file, any other needed helper files, and taking any kind of hydra input), but without adding it to the UI.
    The nix expression of this repository should contain a single job, named `jobsets`.
    The output of the `jobsets` job should be a JSON file containing an object of jobset specifications.
    Each member of the object will become a jobset of the project, configured by the corresponding jobset specification.

2.  In some hydra-fetchable source (potentially, but not necessarily, the same repo you created in step 1), create a JSON file containing a jobset specification that points to the jobset repository you created in the first step, specifying any needed inputs (e.g. nixpkgs) as necessary.

3.  In the project creation/edit page, set declarative input type, declarative input value, and declarative spec file to point to the source and JSON file you created in step 2.

Hydra will create a special jobset named `.jobsets`, which whenever evaluated will go through the steps above in reverse order:

1.  Hydra will fetch the input specified by the declarative input type and value.

2.  Hydra will use the configuration given in the declarative spec file as the jobset configuration for this evaluation.
    In addition to any inputs specified in the spec file, hydra will also pass the `declInput` argument corresponding to the input fetched in step 1 and the `projectName` argument containing the project's name.

3.  As normal, hydra will build the jobs specified in the jobset repository, which in this case is the single `jobsets` job.
    When that job completes, hydra will read the created jobset specifications and create corresponding jobsets in the project, disabling any jobsets that used to exist but are not present in the current spec.
