Jsonnet Engine

Overview

Boilerplate supports Jsonnet as an alternative template engine for generating JSON files. Jsonnet provides functions, imports, and a more structured approach to JSON generation compared to Go templates.

Enabling Jsonnet

Add an engines section to your boilerplate.yml:

engines:
  - path: "**/*.jsonnet"
    template_engine: jsonnet

How It Works

1. Files matching the engine’s path glob are processed with the Jsonnet engine instead of Go templates
2. Boilerplate variables are passed as Jsonnet Top Level Arguments (TLAs)
3. The .jsonnet extension is automatically stripped from output filenames

So config.json.jsonnet becomes config.json in the output.

Example

boilerplate.yml

variables:
  - name: ServiceName
    type: string
  - name: Port
    type: int
    default: 8080
  - name: Replicas
    type: int
    default: 3

engines:
  - path: "**/*.jsonnet"
    template_engine: jsonnet

k8s-deployment.json.jsonnet

function(ServiceName, Port, Replicas)
{
  apiVersion: 'apps/v1',
  kind: 'Deployment',
  metadata: {
    name: ServiceName,
  },
  spec: {
    replicas: std.parseInt(Replicas),
    template: {
      spec: {
        containers: [{
          name: ServiceName,
          port: std.parseInt(Port),
        }],
      },
    },
  },
}

Variable Access

In Jsonnet templates, variables are passed as string TLAs. Use std.parseInt() or std.parseJson() to convert types as needed.

Mixing Engines

You can use both Go templates and Jsonnet in the same template directory. Files matching the Jsonnet engine glob use Jsonnet; all other files use Go templates.

engines:
  - path: "**/*.jsonnet"
    template_engine: jsonnet

# Regular .go, .md, .yaml files still use Go templates