Using Jsonnet

While Tera is excellent for simple text substitution, Jsonnet is a data templating language designed for generating configuration data. It supports local variables, functions, conditionals, list comprehensions, and file imports — making it ideal for complex output generation where Tera’s string-based approach becomes hard to maintain.

For advanced Jsonnet techniques consult the official case studies.

When to prefer Jsonnet over Tera: Use Jsonnet when your output requires list comprehensions, helper functions, conditional object construction, or when you want to split reusable logic into .libsonnet libraries. For straightforward key-value serialisation, Tera template is simpler and sufficient.

Data Context

When using jsonnet, the data context is injected as External Variables accessed with std.extVar().

Example 1: Inline Jsonnet Logic

[[output]]
resource_type = "k8s_manifest"
name = "svc-{{ origin_resource.name }}" # Name generation still uses Tera
jsonnet = """
local origin = std.extVar("origin_resource");
{
  apiVersion: "v1",
  kind: "Service",
  metadata: {
    name: origin.name,
    labels: {
      // Use Jsonnet logic
      app: if std.length(origin.application) > 0 then origin.application[0].name else "unknown"
    }
  }
}
"""

Note: The name, filename, and match_on fields always use Tera even when the

body uses jsonnet. Only the output body (the JSON object that becomes the resource’s properties) is evaluated as Jsonnet.

Example 2: Modular

data/output/k8s.libsonnet:

{
  service(name):: {
    apiVersion: "v1",
    kind: "Service",
    metadata: { name: name }
  }
}

data/output/services.toml:

[[output]]
resource_type = "k8s_manifest"
name = "svc-{{ origin_resource.name }}"
jsonnet = """
local k8s = import 'k8s.libsonnet';
local origin = std.extVar("origin_resource");

k8s.service(origin.name)
"""