# Create a Resource Template

Use a resource template to define the shape of trackable entities. Templates can include typed properties, defaults, property groups, and child templates.

``` python
with namespace.build_resource_template(
    name="Detector File",
    type_names=["file", "detector-data"],
) as builder:
    builder.add_properties({
        "file": [
            {"name": "path", "type": "str", "default": ""},
            {"name": "sha256", "type": "str", "default": ""},
        ]
    })
```


# Define typed property groups

Group related properties so resource values have a stable, readable shape. The mapping form is useful when defining several groups at once:

``` python
builder.add_properties({
    "file": [
        {"name": "path", "type": "str", "default": ""},
        {"name": "size", "type": "int", "default": 0, "unit": "byte"},
    ],
    "provenance": [
        {"name": "instrument", "type": "str", "default": ""},
    ],
})
```

For incremental definitions, open a group and close it when finished:

``` python
properties = builder.prop_group("checksum")
properties.add_attribute("sha256", "str", "", "")
properties.close_group()
```


# Set defaults, units, and metadata

Defaults fill omitted values. Units document how numeric values are interpreted; metadata carries validation hints and other property information:

``` python
builder.add_properties({
    "measurement": [
        {
            "name": "temperature",
            "type": "float",
            "default": 20.0,
            "unit": "degC",
            "metadata": {"min": -273.15, "max": 500.0},
        },
    ],
})
```

Property metadata validates resource values against constraints such as numeric bounds or enum choices. It does not replace the declared property type.


# Constrain values

Use `min` and `max` metadata for numeric properties, or `choices` for an enum:

``` python
builder.add_properties({
    "state": [
        {
            "name": "mode",
            "type": "enum",
            "default": "idle",
            "metadata": {"choices": ["idle", "running", "failed"]},
        },
    ],
})
```

Choose constraints that describe every valid resource instance. Invalid values are rejected when resource properties are validated.


# Choose template child structure

Use `add_child` for hierarchy known when defining the template. Child templates are owned by the parent template and create predictable structure; runtime children are added later to one resource instance. See [Nest resource templates](../../docs/how-to/nested-resource-templates.md) for multi-level examples.