Template Unit

The template unit is a variant of Plainbox unit types. A template is a skeleton for defining additional units, typically job definitions. A template is defined as a typical RFC822-like Plainbox unit (like a typical job definition) with the exception that all the fields starting with the string template- are reserved for the template itself while all the other fields are a definition of all the eventual instances of the template.

Template-Specific Fields

There are four fields that are specific to the template unit:


Name of the unit type this template will generate. By default job definition units are generated (as if the field was specified with the value of job) eventually but other values may be used as well.

This field is optional.


Name of the resource job (if it is a compatible resource identifier) to use to parameterize the template. This must either be a name of a resource job available in the namespace the template unit belongs to or a valid resource identifier matching the definition in the template-imports field.

This field is mandatory.


A resource import statement. It can be used to refer to arbitrary resource job by its full identifier and (optionally) give it a short variable name.

The syntax of each imports line is:

IMPORT_STMT ::  "from" <NAMESPACE> "import" <PARTIAL_ID>
              | "from" <NAMESPACE> "import" <PARTIAL_ID>
                 AS <IDENTIFIER>

The short syntax exposes PARTIAL_ID as the variable name available within all the fields defined within the template unit. If it is not a valid variable name then the second form must be used.

This field is sometimes optional. It becomes mandatory when the resource job definition is from another provider namespace or when it is not a valid resource identifier and needs to be aliased.


A resource program that limits the set of records from which template instances will be made. The syntax of this field is the same as the syntax of typical job definition unit’s requires field, that is, it is a python expression.

When defined, the expression is evaluated once for each resource object and if it evaluates successfully to a True value then that particular resource object is used to instantiate a new unit.

This field is optional.


When a template is instantiated, a single record object is used to fill in the parametric values to all the applicable fields. Each field is formatted using the python formatting language. Within each field the record is exposed as the variable named by the template_resource field. Record data is exposed as attributes of that object.

The special parameter __index__ can be used to iterate over the devices matching the template-filter field.


Basic example

The following example contains a simplified template that instantiates to a simple storage test. The test is only instantiated for devices that are considered physical. In this example we don’t want to spam the user with a long list of loopback devices. This is implemented by exposing that data in the resource job itself:

id: device
plugin: resource
    echo 'path: /dev/sda'
    echo 'has_media: yes'
    echo 'physical: yes'
    echo 'path: /dev/cdrom'
    echo 'has_media: no'
    echo 'physical: yes'
    echo 'path: /dev/loop0'
    echo 'has_media: yes'
    echo 'physical: no'

The template defines a test-storage-XXX test where XXX is replaced by the path of the device. Only devices which are physical according to some definition are considered for testing. This means that the record related to /dev/loop0 will be ignored and will not instantiate a test job for that device. This feature can be coupled with the existing resource requirement to let the user know that we did see their CD-ROM device but it was not tested as there was no inserted media at the time:

unit: template
template-resource: device
template-filter: device.physical == 'yes'
requires: device.has_media == 'yes'
id: test-storage-{path}
plugin: shell
command: perform-testing-on --device {path}

Real life example

Here is a real life example of a template unit that generates a job for each hard drive available on the system:

unit: template
template-resource: device
template-filter: device.category == 'DISK'
plugin: shell
category_id: com.canonical.plainbox::disk
id: disk/stats_{name}
 device.path == "{path}"
 block_device.{name}_state != 'removable'
user: root
command: disk_stats_test {name}
_description: This test checks {name} disk stats, generates some activity and rechecks stats to verify they've changed. It also verifies that disks appear in the various files they're supposed to.

The template-resource used here (device) refers to a resource job using the udev_resource script to get information about the system. The udev_resource script returns a list of items with attributes such as path and name, so we can use these directly in our template.