Creating a New Resource

Creating a New Resource

This guide will walk you through how to add resources for a service.

Naming Conventions

Above all, names across this project conform to Python’s naming standards, as laid out in PEP 8.

The relevant details we need to know are as follows:

  • Module names are lower case, and separated by underscores if more than one word. For example, openstack.object_store

  • Class names are capitalized, with no spacing, and each subsequent word is capitalized in a name. For example, ServerMetadata.

  • Attributes on classes, including methods, are lower case and separated by underscores. For example, allow_list or get_data.

Services

Services in the OpenStack SDK are named after their program name, not their code name. For example, the project often known as “Nova” is always called “compute” within this SDK.

This guide walks through creating service for an OpenStack program called “Fake”. Following our guidelines, the code for its service would live under the openstack.fake namespace. What follows is the creation of a Resource class for the “Fake” service.

Resources

Resources are named after the server-side resource, which is set in the base_path attribute of the resource class. This guide creates a resource class for the /fake server resource, so the resource module is called fake.py and the class is called Fake.

An Example

openstack/fake/fake_service.py

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# Apache 2 header omitted for brevity

from openstack import service_description
from openstack.fake.v2 import _proxy as _proxy_v2


class FakeService(service_description.ServiceDescription):
    """The fake service."""

    supported_versions = {
        '2': _proxy_v2.Proxy,
    }

openstack/fake/v2/fake.py

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# Apache 2 header omitted for brevity

from openstack import resource


class Fake(resource.Resource):
    resource_key = "resource"
    resources_key = "resources"
    base_path = "/fake"

    allow_create = True
    allow_fetch = True
    allow_commit = True
    allow_delete = True
    allow_list = True
    allow_head = True

    #: The transaction date and time.
    timestamp = resource.Header("x-timestamp")
    #: The name of this resource.
    name = resource.Body("name", alternate_id=True)
    #: The value of the resource. Also available in headers.
    value = resource.Body("value", alias="x-resource-value")
    #: Is this resource cool? If so, set it to True.
    #: This is a multi-line comment about cool stuff.
    cool = resource.Body("cool", type=bool)

fake.Fake Attributes

Each service’s resources inherit from Resource, so they can override any of the base attributes to fit the way their particular resource operates.

resource_key and resources_key

These attributes are set based on how your resource responds with data. The default values for each of these are None, which works fine when your resource returns a JSON body that can be used directly without a top-level key, such as {"name": "Ernie Banks", ...}".

However, our Fake resource returns JSON bodies that have the details of the resource one level deeper, such as {"resources": {"name": "Ernie Banks", ...}, {...}}. It does a similar thing with single resources, putting them inside a dictionary keyed on "resource".

By setting Fake.resource_key on line 8, we tell the Resource.create, Resource.get, and Resource.update methods that we’re either sending or receiving a resource that is in a dictionary with that key.

By setting Fake.resources_key on line 9, we tell the Resource.list method that we’re expecting to receive multiple resources inside a dictionary with that key.

base_path

The base_path is the URL we’re going to use to make requests for this resource. In this case, line 10 sets base_path = "/fake", which also corresponds to the name of our class, Fake.

Most resources follow this basic formula. Some cases are more complex, where the URL to make requests to has to contain some extra data. The volume service has several resources which make either basic requests or detailed requests, so they use base_path = "/volumes/%s(detailed)". Before a request is made, if detailed = True, they convert it to a string so the URL becomes /volumes/detailed. If it’s False, they only send /volumes/.

service

Line 11 is an instance of the service we’re implementing. Each resource ties itself to the service through this setting, so that the proper URL can be constructed.

In fake_service.py, we specify the valid versions as well as what this service is called in the service catalog. When a request is made for this resource, the Session now knows how to construct the appropriate URL using this FakeService instance.

Supported Operations

The base Resource disallows all types of requests by default, requiring each resource to specify which requests they support. On lines 14-19, our Fake resource specifies that it’ll work with all of the operations.

In order to have the following methods work, you must allow the corresponding value by setting it to True:

create

allow_create

delete

allow_delete

head

allow_head

list

allow_list

fetch

allow_fetch

commit

allow_commit

An additional attribute to set is commit_method. It defaults to PUT, but some services use POST or PATCH to commit changes back to the remote resource.

Properties

The way resource classes communicate values between the user and the server are prop objects. These act similarly to Python’s built-in property objects, but they share only the name - they’re not the same.

Properties are set based on the contents of a response body or headers. Based on what your resource returns, you should set props to map those those values to ones on your Resource object.

Line 22 sets a prop for timestamp , which will cause the Fake.timestamp attribute to contain the value returned in an X-Timestamp header, such as from a Fake.head request.

Line 24 sets a prop for name, which is a value returned in a body, such as from a Fake.get request. Note from line 12 that name is specified its id attribute, so when this resource is populated from a response, Fake.name and Fake.id are the same value.

Line 26 sets a prop which contains an alias. Fake.value will be set when a response body contains a value, or when a header contains X-Resource-Value.

Line 28 specifies a type to be checked before sending the value in a request. In this case, we can only set Fake.cool to either True or False, otherwise a TypeError will be raised if the value can’t be converted to the expected type.

Documentation

We use Sphinx’s autodoc feature in order to build API documentation for each resource we expose. The attributes we override from Resource don’t need to be documented, but any prop attributes must be. All you need to do is add a comment above the line to document, with a colon following the pound-sign.

Lines 21, 23, 25, and 27-28 are comments which will then appear in the API documentation. As shown in lines 27 & 28, these comments can span multiple lines.

Creative Commons Attribution 3.0 License

Except where otherwise noted, this document is licensed under Creative Commons Attribution 3.0 License. See all OpenStack Legal Documents.