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PythonOpenStackSDK/ClassDesignDiscussion

Introduction

In order to help make this SDK easy to develop, maintain, and test, a clean class design with clearly separated responsibilities is important. This is just one possibility, based on work done on the python-novaclient library. In an attempt to simplify things, the Identity class has been left out, as it is an entire discussion in and of itself. For the purposes of this discussion, assume that Identity is present and can do everything that is needed.

Here is a general overview of the proposed class relationships:

Class Design Overview

NOTE: For this discussion, the term 'Client' has been used to refer to the class that acts as the interface between the application code and the SDK. This is not a universally-loved term, and changing it to something else will not affect the design itself. Similarly, the class that handles all the HTTP traffic has been named 'HTTP', but that is also not critical.

Class Overview

Client

For each service there is a single Client class that contains the interface that the application developer will work with. Any functionality in the SDK should be available as a method in the Client class. Each Client instance has a reference to an instance of its Manager class. There is no logic in the Client class except to pass the request to its manager, and return the result from the manager to the application.

Manager

The Manager class contains the logic to construct the appropriate URIs from the base endpoints (returned from the service catalog), and the particulars of the request. If needed, it also creates the appropriate data to include with the request. It then passes that request to the HTTP class, and handles the response. If there is an exception, the appropriate class of Exception is determined and raised with the information the developer would need to understand and correct the exception. Otherwise, the Manager returns the appropriate response, which can be None, or it can be one or more Resource objects.

Resource

The Resource class represents a resource in the cloud, such as a compute instance, a stored object, a network, etc. Rather than have fixed attributes, its attributes should be populated with the information returned from the API. Every resource should have an id attribute; in the cases where there is not a native id, such as a stored object in Swift, an 'id' property should be created to return the unique identifier for the resource, such as a name.

HTTP

The HTTP class handled all communication between the local system and the API server. This functionality is separated out in order to make testing simpler, so that it can be replaced with a local API 'server' that acts as a test double.


While the Client class is the interface to the developer using this SDK in their applications, there is one exception for convenience: Resource objects may be interacted with directly where appropriate. In general any time you have a method that looks like:

client.some_method(resource)

you should be able to work directly with the resource:

resource.some_method()

For example, if you have a reference server to a compute resource, wish to delete a compute resource, you could call either compute_client.delete(server), or server.delete(), and they would both result in the same action.

Common Methods

The Client class should have methods that correspond to the most common actions across all the service APIs:

  • client.get(id) - Returns the resource corresponding to the specified ID.
  • client.list(pagination_params) - Returns a list of zero or more resources, optionally limited by pagination parameters.
  • client.update(resource_or_id, keyword1=value1, keyword2=value2, ...) - Updates the specified resource with the named values.
  • client.create(list_of_values) - Creates a new resource. The values required will vary between services.
  • client.delete(resource_or_id) - Deletes the specified resource.


Flexibility

Whenever possible, equivalent parameters should be accepted. E.g., when acting on a resource, the user should be able to either supply the ID of that resource, or an instance on the Resource class corresponding to that resource.