Object Dependency Lifecycle¶

bp object-dependency-lifecycle

The way dependency injection (DI) is currently implemented in Keystone is not really using the “dependency injection” pattern. This spec is focused on improving the internal-to-Keystone object lifecycle (including dependencies).

This is based off of a Kilo Summit session: Pre-session Reading

Problem Description¶

Goals:

What should the dependency injection pattern in Keystone look like?
How do we deal with Optional dependencies?
What is the lifecycle of objects (controllers, managers, etc.)?

Motivation

Remove the magic of just adding attributes on to an object without really having them defined in the API
Optional dependencies are currently not implemented in a very object oriented way; we use the existence of the dependency as a sort of feature flag
Better seams for testing; specifically optional dependencies
DI is about dynamically building object graphs where different implementations of some interface can be used. We don’t really do that.

Constraints:

Keep APIs as backward compatible as possible (drivers for sure, managers and controllers to a lesser extent)
Compatibility with Stevedore. We may be looking to use Stevedore for dynamically loading drivers and we should make sure we don’t do anything that would make that harder or impossible.

What is dependency injection?¶

Wikipedia has a good description of dependency injection. In short the pattern is a way of separating the code that implements the business logic from the code that creates objects. Instead of creating dependencies, objects will expect those dependencies to be passed to its constructor. This allows the construction of object graphs (the tree of objects) to vary without changes to the objects themselves.

Proposed Change¶

Phase I - Manual DI

Go through and fix classes to expect dependencies to be provided in via their __init__

This will remove the magic and make dependencies a part of the actual object definition (proof of concept http://paste.openstack.org/show/129171/)

Construct object graphs in a new uber factory (proof of concept http://paste.openstack.org/show/129179/)

This keeps the object lifecycle of the injected objects as singleton

This also keeps a global registry

Keep singleton scope on the current object graphs

Implement optional dependencies using the __init__ as optional kwargs

This will keep the internal code of the objects the same. We would still ‘if self.mydep’

Pitfalls:

We need to add something to the wire up next things. Right now through the magic of the decorator this just happens.

Phase II - Evaluate Manual DI

Is there a need for a framework?

Is there enough of a pain point to use a DI framework?

Is there enough need of some of a DI framework’s features to use it?

Keystone may not be complicated enough to need anything more.

There are a few existing frameworks like inject and snake-guice

Do we need objects that are scoped differently? request scoped, other…

Phase III - Fix up the model of optional dependencies

I think that optional dependencies should be implemented on top of the existing object using composition. Having code doing ‘if self.dep’ is a bad smell.

e.g. The optional thingee_api dependency in keystone.assignment.core.Manager could be implemented as a wrapper around the backend using a decorator pattern.

Other optional dependencies may have to be handled differently