Implement dependency injection

[AndrewEckart]

Story

As a ServiceX developer I want to use dependency injection in API endpoints so I can avoid brittle code

Dependencies such as the TransformerManager, RabbitAdaptor, etc. are currently stored as class attributes on API resource classes. For example:

class SubmitTransformationRequest(ServiceXResource):
    @classmethod
    def make_api(cls, rabbitmq_adaptor, object_store,
                 code_gen_service, lookup_result_processor, docker_repo_adapter):
        cls.rabbitmq_adaptor = rabbitmq_adaptor
        cls.object_store = object_store
        cls.code_gen_service = code_gen_service
        cls.lookup_result_processor = lookup_result_processor
        cls.docker_repo_adapter = docker_repo_adapter
        return cls

There are a few issues with this implementation:

• Lots of boilerplate make_api class methods.
• Web routes, which are simply functions (not classes) cannot store attributes.
• Incentivizes workarounds, such as borrowing dependencies from other resource classes (as in Add transformer deployment status endpoint #90):

class DeploymentStatus(ServiceXResource):
    @auth_required
    def get(self, request_id):
        # todo - improve dependency injection
        manager: TransformerManager = TransformStart.transformer_manager

This is a good use case for the dependency injection pattern.

Acceptance Criteria

1. A new route / API resource can simply declare which dependencies it requires and have them provided automatically:

class SomeResource(ServiceXResource):
    def get(self, rabbitmq_adaptor):
        ...

2. Dependencies can be swapped out or mocked in a modular fashion.

Assumptions

1. We can find a reliable, well-tested implementation or package for dependency injection in Flask. Candidates: Flask-Injector.