Research Dependency Injection in FastAPI

[lasryariel]

• Parent Issue: #771

Overview

As part of our ongoing effort to improve code readability and accessibility for junior developers, we are exploring the use of Dependency Injection (DI) in FastAPI. The goal is to leverage FastAPI’s DI capabilities to create a more modular, testable, and maintainable codebase.

Action Items

1. Understand FastAPI's Dependency Injection System

   • [x] Research how Dependency Injection works in FastAPI.
   • [x] Identify best practices for implementing DI in FastAPI, with a focus on readability and simplicity.

2. Evaluate Current Codebase for DI Opportunities

   • [x] Review the current FastAPI codebase to identify areas where Dependency Injection could simplify or improve the architecture.
   • [x] Document the parts of the code that could benefit from DI, such as services, database connections, or third-party integrations.

3. Decision Record

   • [x] Create Decision Record page: https://github.com/hackforla/HomeUniteUs/wiki/Migrating-from-Flask-to-FastAPI
   • [x] Add findings and decision to Decision Record page

Resources/Instructions

A well-researched assessment of Dependency Injection system in FastAPI and documentation of areas to implement.

[tylerthome]

So far from a brief look at the docs, it looks like the Depends() mechanism available in FastAPI will be handy for abstracting some common processes (along with middlewares), but a bit different than DI within OOP frameworks as the top-level injected Callable's only passed arguments will be parameters equivalent to the Path operations within FastAPI.

Thinking about database, logging, etc dependencies we might consider using global singletons, and allow the ContextManager to handle any open and close operations on e.g. connections.

Overall so far it looks like existing code will port without too much headache, but @paulespinosa may catch something I missed on this read through.

[paulespinosa]

@tylerthome got the basics down. I also second the assertion that porting the existing code base can be done.

The Dependency Injection system in FastAPI is easy to use and capable of doing lots for us. The tutorial is extensive https://fastapi.tiangolo.com/tutorial/dependencies/ with details that provide guidance on injecting database dependencies, security related dependencies, and more. In addition, the Advanced provides another way to make an instance of a class callable and use it as a dependency.

The tutorial tells us FastAPI will create a Context Manager for any dependency with yield and also provides guidance on how to create custom Context Managers. https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/#what-are-context-managers.

The tutorial on writing bigger applications guides us to put dependencies into its own module. https://fastapi.tiangolo.com/tutorial/bigger-applications/#dependencies. This can aid us in maintaining easy to understand code structure.

For logging, it appears the standard is to use Python's logging facility for all logging. The logger can be customized at startup and removes the need for associating DI and logging. https://docs.python.org/3/library/logging.html

import logging
import mylib
logger = logging.getLogger(__name__)

The FastAPI project template also uses this technique for logging. For example: https://github.com/fastapi/full-stack-fastapi-template/blob/master/backend/app/backend_pre_start.py

Regarding, best practices. I think it's too early to know what they are until we have designed the API's architecture(s).

Below is a just a brainstorm for how DI could be used to inject the current user as a guest and case manager.

@router.post("/intake_profile")
def submit_intake_profile(intake_profile: IntakeProfile, guest: Annotated[Guest, Depends(get_current_user_as_guest)], case_manager: Annotated[CaseManager, Depends(get_case_manager)]):
    case = case_manager.caseFor(guest)
    case.submit_intake_profile(intake_profile)

[lasryariel]

Conversation between Ariel and Paul 9/4

As a follow up we need to design for both infrastructure and business objects

Infrastructure

• FastAPI - look at existing objects in HUU software to determine what needs to be added to FastAPI dependency injection
• SQLAlchemy/SQLModel - we currently use SQLAlchemy should we add on the layer of SQLModel? integration with fastapi documentation noted that fastapi will be moving more towards supporting SQLModel, but is that too bleeding edge for HUU context?)
  • SQL Model is just a layer on top of SQLAlchemy
  • discuss in engineering call
• Security

Business objects

• Users/roles, intake profiles, events, tasks, documents, etc.
  • ask Tyler if we will have unmatched case/matched case (case management) type objects
• behaviors and how it updates states of objects
• this can also help us organize the files in the project

Current state of dependency injection in HUU code

• some objects already require dependencies to work (repositories- middleman for when we make db queries and changes)
  • discuss improvements to using repositories, could be an issue for jr developers that it is maybe not being used in the intended way

[lasryariel]

Decision Record Created: https://github.com/hackforla/HomeUniteUs/wiki/Migrating-from-Flask-to-FastAPI