Subgraphs

[Tishka17]

Use case:
Interactor depends on StatisticDbGateway and MainDbGateway. Each of them uses SQLAlchemy Session object, so they have similar init typehints. The difference is that StatisticDbGateway requires Session bound to Clickhouse Engine, while MainDbGateway binds to postgresql Engine. Again both engines have same type, but created differently.

To solve this you can replace Session with some new type (literally NewType("PgSession", Session)), but it requires modification of all code base. Imagine that you took those objects from external library and cannot modify it.

Another solution is again to use NewType, but create factories for all objects instead of registrering classes as depedencies directly. This requries a lot of work.

Proposed solution is to split those objects in 3 groups:

SubGraph1: StatisticDbGateway, Session, Engine
SubGraph2: MainDbGateway, Session, Engine
MainGraph: Interactor, SubGraph1, SubGraph2

Each graph here uses only his factories to solve dependencies or requests specially attached subgraphs. So here, gateways can request Session and they get exactly that session which is declared in his subgraph.

Requirements:

1. (Sub)graphs can have subgraphs
2. All (Sub)graphs are synchronized by scopes
3. Subgraph cannot access depedencies from parent graph
4. When looking for factories subgraphs are requested only if they are not found in current graph itself.

It is an open question whether (sub)graph is just a container or another thing

[Tishka17]

The idea: components

On container creation you need somehow attatch providers to component names. It can be either provider attribute, or separate object or just a dictionary. E.g.

make_container(
   components={
      "": [provider1],
      "C1": [provider2, provider3],
   },
)

What's the effect?

Logic of .get:

• When user calls .get(Class) he receives an object from default ("") component.
• Whe user calls .get(Class, component="C1"), he receives an object from compo1nent named "C1"

Factory dependencies:

• By default objects required by a factory a looked up within the same component
• You can specify Annotated[Class, FromComponent("C1"))] to change lookup component to "C1"

If we want to make all factories from component X to receive Class objects from component "C1" we can create alias: alias(Class, component="C1")

Depends will have component attribute as well.

How does it work?

1. We still have single container, so there is not problem of synchronizing scopes between subgraphs
2. We changer dependency key from just type to a pair (type, component)
3. All internal calls like _get_unlicked will receive that composite depenency key. .get will require type and optional component.
4. Factory dependencies will be those keys as well instead of just types.

What to notify?
We want to reuse same provider with multiple components because part of multiple subgraphs can be the same