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Dependency Injection

What is DI​

As our applications grow and evolves, each one of our code entities will internally require instances of other objects, which are better known as dependencies in the world of software engineering. The action of passing such dependencies to the dependent client is known as injection, and it also entails the participation of another code entity, named the injector. The injector will take responsibility for instantiating and bootstrapping the required dependencies so they are ready for use from the very moment they are successfully injected in the client. This is very important since the client knows nothing about how to instantiate its own dependencies and is only aware of the interface they implement in order to use them.

In short, DI helps you write code in a loosely coupled way and makes your code more testable and reusable.”

How to register​

To achieve DI in our Ap, we are using InversifyJS. This doc will walk you through on how to register your entities such as classes (services,repositories) and constants with the dependency injection. Feature server API exposes two ways to register entities with the DI, which are following using either createContainerFunc and createHemeraContainerFunc.

  • createContainerFunc entities registered via this options will be registered locally inside the gateway
  • createHemeraContainerFuncentities registered via this options will be registered as external to gateway and local to micro service.

Both of these options take a function which has inversify settings as param and expects and Inverisfy ContainerModule as return value (settings) => interfaces.ContainerModule These functions by convention resides inside container/container.ts file

Example​

import { ContainerModule, interfaces } from 'inversify';

export const localContainerModule: (settings) => interfaces.ContainerModule = () =>
    new ContainerModule((bind: interfaces.Bind) => {
        bind(TYPES.UserRepository).to(UserRepository).inSingletonScope().whenTargetIsDefault();
        bind('MongodbMigration').to(DefaultUserMigration).whenTargetNamed(DefaultUserMigration.name);
    });

export const externalContainerModule: (settings) => interfaces.ContainerModule = () =>
    new ContainerModule((bind: interfaces.Bind) => {
        bind(TYPES.UserRepository).to(UserRepository).inSingletonScope().whenTargetIsDefault();
        bind(TYPES.UserService).to(UserService).inSingletonScope().whenTargetIsDefault();
    });

export default new Feature({
    //... Other options
    createContainerFunc: [localContainerModule],
    createHemeraContainerFunc: [externalContainerModule],
})

Once the entities are registered, those can be easily injected anywhere when needed.

Usage​


export class UserService extends BaseService<IUser> {
    constructor(
        @inject(TYPES.UserRepository)
        repository: IUserRepository,
    ) {
        super(repository);
    }
}