This is an example of a service container implementation in Go. The goal is to build it without using any Voodoo magic, code generation at compile time, reflection at runtime and other "Abracadabra": simple, reliable, extensible, easy to understand and use.

See internal/service: this package declares a few interfaces and types described below and implements a few example services.

(comments are omitted, see the source code for more details).

type ShutdownFunc func()

type Container interface {
    RegisterOnShutdown(ShutdownFunc)
    Shutdown()
}

Container interface must be implemented by the application container. See cmd/my-application/app/service/container.go

To make life easier, application-space container can simply embed service.ShutdownHanlder type. It implements Container interface and its methods and so type, which embeds it will implicitly implement Container as well:

type Container struct {
    service.ShutdownHandler
}

type Provider func() error

Provider represents a service provider, and it is responsible for instantiating service and setting it to the container.

Each Provider must be enclosed into a factory function, which must receive a container and services configuration options as its arguments.

An instance of the container and other arguments will be persisted in the Provider function scope.

Also, for each provider there must be an interface defining service getter and setter methods. Container, that is aware of this service, must implement this interface.

type MongoDBAwareContainer interface {
    SetMongoDB(*mongo.Client, ShutdownFunc)
    MongoDB() *mongo.Client
}

func MongoDB(cnt MongoDBAwareContainer, opts *options.ClientOptions) Provider {
    return func() error {
        client, err := mongo.NewClient(opts)
        if err != nil {
            return fmt.Errorf("mongodb service: %w", err)
        }
        cnt.SetMongoDB(client, func() {
            _ = client.Disconnect(context.Background())
        })
        return nil
    }
}

MongoDBAwareContainer interface sets up a contract between the container and the service provider:

• Provider function returned by the MongoDB() will instantiate mongo.Client and set it to the container together with the shutdown function via SetMongoDB(*mongo.Client, ShutdownFunc) method, which container must implement.
• MongoDB() *mongo.Client can be used to retrieve an instance of mongo.Client from the container.

Getter method is generally required to access services on the container, as well as by service providers, which depend on other services.

Note that service provider factory function accepts an instance of MongoDBAwareContainer to enforce the contract.

See advanced example of the Logger service here: internal/service/logger.go

See internal/service/custom_service.go

In this example, CustomService depends on logger, MongoDB client and Google Cloud Storage:

type CustomService struct {
    mgo *mongo.Client
    log *zap.Logger
    bkt *storage.BucketHandle
}

The trick is to specify an interface for it, which embeds dependency interfaces:

type CustomServiceAwareContainer interface {
    MongoDBAwareContainer
    LoggerAwareContainer
    CloudStorageAwareContainer

    SetCustomService(*CustomService)
    CustomService() *CustomService
}

In this case, CustomService provider will know how to retrieve dependencies from the Container.

service.Wire(...) function is responsible to executing service providers. See usage example here: cmd/my-application/app/service/container.go

func NewContainer(cfg *config.Config) (*Container, error) {
    cnt := &Container{cfg: cfg}
    sp := []service.Provider{
        service.Logger(cnt, buildLoggerOptions(cfg)...),
        service.CloudStorage(cnt, context.Background(), option.WithUserAgent(app.ID)),
        service.MongoDB(cnt, cfg.MongoOptions),
        service.CustomServiceFactory(cnt, cfg.Bucket),
        MyServiceFactory(cnt, cfg),
    }
    if err := service.Wire(cnt, sp...); err != nil {
        return nil, err
    } 
    return cnt, nil
}

Note that the order of the service providers passed into the service.Wire(...) function matters. In this case, logger, MongoDB and Cloud Storage clients must be instantiated before CustomService and MyService.