Creating Per-Provider Loggers in Wire Dependency Injection

I'm using github.com/google/wire for dependency injection in an open source example project that I'm working on.

I have the following interfaces in a package named interfaces:

type LoginService interface {
    Login(email, password) (*LoginResult, error)
}

type JWTService interface {
    Generate(user *models.User) (*JWTGenerateResult, error)
    Validate(tokenString string) (*JWTValidateResult, error)
}

type UserDao interface {
    ByEmail(email string) (*models.User, error)
}

I have implementations that look like this:

type LoginServiceImpl struct {
    jwt interfaces.JWTService
    dao interfaces.UserDao
    logger *zap.Logger
}

func NewLoginService(jwt interfaces.JWTService, dao interfaces.UserDao, \
        logger *zap.Logger) *LoginServiceImpl {
    return &LoginServiceImpl{jwt: jwt, dao: dao, logger: logger }
}

type JWTServiceImpl struct {
    key [32]byte
    logger *zap.Logger
}

func NewJWTService(key [32]byte, logger *zap.Logger) (*JWTServiceImpl, error) {
    r := JWTServiceImpl {
        key: key,
        logger: logger,
    }

    if !r.safe() {
        return nil, fmt.Errorf("unable to create JWT service, unsafe key: %s", err)
    }

    return &r, nil
}

type UserDaoImpl struct {
    db: *gorm.DB
    logger: *zap.Logger
}

func NewUserDao(db *gorm.DB, logger *zap.Logger) *UserDao {
    return &UserDaoImpl{ db: db, logger: logger }
}

I'll exclude other factory functions and implementations here because they all look very similar. They may return an error or be infallible.

I have one other interesting factory for creating the database connection, which I'll just show the interface and not the implementation:

func Connect(config interfaces.MySQLConfig) (*gorm.DB, error) { /* ... */ }

Now, onto the problem. In my command-line entry-point, I'm creating a logger:

logger, err := zap.NewDevelopment()

For each of the factory methods above, I need to provide a logger and not the same logger instance, rather as if these methods were called as follows:

logger, err := zap.NewDevelopment()

// check err

db, err := database.Connect(config)

// check err

userDao := dao.NewUserDao(db, logger.Named("dao.user"))
jwtService, err := service.NewJWTService(jwtKey)

// check err

loginService := service.NewLoginService(jwtService, userDao, logger.Named("service.login"))

My wire.ProviderSet construction looks like this:

wire.NewSet(
    wire.Bind(new(interfaces.LoginService), new(*service.LoginServiceImpl)),
    wire.Bind(new(interfaces.JWTService), new(*service.JWTServiceImpl)),
    wire.Bind(new(interfaces.UserDao), new(*dao.UserDaoImpl)),
    service.NewLoginService,
    service.NewJWTService,
    dao.NewUserDao,
    database.Connect,
)

I've read through the user guide, the tutorial, and best practices, and I can't seem to find a way to route a unique zap.Logger to each of these factory methods, and routing a random [32]byte for the JWT service.

Since my root logger is not created at compile time, and since each of these factory methods needs its own unique logger, how do I tell wire to bind these instances to the corresponding factory methods? I'm having a tough time seeing how to route custom instances of the same type to disparate factory methods.

In summary:

> Wire seems to favor doing everything at compile-time, storing the dependency injection configuration in a static package-level variable. For most of my use-case, this is okay.
>
> For the rest of my use-case, I need to create some instances manually before running the dependency injection and the ability to route various *zap.Logger instances to each service that needs it.
>
> Essentially, I need to have wire do services.NewUserDao(Connect(mysqlConfig), logger.Named("dao.user"), but I don't know how to express this in wire and merge variables at runtime with wire's compile-time approach.

How do I do this in wire?

I had to change what I was doing somewhat, as is recommended in the documentation:

> If you need to inject a common type like string, create a new string type to avoid conflicts with other providers. For example:
>
> type MySQLConnectionString string
>

Adding Custom Types

The documentation is admittedly very terse, but what I ended up doing is creating a bunch of types:

type JWTKey [32]byte
type JWTServiceLogger *zap.Logger
type LoginServiceLogger *zap.Logger
type UserDaoLogger *zap.Logger

Updating Producer Functions

I updated my producer methods to accept these types, but did not have to update my structs:

// LoginServiceImpl implements interfaces.LoginService
var _ interfaces.LoginService = (*LoginServiceImpl)(nil)

type LoginServiceImpl struct {
    dao interfaces.UserDao
    jwt interfaces.JWTService
    logger *zap.Logger
}

func NewLoginService(dao interfaces.UserDao, jwt interfaces.JWTService, 
        logger LoginServiceLogger) *LoginServiceImpl {
    return &LoginServiceImpl {
        dao: dao,
        jwt: jwt,
        logger: logger,
    }
}

This above part made sense; giving distinct types meant that wire had less to figure out.

Creating an Injector

Next, I had to create the dummy injector and then use wire to generate the corresponding wire_gen.go. This was not easy and very unintuitive. When following the documentation, things kept breaking and giving me very unhelpful error messages.

I have a cmd/ package and my CLI entrypoint lives in cmd/serve/root.go, which is run as ./api serve from the command-line. I created my injector function in cmd/serve/injectors.go, note that // +build wireinject and the following newline are required to inform Go that this file is used for code generation and not code itself.

I ultimately arrived at the following code after much trial and error:

// +build wireinject

package serve

import /*...*/

func initializeLoginService(
        config interfaces.MySQLConfig,
        jwtKey service.JWTKey,
        loginServiceLogger service.LoginServiceLogger,
        jwtServiceLogger service.JWTServiceLogger,
        userDaoLogger service.UserDaoLogger,
        databaseLogger database.DatabaseLogger,
    ) (interfaces.LoginService, error) {
    
    wire.Build(
        // bind interfaces to implementations
        wire.Bind(new(interfaces.LoginService), new(*service.LoginServiceImpl)),
        wire.Bind(new(interfaces.JWTService), new(*service.JWTServiceImpl)),
        wire.Bind(new(interfaces.UserDao), new(*dao.UserDao)),
        // services
        service.NewLoginService,
        service.NewJWTService,
        // daos
        dao.NewUserDao,
        // database
        database.Connect,
    )

    return nil, nil
}

The wire.Bind calls inform wire which implementation to use for a given interface so it will know that service.NewLoginService which returns a *LoginServiceImpl should be used as the interfaces.LoginService.

The rest of the entities in the call to wire.Build are just factory functions.

Passing Values to an Injector

One of the the issues I ran into was that I was trying to pass values into wire.Build like the documentation describes:

> Occasionally, it is useful to bind a basic value (usually nil) to a type. Instead of having injectors depend on a throwaway provider function, you can add a value expression to a provider set.
>
>
> type Foo struct {
> X int
> }
>
> func injectFoo() Foo {
> wire.Build(wire.Value(Foo{X: 42}))
> return Foo{}
> }
>
>
> ...
>
> It's important to note that the expression will be copied to the injector's package; references to variables will be evaluated during the injector package's initialization. Wire will emit an error if the expression calls any functions or receives from any channels.

This is what confused me; it sounded like you could only really use constant values when trying to run an injector, but there are two lines in the docs in the "injectors" section:

> Like providers, injectors can be parameterized on inputs (which then get sent to providers) and can return errors. Arguments to wire.Build are the same as wire.NewSet: they form a provider set. This is the provider set that gets used during code generation for that injector.

These lines are accompanied by this code:

func initializeBaz(ctx context.Context) (foobarbaz.Baz, error) {
    wire.Build(foobarbaz.MegaSet)
    return foobarbaz.Baz{}, nil
}

This is what I missed and what caused me to lose a lot of time on this. context.Context doesn't seem to be passed anywhere in this code, and it's a common type so I just kind of shrugged it off and didn't learn from it.

I defined my injector function to take arguments for the JWT key, the MySQL config, and the logger types:

func initializeLoginService(
        config interfaces.MySQLConfig,
        jwtKey service.JWTKey,
        loginServiceLogger service.LoginServiceLogger,
        jwtServiceLogger service.JWTServiceLogger,
        userDaoLogger service.UserDaoLogger,
        databaseLogger database.DatabaseLogger,
    ) (interfaces.LoginService, error) {
    // ...
    return nil, nil
}

Then, I attempted to inject them into wire.Build:

wire.Build(
    // ...
    wire.Value(config),
    wire.Value(jwtKey),
    wire.Value(loginServiceLogger),
    // ...
)

When I attempted to run wire, it complained that these types were defined twice. I was very confused by this behavior, but ultimately learned that wire automatically sends all function parameters into wire.Build.

Once again: wire automatically sends all injector function parameters into wire.Build.

This was not intuitive to me, but I learned the hard way that it's the way wire works.

Summary

wire does not provide a way for it to distinguish values of the same type within its dependency injection system. Thus, you need to wrap these simple types with type definitions to let wire know how to route them, so instead of [32]byte, type JWTKey [32]byte.

To inject live values into your wire.Build call, simply change your injector function signature to include those values in the function parameters and wire will automatically inject them into wire.Build.

Run cd pkg/my/package && wire to create wire_gen.go in that directory for your defined injectors. Once this is done, future calls to go generate will automatically update wire_gen.go as changes occur.

I have wire_gen.go files checked-in to my version control system (VCS) which is Git, which feels weird due to these being generated build artifacts, but this seems to be the way that this is typically done. It might be more advantageous to exclude wire_gen.go, but if you do this, you'll need to find every package which includes a file with a // +build wireinject header, run wire in that directory, and then go generate just to be sure.

Hopefully this clears up the way that wire works with actual values: make them type safe with type wrappers, and simply pass them to your injector function, and wire does the rest.