Provide an array of interfaces as a dependency

[NicklasWallgren]

Hey,

I'm trying to setup an array of interfaces ([]HealthChecker) as a dependency to my HealthCheckerManager.
What is the proper way of setting this up with fx?

Thanks.

type HealthChecker interface{}

type DBHealthChecker struct {}
func NewDBHealthChecker(....) HealthChecker {
	return &DBHealthChecker{....}
}

type HealthCheckerManager struct{
  healthCheckers []HealthChecker
}
func NewHealthCheckerManager(healthCheckers []HealthChecker) *HealthCheckerManager {
	return &HealthCheckerManager{healthCheckers: healthCheckers}
}

What I've tried with so far.

var dbChecker = fx.Provide(fx.Annotated{
	Name:   "dbChecker",
	Target: health.NewDBHealthChecker,
})

type Params struct {
	fx.In
	DBChecker health.HealthChecker `name:"dbChecker"`
}

var healthCheckers = fx.Provide(func(healthCheckers Params) []health.HealthChecker {
	return []health.HealthChecker{healthCheckers.DBChecker}
})

var healthCheckerManager = fx.Provide(health.NewHealthCheckerManager)

[abhinav]

Hey there! It looks like what you're looking for is value groups.

Value groups are a feature of Fx that allow:

• producers to produce singular values to a named group
• consumers to consume all instances of those values produces from all producers in a single slice

You can use value groups in one of two forms:

• Parameter and result structs with fx.In and fx.Out
• Annotations with fx.Annotate

Value groups with fx.In and fx.Out

To produce values to a value group, make the constructor for your producer return a struct that embeds fx.Out, and annotate the field for the value with group:"name" where "name" is the name of your value group.

type Result struct {
  fx.Out

  Result HealthChecker `group:"checkers"`
}
 
func NewDBHealthChecker(...) Result {
  return ...
}

To consume a value group, have your constructor accept a struct that embeds fx.In, and annotate the field with group:"name" using the same group name as before. This field should be a slice of values of the type returned by the producer.

type Params struct {
  fx.In

  Checkers []HealthChecker `group:"checkers"`
}

func NewHealthCheckManager(p Params) HealthCheckManager {
  // ...
}

Value groups with annotations

A more concise and conveient form of using value groups is with fx.Annotate.

To produce values to a value group, leave your original constructor unchanged and annotate it when you provide it to Fx—either via fx.Module or directly to fx.New—with fx.ResultTags using the group:"name" annotation.

func NewDBHealthChecker(...) HealthChecker { ... }

// Elsewhere,

fx.Provide(
  fx.Annotate(NewDBHealthChecker, fx.ResultTags(`group:"checkers"`)),
)

// Or,

var Module = fx.Module("DB",
  fx.Provide(
    fx.Annotate(NewDBHealthChecker, fx.ResultTags(`group:"checkers"`)),
    // other provides for the DB module
  ),
)

💡 TIP: If NewDBHealthChecker returns a *DBHealthChecker instead of HealthChecker, you don't need to change it. You can combine it fx.ResultTags with fx.As to turn it into a HealthChecker interface and provide it to the value group like so:

fx.Annotate(NewDBHealthChecker, fx.As(new(HealthChecker)), fx.ResultTags(`group:"checkers"`))

To consume a value group, leave your original constructor similarly unchanged, provided that it accepts a slice of HealthCheckers, and annotate it with fx.ParamTags when you provide it to Fx.

func NewHealthCheckerManager(healthCheckers []HealthChecker) *HealthCheckerManager {
  ...
}

// Elsewhere,

fx.Provide(
  fx.Annotate(NewHealthCheckerManager, fx.ParamTags(`group:"checkers"`)),
)

If your function accepts other parameters, you can pass "" to fx.ParamTags for those:

// Given,
// func NewHealthCheckerManager(cfg *Config, checkers []HealthChecker) *HealthCheckManager

fx.Annotate(NewHealthCheckerManager, fx.ParamTags("", `group:"checkers"`)),

Hope this helps!

[NicklasWallgren]

Thanks!

[NicklasWallgren]

@abhinav Is it possible to order the values associated with a group-tag? Either by index, weight or the order which they have been registered?

It seems as the order of the values differs on each application start.

fx.Provide(
  fx.Annotate(NewDBHealthChecker, fx.ResultTags(`group:"checkers"`)),  // <--- Should always have index 0
  fx.Annotate(NewRabbitMQHealthChecker, fx.ResultTags(`group:"checkers"`)), // <--- Should always have index 1
  fx.Annotate(NewHttpHealthChecker, fx.ResultTags(`group:"checkers"`)), // <--- Should always have index 2
  ....
  fx.Annotate(NewHealthCheckerManager, fx.ParamTags("", `group:"checkers"`)),
)

[NicklasWallgren]

Ah, it has already been asked before. #807