Pattern for fetching multiple fields in parallel

Question

I need to fetch multiple fields in parallel for my system from external services (in this example, simulated by Name(), Age() and CanDrive() methods).

The fetchUser() method does what I want, but it seems too verbose specially if you consider I could have 10+ fields. Are there better ways I can implement this?

playground: https://play.golang.org/p/90sNq1GmrD8

Code (same as in playground):

package main

import (
    "fmt"
    "sync"
)

type User struct {
    Name string
    Age int
    CanDrive *bool
}

func Name() (string, error) {
    return "foobar", nil
}

func Age() (int, error) {
    return 25, nil
}

func CanDrive() (bool, error) {
    return true, nil
}

func fetchUser() (*User, error) {
    var wg sync.WaitGroup
    errs := make(chan error)

    user := &User{}

    wg.Add(1)
    go func() {
        var err error
        defer wg.Done()
        user.Name, err = Name()
        errs <- err
    }()

    wg.Add(1)

    go func() {
        var err error
        defer wg.Done()
        user.Age, err = Age()
        errs <- err
    }()

    wg.Add(1)
    go func() {
        defer wg.Done()
        canDrive, err := CanDrive()
        if err == nil {
            user.CanDrive = &canDrive
        }
        errs <- err
    }()

    // wait until all go-routines are completed successfully
    // if that's the case, close the errs channel
    go func() {
        wg.Wait()
        close(errs)
    }()

    // keep waiting for errors (or for the error channel to be closed
    // if all calls succeed)
    for err := range errs {
        if err != nil {
            return nil, err
        }
    }

    return user, nil
}

func main() {
    user, _ := fetchUser()
    fmt.Println(user)
}

Answer 1

Without knowing more of the specifics of your scenario, my only suggestion would be to separate out the Go routine error handling into another package.

Fortunately, a package already exists that does the same thing, named errgroup. Below is an implementation of your original code using the errgroup package:

package main

import (
    "context"
    "fmt"

    "golang.org/x/sync/errgroup"
)

type User struct {
    Name     string
    Age      int
    CanDrive *bool
}

func Name() (string, error) {
    return "foobar", nil
}

func Age() (int, error) {
    return 25, nil
}

func CanDrive() (bool, error) {
    return true, nil
}

func fetchUser(ctx context.Context) (*User, error) {
    group, ctx := errgroup.WithContext(ctx)

    user := &User{}
    group.Go(func() (err error) {
        user.Name, err = Name()
        return
    })
    group.Go(func() (err error) {
        user.Age, err = Age()
        return
    })
    group.Go(func() error {
        canDrive, err := CanDrive()
        if err == nil {
            user.CanDrive = &canDrive
        }
        return err
    })

    if err := group.Wait(); err != nil {
        return nil, err
    }

    return user, nil
}

func main() {
    user, err := fetchUser(context.Background())
    fmt.Println(user, err)
}