Pilot Tech Talk #12 — Dependency Injection in Go by Justus Perlwitz

About Me
Justus Perlwitz, Software Developer
Email: hello@justus.pw

Big applications have hidden dependencies
Hidden dependencies make code hard to test
High maintenance cost in strong coupling

Mail Delivery Application
package main
type Mail struct {
content string
recipient string
}

Mail Sending Function
// External Mailer API
import "mailbib" // Treat this a black box
func (e *Mail) Send() error {
err := mailbib.Mailer.Send(e.content, e.recipient)
if err != nil {
return fmt.Errorf("MailBib error: %+v", err)
}
return nil
}

MailBib
func (m MailBib) Send(c string, r string) error {
if r == "Santa" {
return fmt.Errorf("Recipient too far away")
}
m.mails = append(m.mails, c)
fmt.Printf("Sent '%s' to %sn", c, r)
return nil
}

Let's use it
mail := Mail{
    content:   "Hello, World!",
    recpient: "Bigfoot",
}
if err := mail.Send(); err != nil {
    fmt.Printf("Error: %+vn", err)
}

This just works fine
Sent 'Hello, World!' to Bigfoot

A Letter to Santa
// Let's try sending the same mail to Santa
mail.recipient = "Santa"
// What if Mailbib cannot deliver mail to the Arctic Circle?
}

Santa is living too far away
Error: MailBib error: Recipient too far away

Let's inject our mailer dependency into the Mail struct:
type MailDelivery interface {
Send(string, string) error
}
The interface requires the mailer to have one function Send()
type Mail struct {
content string
recipient string
delivery MailDelivery
}

Trying Different Dependencies
Now, we have to initialize a Mail as follows
mail := Mail{
    content:   "Hello, World!",
    recipient: "Santa",
    delivery:  mailbib.Mailer,
}

And we adjust the mailing method
func (e *Mail) Send() error {
err := e.delivery.Send(e.content, e.recipient)
if err != nil {
return fmt.Errorf("Error: %+v", err)
}
return nil
}

Of course, MailBib still refuses to deliver our mail:
fmt.Printf("Could not deliver mail ‐> %+vn", err)
}
outputs
Could not deliver mail ‐> Error: Recipient too far away

Injecting a different dependency
Let's pretend, there is a second delivery API, that delivers mail to the
arctic circle
import "mailarctica"
Now we can inject a different service
mail.delivery = mailarctica.Mailer

Let's send the email again
mail.delivery = mailarctica.Mailer
// This time, MailArctica will deliver our letter
}
will output
Sent 'Hello, World!' to Santa

Useful Packages
There are two packages with similar names that make dependency
injection a lot easier:
inject by Facebook
Inject

Pilot Tech Talk #12 — Dependency Injection in Go by Justus Perlwitz

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Pilot Tech Talk #12 — Dependency Injection in Go by Justus Perlwitz