Golang timedout sending emails

Question

I have 708 companies whom I need to send email with login and password for authorization on my Company new web-app, where they can see statistics and other information co-working with our Company. Issue is smtp host we're using set bottleneck on sending emails 30 mail on a minute. And I realy can't think of a good way how to work with this.

I probable need use goroutine with for loop.

 import "gopkg.in/gomail.v2"

 const (
    FROM      = "[email protected]"
    PASSWORD  = "bar"
    SMTP_HOST = "smtp.google.com"
    SMTP_PORT = 465
 )

 type SendEmail struct {
    To      string
    Subject string
    Msg     string
 }

 func (se *SendEmail) Send() error{
    m := gomail.NewMessage()
    m.SetHeader("From", FROM)
    m.SetHeader("To", se.To)
    m.SetHeader("Subject", se.Subject)
    m.SetBody("text/html", se.Msg)
    d := gomail.NewDialer(SMTP_HOST, SMTP_PORT, FROM, PASSWORD)

    // Send the email
    if err := d.DialAndSend(m); err != nil {
        return err
    }

    return nil
 }

 type Company struct{
  Email string
  Login string
  Password string
 }

 func SendNotificartion(companys []Company){
  // Send 30 mail every minute
  
  for _, company := range companys {
   var send SendEmail
   send.To = company.Email
   send.Subject = "Some subject"
   send.Msg = fmt.Sprintf("Your login:%s and password:%s", company.Login, company.Password)

   if err := send.Send(); err != nil{
    log.Println(err)
   }
  }
 }

Answer 1

You can just put here pause for 1 minute, before sending another 30 pack

 func SendNotificartion(companys []Company){
  // Send 30 mail every minute
  var counter = 0
  for _, company := range companys {
   var send SendEmail
   send.To = company.Email
   send.Subject = "Some subject"
   send.Msg = fmt.Sprintf("Your login:%s and password:%s", company.Login, company.Password)

   if err := send.Send(); err != nil{
    log.Println(err)
   }
   counter++
   if counter == 30 {
       counter = 0
       time.Sleep(1 * time.Minute)
   }
  }

Answer 2

The async i/o operator is designed for this scenario. It's a better starting point than a map.

There's also been recent work done to develop a generic async sink, see FLIP-171. This has been merged into master and will be released as part of Flink 1.15.

One of those should be your best way forward. Whatever you do, don't do blocking i/o in your user functions. That causes backpressure and often leads to performance problems and checkpoint failures.