how to limit goroutine

Question

I'm developing a gmail client based on google api.

I have a list of labels obtained through this call

r, err := s.gClient.Service.Users.Labels.List(s.gClient.User).Do()

Then, for every label I need to get details

for _, l := range r.Labels {
    d, err := s.gClient.Service.Users.Labels.Get(s.gClient.User, l.Id).Do()
}

I'd like to handle the loop in a more powerful way so I have implemented a goroutine in the loop:

ch := make(chan label.Label)

for _, l := range r.Labels {

    go func(gmailLabels *gmailclient.Label, gClient *gmail.Client, ch chan<- label.Label) {

        d, err := s.gClient.Service.Users.Labels.Get(s.gClient.User, l.Id).Do()

        if err != nil {
            panic(err)
        }

        // Performs some operation with the label `d`
        preparedLabel := ....

        ch <- preparedLabel

    }(l, s.gClient, ch)
}

for i := 0; i < len(r.Labels); i++ {
    lab := <-ch
    fmt.Printf("Processed %v\n", lab.LabelID)
}

The problem with this code is that gmail api has a rate limit, so, I get this error:

panic: googleapi: Error 429: Too many concurrent requests for user, rateLimitExceeded

What is the correct way to handle this situation?

Answer 1

How about only starting e.g. 10 goroutines and pass the values in from one for loop in another go routine. The channels have a small buffer to decrease synchronisation time.

chIn := make(chan label.Label, 20)
chOut := make(chan label.Label, 20)

for i:=0;i<10;i++ {
    go func(gClient *gmail.Client, chIn chan label.Label, chOut chan<- label.Label) {

        for gmailLabels := range chIn {
            d, err := s.gClient.Service.Users.Labels.Get(s.gClient.User, l.Id).Do()

            if err != nil {
                panic(err)
            }

            // Performs some operation with the label `d`
            preparedLabel := ....

            chOut <- preparedLabel

        }

    }(s.gClient, chIn, chOut)

}

go func(chIn chan label.Label) {
    defer close(chIn)
    for _, l := range r.Labels {
        chIn <- l
    }
}(chIn)

for i := 0; i < len(r.Labels); i++ {
    lab := <-chOut
    fmt.Printf("Processed %v\n", lab.LabelID)
}

EDIT:

Here a playground sample.