Using rate.NewLimiter rate limiter with channels - go

Question

I'm using a rate limiter to throttle the number of requests that are routed
The requests are sent to a channel, and I want to limit the number that are processed per second but i'm struggling to understand if i'm setting this correctly, I don't get an error, but i'm unsure if i'm even using the rate limiter
This is what is being added to the channel:
type processItem struct {
itemString string
}
Here's the channel and limiter:
itemChannel := make(chan processItem, 5)
itemThrottler := rate.NewLimiter(4, 1) //4 a second, no more per second (1)
var waitGroup sync.WaitGroup
Items are added to the channel:
case "newItem":
waitGroup.Add(1)
itemToExec := new(processItem)
itemToExec.itemString = "item string"
itemChannel <- *itemToExec
Then a go routine is used to process everything that is added to the channel:
go func() {
defer waitGroup.Done()
err := itemThrottler.Wait(context.Background())
if err != nil {
fmt.Printf("Error with limiter: %s", err)
return
}
for item := range itemChannel {
execItem(item.itemString) // the processing function
}
defer func() { <-itemChannel }()
}()
waitGroup.Wait()
Can someone confirm that the following occurs:
The execItem function is run on each member of the channel 4 times a second
I don't understand what "err := itemThrottler.Wait(context.Background())" is doing in the code, how is this being invoked?

Answer 1

... i'm unsure if i'm even using the rate limiter
Yes, you are using the rate-limiter. You are rate-limiting the case "newItem": branch of your code.
I don't understand what "err := itemThrottler.Wait(context.Background())" is doing in the code
itemThrottler.Wait(..) will just stagger requests (4/s i.e. every 0.25s) - it does not refuse requests if the rate is exceeded. So what does this mean? If you receive a glut of 1000 requests in 1 second:
4 requests will be handled immediately; but
996 requests will create a backlog of 996 go-routines that will block
The 996 will unblock at a rate of 4/s and thus the backlog of pending go-routines will not clear for another 4 minutes (or maybe longer if more requests come in). A backlog of go-routines may or may not be what you want. If not, you may want to use Limiter.Allow - and if false is returned, then refuse the request (i.e. don't create a go-routine) and return a 429 error (if this is a HTTP request).
Finally, if this is a HTTP request, you should use it's imbedded context when calling Wait e.g.
func (a *app) myHandler(w http.ResponseWriter, r *http.Request) {
// ...
err := a.ratelimiter(r.Context())
if err != nil {
// client http request most likely canceled (i.e. caller disconnected)
}
}