Rate limiting your goroutines

Hey mates!
Just a quick one I like to talk about! Rate limiting goroutines.

This is about controlling the actual amount of concurrent task executions.

Sometimes, we have to process and execute a stream of long-running tasks and we don't know at runtime how many of them are coming out from the task channel. So here, the main concern is not firing all the goroutines together, at the time tasks are ingested. Otherwise, firing many of them concurrently uncontrolled could lead to unpredicted behaviors or memory overflow.

Therefore, a limiter (AKA semaphore) empty struct buffered channel has been added into the mix, capped with the count of tasks to run concurrently.

1. For the first n-count iteration an empty struct will be pushed onto the limiter channel,
2. A goroutine is fired up to run the incoming task.
3. At the n-count + 1 iteration, the limiter channel will be full, hence the current main goroutine will be blocked.
4. Once any of the currently running tasks finish its execution, it will readout of the limiter channel, to make some room for another task to be run. This will unblock the main goroutine.
5. After the main goroutine takes the control back, it will push an empty struct onto the limiter channel and start over the cycle by running a new goroutine for the incoming task.

And so on until the time out is reached, so the for loop brakes and no more tasks are run.

	package main
	import (
	"fmt"
	"time"
	)
	const limit = 2 // how many tasks are able to run concurrently
	var timeoutAt = 75 * time.Millisecond
	func main() {
	limiter := make(chan struct{}, limit)
	defer close(limiter)
	defer (func() {
	fmt.Println("exiting runner")
	})()
	run(limiter)
	}
	func run(limiter chan struct{}) {
	id := 1
	for {
	select {
	// fill up limiter channel till reach its capacity
	case limiter <- struct{}{}:
	// fire up a new goroutine for the long running function
	go longRunningTask(limiter, id)
	id++
	// when time out is reached, it breaks the for loop
	case <-time.After(timeoutAt):
	fmt.Println("task runner has timed out")
	return
	default:
	}
	}
	}
	func longRunningTask(limiter chan struct{}, id int) {
	defer func() { <-limiter }() // free up after the function execution has run
	fmt.Printf("executing task with ID [%v] at [%v]\n", id, time.Now())
	time.Sleep(1 * time.Second) // simulate some long running task
	fmt.Printf("finished task with ID [%v] at [%v]\n", id, time.Now())
	}

view raw limiter.go hosted with ❤ by GitHub

To sum up, this is how we can limit goroutines to come up all together by controlling how many of them could be up and running concurrently by using a limiter buffered channel for it. Avoiding causing a memory overflow and unpredicted behavior.

Comments

[Kishan B]

A simpler version of the above
github.com/aviddiviner/gin-limit/b...

[Kishan B]

When using the dark theme, the github gist embed does not adapt itself(screenshot below) this hurts the eye.

My suggestion,
Use markdown code blocks with go. Example below
This adapts well with the theme dark/white and has code coloring

import fmt

func main() {
    fmt.Println("Hello world")
}

Screenshot