Introduction

WaitGroups solve the problem: "How do I wait for multiple goroutines to finish?"

The Problem

func main() {
    go doWork("Job 1")
    go doWork("Job 2")
    go doWork("Job 3")

    fmt.Println("All done!")  // Prints immediately, jobs still running
}

Main function exits before goroutines finish. You need to wait for them.

Bad Solution

func main() {
    go doWork("Job 1")
    go doWork("Job 2")
    go doWork("Job 3")

    time.Sleep(5 * time.Second)  // Guess how long to wait
    fmt.Println("All done!")
}

What if jobs take 6 seconds? What if they take 1 second? You're either waiting too long or not long enough.

Good Solution -> WaitGroup

func main() {
    var wg sync.WaitGroup

    wg.Add(3)  // "I'm expecting 3 goroutines to finish"

    go doWork("Job 1", &wg)
    go doWork("Job 2", &wg)
    go doWork("Job 3", &wg)

    wg.Wait()  // Block until all 3 call Done()
    fmt.Println("All done!")
}

func doWork(name string, wg *sync.WaitGroup) {
    defer wg.Done()  // "I'm finished" (decrements counter)

    fmt.Printf("Starting %s\n", name)
    time.Sleep(2 * time.Second)  // Simulate work
    fmt.Printf("Finished %s\n", name)
}

How WaitGroup works

Add(3) sets internal counter to 3
Each Done() decrements the counter (3→2→1→0)
Wait() blocks until counter reaches 0

Common pattern

var wg sync.WaitGroup

for i := 0; i < 10; i++ {
    wg.Add(1)
    go func(jobID int) {
        defer wg.Done()
        processJob(jobID)
    }(i)
}

wg.Wait()  // Wait for all 10 jobs

Important rules

Always call Add() before starting goroutines
Always call Done() exactly once per goroutine
Use defer wg.Done() to ensure it runs even if goroutine panics

WaitGroups are like taking attendance -> you know how many people you're expecting, and you wait until everyone checks in.

Golang Concurrency #7 - WaitGroup

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Introduction

The Problem

Bad Solution

Good Solution -> WaitGroup

How WaitGroup works

Common pattern

Important rules

Command Palette

Comments

More from this blog

Introduction

The Problem

Bad Solution

Good Solution -> WaitGroup

How WaitGroup works

Common pattern

Important rules