Wait Groups

A wait group is an object that tracks the number of goroutines to wait for. We increment its counter when starting a goroutine and decrement it when a goroutine finishes. The main goroutine can then block until the counter drops to zero.

• Provided by the sync package: sync.WaitGroup
• Tracks active goroutines
• Blocks until all goroutines signal completion

Basic Usage

• Add(n): Increment the counter by n (usually 1 per goroutine)
• Done(): Decrement the counter by 1 (called by each goroutine when finished)
• Wait(): Block until the counter is zero

Example: Waiting for Multiple Goroutines

package main

import (
    "fmt"
    "sync"
)

func worker(id int, wg *sync.WaitGroup) {
    defer wg.Done() // Signal completion
    fmt.Printf("Worker %d starting\n", id)
    // Simulate work
    fmt.Printf("Worker %d done\n", id)
}

func main() {
    var wg sync.WaitGroup
    for i := 1; i <= 3; i++ {
        wg.Add(1)
        go worker(i, &wg)
    }
    wg.Wait() // Wait for all workers to finish
    fmt.Println("All workers done!")
}

Why Use Wait Groups?

• Ensure all goroutines complete before continuing
• Avoid race conditions and premature exits
• Useful for parallel processing, web scraping, batch jobs, etc.

Common Patterns

Example: Parallel Data Processing

package main

import (
    "fmt"
    "sync"
)

func process(data int, wg *sync.WaitGroup, results chan<- int) {
    defer wg.Done()
    results <- data * 2 // Simulate processing
}

func main() {
    var wg sync.WaitGroup
    results := make(chan int, 5)
    data := []int{1, 2, 3, 4, 5}
    for _, d := range data {
        wg.Add(1)
        go process(d, &wg, results)
    }
    wg.Wait()
    close(results)
    for r := range results {
        fmt.Println(r)
    }
}

Example: Web Scraping (Simplified)

package main

import (
    "fmt"
    "sync"
)

func scrape(url string, wg *sync.WaitGroup) {
    defer wg.Done()
    fmt.Printf("Scraping %s\n", url)
    // Simulate scraping
}

func main() {
    var wg sync.WaitGroup
    urls := []string{"https://site1.com", "https://site2.com", "https://site3.com"}
    for _, url := range urls {
        wg.Add(1)
        go scrape(url, &wg)
    }
    wg.Wait()
    fmt.Println("All scraping done!")
}

Best Practices

• Always call Done() in a deferred statement to ensure it's executed
• Match every Add(1) with a corresponding Done()
• Never copy a WaitGroup (always pass by pointer)
• Use with channels for collecting results

Limitations & Pitfalls

• Calling Add() after Wait() has started can cause a panic
• Forgetting to call Done() leads to deadlocks
• Not thread-safe for copying