Explain the fan-out and fan-in concurrency pattern with a practical example.

Question

Accepted Answer

func producer(urls []string) <-chan string {
    out := make(chan string)
    go func() {
        defer close(out)
        for _, u := range urls { out <- u }
    }()
    return out
}

func worker(id int, in <-chan string) <-chan Result {
    out := make(chan Result)
    go func() {
        defer close(out)
        for url := range in {
            resp, err := http.Get(url)
            out <- Result{URL: url, StatusCode: resp.StatusCode, Err: err}
        }
    }()
    return out
}

// Fan-in: merge multiple channels into one
func merge(channels ...<-chan Result) <-chan Result {
    var wg sync.WaitGroup
    merged := make(chan Result, len(channels))

output := func(c <-chan Result) {
        defer wg.Done()
        for r := range c { merged <- r }
    }
    wg.Add(len(channels))
    for _, c := range channels { go output(c) }

go func() { wg.Wait(); close(merged) }()
    return merged
}

func main() {
    urls := []string{"https://a.com", "https://b.com", "https://c.com"}
    jobs := producer(urls)

// Fan out to 3 workers
    workers := make([]<-chan Result, 3)
    for i := range workers {
        workers[i] = worker(i, jobs)
    }

// Fan in results
    for result := range merge(workers...) {
        fmt.Println(result.URL, result.StatusCode)
    }
} Always close channels when producers are done — ranging goroutines exit cleanly Use context cancellation to stop all workers when one fails or timeout occurs Buffer the merged channel to avoid one slow worker blocking others WaitGroup in fan-in ensures merged channel is closed only after all workers finish For CPU-bound work, limit fan-out to runtime.NumCPU() workers

Explain the fan-out and fan-in concurrency pattern with a practical example.

Fan-out distributes work across multiple goroutines reading from a shared input channel. Fan-in merges results from multiple output channels into a single channel for the consumer.

Explain the fan-out and fan-in concurrency pattern with a practical example.

Fan-out distributes work across multiple goroutines reading from a shared input channel. Fan-in merges results from multiple output channels into a single channel for the consumer.

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