1# Throttler - intelligent WaitGroups
2
3[](http://godoc.org/github.com/nozzle/throttler) [](https://coveralls.io/r/nozzle/throttler?branch=master)
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5
6 Throttler fills the gap between sync.WaitGroup and manually monitoring your goroutines with channels. The API is almost identical to Wait Groups, but it allows you to set a max number of workers that can be running simultaneously. It uses channels internally to block until a job completes by calling Done() or until all jobs have been completed. It also provides a built in error channel that captures your goroutine errors and provides access to them as `[]error` after you exit the loop.
7
8See a fully functional example on the playground at http://bit.ly/throttler-v3
9
10Compare the Throttler example to the sync.WaitGroup example from http://golang.org/pkg/sync/#example_WaitGroup
11
12### How to use Throttler
13
14```
15// This example fetches several URLs concurrently,
16// using a Throttler to block until all the fetches are complete.
17// Compare to http://golang.org/pkg/sync/#example_WaitGroup
18func ExampleThrottler() {
19 var urls = []string{
20 "http://www.golang.org/",
21 "http://www.google.com/",
22 "http://www.somestupidname.com/",
23 }
24 // Create a new Throttler that will get 2 urls at a time
25 t := throttler.New(2, len(urls))
26 for _, url := range urls {
27 // Launch a goroutine to fetch the URL.
28 go func(url string) {
29 // Fetch the URL.
30 err := http.Get(url)
31 // Let Throttler know when the goroutine completes
32 // so it can dispatch another worker
33 t.Done(err)
34 }(url)
35 // Pauses until a worker is available or all jobs have been completed
36 // Returning the total number of goroutines that have errored
37 // lets you choose to break out of the loop without starting any more
38 errorCount := t.Throttle()
39 }
40}
41```
42
43### vs How to use a sync.WaitGroup
44
45```
46// This example fetches several URLs concurrently,
47// using a WaitGroup to block until all the fetches are complete.
48func ExampleWaitGroup() {
49 var wg sync.WaitGroup
50 var urls = []string{
51 "http://www.golang.org/",
52 "http://www.google.com/",
53 "http://www.somestupidname.com/",
54 }
55 for _, url := range urls {
56 // Increment the WaitGroup counter.
57 wg.Add(1)
58 // Launch a goroutine to fetch the URL.
59 go func(url string) {
60 // Decrement the counter when the goroutine completes.
61 defer wg.Done()
62 // Fetch the URL.
63 http.Get(url)
64 }(url)
65 }
66 // Wait for all HTTP fetches to complete.
67 wg.Wait()
68}
69```View as plain text