Colly is a highly customizable scraping framework. It has sane defaults and provides plenty of options to change them.

Collector configuration

Full list of collector attributes can be found here. The recommended way to initialize a collector is using colly.NewCollector(options...).

Create a collector with default settings:

c1 := colly.NewCollector()

Create another collector and change User-Agent and url revisit options:

c2 := colly.NewCollector(


c2 := colly.NewCollector()
c2.UserAgent = "xy"
c2.AllowURLRevisit = true

Configuration can be changed at any point of a scraping job by overwriting the attributes of the collectors.

A good example is a User-Agent switcher which changes User-Agent on every request:

const letterBytes = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

func RandomString() string {
	b := make([]byte, rand.Intn(10)+10)
	for i := range b {
		b[i] = letterBytes[rand.Intn(len(letterBytes))]
	return string(b)

c := colly.NewCollector()

c.OnRequest(func(r *colly.Request) {
	r.Headers.Set("User-Agent", RandomString())

Configuration via environment variables

Collector’s default configuration can be changed via environment variables. This allows us to fine-tune collectors without recompile. Environment parsing is the last step of the collector initialization, so every configuration change after the initialization overrides the config parsed from environment.

Environment config variables

  • ALLOWED_DOMAINS (comma separated list of domains)
  • CACHE_DIR (string)
  • DISALLOWED_DOMAINS (comma separated list of domains)
  • MAX_BODY_SIZE (int)
  • MAX_DEPTH (int - 0 means infinite)
  • USER_AGENT (string)

HTTP configuration

Colly uses Golang’s default http client as networking layer. HTTP options can be tweaked by changing the default HTTP roundtripper.

c := colly.NewCollector()
	Proxy: http.ProxyFromEnvironment,
	DialContext: (&net.Dialer{
		Timeout:   30 * time.Second,
		KeepAlive: 30 * time.Second,
		DualStack: true,
	MaxIdleConns:          100,
	IdleConnTimeout:       90 * time.Second,
	TLSHandshakeTimeout:   10 * time.Second,
	ExpectContinueTimeout: 1 * time.Second,