Golang crawler implementation principle

In recent years, the application of crawler technology has become more and more widespread, involving various fields such as artificial intelligence and big data. As a high-concurrency and high-performance programming language, Golang is also used by more and more crawler programmers. favor. This article will introduce you to the implementation principle of golang crawler.

1. HTTP request

When using golang for crawler development, the most important task is to initiate an HTTP request and obtain the response result. The Golang standard library has provided a wealth of HTTP client-related functions and types, allowing us to easily complete the sending and processing of HTTP requests.

For example, we can use the http.Get() function to directly send a GET request. This function will send an HTTP GET request to the specified URL and return a resp object of *http.Response type, containing the response. Status code, header information and response data:

response, err := http.Get("https://www.baidu.com")
if err != nil {
     log.Fatalln(err)
}
defer response.Body.Close()

If you need to send a POST request, you can use the http.Post() function to send it. The usage method is similar, except that you need to add the parameters of the request body:

form := url.Values{
    "key":   {"value"},
}
response, err := http.PostForm("https://www.example.com/login", form)
if err != nil {
    log.Fatalln(err)
}
defer response.Body.Close()

In addition, the Golang standard library also provides other types of HTTP clients, such as http.Client, http.Transport, etc., all of which are available Very good to meet a variety of needs. When some special parameters need to be customized, HTTP client parameters can be customized.

2. Parse HTML

After obtaining the web page content, the next step is to extract the required information. Generally, web page content is returned in HTML form, so we need to use an HTML parser to parse the web page and extract information. The Golang standard library provides an html package that can easily implement HTML parsing. We can use the html.Parse() function to parse HTML text into an AST (Abstract Syntax Tree) object.

For example, we can parse out all the links in an HTML text:

resp, err := http.Get("https://www.example.com")
if err != nil {
    log.Fatalln(err)
}
defer resp.Body.Close()

doc, err := html.Parse(resp.Body)
if err != nil {
    log.Fatalln(err)
}

var links []string
findLinks(doc, &links)

func findLinks(n *html.Node, links *[]string) {
    if n.Type == html.ElementNode && n.Data == "a" {
        for _, a := range n.Attr {
            if a.Key == "href" {
                *links = append(*links, a.Val)
                break
            }
        }
    }
    for c := n.FirstChild; c != nil; c = c.NextSibling {
        findLinks(c, links)
    }
}

In the above function findLinks(), we traverse the entire AST recursively and find all HTML node, if the node is an a tag, find the attribute href of the node, and then add it to the links slice.

Similarly, we can extract article content, image links, etc. in a similar way.

3. Parse JSON

Some websites will also return data in JSON format (RESTful API), and Golang also provides a JSON parser, which is very convenient.

For example, we can parse a set of objects from a JSON format response result, the code is as follows:

type User struct {
    ID       int    `json:"id"`
    Name     string `json:"name"`
    Username string `json:"username"`
    Email    string `json:"email"`
    Phone    string `json:"phone"`
    Website  string `json:"website"`
}

func main() {
    response, err := http.Get("https://jsonplaceholder.typicode.com/users")
    if err != nil {
        log.Fatalln(err)
    }
    defer response.Body.Close()

    var users []User
    if err := json.NewDecoder(response.Body).Decode(&users); err != nil {
        log.Fatalln(err)
    }

    fmt.Printf("%+v", users)
}

In the above code, we use the json.NewDecoder() function to The body content is decoded into a slice of type []User, and then all user information is printed out.

4. Anti-crawlers

In the field of web crawlers, anti-crawlers are the norm. Websites will use various methods to anti-crawl, such as IP bans, verification codes, User-Agent detection, request frequency limits, etc.

We can also use various methods to circumvent these anti-crawler measures, such as:

1. Use a proxy pool: walk between various proxies to crawl.
2. Use User-Agent pool: Use random User-Agent request header.
3. Frequency limit: Limit the frequency of requests, or use delayed submission.
4. Connect to the browser’s anti-crawler filter.

The above are just a few of the countermeasures. Crawlers engineers also need to customize the implementation as needed during actual development.

5. Summary

This article summarizes the key points of implementing web crawlers in Golang based on four aspects: HTTP client, HTML, JSON parsing and anti-crawler. Golang utilizes concurrency and lightweight coroutines, which is very suitable for concurrent crawling of data. Of course, web crawlers are an application with special needs and need to be designed based on business scenarios, use technical means rationally, and avoid being opened and used at will.

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