原题目
- Exercise: Web Crawler
- In this exercise you'll use Go's concurrency features to parallelize a web crawler.
- Modify the Crawl function to fetch URLs in parallel without fetching the same URL twice.
- Hint: you can keep a cache of the URLs that have been fetched on a map, but maps alone are not safe for concurrent use!
中文版题目
- 练习：Web 爬虫
- 在这个练习中，我们将会使用 Go 的并发特性来并行化一个 Web 爬虫。
- 修改 Crawl 函数来并行地抓取 URL，并且保证不重复。
- 提示：你可以用一个 map 来缓存已经获取的 URL，但是要注意 map 本身并不是并发安全的！
初始代码
package main
import (
"fmt"
)
type Fetcher interface {
// Fetch returns the body of URL and
// a slice of URLs found on that page.
Fetch(url string) (body string, urls []string, err error)
}
// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
// TODO: Fetch URLs in parallel.
// TODO: Don't fetch the same URL twice.
// This implementation doesn't do either:
if depth <= 0 {
return
}
body, urls, err := fetcher.Fetch(url)
if err != nil {
fmt.Println(err)
return
}
fmt.Printf("found: %s %q\n", url, body)
for _, u := range urls {
Crawl(u, depth-1, fetcher)
}
return
}
func main() {
Crawl("https://golang.org/", 4, fetcher)
}
// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult
type fakeResult struct {
body string
urls []string
}
func (f fakeFetcher) Fetch(url string) (string, []string, error) {
if res, ok := f[url]; ok {
return res.body, res.urls, nil
}
return "", nil, fmt.Errorf("not found: %s", url)
}
// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
"https://golang.org/": &fakeResult{
"The Go Programming Language",
[]string{
"https://golang.org/pkg/",
"https://golang.org/cmd/",
},
},
"https://golang.org/pkg/": &fakeResult{
"Packages",
[]string{
"https://golang.org/",
"https://golang.org/cmd/",
"https://golang.org/pkg/fmt/",
"https://golang.org/pkg/os/",
},
},
"https://golang.org/pkg/fmt/": &fakeResult{
"Package fmt",
[]string{
"https://golang.org/",
"https://golang.org/pkg/",
},
},
"https://golang.org/pkg/os/": &fakeResult{
"Package os",
[]string{
"https://golang.org/",
"https://golang.org/pkg/",
},
},
}
先看懂原始代码
- 原始代码本来就可以运行
- 结果也没有错
- 但是有两点缺陷：
- 其一就是没有并发
- 其二是结果有重复值，而操作上也的确是做了重复的工作
- 这些导致了低效率
我的改进方法
- 使用channel quit来确保当前goroutine会在执行完其他goroutine后才结束
- 使用visited[]来标记哪些已经访问，哪些没有
- visited是各个goroutine竞争的变量，因此使用Mutex来限制visited的访问
- 代码：
ackage main
import (
"fmt"
"sync"
)
type Fetcher interface {
// Fetch returns the body of URL and
// a slice of URLs found on that page.
Fetch(url string) (body string, urls []string, err error)
}
type Visitor struct {
visited map[string]bool
mux sync.Mutex
}
// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
// TODO: Fetch URLs in parallel.
// TODO: Don't fetch the same URL twice.
// This implementation doesn't do either:
quit := make(chan bool)
var v Visitor
v.visited = make(map[string]bool)
go MyCrawl(url, depth, fetcher, quit, &v)
<-quit
}
func MyCrawl(url string, depth int, fetcher Fetcher, quit chan bool, v *Visitor) {
if depth <= 0 {
quit <- true
return
}
v.mux.Lock()
if !v.visited[url] {
body, urls, err := fetcher.Fetch(url)
if err != nil {
fmt.Println(err)
v.visited[url] = true
v.mux.Unlock()
quit <- true
return
}
v.visited[url] = true
fmt.Printf("found: %s %q\n", url, body)
v.mux.Unlock()
for _, u := range urls {
childquit := make(chan bool)
go MyCrawl(u, depth-1, fetcher, childquit, v)
<-childquit
}
} else {
v.mux.Unlock()
}
quit <- true
}
func main() {
Crawl("https://golang.org/", 4, fetcher)
}
// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult
type fakeResult struct {
body string
urls []string
}
func (f fakeFetcher) Fetch(url string) (string, []string, error) {
if res, ok := f[url]; ok {
return res.body, res.urls, nil
}
return "", nil, fmt.Errorf("not found: %s", url)
}
// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
"https://golang.org/": &fakeResult{
"The Go Programming Language",
[]string{
"https://golang.org/pkg/",
"https://golang.org/cmd/",
},
},
"https://golang.org/pkg/": &fakeResult{
"Packages",
[]string{
"https://golang.org/",
"https://golang.org/cmd/",
"https://golang.org/pkg/fmt/",
"https://golang.org/pkg/os/",
},
},
"https://golang.org/pkg/fmt/": &fakeResult{
"Package fmt",
[]string{
"https://golang.org/",
"https://golang.org/pkg/",
},
},
"https://golang.org/pkg/os/": &fakeResult{
"Package os",
[]string{
"https://golang.org/",
"https://golang.org/pkg/",
},
},
}
- 笔者曾查过许多网上的代码，但大多都不能满足，并发，去重的要求，许多都只是为了用上channel和Mutex而去编写的代码，所以就只能自己写了
- 这道问题涉及程序中的许多细节问题，欢迎读者质疑我，我也找个时间放上github
