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crawler.go
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crawler.go
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package crawler
import (
"log"
"net/http"
"strconv"
"time"
"github.com/patrickmn/go-cache"
"github.com/wikidistance/wikidist/pkg/db"
"github.com/wikidistance/wikidist/pkg/metrics"
)
// ratio to nWorkers
const queueSizeFactor = 400
const resultQueueSizeFactor = 2
const refillFactor = 3 * queueSizeFactor / 2
// requests per minute
const rateLimit = 100000
type Crawler struct {
nWorkers int
startTitle string
prefix string
queue chan string
results chan db.Article
notifyDequeued chan struct{}
canMakeRequest chan struct{}
seen *cache.Cache
database db.DB
}
func NewCrawler(nWorkers int, prefix string, startTitle string, database db.DB) *Crawler {
c := Crawler{}
c.database = database
c.canMakeRequest = make(chan struct{}, 10)
c.nWorkers = nWorkers
c.startTitle = startTitle
c.prefix = prefix
c.notifyDequeued = make(chan struct{}, 1)
c.queue = make(chan string, queueSizeFactor*nWorkers)
c.results = make(chan db.Article, resultQueueSizeFactor*nWorkers)
c.seen = cache.New(30*time.Minute, 30*time.Minute)
return &c
}
// until we find a better solution
func (c *Crawler) AlwaysRefill() {
for {
c.notifyDequeued <- struct{}{}
}
}
func (c *Crawler) Start() {
for i := 1; i <= c.nWorkers; i++ {
go c.addWorker()
}
for i := 1; i <= c.nWorkers; i++ {
go c.addRegisterer()
}
go c.metrics()
go c.rateLimit()
go c.refillQueue()
go c.AlwaysRefill()
c.queue <- c.startTitle
}
func (c *Crawler) metrics() {
for range time.Tick(10 * time.Second) {
metrics.Statsd.Gauge("wikidist.crawler.queue.length", float64(len(c.queue)), nil, 1)
metrics.Statsd.Gauge("wikidist.crawler.results.length", float64(len(c.results)), nil, 1)
}
}
func (c *Crawler) refillQueue() {
for {
<-c.notifyDequeued
if len(c.queue) <= refillFactor*c.nWorkers {
titles, err := c.database.NextsToVisit(queueSizeFactor * c.nWorkers)
if err != nil {
log.Println(err)
}
var newTitles int64
for _, title := range titles {
if _, ok := c.seen.Get(title); ok {
continue
}
if len(c.queue) >= cap(c.queue) {
break
}
c.seen.Set(title, struct{}{}, cache.DefaultExpiration)
newTitles++
c.queue <- title
}
}
}
}
func (c *Crawler) addRegisterer() {
for {
result := <-c.results
resultCopy := result
log.Println("Registering", result.Title, len(result.LinkedArticles), result.Missing)
c.database.AddVisited(&resultCopy)
metrics.Statsd.Count("wikidist.articles.registered", 1, []string{"missing:" + strconv.FormatBool(result.Missing)}, 1)
}
}
type httpGet struct{}
func (hg httpGet) Get(url string) (*http.Response, error) {
return http.Get(url)
}
func (c *Crawler) addWorker() {
for {
title := <-c.queue
// non-blocking write to channel
select {
case c.notifyDequeued <- struct{}{}:
default:
}
if title == "" {
continue
}
<-c.canMakeRequest
article, err := CrawlArticle(title, c.prefix, httpGet{})
if err != nil {
log.Println(err)
// try putting the title back in the queue
select {
case c.queue <- title:
default:
}
continue
}
c.results <- article
}
}
func (c *Crawler) rateLimit() {
for range time.Tick(time.Minute / rateLimit) {
c.canMakeRequest <- struct{}{}
}
}