// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package main

import (
	"log"
	"net/http"
	"time"
)

const (
	numPollers     = 2                // number of Poller goroutines to launch
	pollInterval   = 60 * time.Second // how often to poll each URL
	statusInterval = 10 * time.Second // how often to log status to stdout
	errTimeout     = 10 * time.Second // back-off timeout on error
)

var urls = []string{
	"http://www.google.com/",
	"http://golang.org/",
	"http://blog.golang.org/",
}

// State represents the last-known state of a URL. type State struct { url string status string }
// StateMonitor maintains a map that stores the state of the URLs being // polled, and prints the current state every updateInterval nanoseconds. // It returns a chan State to which resource state should be sent. func StateMonitor(updateInterval time.Duration) chan<- State { updates := make(chan State) urlStatus := make(map[string]string) ticker := time.NewTicker(updateInterval) go func() { for { select { case <-ticker.C: logState(urlStatus) case s := <-updates: urlStatus[s.url] = s.status } } }() return updates } // logState prints a state map. func logState(s map[string]string) { log.Println("Current state:") for k, v := range s { log.Printf(" %s %s", k, v) } } // Resource represents an HTTP URL to be polled by this program. type Resource struct { url string errCount int } // Poll executes an HTTP HEAD request for url // and returns the HTTP status string or an error string. func (r *Resource) Poll() string { resp, err := http.Head(r.url) if err != nil { log.Println("Error", r.url, err) r.errCount++ return err.Error() } r.errCount = 0 return resp.Status } // Sleep sleeps for an appropriate interval (dependent on error state) // before sending the Resource to done. func (r *Resource) Sleep(done chan<- *Resource) { time.Sleep(pollInterval + errTimeout*time.Duration(r.errCount)) done <- r } func Poller(in <-chan *Resource, out chan<- *Resource, status chan<- State) { for r := range in { s := r.Poll() status <- State{r.url, s} out <- r } } func main() { // Create our input and output channels. pending, complete := make(chan *Resource), make(chan *Resource) // Launch the StateMonitor. status := StateMonitor(statusInterval) // Launch some Poller goroutines. for i := 0; i < numPollers; i++ { go Poller(pending, complete, status) } // Send some Resources to the pending queue. go func() { for _, url := range urls { pending <- &Resource{url: url} } }() for r := range complete { go r.Sleep(pending) } }