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51 lines
1.1 KiB
Go
51 lines
1.1 KiB
Go
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 130.
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// The wait program waits for an HTTP server to start responding.
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package main
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import (
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"fmt"
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"log"
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"net/http"
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"os"
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"time"
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)
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//!+
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// WaitForServer attempts to contact the server of a URL.
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// It tries for one minute using exponential back-off.
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// It reports an error if all attempts fail.
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func WaitForServer(url string) error {
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const timeout = 1 * time.Minute
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deadline := time.Now().Add(timeout)
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for tries := 0; time.Now().Before(deadline); tries++ {
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_, err := http.Head(url)
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if err == nil {
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return nil // success
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}
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log.Printf("server not responding (%s); retrying...", err)
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time.Sleep(time.Second << uint(tries)) // exponential back-off
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}
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return fmt.Errorf("server %s failed to respond after %s", url, timeout)
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}
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//!-
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func main() {
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if len(os.Args) != 2 {
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fmt.Fprintf(os.Stderr, "usage: wait url\n")
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os.Exit(1)
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}
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url := os.Args[1]
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//!+main
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// (In function main.)
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if err := WaitForServer(url); err != nil {
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fmt.Fprintf(os.Stderr, "Site is down: %v\n", err)
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os.Exit(1)
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}
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//!-main
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}
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