An exercise to the viewer

01_query.go

package main

import (
  "fmt"
  "math/rand"
  "time"
)

type Conn struct {
  num int
}

type Result struct {
  msg string
}

func (c Conn) DoQuery(query string) Result {
  time.Sleep(time.Duration(rand.Intn(10)) * time.Second)
  return Result{fmt.Sprintf("Conn #%d was first!", c.num)}
}

func Query(conns []Conn, query string) Result {
    ch := make(chan Result, len(conns))  // buffered
    for _, conn := range conns {
        go func(c Conn) {
            ch <- c.DoQuery(query)
        }(conn)
    }
    return <-ch
}

func main() {
  rand.Seed(int64(time.Now().UTC().Nanosecond()))
  conns := make([]Conn, 10)
  for i, _ := range conns {
    conns[i].num = i
  }
  fmt.Println(Query(conns, "foo bar"))
}

02_cancel_channel.go

package main

import (
  "fmt"
  "math/rand"
  "time"
)

type Conn struct {
  num int
}

type Result struct {
  msg string
}

func (c Conn) DoQuery(query string, cancel chan bool) Result {
  select {
    case <-time.After(time.Duration(rand.Intn(10)) * time.Second):
      return Result{fmt.Sprintf("Conn #%d was first!", c.num)}
    case <-cancel:
      return Result{fmt.Sprintf("Conn #%d was cancelled!", c.num)}
  }
}

func Query(conns []Conn, query string) Result {
    ch := make(chan Result)  // buffered
    cancel := make(chan bool, len(conns) - 1)
    for _, conn := range conns {
        go func(c Conn) {
            ch <- c.DoQuery(query, cancel)
        }(conn)
    }
    winner := <-ch
    for i := 0; i < len(conns) - 1; i++ {
      cancel <- true
      fmt.Println((<-ch).msg)
    }
    return winner
}

func main() {
  rand.Seed(int64(time.Now().UTC().Nanosecond()))
  conns := make([]Conn, 10)
  for i, _ := range conns {
    conns[i].num = i
  }
  fmt.Println(Query(conns, "foo bar"))
}