04/demos/07_select_statements/after/main.go

package main

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

var cache = map[int]Book{}
var rnd = rand.New(rand.NewSource(time.Now().UnixNano()))

func main() {
        wg := &sync.WaitGroup{}
        m := &sync.RWMutex{}
        cacheCh := make(chan Book)
        dbCh := make(chan Book)

        for i := 0; i < 10; i++ {
                id := rnd.Intn(10) + 1
                wg.Add(2)
                go func(id int, wg *sync.WaitGroup, m *sync.RWMutex, ch chan<- Book) {
                        if b, ok := queryCache(id, m); ok {
                                ch <- b
                        }
                        wg.Done()
                }(id, wg, m, cacheCh)
                go func(id int, wg *sync.WaitGroup, m *sync.RWMutex, ch chan<- Book) {
                        if b, ok := queryDatabase(id); ok {
                                m.Lock()
                                cache[id] = b
                                m.Unlock()
                                ch <- b
                        }
                        wg.Done()
                }(id, wg, m, dbCh)

                wg.Add(1)
                go func(cacheCh, dbCh <-chan Book) {
                        defer wg.Done()
                        select {
                        case b := <-cacheCh:
                                fmt.Println("from cache")
                                fmt.Println(b)
                                <-dbCh
                        case b := <-dbCh:
                                fmt.Println("from database")
                                fmt.Println(b)
                        }
                }(cacheCh, dbCh)
        }
        wg.Wait()
}

func queryCache(id int, m *sync.RWMutex) (Book, bool) {
        m.RLock()
        b, ok := cache[id]
        m.RUnlock()
        return b, ok
}

func queryDatabase(id int) (Book, bool) {
        time.Sleep(100 * time.Millisecond)
        for _, b := range books {
                if b.ID == id {
                        return b, true
                }
        }

        return Book{}, false
}