doron2402 · April 7, 2020 05:41 · Apr 7, 2020 · Apr 7, 2020 · Apr 7, 2020
diff --git a/main.go b/main.go
@@ -1,5 +1,8 @@
 package main
-
+/**
+A cool way to find out if there are any race condition is using 
+`go run --race .`
+**/
 import (
 	"fmt"
 	"math/rand"

diff --git a/main.go b/main.go
@@ -7,7 +7,9 @@ import (
 	"time"
 )
 
+// cache - key value (key = book id, value = book object)
 var cache = map[int]Book{}
+// get random instance
 var rnd = rand.New(rand.NewSource(time.Now().UnixNano()))
 
 func main() {
@@ -17,26 +19,30 @@ func main() {
 
 	for i := 0; i < 10; i++ {
 		id := rnd.Intn(10) + 1
+		// set 2 waiting group (1. cache, 2. db)
 		wg.Add(2)
 		go func(wg *sync.WaitGroup, m *sync.RWMutex) {
 			if b, ok := queryCache(id, m); ok {
 				fmt.Println("from cache")
 				fmt.Println(b)
 			}
+			// if the book found in cache finish the waiting group 
 			wg.Done()
 		}(wg, m)
 
 		go func(wg *sync.WaitGroup, m *sync.RWMutex) {
+			// get book from db
 			if b, ok := queryDatabase(id, m); ok {
 				fmt.Println("from database")
 				fmt.Println(b)
 			}
+			// finish waiting group
 			wg.Done()
 		}(wg, m)
 
 		// time.Sleep(150 * time.Millisecond)
 	}
-
+	// wait for all concurrent functions 
 	wg.Wait()
 }
 

diff --git a/main.go b/main.go
@@ -0,0 +1,150 @@
+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{}
+
+	for i := 0; i < 10; i++ {
+		id := rnd.Intn(10) + 1
+		wg.Add(2)
+		go func(wg *sync.WaitGroup, m *sync.RWMutex) {
+			if b, ok := queryCache(id, m); ok {
+				fmt.Println("from cache")
+				fmt.Println(b)
+			}
+			wg.Done()
+		}(wg, m)
+
+		go func(wg *sync.WaitGroup, m *sync.RWMutex) {
+			if b, ok := queryDatabase(id, m); ok {
+				fmt.Println("from database")
+				fmt.Println(b)
+			}
+			wg.Done()
+		}(wg, m)
+
+		// time.Sleep(150 * time.Millisecond)
+	}
+
+	wg.Wait()
+}
+
+func insertBook(books Book[], b Book) Book[] {
+	books = append(books, book)
+	return books
+}
+
+func queryCache(id int, m *sync.RWMutex) (Book, bool) {
+	m.RLock()
+	b, ok := cache[id]
+	m.RUnlock()
+	return b, ok
+}
+
+func queryDatabase(id int, m *sync.RWMutex) (Book, bool) {
+	time.Sleep(100 * time.Millisecond)
+	for _, b := range books {
+		if b.ID == id {
+			m.Lock()
+			cache[id] = b
+			m.Unlock()
+			return b, true
+		}
+	}
+
+	return Book{}, false
+}
+
+// books.go
+package main
+
+import "fmt"
+
+type Book struct {
+	ID            int
+	Title         string
+	Author        string
+	YearPublished int
+}
+
+func (b Book) String() string {
+	return fmt.Sprintf(
+		"Title:\t\t%q\n"+
+			"Author:\t\t%q\n"+
+			"Published:\t%v\n", b.Title, b.Author, b.YearPublished)
+}
+
+var books = []Book{
+	Book{
+		ID:            1,
+		Title:         "The Hitchhiker's Guide to the Galaxy",
+		Author:        "Douglas Adams",
+		YearPublished: 1979,
+	},
+	Book{
+		ID:            2,
+		Title:         "The Hobbit",
+		Author:        "J.R.R. Tolkien",
+		YearPublished: 1937,
+	},
+	Book{
+		ID:            3,
+		Title:         "A Tale of Two Cities",
+		Author:        "Charles Dickens",
+		YearPublished: 1859,
+	},
+	Book{
+		ID:            4,
+		Title:         "Les Misérables",
+		Author:        "Victor Hugo",
+		YearPublished: 1862,
+	},
+	Book{
+		ID:            5,
+		Title:         "Harry Potter and the Philosopher's Stone",
+		Author:        "J.K. Rowling",
+		YearPublished: 1997,
+	},
+	Book{
+		ID:            6,
+		Title:         "I, Robot",
+		Author:        "Isaac Asimov",
+		YearPublished: 1950,
+	},
+	Book{
+		ID:            7,
+		Title:         "The Gods Themselves",
+		Author:        "Isaac Asimov",
+		YearPublished: 1973,
+	},
+	Book{
+		ID:            8,
+		Title:         "The Moon is a Harsh Mistress",
+		Author:        "Robert A. Heinlein",
+		YearPublished: 1966,
+	},
+	Book{
+		ID:            9,
+		Title:         "On Basilisk Station",
+		Author:        "David Weber",
+		YearPublished: 1993,
+	},
+	Book{
+		ID:            10,
+		Title:         "The Android's Dream",
+		Author:        "John Scalzi",
+		YearPublished: 2006,
+	},
+}
+