Add background refresh functionality

cache.go

@@ -79,6 +79,12 @@ type Config struct {
 	OnEviction func(key, value interface{})
 	// Optional callback invoked when an item expired
 	OnExpiration func(key, value interface{})
+	// Optional refresh interval after which all items in the cache expires.
+	// If zero, refreshing cache is disabled.
+	RefreshInterval time.Duration
+	// Optional on refresh callback invoked when the cache is refreshed
+	// Both RefreshInterval and OnRefresh must be provided to enable background cache refresh
+	OnRefresh func() map[interface{}]interface{}
 }
 
 // Entry pointed to by each list.Element
@@ -133,6 +139,10 @@ func New(config Config) *Cache {
 		panic("config.MinAge must be less than or equal to config.MaxAge")
 	}
 
+	if config.RefreshInterval < 0 {
+		panic("Must supply a zero or positive config.RefreshInterval")
+	}
+
 	minAge := config.MinAge
 	if minAge == 0 {
 		minAge = config.MaxAge
@@ -166,6 +176,18 @@ func New(config Config) *Cache {
 		}()
 	}
 
+	if config.RefreshInterval > 0 && config.OnRefresh != nil {
+		cache.RefreshCache(config.OnRefresh())
+		go func() {
+			t := time.NewTicker(config.RefreshInterval)
+			defer t.Stop()
+			for {
+				<-t.C
+				cache.RefreshCache(config.OnRefresh())
+			}
+		}()
+	}
+
 	return cache
 }
 
@@ -232,9 +254,7 @@ func (cache *Cache) RefreshCache(items map[interface{}]interface{}) {
 	cache.mutex.Lock()
 	defer cache.mutex.Unlock()
 
-	for _, val := range cache.items {
-		cache.deleteElement(val)
-	}
+	cache.items = make(map[interface{}]*list.Element)
 	cache.evictionList.Init()
 
 	for key, value := range items {

cache_test.go

@@ -34,6 +34,12 @@ func TestInvalidMinAge(t *testing.T) {
 	})
 }
 
+func TestInvalidRefreshInterval(t *testing.T) {
+	assert.Panics(t, func() {
+		New(Config{Capacity: 1, RefreshInterval: -1 * time.Hour})
+	})
+}
+
 func TestBasicSetGet(t *testing.T) {
 	cache := New(Config{Capacity: 2})
 	cache.Set("foo", 1)
@@ -109,6 +115,34 @@ func TestExpiration(t *testing.T) {
 	assert.False(t, eviction)
 }
 
+func TestCacheBackgroundRefresh(t *testing.T) {
+	count := 0
+	cache := New(Config{
+		Capacity:        1,
+		RefreshInterval: 3 * time.Second,
+		OnRefresh: func() map[interface{}]interface{} {
+			count++
+			return map[interface{}]interface{}{"key": count}
+		},
+	})
+
+	value, ok := cache.Get("key")
+	assert.Equal(t, true, ok)
+	assert.Equal(t, 1, value)
+
+	time.Sleep(4 * time.Second) // wait for the refresh loop to run
+
+	value, ok = cache.Get("key")
+	assert.Equal(t, true, ok)
+	assert.Equal(t, 2, value)
+
+	time.Sleep(4 * time.Second)
+	value, ok = cache.Get("key")
+	assert.Equal(t, true, ok)
+	assert.Equal(t, 3, value)
+
+}
+
 type MockRandGenerator struct {
 	startAt int64
 	incr    int64