diff --git a/os/gcache/gcache_adapter.go b/os/gcache/gcache_adapter.go
index 1131e5d42..944735191 100644
--- a/os/gcache/gcache_adapter.go
+++ b/os/gcache/gcache_adapter.go
@@ -66,6 +66,10 @@ type Adapter interface {
 	// safety purpose.
 	GetOrSetFuncLock(key interface{}, f func() interface{}, duration time.Duration) interface{}
 
+	// GetExpire retrieves and returns the expiration of <key> in the cache.
+	// It returns -1 if the <key> does not exist in the cache.
+	GetExpire(key interface{}) time.Duration
+
 	// Remove deletes one or more keys from cache, and returns its value.
 	// If multiple keys are given, it returns the value of the last deleted item.
 	Remove(keys ...interface{}) (value interface{})
@@ -81,10 +85,6 @@ type Adapter interface {
 	// Contains checks and returns whether given <key> exists in the cache.
 	Contains(key interface{}) bool
 
-	// GetExpire retrieves and returns the expiration of <key> in the cache.
-	// It returns -1 if the <key> does not exist in the cache.
-	GetExpire(key interface{}) time.Duration
-
 	// Size returns the number of items in the cache.
 	Size() (size int)
 
diff --git a/os/gcache/gcache_adapter_memory.go b/os/gcache/gcache_adapter_memory.go
index 7bfe36f80..e606401e9 100644
--- a/os/gcache/gcache_adapter_memory.go
+++ b/os/gcache/gcache_adapter_memory.go
@@ -95,8 +95,9 @@ func newAdapterMemory(lruCap ...int) *adapterMemory {
 }
 
 // Set sets cache with <key>-<value> pair, which is expired after <duration>.
+//
 // It does not expire if <duration> == 0.
-// It deletes the <key> if <duration> < 0 or given <value> is nil.
+// It deletes the <key> if <duration> < 0.
 func (c *adapterMemory) Set(key interface{}, value interface{}, duration time.Duration) {
 	expireTime := c.getInternalExpire(duration)
 	c.dataMu.Lock()
@@ -112,7 +113,7 @@ func (c *adapterMemory) Set(key interface{}, value interface{}, duration time.Du
 }
 
 // Update updates the value of <key> without changing its expiration and returns the old value.
-// The returned <exist> value is false if the <key> does not exist in the cache.
+// The returned value <exist> is false if the <key> does not exist in the cache.
 func (c *adapterMemory) Update(key interface{}, value interface{}) (oldValue interface{}, exist bool) {
 	c.dataMu.Lock()
 	defer c.dataMu.Unlock()
@@ -146,7 +147,7 @@ func (c *adapterMemory) UpdateExpire(key interface{}, duration time.Duration) (o
 	return -1
 }
 
-// GetExpire retrieves and returns the expiration of <key>.
+// GetExpire retrieves and returns the expiration of <key> in the cache.
 // It returns -1 if the <key> does not exist in the cache.
 func (c *adapterMemory) GetExpire(key interface{}) time.Duration {
 	c.dataMu.RLock()