WIP: kvmemcache

kvmemcache/cache.go

@@ -34,39 +34,34 @@ type Config struct {
 	Src     func(string) (interface{}, error)
 }
 
-type Stats struct {
-	Hits    uint64
-	Misses  uint64
-	Expired uint64
-}
-
 func New(conf Config) *Cache {
 	return &Cache{
-		lock:       sync.Mutex{},
 		updateLock: keyedmutex.New(),
 		src:        conf.Src,
 		ttl:        conf.TTL,
 		maxSize:    conf.MaxSize,
-		cache:      make(map[string]*list.Element),
+		lock:       sync.Mutex{},
+		cache:      make(map[string]*list.Element, conf.MaxSize+1),
 		ll:         list.New(),
 	}
 }
 
 func (c *Cache) Get(key string) (interface{}, error) {
-	item := c.getItem(key)
+	val, err, ok := c.get(key)
-	if item == nil {
+	if ok {
-		item = c.loadItemFromSource(key)
+		return val, err
 	}
-	return item.value, item.err
+
+	return c.load(key)
 }
 
 func (c *Cache) Evict(key string) {
 	c.lock.Lock()
 	defer c.lock.Unlock()
-	c.evictKey(key)
+	c.evict(key)
 }
 
-func (c Cache) Stats() Stats {
+func (c *Cache) Stats() Stats {
 	c.lock.Lock()
 	defer c.lock.Unlock()
 	return c.stats

kvmemcache/cache_test.go

@@ -1,98 +1,149 @@
 package kvmemcache
 
 import (
+	"errors"
 	"fmt"
-	"math/rand"
 	"testing"
 	"time"
 )
 
-func testRunner(c *Cache, t *testing.T, done chan bool) {
+type State struct {
-	rand.Seed(time.Now().UnixNano())
+	Keys  []string
-	for i := 0; i < 200001; i++ {
+	Stats Stats
-		x := rand.Int31n(50)
-		if rand.Float64() < 0.01 {
-			x = rand.Int31n(9999)
 }
 
-		key := fmt.Sprintf("key-%v", x)
+func (c *Cache) assert(state State) error {
-		expectedVal := "value for " + key
+	c.lock.Lock()
+	defer c.lock.Unlock()
 
-		val, err := c.Get(key)
+	if len(c.cache) != len(state.Keys) {
-
+		return fmt.Errorf(
-		if err != nil {
+			"Expected %d keys but found %d.",
-			t.Fatal(err)
+			len(state.Keys),
+			len(c.cache))
 	}
 
-		if val.(string) != expectedVal {
+	for _, k := range state.Keys {
-			t.Fatal(val)
+		if _, ok := c.cache[k]; !ok {
+			return fmt.Errorf(
+				"Expected key %s not found.",
+				k)
 		}
 	}
-	done <- true
+
+	if c.stats.Hits != state.Stats.Hits {
+		return fmt.Errorf(
+			"Expected %d hits, but found %d.",
+			state.Stats.Hits,
+			c.stats.Hits)
 	}
 
-func testCache(name string, c *Cache, t *testing.T) {
+	if c.stats.Misses != state.Stats.Misses {
-	N := 8
+		return fmt.Errorf(
-	done := make(chan bool, N)
+			"Expected %d misses, but found %d.",
-
+			state.Stats.Misses,
-	for i := 0; i < N; i++ {
+			c.stats.Misses)
-		go testRunner(c, t, done)
 	}
 
-	for i := 0; i < N; i++ {
+	return nil
-		<-done
 }
 
-	stats := c.Stats()
+var TestError = errors.New("Hello")
 
-	fmt.Println(name)
+func TestCache_Basic(t *testing.T) {
-	fmt.Printf("  Hits:    %d\n", stats.Hits)
-	fmt.Printf("  Misses:  %d\n", stats.Misses)
-	fmt.Printf("  Expired: %d\n", stats.Expired)
-	fmt.Printf("  Hit-rate: %.2f%%\n",
-		100*float64(stats.Hits)/float64(stats.Hits+stats.Misses))
-}
-
-func TestCache(t *testing.T) {
 	c := New(Config{
-		MaxSize: 2048,
+		MaxSize: 4,
-		TTL:     100 * time.Millisecond,
+		TTL:     50 * time.Millisecond,
 		Src: func(key string) (interface{}, error) {
-			return fmt.Sprintf("value for %s", key), nil
+			if key == "err" {
-		},
+				return nil, TestError
-	})
-	testCache("LRUTimeout", c, t)
 			}
-
-func TestCacheTTL(t *testing.T) {
-	c := New(Config{
-		MaxSize: 2048,
-		TTL:     100 * time.Millisecond,
-		Src: func(key string) (interface{}, error) {
 			return key, nil
 		},
 	})
 
-	c.Get("a")
+	type testCase struct {
-	c.Get("b")
+		name  string
-	time.Sleep(50 * time.Millisecond)
+		sleep time.Duration
-	c.Get("a")
+		key   string
-	c.Get("b")
+		state State
-	c.Get("c")
-
-	stats := c.Stats()
-	if stats.Expired != 0 {
-		t.Fatal(stats.Expired)
 	}
 
-	time.Sleep(50 * time.Millisecond)
+	cases := []testCase{
-	c.Get("a")
+		{
-	c.Get("b")
+			name: "get a",
-	c.Get("c")
+			key:  "a",
-
+			state: State{
-	stats = c.Stats()
+				Keys:  []string{"a"},
-	if stats.Expired != 2 {
+				Stats: Stats{Hits: 0, Misses: 1},
-		t.Fatal(stats.Expired)
+			},
+		}, {
+			name: "get a again",
+			key:  "a",
+			state: State{
+				Keys:  []string{"a"},
+				Stats: Stats{Hits: 1, Misses: 1},
+			},
+		}, {
+			name:  "sleep, then get a again",
+			sleep: 55 * time.Millisecond,
+			key:   "a",
+			state: State{
+				Keys:  []string{"a"},
+				Stats: Stats{Hits: 1, Misses: 2},
+			},
+		}, {
+			name: "get b",
+			key:  "b",
+			state: State{
+				Keys:  []string{"a", "b"},
+				Stats: Stats{Hits: 1, Misses: 3},
+			},
+		}, {
+			name: "get c",
+			key:  "c",
+			state: State{
+				Keys:  []string{"a", "b", "c"},
+				Stats: Stats{Hits: 1, Misses: 4},
+			},
+		}, {
+			name: "get d",
+			key:  "d",
+			state: State{
+				Keys:  []string{"a", "b", "c", "d"},
+				Stats: Stats{Hits: 1, Misses: 5},
+			},
+		}, {
+			name: "get e",
+			key:  "e",
+			state: State{
+				Keys:  []string{"b", "c", "d", "e"},
+				Stats: Stats{Hits: 1, Misses: 6},
+			},
+		}, {
+			name: "get c again",
+			key:  "c",
+			state: State{
+				Keys:  []string{"b", "c", "d", "e"},
+				Stats: Stats{Hits: 2, Misses: 6},
+			},
+		},
 	}
 
+	for _, tc := range cases {
+		time.Sleep(tc.sleep)
+		val, err := c.Get(tc.key)
+		if tc.key == "err" && err != TestError {
+			t.Fatal(tc.name, val)
 		}
+		if tc.key != "err" && val.(string) != tc.key {
+			t.Fatal(tc.name, tc.key, val)
+		}
+
+		if err := c.assert(tc.state); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+// TODO: Test thundering herd mitigation.

kvmemcache/internal.go

@@ -4,70 +4,64 @@ import (
 	"time"
 )
 
-func (c *Cache) getItem(key string) *lruItem {
+func (c *Cache) put(key string, value interface{}, err error) {
 	c.lock.Lock()
 	defer c.lock.Unlock()
-
-	elem, ok := c.cache[key]
-	if !ok {
 	c.stats.Misses++
-		return nil
+	c.cache[key] = c.ll.PushFront(lruItem{
-	}
-
-	item := elem.Value.(*lruItem)
-	if c.ttl != 0 && time.Since(item.createdAt) > c.ttl {
-		c.stats.Expired++
-		c.evictKey(key)
-		return nil
-	}
-
-	c.stats.Hits++
-	c.ll.MoveToFront(elem)
-	return item
-}
-
-func (c *Cache) loadItemFromSource(key string) *lruItem {
-	c.updateLock.Lock(key)
-	defer c.updateLock.Unlock(key)
-
-	// May have lost update race.
-	if item := c.getItem(key); item != nil {
-		return item
-	}
-
-	val, err := c.src(key)
-	item := &lruItem{
 		key:       key,
-		value: val,
+		createdAt: time.Now(),
+		value:     value,
 		err:       err,
-	}
+	})
 
-	if c.ttl != 0 {
+	if c.maxSize != 0 && len(c.cache) > c.maxSize {
-		item.createdAt = time.Now()
+		li := c.ll.Back()
-	}
+		c.ll.Remove(li)
-
+		delete(c.cache, li.Value.(lruItem).key)
-	c.putItem(key, item)
-	return item
-}
-
-func (c *Cache) putItem(key string, item *lruItem) {
-	c.lock.Lock()
-	defer c.lock.Unlock()
-
-	c.cache[key] = c.ll.PushFront(item)
-
-	if c.maxSize > 0 && len(c.cache) > c.maxSize {
-		elem := c.ll.Back()
-		c.ll.Remove(elem)
-		delete(c.cache, elem.Value.(*lruItem).key)
 	}
 }
 
-func (c *Cache) evictKey(key string) {
+func (c *Cache) evict(key string) {
-	elem, ok := c.cache[key]
+	elem := c.cache[key]
-	if !ok {
+	if elem != nil {
-		return
-	}
 		delete(c.cache, key)
 		c.ll.Remove(elem)
 	}
+}
+
+func (c *Cache) get(key string) (val interface{}, err error, ok bool) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	li := c.cache[key]
+	if li == nil {
+		return nil, nil, false
+	}
+
+	item := li.Value.(lruItem)
+	// Maybe evict.
+	if c.ttl != 0 && time.Since(item.createdAt) > c.ttl {
+		c.evict(key)
+		return nil, nil, false
+	}
+
+	c.stats.Hits++
+	return item.value, item.err, true
+}
+
+func (c *Cache) load(key string) (interface{}, error) {
+	c.updateLock.Lock(key)
+	defer c.updateLock.Unlock(key)
+
+	// Check again in case we lost the update race.
+	val, err, ok := c.get(key)
+	if ok {
+		return val, err
+	}
+
+	// Won the update race.
+	val, err = c.src(key)
+	c.put(key, val, err)
+	return val, err
+}

kvmemcache/stats.go

@@ -0,0 +1,6 @@
+package kvmemcache
+
+type Stats struct {
+	Hits   uint64
+	Misses uint64
+}