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feat: expire messages from the cache based on last seen time (#513)
* feat: expire messages from the cache based on last seen time * chore: minor renaming * fix: messages should not be found after expiration * chore: editorial * fix: use new time cache strategy consistently * fix: default to old time cache and add todo for background gc
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,71 @@ | ||
| package timecache | ||
|
|
||
| import ( | ||
| "container/list" | ||
| "sync" | ||
| "time" | ||
| ) | ||
|
|
||
| // FirstSeenCache is a thread-safe copy of https://github.com/whyrusleeping/timecache. | ||
| type FirstSeenCache struct { | ||
| q *list.List | ||
| m map[string]time.Time | ||
| span time.Duration | ||
| guard *sync.RWMutex | ||
| } | ||
|
|
||
| func newFirstSeenCache(span time.Duration) TimeCache { | ||
| return &FirstSeenCache{ | ||
| q: list.New(), | ||
| m: make(map[string]time.Time), | ||
| span: span, | ||
| guard: new(sync.RWMutex), | ||
| } | ||
| } | ||
|
|
||
| func (tc FirstSeenCache) Add(s string) { | ||
| tc.guard.Lock() | ||
| defer tc.guard.Unlock() | ||
|
|
||
| _, ok := tc.m[s] | ||
| if ok { | ||
| panic("putting the same entry twice not supported") | ||
| } | ||
|
|
||
| // TODO(#515): Do GC in the background | ||
| tc.sweep() | ||
|
|
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| tc.m[s] = time.Now() | ||
| tc.q.PushFront(s) | ||
| } | ||
|
|
||
| func (tc FirstSeenCache) sweep() { | ||
| for { | ||
| back := tc.q.Back() | ||
| if back == nil { | ||
| return | ||
| } | ||
|
|
||
| v := back.Value.(string) | ||
| t, ok := tc.m[v] | ||
| if !ok { | ||
| panic("inconsistent cache state") | ||
| } | ||
|
|
||
| if time.Since(t) > tc.span { | ||
| tc.q.Remove(back) | ||
| delete(tc.m, v) | ||
| } else { | ||
| return | ||
| } | ||
| } | ||
| } | ||
|
|
||
| func (tc FirstSeenCache) Has(s string) bool { | ||
| tc.guard.RLock() | ||
| defer tc.guard.RUnlock() | ||
|
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| ts, ok := tc.m[s] | ||
| // Only consider the entry found if it was present in the cache AND hadn't already expired. | ||
| return ok && time.Since(ts) <= tc.span | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,39 @@ | ||
| package timecache | ||
|
|
||
| import ( | ||
| "fmt" | ||
| "testing" | ||
| "time" | ||
| ) | ||
|
|
||
| func TestFirstSeenCacheFound(t *testing.T) { | ||
| tc := newFirstSeenCache(time.Minute) | ||
|
|
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| tc.Add("test") | ||
|
|
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| if !tc.Has("test") { | ||
| t.Fatal("should have this key") | ||
| } | ||
| } | ||
|
|
||
| func TestFirstSeenCacheExpire(t *testing.T) { | ||
| tc := newFirstSeenCache(time.Second) | ||
| for i := 0; i < 11; i++ { | ||
| tc.Add(fmt.Sprint(i)) | ||
| time.Sleep(time.Millisecond * 100) | ||
| } | ||
|
|
||
| if tc.Has(fmt.Sprint(0)) { | ||
| t.Fatal("should have dropped this from the cache already") | ||
| } | ||
| } | ||
|
|
||
| func TestFirstSeenCacheNotFoundAfterExpire(t *testing.T) { | ||
| tc := newFirstSeenCache(time.Second) | ||
| tc.Add(fmt.Sprint(0)) | ||
| time.Sleep(1100 * time.Millisecond) | ||
|
|
||
| if tc.Has(fmt.Sprint(0)) { | ||
| t.Fatal("should have dropped this from the cache already") | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,84 @@ | ||
| package timecache | ||
|
|
||
| import ( | ||
| "sync" | ||
| "time" | ||
|
|
||
| "github.com/emirpasic/gods/maps/linkedhashmap" | ||
| ) | ||
|
|
||
| // LastSeenCache is a LRU cache that keeps entries for up to a specified time duration. After this duration has elapsed, | ||
| // "old" entries will be purged from the cache. | ||
| // | ||
| // It's also a "sliding window" cache. Every time an unexpired entry is seen again, its timestamp slides forward. This | ||
| // keeps frequently occurring entries cached and prevents them from being propagated, especially because of network | ||
| // issues that might increase the number of duplicate messages in the network. | ||
| // | ||
| // Garbage collection of expired entries is event-driven, i.e. it only happens when there is a new entry added to the | ||
| // cache. This should be ok - if existing entries are being looked up then the cache is not growing, and when a new one | ||
| // appears that would grow the cache, garbage collection will attempt to reduce the pressure on the cache. | ||
| // | ||
| // This implementation is heavily inspired by https://github.com/whyrusleeping/timecache. | ||
| type LastSeenCache struct { | ||
| m *linkedhashmap.Map | ||
| span time.Duration | ||
| guard *sync.Mutex | ||
| } | ||
|
|
||
| func newLastSeenCache(span time.Duration) TimeCache { | ||
| return &LastSeenCache{ | ||
| m: linkedhashmap.New(), | ||
| span: span, | ||
| guard: new(sync.Mutex), | ||
| } | ||
| } | ||
|
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| func (tc *LastSeenCache) Add(s string) { | ||
| tc.guard.Lock() | ||
| defer tc.guard.Unlock() | ||
|
|
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| tc.add(s) | ||
|
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| // Garbage collect expired entries | ||
| // TODO(#515): Do GC in the background | ||
| tc.gc() | ||
| } | ||
|
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| func (tc *LastSeenCache) add(s string) { | ||
| // We don't need a lock here because this function is always called with the lock already acquired. | ||
|
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| // If an entry already exists, remove it and add a new one to the back of the list to maintain temporal ordering and | ||
| // an accurate sliding window. | ||
| tc.m.Remove(s) | ||
| now := time.Now() | ||
| tc.m.Put(s, &now) | ||
| } | ||
|
|
||
| func (tc *LastSeenCache) gc() { | ||
| // We don't need a lock here because this function is always called with the lock already acquired. | ||
| iter := tc.m.Iterator() | ||
| for iter.Next() { | ||
| key := iter.Key() | ||
| ts := iter.Value().(*time.Time) | ||
| // Exit if we've found an entry with an unexpired timestamp. Since we're iterating in order of insertion, all | ||
| // entries hereafter will be unexpired. | ||
| if time.Since(*ts) <= tc.span { | ||
| return | ||
| } | ||
| tc.m.Remove(key) | ||
| } | ||
| } | ||
|
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| func (tc *LastSeenCache) Has(s string) bool { | ||
| tc.guard.Lock() | ||
| defer tc.guard.Unlock() | ||
|
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| // If the entry exists and has not already expired, slide it forward. | ||
| if ts, found := tc.m.Get(s); found { | ||
| if t := ts.(*time.Time); time.Since(*t) <= tc.span { | ||
| tc.add(s) | ||
| return true | ||
| } | ||
| } | ||
| return false | ||
| } |
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