github.com/ndau/noms@v1.0.5/go/nbs/manifest_cache.go (about) 1 // Copyright 2016 Attic Labs, Inc. All rights reserved. 2 // Licensed under the Apache License, version 2.0: 3 // http://www.apache.org/licenses/LICENSE-2.0 4 5 package nbs 6 7 import ( 8 "container/list" 9 "time" 10 11 "sync" 12 13 "github.com/ndau/noms/go/d" 14 ) 15 16 func newManifestCache(maxSize uint64) *manifestCache { 17 return &manifestCache{ 18 maxSize: maxSize, 19 cache: map[string]manifestCacheEntry{}, 20 mu: &sync.Mutex{}, 21 } 22 } 23 24 type manifestCacheEntry struct { 25 lruEntry *list.Element 26 contents manifestContents 27 t time.Time 28 } 29 30 type manifestCache struct { 31 totalSize uint64 32 maxSize uint64 33 mu *sync.Mutex 34 lru list.List 35 cache map[string]manifestCacheEntry 36 } 37 38 // Get() checks the searches the cache for an entry. If it exists, it moves it's 39 // lru entry to the back of the queue and returns (value, true). Otherwise, it 40 // returns (nil, false). 41 func (mc *manifestCache) Get(db string) (contents manifestContents, t time.Time, present bool) { 42 mc.mu.Lock() 43 defer mc.mu.Unlock() 44 45 if entry, ok := mc.entry(db); ok { 46 contents, t, present = entry.contents, entry.t, true 47 } 48 return 49 } 50 51 // entry() checks if the value is in the cache. If not in the cache, it returns an 52 // empty manifestCacheEntry and false. It it is in the cache, it moves it to 53 // to the back of lru and returns the entry and true. 54 func (mc *manifestCache) entry(key string) (manifestCacheEntry, bool) { 55 entry, ok := mc.cache[key] 56 if !ok { 57 return manifestCacheEntry{}, false 58 } 59 mc.lru.MoveToBack(entry.lruEntry) 60 return entry, true 61 } 62 63 // Put inserts |contents| into the cache with the key |db|, replacing any 64 // currently cached value. Put() will add this element to the cache at the 65 // back of the queue as long it's size does not exceed maxSize. If the 66 // addition of this entry causes the size of the cache to exceed maxSize, the 67 // necessary entries at the front of the queue will be deleted in order to 68 // keep the total cache size below maxSize. |t| must be *prior* to initiating 69 // the call which read/wrote |contents|. 70 func (mc *manifestCache) Put(db string, contents manifestContents, t time.Time) { 71 mc.mu.Lock() 72 defer mc.mu.Unlock() 73 74 if entry, ok := mc.entry(db); ok { 75 mc.totalSize -= entry.contents.size() 76 mc.lru.Remove(entry.lruEntry) 77 delete(mc.cache, db) 78 } 79 80 if contents.size() <= mc.maxSize { 81 newEl := mc.lru.PushBack(db) 82 ce := manifestCacheEntry{lruEntry: newEl, contents: contents, t: t} 83 mc.cache[db] = ce 84 mc.totalSize += ce.contents.size() 85 for el := mc.lru.Front(); el != nil && mc.totalSize > mc.maxSize; { 86 key1 := el.Value.(string) 87 ce, ok := mc.cache[key1] 88 if !ok { 89 d.Panic("manifestCache is missing expected value") 90 } 91 next := el.Next() 92 delete(mc.cache, key1) 93 mc.totalSize -= ce.contents.size() 94 mc.lru.Remove(el) 95 el = next 96 } 97 } 98 }