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cacheMergeIterator #298

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Dec 12, 2017
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255 changes: 255 additions & 0 deletions store/cacheiterkvstore.go
Original file line number Diff line number Diff line change
@@ -0,0 +1,255 @@
package store

// TODO: Consider merge w/ tendermint/tmlibs/db/cachedb.go.
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make an issue & link it in the code

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I'll actually just do the whole thing. 1 moment...


import (
"bytes"
"sort"
"sync"
"sync/atomic"
)

// If value is nil but deleted is false, it means the parent doesn't have the
// key. (No need to delete upon Write())
type cValue struct {
value []byte
deleted bool
dirty bool
}

// cacheIterKVStore wraps an in-memory cache around an underlying IterKVStore.
type cacheIterKVStore struct {
mtx sync.Mutex
cache map[string]cValue
parent IterKVStore
lockVersion interface{}

cwwMutex
}

var _ CacheIterKVStore = (*cacheIterKVStore)(nil)

// Users should typically not be required to call NewCacheIterKVStore directly, as the
// IterKVStore implementations here provide a .CacheIterKVStore() function already.
// `lockVersion` is typically provided by parent.GetWriteLockVersion().
func NewCacheIterKVStore(parent IterKVStore, lockVersion interface{}) *cacheIterKVStore {
ci := &cacheIterKVStore{
cache: make(map[string]cValue),
parent: parent,
lockVersion: lockVersion,
cwwMutex: NewCWWMutex(),
}
return ci
}

func (ci *cacheIterKVStore) Get(key []byte) (value []byte) {
ci.mtx.Lock()
defer ci.mtx.Unlock()
ci.assertValidKey(key)

cacheValue, ok := ci.cache[string(key)]
if !ok {
value = ci.parent.Get(key)
ci.setCacheValue(key, value, false, false)
} else {
value = cacheValue.value
}

return value
}

func (ci *cacheIterKVStore) Set(key []byte, value []byte) {
ci.mtx.Lock()
defer ci.mtx.Unlock()
ci.assertValidKey(key)

ci.setCacheValue(key, value, false, true)
}

func (ci *cacheIterKVStore) Has(key []byte) bool {
value := ci.Get(key)
return value != nil
}

func (ci *cacheIterKVStore) Remove(key []byte) {
ci.mtx.Lock()
defer ci.mtx.Unlock()
ci.assertValidKey(key)

ci.setCacheValue(key, nil, true, true)
}

// Write writes pending updates to the parent database and clears the cache.
// NOTE: Not atomic.
func (ci *cacheIterKVStore) Write() {
ci.mtx.Lock()
defer ci.mtx.Unlock()

// Optional sanity check to ensure that cacheIterKVStore is valid
if parent, ok := ci.parent.(WriteLocker); ok {
if parent.TryWriteLock(ci.lockVersion) {
// All good!
} else {
panic("parent.Write() failed. Did this CacheIterKVStore expire?")
}
}

// We need a copy of all of the keys.
// Not the best, but probably not a bottleneck depending.
keys := make([]string, 0, len(ci.cache))
for key, dbValue := range ci.cache {
if dbValue.dirty {
keys = append(keys, key)
}
}
sort.Strings(keys)

// TODO in tmlibs/db we use Batch to write atomically.
// Consider locking the underlying IterKVStore during write.
for _, key := range keys {
cacheValue := ci.cache[key]
if cacheValue.deleted {
ci.parent.Remove([]byte(key))
} else if cacheValue.value == nil {
// Skip, it already doesn't exist in parent.
} else {
ci.parent.Set([]byte(key), cacheValue.value)
}
}

// Clear the cache
ci.cache = make(map[string]cValue)
}

//----------------------------------------
// To cache-wrap this cacheIterKVStore further.

func (ci *cacheIterKVStore) CacheWrap() CacheWrap {
return ci.CacheIterKVStore()
}

func (ci *cacheIterKVStore) CacheKVStore() CacheKVStore {
return ci.CacheIterKVStore()
}

func (ci *cacheIterKVStore) CacheIterKVStore() CacheIterKVStore {
return NewCacheIterKVStore(ci, ci.GetWriteLockVersion())
}

// If the parent parent DB implements this, (e.g. such as a cacheIterKVStore
// parent to a cacheIterKVStore child), cacheIterKVStore will call
// `parent.TryWriteLock()` before attempting to write.
// This prevents multiple siblings from Write'ing to the parent.
type WriteLocker interface {
GetWriteLockVersion() (lockVersion interface{})
TryWriteLock(lockVersion interface{}) bool
}

// Implements TryWriteLocker. Embed this in DB structs if desired.
type cwwMutex struct {
mtx sync.Mutex
// CONTRACT: reading/writing to `*written` should use `atomic.*`.
// CONTRACT: replacing `written` with another *int32 should use `.mtx`.
written *int32
}

func NewCWWMutex() cwwMutex {
return cwwMutex{
written: new(int32),
}
}

func (cww *cwwMutex) GetWriteLockVersion() interface{} {
cww.mtx.Lock()
defer cww.mtx.Unlock()

// `written` works as a "version" object because it gets replaced upon
// successful TryWriteLock.
return cww.written
}

func (cww *cwwMutex) TryWriteLock(version interface{}) bool {
cww.mtx.Lock()
defer cww.mtx.Unlock()

if version != cww.written {
return false // wrong "WriteLockVersion"
}
if !atomic.CompareAndSwapInt32(cww.written, 0, 1) {
return false // already written
}

// New "WriteLockVersion"
cww.written = new(int32)
return true
}

//----------------------------------------
// Iteration

func (ci *cacheIterKVStore) Iterator(start, end []byte) Iterator {
return ci.iterator(start, end, true)
}

func (ci *cacheIterKVStore) ReverseIterator(start, end []byte) Iterator {
return ci.iterator(start, end, false)
}

func (ci *cacheIterKVStore) iterator(start, end []byte, ascending bool) Iterator {
var parent, cache Iterator
if ascending {
parent = ci.parent.Iterator(start, end)
} else {
parent = ci.parent.ReverseIterator(start, end)
}
items := ci.dirtyItems(ascending)
cache = newMemIterator(start, end, items)
return newCacheMergeIterator(parent, cache, ascending)
}

func (ci *cacheIterKVStore) First(start, end []byte) (kv KVPair, ok bool) {
return iteratorFirst(ci, start, end)
}

func (ci *cacheIterKVStore) Last(start, end []byte) (kv KVPair, ok bool) {
return iteratorLast(ci, start, end)
}

// Constructs a slice of dirty items, to use w/ memIterator.
func (ci *cacheIterKVStore) dirtyItems(ascending bool) []KVPair {
items := make([]KVPair, 0, len(ci.cache))
for key, cacheValue := range ci.cache {
if !cacheValue.dirty {
continue
}
items = append(items,
KVPair{[]byte(key), cacheValue.value})
}
sort.Slice(items, func(i, j int) bool {
if ascending {
return bytes.Compare(items[i].Key, items[j].Key) < 0
} else {
return bytes.Compare(items[i].Key, items[j].Key) > 0
}
})
return items
}

//----------------------------------------
// etc

func (ci *cacheIterKVStore) assertValidKey(key []byte) {
if key == nil {
panic("key is nil")
}
}

// Only entrypoint to mutate ci.cache.
func (ci *cacheIterKVStore) setCacheValue(key, value []byte, deleted bool, dirty bool) {
cacheValue := cValue{
value: value,
deleted: deleted,
dirty: dirty,
}
ci.cache[string(key)] = cacheValue
}
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