// newDbCache returns a new database cache instance backed by the provided
// leveldb instance. The cache will be flushed to leveldb when the max size
// exceeds the provided value or it has been longer than the provided interval
// since the last flush.
func newDbCache(ldb *leveldb.DB, store *blockStore, maxSize uint64, flushIntervalSecs uint32) *dbCache {
return &dbCache{
ldb: ldb,
store: store,
maxSize: maxSize,
flushInterval: time.Second * time.Duration(flushIntervalSecs),
lastFlush: time.Now(),
cachedKeys: treap.NewImmutable(),
cachedRemove: treap.NewImmutable(),
}
}
// flush flushes the database cache to persistent storage. This involes syncing
// the block store and replaying all transactions that have been applied to the
// cache to the underlying database.
//
// This function MUST be called with the database write lock held.
func (c *dbCache) flush() error {
c.lastFlush = time.Now()
// Sync the current write file associated with the block store. This is
// necessary before writing the metadata to prevent the case where the
// metadata contains information about a block which actually hasn't
// been written yet in unexpected shutdown scenarios.
if err := c.store.syncBlocks(); err != nil {
return err
}
// Since the cached keys to be added and removed use an immutable treap,
// a snapshot is simply obtaining the root of the tree under the lock
// which is used to atomically swap the root.
c.cacheLock.RLock()
cachedKeys := c.cachedKeys
cachedRemove := c.cachedRemove
c.cacheLock.RUnlock()
// Nothing to do if there is no data to flush.
if cachedKeys.Len() == 0 && cachedRemove.Len() == 0 {
return nil
}
// Perform all leveldb updates using an atomic transaction.
if err := c.commitTreaps(cachedKeys, cachedRemove); err != nil {
return err
}
// Clear the cache since it has been flushed.
c.cacheLock.Lock()
c.cachedKeys = treap.NewImmutable()
c.cachedRemove = treap.NewImmutable()
c.cacheLock.Unlock()
return nil
}