// 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
}
// Nothing to do if there are no transactions to flush.
if len(c.txLog) == 0 {
return nil
}
// Perform all leveldb updates using batches for atomicity.
batchLen := 0
batchTxns := 0
batch := new(leveldb.Batch)
for logTxNum, txLogEntries := range c.txLog {
// Replay the transaction from the log into the current batch.
for _, logEntry := range txLogEntries {
switch logEntry.entryType {
case entryTypeUpdate:
batch.Put(logEntry.key, logEntry.value)
case entryTypeRemove:
batch.Delete(logEntry.key)
}
}
batchTxns++
// Write and reset the current batch when the number of items in
// it exceeds the the batch threshold or this is the last
// transaction in the log.
batchLen += len(txLogEntries)
if batchLen > batchThreshold || logTxNum == len(c.txLog)-1 {
if err := c.ldb.Write(batch, nil); err != nil {
return convertErr("failed to write batch", err)
}
batch.Reset()
batchLen = 0
// Clear the transactions that were written from the
// log so the memory can be reclaimed.
for i := logTxNum - (batchTxns - 1); i <= logTxNum; i++ {
c.txLog[i] = nil
}
batchTxns = 0
}
}
c.txLog = c.txLog[:]
// Clear the cache since it has been flushed.
c.cacheLock.Lock()
c.cachedKeys = treap.NewImmutable()
c.cachedRemove = treap.NewImmutable()
c.cacheLock.Unlock()
return nil
}