// commitTx atomically adds all of the pending keys to add and remove into the
// database cache. When adding the pending keys would cause the size of the
// cache to exceed the max cache size, or the time since the last flush exceeds
// the configured flush interval, the cache will be flushed to the underlying
// persistent database.
//
// This is an atomic operation with respect to the cache in that either all of
// the pending keys to add and remove in the transaction will be applied or none
// of them will.
//
// The database cache itself might be flushed to the underlying persistent
// database even if the transaction fails to apply, but it will only be the
// state of the cache without the transaction applied.
//
// This function MUST be called during a database write transaction which in
// turn implies the database write lock will be held.
func (c *dbCache) commitTx(tx *transaction) error {
// Flush the cache and write directly to the database if a flush is
// needed.
if c.needsFlush(tx) {
if err := c.flush(); err != nil {
return err
}
// Perform all leveldb update operations using a batch for
// atomicity.
batch := new(leveldb.Batch)
tx.pendingKeys.ForEach(func(k, v []byte) bool {
batch.Put(k, v)
return true
})
tx.pendingKeys = nil
tx.pendingRemove.ForEach(func(k, v []byte) bool {
batch.Delete(k)
return true
})
tx.pendingRemove = nil
if err := c.ldb.Write(batch, nil); err != nil {
return convertErr("failed to commit transaction", err)
}
return nil
}
// At this point a database flush is not needed, so atomically commit
// the transaction to the cache.
// Create a slice of transaction log entries large enough to house all
// of the updates and add it to the list of logged transactions to
// replay on flush.
numEntries := tx.pendingKeys.Len() + tx.pendingRemove.Len()
txLogEntries := make([]txLogEntry, numEntries)
c.txLog = append(c.txLog, txLogEntries)
// 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()
newCachedKeys := c.cachedKeys
newCachedRemove := c.cachedRemove
c.cacheLock.RUnlock()
// Apply every key to add in the database transaction to the cache.
// Also create a transaction log entry for each one at the same time so
// the database transaction can be replayed during flush.
logEntryNum := 0
tx.pendingKeys.ForEach(func(k, v []byte) bool {
newCachedRemove = newCachedRemove.Delete(k)
newCachedKeys = newCachedKeys.Put(k, v)
logEntry := &txLogEntries[logEntryNum]
logEntry.entryType = entryTypeUpdate
logEntry.key = k
logEntry.value = v
logEntryNum++
return true
})
tx.pendingKeys = nil
// Apply every key to remove in the database transaction to the cache.
// Also create a transaction log entry for each one at the same time so
// the database transaction can be replayed during flush.
tx.pendingRemove.ForEach(func(k, v []byte) bool {
newCachedKeys = newCachedKeys.Delete(k)
newCachedRemove = newCachedRemove.Put(k, nil)
logEntry := &txLogEntries[logEntryNum]
logEntry.entryType = entryTypeRemove
logEntry.key = k
logEntryNum++
return true
})
tx.pendingRemove = nil
// Atomically replace the immutable treaps which hold the cached keys to
// add and delete.
c.cacheLock.Lock()
c.cachedKeys = newCachedKeys
c.cachedRemove = newCachedRemove
c.cacheLock.Unlock()
return nil
}
// 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
}