// getInternal implements the actual logic of get function.
// The values of multiple versions for the given key should
// be organized as follows:
// ...
// keyA : keyMetatata of keyA
// keyA_Timestamp_n : value of version_n
// keyA_Timestamp_n-1 : value of version_n-1
// ...
// keyA_Timestamp_0 : value of version_0
// keyB : keyMetadata of keyB
// ...
func (mvcc *MVCC) getInternal(key Key, timestamp hlc.HLTimestamp, txnID string) ([]byte, hlc.HLTimestamp, string, error) {
keyMetadata := &keyMetadata{}
ok, err := GetI(mvcc.engine, key, keyMetadata)
if err != nil || !ok {
return nil, hlc.HLTimestamp{}, "", err
}
// If the read timestamp is greater than the latest one, we can just
// fetch the value without a scan.
if !timestamp.Less(keyMetadata.Timestamp) {
if len(keyMetadata.TxnID) > 0 && (len(txnID) == 0 || keyMetadata.TxnID != txnID) {
return nil, hlc.HLTimestamp{}, "", &writeIntentError{TxnID: keyMetadata.TxnID}
}
latestKey := mvccEncodeKey(key, keyMetadata.Timestamp)
val, err := mvcc.engine.Get(latestKey)
return val, keyMetadata.Timestamp, keyMetadata.TxnID, err
}
nextKey := mvccEncodeKey(key, timestamp)
// We use the PrefixEndKey(key) as the upper bound for scan.
// If there is no other version after nextKey, it won't return
// the value of the next key.
kvs, err := mvcc.engine.Scan(nextKey, PrefixEndKey(key), 1)
if len(kvs) > 0 {
_, ts := mvccDecodeKey(kvs[0].Key)
return kvs[0].Value, ts, "", err
}
return nil, hlc.HLTimestamp{}, "", err
}
func (mvcc *MVCC) putInternal(key Key, timestamp hlc.HLTimestamp, value []byte, txnID string) error {
keyMeta := &keyMetadata{}
ok, err := GetI(mvcc.engine, key, keyMeta)
if err != nil {
return err
}
// In case the key metadata exists.
if ok {
// There is an uncommitted write intent and the current Put
// operation does not come from the same transaction.
// This should not happen since range should check the existing
// write intent before executing any Put action at MVCC level.
if len(keyMeta.TxnID) > 0 && (len(txnID) == 0 || keyMeta.TxnID != txnID) {
return &writeIntentError{TxnID: keyMeta.TxnID}
}
if keyMeta.Timestamp.Less(timestamp) ||
(timestamp.Equal(keyMeta.Timestamp) && txnID == keyMeta.TxnID) {
// Update key metadata.
PutI(mvcc.engine, key, &keyMetadata{TxnID: txnID, Timestamp: timestamp})
} else {
// In case we receive a Put request to update an old version,
// it must be an error since raft should handle any client
// retry from timeout.
return &writeTimestampTooOldError{Timestamp: keyMeta.Timestamp}
}
} else { // In case the key metadata does not exist yet.
// Create key metadata.
PutI(mvcc.engine, key, &keyMetadata{TxnID: txnID, Timestamp: timestamp})
}
// Save the value with the given version (Key + Timestamp).
return mvcc.engine.Put(mvccEncodeKey(key, timestamp), value)
}