Method BeginReadOnlyTransaction

session.go:180–216

BeginReadOnlyTransaction starts read-only transaction and returns the snapshot timestamp for the transaction if successful.

(ctx context.Context, typ timestampBoundType, staleness time.Duration, timestamp time.Time, priority pb.RequestOptions_Priority, tag string)

// BeginReadOnlyTransaction starts read-only transaction and returns the snapshot timestamp for the transaction if successful.
func (s *Session) BeginReadOnlyTransaction(ctx context.Context, typ timestampBoundType, staleness time.Duration, timestamp time.Time, priority pb.RequestOptions_Priority, tag string) (time.Time, error) {
	if s.InReadOnlyTransaction() {
		return time.Time{}, errors.New("read-only transaction is already running")
	}

	txn := s.client.ReadOnlyTransaction()
	switch typ {
	case strong:
		txn = txn.WithTimestampBound(spanner.StrongRead())
	case exactStaleness:
		txn = txn.WithTimestampBound(spanner.ExactStaleness(staleness))
	case readTimestamp:
		txn = txn.WithTimestampBound(spanner.ReadTimestamp(timestamp))
	}

	// Use session's priority if transaction priority is not set.
	if priority == pb.RequestOptions_PRIORITY_UNSPECIFIED {
		priority = s.defaultPriority
	}

	// Because google-cloud-go/spanner defers calling BeginTransaction RPC until an actual query is run,
	// we explicitly run a "SELECT 1" query so that we can determine the timestamp of read-only transaction.
	opts := spanner.QueryOptions{Priority: priority}
	if err := txn.QueryWithOptions(ctx, spanner.NewStatement("SELECT 1"), opts).Do(func(r *spanner.Row) error {
		return nil
	}); err != nil {
		return time.Time{}, err
	}

	s.tc = &transactionContext{
		tag:      tag,
		priority: priority,
		roTxn:    txn,
	}

	return txn.Timestamp()
}

// CloseReadOnlyTransaction closes a running read-only transaction.
func (s *Session) CloseReadOnlyTransaction() error {