in cloud-spanner-r2dbc/src/main/java/com/google/cloud/spanner/r2dbc/v2/DatabaseClientTransactionManager.java [79:86]
TransactionContextFuture beginTransaction() {
if (this.isInTransaction()) {
throw new TransactionInProgressException(this.isInReadWriteTransaction());
}
this.transactionManager = this.dbClient.transactionManagerAsync();
this.txnContextFuture = this.transactionManager.beginAsync();
return this.txnContextFuture;
}