Connect mongodb://localhost:27017/?w=majority&retryWrites=false.

Flush clears all records of collection and return number of deleted records.

REPLACE.

Tx -> transaction.

TxOptimal will do the transaction with majority write-concern snapshot read-concern, primary read preference This should be used when transaction does not contain any read.

TransactionConfig, you can just define the func, the transaction options are not really need with simple application.