A payments engine which holds account and transaction ledgers and execute operations like deposits, withdrawals, disputes for previous deposits, resolving and chargebacks for disputes.

Transactions are read/processed from a CSV input as a stream through an asynchronous multi-threaded runtime, by using tokio, csv-async and serde.

All operations over numbers with decimals are done by using BigDecimal struct from bigdecimal crate. We shouldn't be concerned with overflow because of this, but underflow can be possible, so we defend against it by emitting an error and gracefully handling it so that we don't error out and stop the stream processing because of an invalid transaction.

The code base relies on a few abstractions:

• AccountsDal - a data access layer which provides an interface over all clients' accounts by not being concerned with the underlying storage solution.
• TxsDal - a data access layer similar to the AccountsDal but for transactions storage.
• The Engine::handle_txs method which processes TXs by consuming them from a stream received as a parameter, that an implementation for AsyncRead + Send + Unpin. This is the basis for consuming TXs from both a tokio::io::File and tokio::io::TcpStream, so the business logic should be usable inside an asynchronous multi-threaded web server.

The payments engine main logic is tested through unit tests for every transaction and the majority of corner cases worth testing.