Toy Payment Engine

A simple payment engine implementation in Rust that accepts an input transaction file (provided as an .csv) and outputs the final clients and their balance.

Assumptions:

• Only Deposits are disputable
• All TransactionIds are globally unique and not provided mulitple times (replays)
• A Deposit can only be disputed once. Any further operations are ignored
• A client has only one single asset account
• If a deposit references a non-existing client it will be created beforehand.
• Amount is less than u64::MAX / 10000
• Clients with a zero-balance are still required to be exported
• dispute, resolve, chargeback are passed as xxxx, 2, 8,

Focus

Don't Panic
As this simulates a financial (possibly high-performing) environment its important that all transactions are resulting in a predictable state and must not terminate an execution.

Atomicity
If any part of the transaction fails, no state will be updated.

Performance
Input files are processed as a stream so that the execution starts immediately.
This allows for a (future) feature that could resume a previously processed file (e.g. if the process is killed).

Input Parsing

While the input provides the amounts as a fixed point number the engine uses u64 internally where the input is multiplied by 10000.
This is to ensure we will never have any issues with decimal places (even though this would not be the case here with only additions and subtractions). Because of that, the amount is maxed at u64::MAX / 10000.

An alternative, parallel solution

The processing procedure can be parallelized because the balance is stored on the client object. So every transaction could be grouped by the client id into a separate queue that then can be processed in its own thread. Of course, sometimes it doesn't make sense to have a thread per clientId (as its a costly operation) so a threadpool might also be an interesting solution

Provided files

• input.csv an example CSV
• output.csv output generated for input.csv
• large_input.zip a zipped input csv with 10_000_000 rows

Logging

I've added the env_logger crate to output possible parsing or execution warnings.
They are disabled by default but can be enabled by passing the RUST_LOG env var.