Payment Engine

For an in-depth explanation, please read the code, associated comments, and tests.

Invariants

• Up to 4 digits after amount decimal point.
• Amount * 10000.0 <= i64::MAX (for normalization of f64 <=> i64).
• CSV transaction order = logical order.

Assumptions

• Disputes, resolves, and chargebacks only apply to deposits/withdrawals.
  • Withdrawals do the inverse of deposits; not sure if it makes sense to lock the account on chargeback (e.g., the user reverses a withdrawal by giving money back to the system...)

Correctness

• Amount invariants are enforced via the Amount type when a CSV row is parsed.
  • Unit tests for both the amount parsing and serde parsing
• Integration tests for some transaction scenarios
  • Not comprehensive, but I hope you get a good idea of the things I think about based on the included tests

Implementations

• SerialPaymentEngine

  • Default implementation used.
  • Processes the transactions serially and syncronously as they're parsed.

• StreamPaymentEngine

  • Naively spawns 1 worker thread per user and sends transactions to the worker thread via a channel/queue. Obviously, this won't scale, but in a real system we'd use a distributed queue + worker nodes anyway, so I didn't bother fixing this.