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Implement banking ops: transfer, top-k, cashback, merge

CoinbaseSoftware Engineer · Online Assessment
MediumSoftware EngineerOnline AssessmentCoding & Algorithms

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This question evaluates a candidate's ability to design a robust in-memory banking system, covering transactional correctness, idempotency, concurrency control, delayed cashback scheduling, aggregate top-k queries, and data structure and complexity trade-offs within the Coding & Algorithms domain.

Design and implement an in-memory banking system that supports: (

  1. CreateAccount(id, initialBalance) and Transfer(fromId, toId, amount) with validation for nonnegative amounts, sufficient funds, account existence, idempotency via transaction IDs, and atomic balance updates; (
  2. a query to return the accounts with the top k total outgoing funds across all transfers, specifying output format, tie-breaking, and expected time/space complexity (e.g., maintain running aggregates for O(log n) updates and O(k log n) queries); (
  3. a Purchase(accountId, amount, txnId) that awards 2% cashback credited exactly 24 hours after the purchase, requiring a structure for pending rewards, a scheduler or sweep to post credits, and idempotent, crash-safe processing; (
  4. MergeAccount(primaryId, duplicateId) to combine balances and transaction histories, redirect future operations from duplicateId to primaryId, and reconcile pending cashback and duplicate transaction IDs. Discuss chosen data structures, concurrency assumptions (single-threaded vs. multi-threaded), and provide unit tests for edge cases (insufficient funds, duplicate transfers, simultaneous merges, and time-bound rewards).