Implement held transfers with accept/cancel

Q: Implement held transfers with accept/cancel

This question evaluates system-design and concurrent-systems competencies, including two-phase fund transfer semantics, holds and TTLs, idempotency, atomicity, error handling, efficient hash-map data structures, and time/space complexity reasoning, and it belongs to the System Design domain.

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Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Take-home Project rounds at Meta.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Meta during technical interviews.

Question

Two‑Phase Funds Transfer With Holds (Design + Implementation)

You are building an in‑memory wallet/ledger component that supports two‑phase transfers using holds. Assume the caller is authenticated and acts on behalf of the source account for transfer(). The system runs in a single process; persistence/durability is out of scope.

API to Add

transfer(timestamp, targetAccountId, amount) -> String
- Places a hold on the source account’s funds and returns a transferId.
accept(timestamp, accountId, transferId) -> boolean
- Finalizes the transfer to the target account; should be callable only by the target account (accountId must match the transfer’s target).
cancel(transferId) -> boolean
- Cancels the transfer, releasing the hold and restoring availability to the source.

Requirements

Define clear semantics for:
- Available vs. ledger balance.
- Duplicate requests and idempotency (for transfer, accept, cancel).
- Expired holds and how to handle them.
- Error handling for invalid transferIds and wrong accountId in accept().
- Atomicity across updates (accounts, holds, transfer status) with concurrency safety.
Data structures: Use hash maps to track all transfers and pending holds; avoid list scans.
Provide time and space complexity for each operation.

Notes/Assumptions to Clarify

Source account for transfer() is the authenticated caller; you may reference it as sourceAccountId.
Choose a reasonable hold TTL and define how timestamp is used (e.g., for auditing vs. TTL).
Amounts are positive, integer cents (or smallest currency unit).

Implement held transfers with accept/cancel

Quick Overview

Two‑Phase Funds Transfer With Holds (Design + Implementation)

API to Add

Requirements

Notes/Assumptions to Clarify

Solution

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