Meta·Software Engineer·Onsite - System Design / Architecture·Senior

Senior

May 2026

Summary

Meta SWE interview that was basically one big object-oriented design problem for an in-memory banking system. More moving parts than I expected for a single question, took up the whole session.

Questions Asked (1)

Design an in-memory banking system that supports account creation, deposits and withdrawals, immediate transfers, scheduled transfers with cancellation, querying top spenders, and merging accounts.

System DesignData ModelingTechnical Trade-offs

Author's notes

This is a lot of surface area for one question.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints (e.g., consistency, concurrency, scale) with the interviewer. Then design the core data model and operations, focusing on correctness and efficiency, and discuss trade-offs for advanced features like scheduled transfers and top spenders.

Pro tip: Emphasize thread-safety and idempotency for transfers, as these are critical in banking systems. Also, proactively discuss how you would handle failures and ensure data consistency.

1. Clarify Requirements

Ask questions to understand expected scale, consistency needs, and specific behaviors (e.g., what happens if a scheduled transfer fails due to insufficient funds?).

2. Design Core Data Model

Define Account and Transaction classes, and choose appropriate data structures (e.g., HashMap for accounts, priority queue for top spenders).

3. Implement Basic Operations

Detail account creation, deposit, withdrawal, and immediate transfer, ensuring atomicity and thread-safety (e.g., using locks or synchronized methods).

4. Handle Scheduled Transfers

Design a scheduler (e.g., using a priority queue or timer) to execute transfers at specified times, with support for cancellation.

5. Optimize Queries and Merging

Discuss efficient ways to query top spenders (e.g., maintaining a sorted structure) and merge accounts (e.g., transferring balances and updating references).

Key Points to Mention

Thread-safety and concurrency control (e.g., locks, atomic operations)
Data structures for efficient top spenders query (e.g., heap, balanced tree)
Scheduled transfer implementation (e.g., priority queue, background thread)
Cancellation mechanism for scheduled transfers (e.g., unique IDs, removal from queue)
Account merging logic (e.g., transfer balance, update transaction history)
Trade-offs between consistency and performance (e.g., locking granularity)

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.