Citadel·Software Engineer·Onsite - System Design / Architecture·Intermediate

Intermediate

Jun 2026

Summary

Citadel system design question focused on shared state across processes on a single machine. Short but surprisingly tricky if you haven't thought about IPC much.

Questions Asked (1)

You have a counter running on a single host, and multiple separate processes need to read and update it. How do you design this so it works correctly across process boundaries?

System DesignTechnical Trade-offs

Author's notes

My first instinct was shared memory with a mutex and I said it pretty quickly, but then they pushed on what happens when a process crashes mid-write.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements: what consistency guarantees are needed, expected throughput, and whether the counter is a simple integer or more complex. Then propose a design using inter-process communication (IPC) mechanisms like shared memory with atomic operations or file locks, or a centralized server process. Discuss trade-offs between performance, complexity, and correctness.

Pro tip: Mention that while shared memory with atomics is fast, it requires careful handling of synchronization and is limited to a single host; for scalability across hosts, a distributed approach like a centralized service or consensus protocol is needed. Also, consider using OS-level primitives like file locks for simplicity if performance is not critical.

1. Clarify Requirements

Ask about consistency requirements (e.g., linearizability, eventual consistency), expected read/write ratio, and performance needs. Determine if the counter is a simple integer or requires more complex operations.

2. Evaluate IPC Options

Consider mechanisms like shared memory with atomic operations, file locks, message queues, or a centralized server process. Discuss the trade-offs of each in terms of performance, complexity, and fault tolerance.

3. Design the Solution

Propose a concrete design. For example, use shared memory with atomic compare-and-swap for high performance, or a server process that serializes updates via a mutex. Explain how processes will coordinate.

4. Address Correctness and Edge Cases

Discuss how to handle process crashes, partial updates, and synchronization issues. Mention the need for atomicity and durability if required.

5. Discuss Trade-offs and Alternatives

Compare your chosen approach with alternatives, highlighting pros and cons. Mention scalability limitations and potential extensions for distributed systems.

Key Points to Mention

Shared memory with atomic operations (e.g., compare-and-swap) for lock-free updates.
File locking (e.g., flock) for simplicity, but potential performance bottlenecks.
Centralized server process using sockets or message queues to serialize updates.
Memory-mapped files for persistence and inter-process communication.
Trade-offs: performance vs. complexity, single-host vs. distributed, consistency guarantees.
Handling process crashes and ensuring atomicity (e.g., using robust mutexes or transactions).

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