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

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Jun 2026

Summary

Roku system design round for a software engineer position. The main problem was building a thread-safe LRU cache from scratch, which sounds manageable until they start pushing on concurrency trade-offs and you realize how much there is to actually say.

Questions Asked (1)

Design and implement a thread-safe LRU cache with get and put operations that both run in O(1) time and can be called concurrently from multiple threads.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

I went with a hashmap plus doubly linked list, slapped a global mutex around everything, and called it thread-safe.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then describe the standard LRU design using a hash map and doubly linked list to achieve O(1) operations. Address thread safety by discussing synchronization strategies, trade-offs between coarse and fine-grained locking, and potential optimizations like lock striping or read-write locks.

Pro tip: Mention that you would first implement a single-threaded version and then add thread safety, as this shows incremental development and helps isolate concurrency bugs. Also, discuss how you would test the concurrent implementation with stress tests and race condition detectors.

1. Clarify Requirements and Constraints

Ask about expected read/write ratio, cache size limits, eviction policy specifics, and performance goals to tailor the design.

2. Design Single-Threaded LRU

Explain the hash map + doubly linked list approach for O(1) get and put, and how to maintain recency order.

3. Add Thread Safety

Discuss synchronization options: coarse-grained lock, fine-grained locks per bucket, read-write locks, and lock-free approaches. Analyze trade-offs.

4. Optimize for Concurrency

Propose improvements like lock striping, using ConcurrentHashMap with a custom eviction policy, or employing a concurrent linked list.

5. Test and Validate

Outline testing strategies: unit tests, stress tests with multiple threads, and using tools like ThreadSanitizer to detect races.

Key Points to Mention

Hash map provides O(1) access to nodes; doubly linked list maintains recency order for O(1) eviction.
Thread safety can be achieved with a mutex, but this serializes access; consider finer-grained locking for better concurrency.
Lock striping or partitioning the cache can reduce contention, but complicates eviction and may affect LRU semantics.
Read-write locks can improve read-heavy workloads but may not help if writes are frequent.
ConcurrentHashMap alone is not sufficient for LRU because eviction order requires a linked structure.
Testing should include race condition detection and performance benchmarks under concurrent load.

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