Datadog·Software Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

Datadog SWE interview with a low-level systems design question about buffered I/O. The problem was more involved than it looked and had a lot of moving parts to keep track of.

Questions Asked (1)

Design and implement a BufferedWriter class that wraps a constrained device API. The API accepts at most M bytes per call and has high per-call overhead. Your class needs write(), flush(), and close() methods, must preserve byte order across calls, minimize total calls to the device, handle inputs larger than the internal buffer, and guarantee no data loss. Walk through your buffer management strategy, tricky edge cases between consecutive writes, time/space complexity, and optionally how you'd make it thread-safe.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

I liked this question more than I expected to.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then describe a buffer management strategy that accumulates writes and flushes in chunks of up to M bytes. Walk through edge cases like partial writes and buffer overflow, and analyze time/space complexity. Optionally discuss thread-safety with locks or thread-local buffers.

Pro tip: Emphasize that minimizing device calls is the primary goal, so always fill the buffer to M before flushing, and handle the final flush on close. Mention that you'd test with boundary conditions like exactly M bytes and M+1 bytes to ensure correctness.

1. Clarify Requirements and Constraints

Ask about the device API's behavior (e.g., does it guarantee full writes? What happens on error?), the expected write patterns, and whether thread-safety is required. Confirm that M is the maximum bytes per call and that per-call overhead is high.

2. Design Buffer Management Strategy

Propose an internal buffer (e.g., byte array) of size M. On write(), append data to the buffer; if the buffer fills, flush it to the device. For inputs larger than M, bypass the buffer and write directly in M-sized chunks to minimize copies.

3. Handle Edge Cases and Guarantees

Address partial writes from the device (retry remaining bytes), buffer overflow (flush before appending), and data loss (ensure flush on close). Discuss how to preserve byte order across calls by maintaining a single buffer and flushing in order.

4. Analyze Complexity and Trade-offs

Explain that time complexity is O(n) for n bytes written, with amortized O(1) per byte, and space complexity is O(M) for the buffer. Trade-offs include buffer size vs. memory usage and the cost of flushing vs. latency.

5. Discuss Thread-Safety (Optional)

If required, propose using a mutex to synchronize write(), flush(), and close(). Alternatively, use thread-local buffers if writes from different threads can be interleaved, but note that this may increase device calls.

Key Points to Mention

Buffer size should be exactly M to maximize device call efficiency.
For inputs larger than M, write directly in M-sized chunks to avoid unnecessary buffering.
Handle partial writes by looping until all bytes are written, and track the number of bytes successfully written.
Ensure flush() and close() are idempotent and that close() flushes any remaining data.
Time complexity is O(n) for n bytes, space complexity is O(M).
Thread-safety can be achieved with a mutex, but consider performance implications.

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