Implement a robust socket message reader

Q: Implement a robust socket message reader

This question evaluates understanding of socket programming, stream message framing, internal buffer management, EOF and error handling, input size limits, unit testing, and time/space complexity analysis.

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System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a medium difficulty System Design question, commonly asked during Technical Screen rounds at Apple.

Q: What role is this question designed for?

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

Question

Implement Message Framing Readers for TCP Sockets

Context

You are given a TCP socket. Each call to recv(k) may return any number of bytes up to k; in blocking mode, a return of 0 indicates EOF (peer closed). You must not assume a recv will return the full amount requested. Messages may arrive in pieces or back-to-back. Implement a reader that maintains an internal buffer across calls and extracts exactly one message per API call.

Use any mainstream language; Python 3 is acceptable for clarity and unit-testing.

Requirements

Build two variants backed by an internal buffer that persists across calls:

Length-prefixed messages
- The protocol is: a 4-byte big-endian unsigned integer N, followed by exactly N bytes of payload.
- API: readMessage() returns exactly one payload as bytes (or string, if you choose), blocking until complete or raising on EOF/error.
Newline-delimited messages
- Messages are delimited by '\n'. Treat an optional preceding '\r' (CRLF) as part of the delimiter and return the line without it.
- API: readLine() returns exactly one line (without the delimiter), blocking until complete or raising on EOF/error.

Both variants must:

Maintain an internal buffer across calls.
Correctly handle partial reads, back-to-back messages, and EOF.
Provide reasonable guardrails (e.g., a max message/line size to avoid unbounded memory growth) and clear error signaling when constraints are violated.

Also provide:

Unit tests covering partial reads, back-to-back messages, empty message/line, EOF during header/body, and size limit violations.
Time and space complexity analysis, including any amortization arguments.

Implement a robust socket message reader

Quick Overview

Implement a robust socket message reader

Implement Message Framing Readers for TCP Sockets

Context

Requirements

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