Design an asynchronous logger with formatting

Q: Design an asynchronous logger with formatting

This question evaluates expertise in concurrent programming, lock-free MPMC queue usage, producer–consumer asynchronous logging, variadic formatting APIs, and robustness in shutdown/flush semantics and error handling.

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Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Technical Screen rounds at Qube Research & Technologies.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Qube Research & Technologies during technical interviews.

Question

Design an Asynchronous C++ Logger Using a Lock-Free Queue

Context

You are asked to design and implement an asynchronous logger suitable for a technical screen in a systems/engineering interview. The logger must:

Accept variadic arguments using fmt-style format strings.
Use a background thread to perform formatting and writing.
Pass log events through a provided lock-free queue.
Not optimize for heap usage or memory footprint (allocations are acceptable).

Assume a generic lock-free MPMC queue is provided with a minimal try_enqueue/try_dequeue API. You should:

Specify the public API.
Define the log event structure.
Describe the producer–consumer flow.
Demonstrate shutdown/flush behavior.
Describe error handling and overflow policy.

Requirements

Public API that supports:
- Variadic, fmt-style logging (compile-time format checking preferred).
- Flush and shutdown semantics.
- Configurable sink (e.g., file or stderr) and error handling hooks.
Log events must be enqueued by producers and formatted/written by a background consumer thread.
Messages should be passed as events through the lock-free queue; formatting happens only on the consumer side.
Show robust shutdown/flush that drains the queue and avoids message loss.
Explain error handling (enqueue overflow, formatting errors, I/O failures).

Design an asynchronous logger with formatting

Quick Overview

Design an asynchronous logger with formatting

Design an Asynchronous C++ Logger Using a Lock-Free Queue

Context

Requirements

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