Identify and handle race conditions

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Quick Overview

This question evaluates understanding of concurrency, race conditions, synchronization mechanisms, and performance trade-offs in designing thread-safe in-memory data structures like key-value stores and hit counters.

Identify and handle race conditions

Company: Databricks

Role: Software Engineer

Category: System Design

Difficulty: hard

Interview Round: Technical Screen

Identify the race conditions that could occur in your key-value store and hit counter under concurrent access. Propose thread-safety mechanisms (e.g., coarse- vs fine-grained locks, read/write locks, atomic operations, lock-free designs) and discuss their correctness and performance trade-offs. Outline how you would test and validate concurrency behavior.

Overview: This question evaluates understanding of concurrency, race conditions, synchronization mechanisms, and performance trade-offs in designing thread-safe in-memory data structures like key-value stores and hit counters.

Read the full Databricks Software Engineer interview experience this question came from

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Databricks
Sep 6, 2025
hardSoftware EngineerTechnical ScreenSystem Design
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Concurrency in an In-Memory Key-Value Store and Hit Counter

Context and Assumptions

Assume you are implementing an in-memory key-value store and a hit counter within a single process, shared by multiple threads:

  • KeyValueStore supports: get(k), put(k, v), delete(k), and optionally compareAndSet(k, expected, new).
  • HitCounter supports: increment([key]), getCount([key]) — either a single global counter or per-key counters.
  • Data structure backing the store is a hash map with buckets and possible resizing.
  • High read-to-write ratio is common, but hot keys and heavy write bursts can occur.

Tasks

  1. Identify the race conditions that can occur under concurrent access.
  2. Propose thread-safety mechanisms (coarse-/fine-grained locks, read/write locks, atomic operations, lock-free designs) and explain their correctness.
  3. Discuss performance trade-offs of the chosen mechanisms.
  4. Outline how to test and validate concurrency behavior.

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