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Design distributed median and mode

Last updated: Apr 24, 2026

Quick Overview

This question evaluates skills in distributed systems and large-scale analytics, covering data partitioning, streaming and sliding-window processing, quantile and heavy-hitter summarization, fault tolerance, and trade-off analysis for time, space, and network I/O.

  • hard
  • Anthropic
  • System Design
  • Software Engineer

Design distributed median and mode

Company: Anthropic

Role: Software Engineer

Category: System Design

Difficulty: hard

Interview Round: Onsite

You have a dataset with hundreds of billions of records spread across many machines. Design how to compute the global median and the global mode. Specify the data-partitioning strategy, local sketches or summaries, how reducers aggregate partial results, and how you minimize network I/O. Compare exact vs approximate approaches (e.g., quantile sketches, heavy-hitter sketches), discuss skew handling, fault tolerance, and incremental/streaming updates. Provide time, space, and communication complexity, and extend the design to a sliding-window median/mode over an unbounded stream.

Quick Answer: This question evaluates skills in distributed systems and large-scale analytics, covering data partitioning, streaming and sliding-window processing, quantile and heavy-hitter summarization, fault tolerance, and trade-off analysis for time, space, and network I/O.

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Anthropic
Sep 6, 2025, 12:00 AM
Software Engineer
Onsite
System Design
102
0

Distributed System Design: Global Median and Global Mode at Massive Scale

Context

You are designing a distributed analytics system that must compute the global median and the global mode over a dataset with hundreds of billions of records stored across many machines. The system should support both batch and streaming (including sliding windows) and be efficient, fault-tolerant, and scalable.

Assume records each contain a single value x (numeric for median; categorical or integer for mode). You may make minimal, explicit assumptions as needed.

Tasks

  1. Specify data partitioning strategies (batch and streaming) to compute:
    • Global median
    • Global mode
  2. Define local sketches/summaries computed on each worker.
  3. Describe how reducers aggregate partial results.
  4. Minimize network I/O (explain how and why).
  5. Compare exact vs approximate approaches (quantile sketches, heavy-hitter sketches) with error/resource trade-offs.
  6. Discuss handling data skew and hot keys.
  7. Address fault tolerance and recovery.
  8. Support incremental/streaming updates.
  9. Provide time, space, and communication complexity.
  10. Extend the design to a sliding-window median and mode over an unbounded stream.

Solution

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