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DoorDash·Software Engineer·Onsite - System Design / Architecture·Senior

Senior
Jul 2026

Summary

DoorDash system design round focused entirely on building a real-time driver assignment system from scratch. Pretty intense scope for a single session, covering everything from objectives and constraints to algorithms and failure modes.

Questions Asked (1)

Q1

Design a real-time system that assigns delivery drivers to incoming orders. Walk through your objectives, constraints, data structures, algorithm choices, and how you'd handle scale and partial information.

System DesignAlgorithms & Data StructuresTechnical Trade-offs
Author's notes

This one sprawled fast.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and defining objectives (e.g., minimize delivery time, balance driver load) and constraints (real-time, partial information, scale). Then propose a high-level architecture with data structures (e.g., geospatial index, priority queue) and algorithms (e.g., greedy matching with rebalancing), and discuss trade-offs and scaling strategies.

Pro tip: Emphasize that perfect assignment is impossible with partial information; instead, focus on a robust, incremental approach that handles uncertainty and adapts as new data arrives, which mirrors real-world systems.

1. Clarify Requirements and Objectives

Ask questions to understand scale (orders/drivers per second), latency requirements, and business goals (e.g., minimize delivery time, maximize driver utilization). Define clear objectives and constraints.

2. Design Data Structures

Propose data structures to efficiently store and query driver locations (e.g., geohash, quadtree) and orders (e.g., priority queue by time). Consider real-time updates and spatial indexing.

3. Choose Matching Algorithm

Describe a matching algorithm (e.g., greedy nearest driver, Hungarian algorithm for batch) and justify choice based on trade-offs (optimality vs. latency). Discuss handling partial information (e.g., driver availability, traffic).

4. Address Scale and Reliability

Explain how to scale horizontally (sharding by region, using message queues) and ensure fault tolerance (replication, fallbacks). Discuss monitoring and dynamic rebalancing.

5. Discuss Trade-offs and Extensions

Summarize key trade-offs (e.g., optimality vs. speed, centralization vs. decentralization) and potential improvements (e.g., machine learning for demand prediction).

Key Points to Mention

  • Use of geospatial indexing (e.g., geohash, quadtree) for efficient nearest-driver queries.
  • Matching algorithms: greedy vs. batch (e.g., Hungarian) and their trade-offs in latency and optimality.
  • Handling partial information: incorporating probabilistic estimates, driver acceptance likelihood, and real-time traffic.
  • Scalability: sharding by geographic region, using distributed queues (e.g., Kafka), and load balancing.
  • Fault tolerance: replication, graceful degradation, and fallback strategies.
  • Real-time constraints: low-latency decision making, possibly using in-memory data stores and stream processing.

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