Make Rider Addition and Dispatch Safe Under Concurrency
DoorDashSoftware Engineer · Onsite
Last updated
Make concurrent rider addition and dispatch safe with atomic availability-to-assignment transitions, idempotent adds, narrow locks, and explicit distributed-state limits.
A rider-assignment system supports adding available riders and dispatching a rider to work. Identify and fix the concurrency problems that can occur when these operations run simultaneously.
Constraints & Assumptions
The original implementation is not supplied. For a concrete practice model, dispatch any available rider, never assign the same availability entry twice, and make duplicate add calls idempotent while a rider is already available or assigned. Riders are not re-added after assignment in this limited model; completion/reavailability would need an additional lifecycle operation.
Clarifying Questions
What is the rider identity and lifecycle? Is dispatch selection ordered or arbitrary? Can two workers dispatch concurrently? Which state changes must be atomic? Does assignment trigger an external operation?
Follow-up questions
- Why is checking that a queue is nonempty before locking unsafe? What happens if a rider is removed from the queue but assignment persistence fails? How would the design change across several service instances?
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