Instacart·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026Remote

Summary

Instacart coding screen with a concurrency-flavored design problem. Not what I expected from a product company but the problem was actually pretty interesting once I stopped panicking.

Questions Asked (1)

Design a promotion system where an employee's title and salary can be updated, but if they're currently in the office, the promotion should be queued and applied the next time they leave and re-enter. If multiple promotions are queued, only the most recent one should take effect.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

I spent the first few minutes trying to figure out if this was a system design question or a coding question and kind of wasted that time.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and assumptions, then design a state machine that tracks employee location and pending promotions. Use a queue or stack to store promotions, ensuring only the latest is applied when the employee re-enters, and discuss trade-offs of different data structures and concurrency handling.

Pro tip: Emphasize idempotency and atomicity: promotions should be applied exactly once, and the system must handle concurrent updates safely. Mention using a version number or timestamp to resolve conflicts.

1. Clarify Requirements and Assumptions

Ask questions to understand the scope: Is the system distributed? What defines 'in the office'? How are promotions triggered? Confirm that only the latest queued promotion applies and that updates are atomic.

2. Define Data Model and State

Model Employee with fields: current title, salary, location status (in/out), and a pending promotion (or queue). Decide whether to store a single pending promotion (since only latest matters) or a queue with overwrite.

3. Design State Transitions

Define events: promotion request, enter office, leave office. On promotion request: if out of office, apply immediately; if in office, store as pending (overwriting any previous). On leave: clear pending? On re-enter: apply pending if exists.

4. Handle Concurrency and Consistency

Use locks or optimistic concurrency to ensure atomic updates. Consider distributed scenarios: use a centralized service or consensus protocol. Ensure promotions are applied exactly once and are idempotent.

5. Discuss Trade-offs and Extensions

Compare using a single pending promotion vs. a queue (memory vs. correctness). Discuss scalability, fault tolerance, and how to handle multiple promotions in quick succession. Mention monitoring and auditing.

Key Points to Mention

State machine design for employee location and promotion status
Using a single pending promotion (or queue with overwrite) to ensure only latest applies
Atomicity and idempotency of promotion application
Concurrency control (locks, optimistic concurrency, versioning)
Trade-offs between in-memory vs. persistent storage and distributed vs. single-node
Edge cases: multiple promotions while in office, promotion during transition, failure recovery

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