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

Senior
May 2026

Summary

System design round at Grammarly for a software engineer role. The problem was a read-once messaging system where one sender fans out to multiple recipients, each getting their own isolated copy that disappears after being viewed. Pretty involved question with a lot of moving parts.

Questions Asked (1)

Q1

Design a messaging system where a sender can address the same message to multiple recipients at once, but each recipient gets an independent copy that is permanently deleted or made inaccessible after they read it once. How would you handle the APIs, data model, fan-out, read-once enforcement, deletion, scaling, and failure recovery?

System DesignData ModelingTechnical Trade-offs
Author's notes

This one is deceptively wide.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then propose a high-level design that separates message metadata from per-recipient copies, using unique tokens for access and atomic operations for read-once enforcement. Discuss trade-offs between consistency, availability, and scalability, and outline failure recovery strategies.

Pro tip: Emphasize idempotency and atomicity in read-once enforcement to prevent race conditions, and consider using a distributed lock or conditional write with a TTL for deletion. Also, mention that you would monitor deletion lag and have a fallback mechanism to ensure data is not accessible after read.

1. Clarify Requirements and Constraints

Ask about scale (number of users, messages per second), latency requirements, consistency needs, and security/compliance constraints. Confirm that 'read once' means the message is inaccessible after the first read, and deletion should be permanent.

2. Design APIs and Data Model

Define APIs for sending a message to multiple recipients, retrieving a message, and possibly acknowledging read. Design a data model with a message table (metadata) and a recipient table (per-recipient copy with status, unique access token, and TTL).

3. Plan Fan-out and Read-Once Enforcement

On send, fan out by creating individual recipient records asynchronously. For read-once, use an atomic operation (e.g., conditional update or distributed lock) to mark the message as read and return the content, ensuring only one reader succeeds.

4. Handle Deletion and Scaling

After read, delete the message content immediately or mark for deletion with a background job. Scale by sharding recipient data, using a distributed store like Cassandra or DynamoDB, and employing a queue for fan-out.

5. Address Failure Recovery and Trade-offs

Discuss handling failures: if deletion fails, retry with exponential backoff; if read succeeds but deletion fails, ensure the message is not accessible (e.g., by marking as read). Trade-offs: strong consistency vs. availability, synchronous vs. asynchronous fan-out.

Key Points to Mention

  • Use of unique per-recipient tokens to prevent unauthorized access and enable independent read-once semantics.
  • Atomic read-and-delete operation using conditional writes or transactions to avoid race conditions.
  • Asynchronous fan-out with a message queue to handle high throughput and decouple send from delivery.
  • Data model separation: message metadata (shared) vs. recipient copies (individual) to optimize storage and access.
  • Deletion strategies: immediate deletion vs. soft delete with TTL, and background cleanup for reliability.
  • Scaling considerations: sharding by recipient ID, using NoSQL for scalability, and caching for hot messages.
  • Failure recovery: idempotent operations, retries, and monitoring for deletion lag.

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