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PayPal·Machine Learning Engineer·Onsite - System Design / Architecture·Senior

Senior
Apr 2026

Summary

ML engineer system design round at PayPal, one question about building a recommendation system for Confluence pages. Pretty open-ended, which I wasn't fully prepared for.

Questions Asked (1)

Q1

Design a system that recommends the next Confluence pages a user should view.

System DesignTechnical Trade-offsProduct Sense & Ideation
Author's notes

I jumped straight into collaborative filtering and probably spent too long there before they nudged me toward cold start.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem scope and requirements, then outline a high-level system design covering data collection, feature engineering, model selection, and serving. Emphasize the trade-offs between different approaches and how you would evaluate and iterate on the system.

Pro tip: Focus on the unique aspects of Confluence data, such as hierarchical page structures and user collaboration patterns, and discuss how to handle cold-start and scalability in an enterprise setting.

1. Clarify Requirements

Ask questions to understand the goal: Is it to increase engagement, reduce search time, or surface relevant content? Determine scale, latency, and personalization needs.

2. Data and Features

Identify data sources: user interactions (views, edits, comments), page content (text, metadata), and organizational graph. Discuss feature engineering for user, page, and context.

3. Modeling Approach

Choose between collaborative filtering, content-based, or hybrid models. Consider deep learning for sequence modeling (e.g., RNNs, transformers) to capture user behavior over time.

4. System Architecture

Design offline training pipeline and online serving with low latency. Discuss candidate generation, ranking, and re-ranking, and how to handle updates and feedback loops.

5. Evaluation and Iteration

Define offline metrics (precision@k, recall, NDCG) and online metrics (CTR, dwell time). Plan A/B testing and monitoring for model drift.

Key Points to Mention

  • Cold-start problem for new users and pages
  • Scalability to millions of users and pages
  • Privacy and access control (permissions)
  • Real-time vs batch processing
  • Explainability and user trust
  • Feedback loops and continuous learning

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