curiositech/ml-system-design-interview

ML System Design Interview

End-to-end ML pipeline design coaching for staff+ engineers. Covers the full arc from problem definition through production monitoring -- the scope expected at L6+ interviews at top-tier ML organizations.

Decision Points

Serving Architecture Selection

Query: What inference pattern should we use?

1. Input Predictability?
   ├─ Finite/enumerable → BATCH PREDICTION
   │  └─ Example: recommend videos for all users nightly
   └─ Infinite/dynamic → ONLINE INFERENCE
      └─ Go to #2

2. Latency Requirement?
   ├─ <100ms → ONLINE INFERENCE (cached features)
   │  └─ Example: search ranking, fraud detection
   ├─ 100ms-1s → NEAR-REAL-TIME
   │  └─ Example: feed ranking with fresh user signals
   └─ >1s → STREAMING COMPUTATION
       └─ Example: complex multi-modal analysis

3. Feature Freshness?
   ├─ Static features only → Precompute embeddings
   ├─ Daily updates acceptable → Feature store + cache
   └─ Real-time required → Online feature computation

Problem Scoping Strategy

Given a vague prompt like "design a recommendation system":

1. Business Objective Clarification
   ├─ Revenue optimization? → Focus on purchase conversion metrics
   ├─ Engagement optimization? → Focus on time-spent, retention metrics  
   └─ Exploration/discovery? → Focus on diversity, serendipity metrics

2. Scale Assessment
   ├─ <1M users → Simple collaborative filtering acceptable
   ├─ 1-100M users → Need distributed training, caching layer
   └─ >100M users → Multi-stage retrieval mandatory (retrieval→ranking)

3. Cold Start Criticality
   ├─ High new user/item churn → Content-based features essential
   ├─ Stable catalog → Collaborative filtering sufficient
   └─ Mixed → Hybrid approach with exploration bonus

Model Complexity Progression

Start simple, justify complexity upgrades:

1. Baseline Decision
   ├─ Tabular features → XGBoost/LightGBM
   ├─ Text/sequences → BERT/RoBERTa fine-tuned
   └─ Multi-modal → Pre-trained encoders + MLP

2. Production Upgrade Triggers
   ├─ Baseline hits accuracy ceiling → Deep architecture
   ├─ Cold start problems → Content-based + collaborative hybrid
   ├─ Real-time features critical → Online learning/streaming updates
   └─ Multi-objective conflicts → Multi-task architecture

3. Scaling Triggers  
   ├─ Inference latency >SLA → Model distillation/quantization
   ├─ Training time >acceptable → Distributed training
   └─ Serving cost >budget → Cheaper architecture or caching

Failure Modes

Cold Start Death Spiral

Symptoms: New users see irrelevant recommendations, bounce immediately, never generate engagement signals for learning. Root Cause: System relies entirely on collaborative filtering without content-based fallback. Detection Rule: If new user retention <50% of established user retention, you have cold start problems. Fix: Implement content-based features (item metadata, user demographics), exploration bonus for new users, and onboarding questionnaire to bootstrap preferences.

Batch-Online Training Skew

Symptoms: Model performs well offline but poorly in production. Offline metrics improve but online metrics stagnate. Root Cause: Training data differs from serving data (features computed differently, timing gaps, selection bias). Detection Rule: If offline metric improves >5% but online metric shows <1% gain, suspect training-serving skew. Fix: Log exact serving features for training, implement feature validation pipeline, use identical preprocessing code paths.

Metric Misalignment Trap

Symptoms: Optimizing for clicks but business wants purchases. Model achieves high CTR but low conversion. Root Cause: Proxy metric (clicks) diverges from business metric (revenue) due to clickbait or low-intent traffic. Detection Rule: If primary metric improves but business KPI degrades or stagnates, your metrics are misaligned. Fix: Multi-objective optimization with business metric as constraint, or switch to business metric despite noisiness.

Serving Latency Creep

Symptoms: Model performance is good but p99 latency gradually increases over time, eventually hitting SLA violations. Root Cause: Feature computation becomes more expensive, model complexity grows, cache hit rates degrade. Detection Rule: If p99 latency increases >20% over 3 months without architecture changes, you have latency creep. Fix: Implement latency budgeting, feature computation timeout, model complexity governance, and cache performance monitoring.

Data Drift Blindness

Symptoms: Model performance silently degrades over weeks/months. No alerts fire until business impact is severe. Root Cause: Input data distribution shifts but model continues making confident predictions on out-of-distribution examples. Detection Rule: If model confidence remains high (>0.8) but accuracy drops >10%, suspect distribution shift. Fix: Population Stability Index monitoring, feature distribution alerts, model uncertainty calibration, and automatic retraining triggers.

Worked Examples

Example 1: E-commerce Fraud Detection System

Context: Design real-time fraud detection for payment processing. 10M transactions/day, must decide approve/deny in <100ms, false positive cost $50/transaction, fraud loss $500/transaction on average.

Requirements Analysis:

• Latency SLA: 100ms (hard constraint - payment flow blocks)
• Scale: 10M/day = 116 QPS, 1000 QPS peak
• Cost structure: 10:1 ratio favors precision over recall (better to block legitimate than approve fraud)

Serving Architecture Decision:

Option 1: Online ML model only
├─ Latency: 50-80ms (acceptable)  
├─ Accuracy: High (full feature set)
└─ Risk: Model failure = no fraud protection

Option 2: Rules + ML model
├─ Latency: 30ms rules + 50ms ML = 80ms  
├─ Accuracy: Rules catch obvious fraud, ML handles edge cases
└─ Risk: Rule maintenance overhead

DECISION: Option 2 - Rules as first line, ML for nuanced cases
REASONING: 80ms meets SLA, rules provide failure safety, ML adds precision

Feature Strategy:

• Real-time: Transaction amount, merchant, device fingerprint, IP geo (cached)
• Historical: User velocity features (transactions/hour last 4 hours)
• Engineered: Amount deviation from user's typical spending, merchant category risk score
• Trade-off: Skip computationally expensive graph features (user-merchant network) due to latency constraint

Model Selection:

• Baseline: Rule-based thresholds (amount >$1000, international merchant, new device)
• Production: XGBoost ensemble (fast inference, handles feature interactions, interpretable for compliance)
• Rejected: Deep neural networks (unnecessary complexity, harder to debug, longer inference time)

Final Architecture: Rules Engine (30ms) → If uncertain → XGBoost Model (50ms) → Decision Total latency: 80ms (within 100ms SLA)

Example 2: Document RAG System

Context: Build RAG system for company knowledge base. 50k documents, 1000 employees, must handle complex multi-hop questions about policies, procedures, and project history.

Requirements Analysis:

• Users: Employees asking policy/procedure questions
• Scale: 1000 users, ~10 queries/user/day = 10k queries/day
• Latency: <3 seconds acceptable for knowledge work
• Accuracy: High precision required (wrong policy advice costly)

Retrieval Strategy Decision:

Question: How to find relevant documents for complex queries?

Option 1: Keyword search only (BM25)
├─ Pros: Fast, interpretable, good for exact matches
├─ Cons: Misses semantic similarity, poor on multi-hop
└─ Verdict: Insufficient for complex queries

Option 2: Embedding search only  
├─ Pros: Semantic understanding, handles paraphrasing
├─ Cons: May miss exact keyword matches, harder to debug
└─ Verdict: Good but incomplete

Option 3: Hybrid search (BM25 + embeddings)
├─ Pros: Gets both exact matches AND semantic similarity
├─ Cons: More complex, need score fusion strategy
└─ CHOSEN: Best of both worlds

Fusion Strategy: RRF (Reciprocal Rank Fusion)
Score = 1/(rank_bm25 + k) + 1/(rank_embedding + k) where k=60

Chunking Strategy:

• Challenge: Documents vary from 1-page forms to 100-page manuals
• Strategy: Hierarchical chunking
  • Level 1: Sections (preserve context boundaries)
  • Level 2: Paragraphs (if section >1000 tokens)
  • Overlap: 100 tokens between chunks
• Reasoning: Preserve logical document structure while staying within embedding model limits

Reranking Decision:

After retrieval, should we rerank results?

Raw retrieval quality assessment:
├─ Top-5 contains answer: 75% of time
├─ Top-1 contains answer: 40% of time  
└─ Conclusion: Retrieval recall good, precision needs work

Reranking options:
├─ Cross-encoder (BERT-based): +15% top-1 precision, +200ms latency
├─ LLM-based: +25% top-1 precision, +800ms latency
└─ CHOSEN: Cross-encoder (better precision/latency trade-off)

Final pipeline: Hybrid retrieval → Cross-encoder reranking → LLM generation

Example 3: Real-time Recommendation System

Context: Design homepage recommendations for social media app. 100M monthly users, infinite scroll feed, optimize for time spent and user satisfaction.

Cold Start Strategy:

• New users: No historical interactions
• Solution: Content-based onboarding
  1. Interest survey during signup (5 topics)
  2. Seed recommendations from popular content in chosen topics
  3. Switch to collaborative filtering after 10 interactions
  4. Exploration bonus: 20% of recommendations are random (decreases to 5% over time)

Multi-Stage Architecture Decision:

Scale constraint: Cannot run complex model on all user-item pairs

Stage 1: Candidate Retrieval  
├─ Input: User profile
├─ Output: 10k candidate items (from 100M+ total)
├─ Methods: Collaborative filtering, content similarity, trending
├─ Latency budget: 50ms

Stage 2: Ranking
├─ Input: 10k candidates  
├─ Output: Ranked list of 100 items
├─ Model: Deep neural network with rich features
├─ Latency budget: 100ms

Stage 3: Diversification
├─ Input: Ranked 100 items
├─ Output: Final 20 items for feed
├─ Logic: MMR algorithm (maximal marginal relevance)
├─ Latency budget: 20ms

Total: 170ms (within 200ms SLA)

Quality Gates

• [ ] Requirements defined: Business metric, user scale, latency SLA, and constraints are explicitly stated
• [ ] Metrics aligned: Offline metrics connect to online metrics with explanation of gaps/alignment issues
• [ ] Data pipeline complete: Sources, labeling strategy, ETL, quality checks, and freshness requirements covered
• [ ] Feature architecture specified: Online/offline split, feature store design, freshness requirements, and computation costs
• [ ] Baseline established: Simple model proposed first with performance expectations before complex architecture
• [ ] Serving pattern justified: Batch vs online vs streaming decision made with latency/cost/freshness trade-offs
• [ ] Monitoring strategy: Data drift detection, model performance alerts, A/B testing, and rollback plan specified
• [ ] Failure modes addressed: At least 3 potential failure points identified with detection and mitigation strategies
• [ ] Organizational constraints: Team size, on-call burden, compliance, migration path, and build-vs-buy decisions discussed
• [ ] End-to-end latency calculated: All components have latency estimates that sum to meet SLA

NOT-FOR Boundaries

This skill should NOT be used for:

• Coding interviews: For algorithm/data structure problems, use senior-coding-interview instead
• ML theory questions: For mathematical derivations, optimization theory, or paper explanations, delegate to domain expert
• Behavioral interviews: For leadership scenarios, conflict resolution, or career progression questions, use interview-loop-strategist
• Implementation details: For actual code writing, debugging, or framework-specific syntax, this is whiteboard design only
• Research paper review: For novel architecture analysis or cutting-edge technique evaluation, focus on production-proven approaches
• Data science exploratory work: For hypothesis generation, statistical analysis, or experimental design, this assumes production deployment context

Where to delegate:

• Deep technical coding → senior-coding-interview
• Leadership and team dynamics → interview-loop-strategist
• Company-specific technical deep dives → anthropic-technical-deep-dive
• General interview strategy and loop navigation → interview-simulator