Bài 4: Feature Engineering & Vertex AI Feature Store

1. Feature Engineering Techniques

Technique	When to Use	Example
Normalization (Min-Max)	Bounded range required (0-1)	Image pixels, probabilities
Standardization (Z-score)	Normal-ish distribution, no bounds	Customer age, transaction amount
Log Transform	Skewed distributions (price, salary)	Log(price) for housing
One-Hot Encoding	Nominal categorical (no order)	Country, brand, color
Label Encoding	Ordinal categorical (has order)	Low/Medium/High → 0/1/2
Feature Crossing	Capture interaction between features	city × day_of_week
Bucketizing	Convert continuous to categorical	Age → age_group
Embeddings	High-cardinality categorical	UserID, ProductID

2. Handling Missing Values

Strategy	When
Mean/Median imputation	Numerical, low missingness rate
Mode imputation	Categorical features
Model-based imputation	High missingness, complex patterns
Indicator variable	Missingness itself is informative (add is_missing flag)
Drop rows	Missing target / very few rows affected
Drop column	>80% missing

3. Training-Serving Skew

Training-serving skew là vấn đề nghiêm trọng: features được compute khác nhau giữa training và serving, khiến model hoạt động kém trong production dù test metrics tốt.

Training-Serving Skew Example:

TRAINING TIME:
  avg_purchase_last_30d = mean(all purchases in batch)  ← computed over full period

SERVING TIME:
  avg_purchase_last_30d = mean(last 5 purchases)        ← computed differently!

Result: Feature distribution mismatch → poor predictions

SOLUTION: Vertex AI Feature Store
  Same feature serve logic used at training AND serving time

4. Vertex AI Feature Store

Component	Description
Feature Store	Centralized repository for ML features
Entity Type	Category of things you track (User, Product)
Feature	Named attribute of an entity (user.avg_spend)
Online Store	Low-latency serving (ms) for real-time predictions
Offline Store	BigQuery-backed, for batch training data retrieval

Vertex AI Feature Store Architecture:

Feature Ingestion (Batch or Streaming)
        ↓
┌──── Feature Store ────────────────┐
│  Offline Store (BigQuery)          │  ← Training data export
│  Online Store (Bigtable-backed)    │  ← Serving (ms latency)
└───────────────────────────────────┘
        ↑ Same features ↑
  Training      Inference
  Pipeline      Endpoint

5. BigQuery for Feature Engineering

BigQuery là công cụ tốt nhất trên GCP để compute aggregate features từ large datasets.

Feature Pattern	BigQuery Approach
Rolling window aggregates	Window functions: AVG() OVER (PARTITION BY ... ORDER BY ... ROWS BETWEEN ...)
User activity counts	COUNT() GROUP BY user_id
Categorical encoding	CASE WHEN ... or ML.ONE_HOT_ENCODE()
Hash embedding (high cardinality)	FARM_FINGERPRINT() mod N
Feature normalization	ML.STANDARD_SCALER() in BigQuery ML

Exam tip: Khi câu hỏi nhắc đến "training-serving consistency" hoặc "feature reuse across multiple models" → Vertex AI Feature Store. Khi nhắc đến "compute features from BigQuery data at scale" → BigQuery window functions + scheduled queries.

6. Feature Drift Monitoring

Type	What Changes	Detection Method
Feature Skew	Training vs serving feature distribution differs	Compare training baseline vs serving stats
Feature Drift	Serving features change over time	Monitor serving feature distributions daily
Label Drift	Target variable distribution changes	Track prediction distribution shifts

7. Practice Questions

Q1: A team's ML model has excellent accuracy during testing but performs poorly in production. Investigations reveal that the average purchase feature is calculated differently in training (using historical batch data) vs. serving (using real-time lookups). What is this problem called and how should it be solved?

• A) Model drift — retrain the model more frequently
• B) Training-serving skew — use Vertex AI Feature Store ✓
• C) Data leakage — remove the purchase feature
• D) Overfitting — add dropout layers

Explanation: Training-serving skew occurs when features are computed differently at training and serving time. Vertex AI Feature Store solves this by providing a single source of truth for feature computation, ensuring the same logic is used for both training data export and online serving.

Q2: A feature has values ranging from $10 to $10,000,000 with a heavily right-skewed distribution. Which transformation is MOST appropriate before using this feature in a linear model?

• A) One-Hot Encoding
• B) Min-Max Normalization
• C) Log transformation ✓
• D) Label Encoding

Explanation: Log transformation compresses the scale of highly skewed distributions, making them more normal-like and suitable for linear models. Min-Max normalization would still preserve the skew. One-hot encoding is for categorical data.

Q3: Which Vertex AI Feature Store store type is optimized for serving features to real-time prediction endpoints with millisecond latency?

• A) Offline Store (BigQuery)
• B) Online Store (Bigtable-backed) ✓
• C) Feature Catalog
• D) Cloud Memorystore

Explanation: The Online Store in Vertex AI Feature Store is backed by Bigtable and designed for sub-100ms latency lookups, serving fresh feature values to real-time prediction endpoints. The Offline Store uses BigQuery and is for batch training data retrieval.