Hybrid Recommendation System

“This project wasn’t just about building a recommender. It was about making user engagement meaningful — blending machine learning, scalable architecture, and real-time analytics to surface what truly matters.”

This case study walks through my approach to designing, developing, and deploying a production-grade Hybrid Recommendation System that powers personalized content delivery using text, image, and interaction signals.

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Backstory

Ever since I realized how hooked I was to platforms like Instagram, Pinterest, and Reddit, I started asking myself why. What was it that kept pulling me back? The answer: relevance. The posts that showed up were eerily well-aligned with my preferences. Sometimes they looked like the ones I liked, sometimes they were liked by people I follow — and sometimes, both.

That’s when I dove into the world of recommendation engines.

There are three core types:

1. Content-Based Filtering
2. Collaborative Filtering
3. Hybrid Systems

Most tutorials on the internet walk through movie or music recommendations. But I wanted to go deeper — to simulate a social media recommendation system.

So, I curated and cleaned datasets mimicking real-world post interactions. Posts, users, likes — all injected into MongoDB with realistic object IDs. My goal was to replicate how Instagram or Pinterest might structure their backend — just at a smaller scale.

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Architecture Overview

ML Pipeline Components

🔍 1. Content-Based Recommendations

This strategy focuses on the content itself — posts that are textually and visually similar to what a user has already liked.

Why is this important? Because human attention is drawn to familiarity — we gravitate toward content that matches our interests and past behavior.

🔧 Implementation Details:

• Text Embeddings:

  • Model Used: all-MiniLM-L6-v2 from Sentence Transformers (384-dim output)

  • Chosen for its excellent trade-off between speed, size, and semantic similarity accuracy. Larger models like all-mpnet-base-v2 offer slightly better performance but are computationally expensive.

  • Converts post captions, hashtags, and metadata into dense vectors:

    from sentence_transformers import SentenceTransformer
    model = SentenceTransformer("all-MiniLM-L6-v2")
    emb = model.encode(text, normalize_embeddings=True)
    

• Image Embeddings:

  • Model Used: resnet50 (from torchvision)

  • Chosen over ViT due to faster inference and availability on CPU/GPU out-of-the-box

  • We used the feature vector from the penultimate layer (2048-dim), then reduced using PCA to match text embedding size

    from torchvision.models import resnet50
    from torchvision import transforms
    model = resnet50(pretrained=True)
    model = nn.Sequential(*list(model.children())[:-1])  # remove final classifier
    emb = model(image).flatten()
    

• Final Post Embedding:

  • After PCA reduction, we concatenate both: $v_{post} = [v_{text}; v_{image}] \Rightarrow 768 \text{ dim}$

• User Vector:

  • Mean of embeddings of liked posts:

    user_vector = np.mean(liked_post_vectors, axis=0)
    

• Similarity Score:

  • Cosine similarity to all other post vectors:

    cosine_scores = cosine_similarity(user_vector.reshape(1, -1), post_matrix)
    

This allowed us to recommend posts that looked and read like what the user already enjoyed — even if no one else had liked them.

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🤝 2. Collaborative Filtering

This approach leverages implicit interactions — likes — to map behavioral similarity between users and posts.

Psychologically, it taps into social proof — what others like you enjoy.

🔧 Implementation Details:

• Data Format: CSV with user_id, post_id, interaction (1 or 0)

• Matrix Factorization Model: Surprise SVD

  • Formula: $\hat{r}_{ui} = \mu + b_u + b_i + q_i^T p_u$ Where:

    • $\mu$: global average
    • $b_u, b_i$: user/item biases
    • $q_i, p_u$: latent factors

• Loss Function: $L = \sum_{(u,i) \in R} (r_{ui} - \hat{r}_{ui})^2 + \lambda (|p_u|^2 + |q_i|^2)$

• Training Pipeline:

  from surprise import SVD, Dataset, Reader
  reader = Reader(rating_scale=(0, 1))
  data = Dataset.load_from_df(df[['user_id', 'post_id', 'interaction']], reader)
  trainset = data.build_full_trainset()
  model = SVD()
  model.fit(trainset)
  

• Model Evaluation: RMSE metric on test split

• Caching:

  • Precompute top-5 post recommendations per user
  • Store as user_recommendations.json
  • Serve via FastAPI endpoint for quick access

We use joblib to persist model (cf_model.pkl) and MLflow to track metrics, parameters, and artifacts.

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🔀 3. Hybrid Recommendation System

This combines the semantic richness of content-based embeddings with the behavioral accuracy of collaborative filtering.

🔧 Fusion Formula:

• Given:

  • c_score: cosine similarity (normalized)
  • cf_score: collaborative prediction (also normalized to [0,1])

• Compute Final Score: $\text{final_score} = \alpha \cdot c_score + (1 - \alpha) \cdot cf_score$

  • where $\alpha = 0.6$

• This score is used to re-rank all available posts:

  final_scores = 0.6 * content_scores + 0.4 * collab_scores
  

📉 Why Weighted Fusion?

• Combats cold-start problem
• Prioritizes meaningful unseen posts (from content) but doesn’t ignore social signals
• α is tunable — can be auto-optimized using a held-out validation set

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Production-Ready API with FastAPI

Initially, I used Flask — but as the project grew, I migrated to FastAPI for:

• Faster async support
• Built-in validation with Pydantic
• Cleaner route management

I built endpoints like:

• /likes/{user_id} → fetch liked posts
• /recommend/content → content-based
• /recommend/collab → collaborative
• /recommend/hybrid → hybrid

Performance Bottlenecks & Fixes

When I tested the system, the initial API responses were 30–40 seconds. Not acceptable.

Here’s what I did:

• Pagination — to serve results in chunks
• Reduced embedding recomputation — used cached values
• Optimized queries with $nin in MongoDB
• Improved sorting and slicing logic

Current average response time? Under 5 seconds — even with image and text embeddings running in-memory.

Total dataset: 132 users, 481 posts, 1.5K likes

This is still a small dataset. The next version will focus on large-scale, distributed computation, perhaps with Faiss, Redis caching, and batch inference pipelines.

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Frontend: Designed for Clarity

Built using:

• React
• Material UI (MUI)
• React Router
• Vite

UX Flow:

1. User List Page → All users shown (paginated)

2. Click a user → Navigate to UserDetail

3. Tabs show:

   • Liked Posts
   • Content-Based
   • Collaborative
   • Hybrid

Each tab shows paginated results and uses loading states with MUI CircularProgress.

View the Frontend Code → src/pages/UserDetail.jsx

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Final Summary Table

Recommendation Type	Method	Model Used	Source Data	Pagination	Explainability
Liked Posts	Query by user_id	Mongo Aggregates	Likes Collection	✅	❌
Content-Based	Cosine + Embeddings	MiniLM + ResNet	Posts Collection	✅	✅
Collaborative	SVD	Surprise	interactions.csv	❌ (cached)	❌
Hybrid	Weighted Scoring	Fusion	Merged from above	✅	✅

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What I Learned

This project made me fall in love with ML engineering — not just the models, but the plumbing:

• Embedding engineering
• Model serialization
• API design
• Real-world latency constraints
• UI that makes AI interpretable

It taught me the math behind relevance — and how good engineering makes AI actually usable.

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What’s Next?

• Use Faiss or Annoy for faster vector retrieval
• Add Redis caching for hot queries
• Integrate user feedback loop for model retraining
• Experiment with session-based recommendations
• Extend to audio/video posts

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GitHub Repo → hybrid-recommendation-system

Want a system like this built for your team? Reach out → rishwanth.perumandla@hotmail.com