How do you build a real-time recommender system?

Building a real-time recommender system involves three core components: data ingestion and processing, model design, and serving infrastructure. The goal is to capture user interactions instantly, generate recommendations using up-to-date data, and deliver results with low latency. This requires a combination of streaming frameworks, machine learning models optimized for speed, and scalable APIs to handle requests.

First, data ingestion and processing must handle real-time user activity. Tools like Apache Kafka or Amazon Kinesis can stream clickstream data, search queries, or purchase events as they occur. This data is cleaned and transformed into features (e.g., user preferences, item metadata) using frameworks like Apache Flink or Spark Streaming. For example, if a user clicks on a product, the system immediately logs this interaction and updates their profile. To reduce latency, consider storing frequently accessed data (e.g., user profiles) in an in-memory database like Redis. This ensures the model has the latest context when generating recommendations.

Next, the model must balance accuracy with speed. Traditional batch-trained models (e.g., matrix factorization) can’t adapt quickly to new data, so use online learning techniques. Algorithms like incremental collaborative filtering or simplified neural networks (e.g., shallow embeddings) update in real time as new data arrives. For example, a hybrid model might combine collaborative filtering (based on user-item interactions) with content-based filtering (using item attributes like category or price) to handle cold-start scenarios. Approximate nearest neighbor (ANN) libraries like FAISS or Spotify’s Annoy can quickly retrieve similar items from large catalogs. Precompute candidate recommendations periodically (e.g., every 5 minutes) and fine-tune them in real time using fresh user data.

Finally, the serving layer must deliver recommendations with minimal delay. Deploy the model as a REST or gRPC API using frameworks like FastAPI or TensorFlow Serving. Cache precomputed recommendations (e.g., “users who liked X also bought Y”) in Redis to reduce compute overhead. For personalization, use a two-stage approach: retrieve a broad set of candidates from the cache, then rerank them using real-time user context (e.g., current session data). Optimize the pipeline by profiling latency at each step—database queries, model inference, network calls—and eliminate bottlenecks. Use load balancers and autoscaling (e.g., Kubernetes) to handle traffic spikes. Monitor metrics like throughput, latency, and recommendation relevance to ensure performance stays consistent.