How do you design low-latency video search systems?

Designing a low-latency video search system requires a combination of efficient indexing, optimized storage, and streamlined query processing. The core challenge is balancing speed with accuracy when searching large video datasets. To achieve this, the system must preprocess video content to create searchable metadata, use scalable databases, and minimize processing steps during queries. Let’s break this into three key areas: indexing, storage/retrieval, and query optimization.

First, efficient indexing is critical. Videos are split into smaller segments (e.g., 5-second clips) or keyframes to reduce computational overhead. Each segment is analyzed using lightweight computer vision models to extract features like object detection (e.g., “car,” “person”) or scene descriptors (e.g., “outdoor,” “night”). These features are stored as metadata in a structured format, such as JSON or binary embeddings. For example, a video of a city street might be tagged with “traffic,” “pedestrians,” and “sunset.” To speed up indexing, parallel processing frameworks like Apache Spark or serverless functions (AWS Lambda) can distribute tasks across multiple workers. Indexing at ingest time ensures queries don’t need to process raw video during searches.

Next, optimized storage and retrieval are essential. Metadata and embeddings are stored in databases optimized for low-latency lookups, such as Elasticsearch (for text-based tags) or vector databases like Milvus (for similarity searches). For instance, a search for “red car” would query Elasticsearch for matching tags, while a “find similar videos” request would use vector similarity in Milvus. Caching frequently accessed results (using Redis or Memcached) reduces repeated computation. To handle geographic latency, content delivery networks (CDNs) store pre-rendered thumbnails or video previews closer to users. Sharding databases by region or content type further improves query speed by reducing dataset sizes per node.

Finally, query processing must minimize latency. When a user submits a search, the system first checks caches for precomputed results. If unavailable, it parses the query into structured terms (e.g., converting “sunset beach” to ["sunset", “beach”]) and searches the indexed metadata. For complex queries, like finding a specific action within a video, frame-level timestamps allow direct jumps to relevant segments without scanning the entire file. Network optimizations, such as using HTTP/2 for faster connections and compressing responses (e.g., gzip), reduce transfer times. Edge computing can offload tasks like thumbnail generation to devices closer to users. For example, a mobile app might use on-device ML models to filter results before sending refined queries to the backend. Together, these steps ensure responses are delivered in milliseconds, even for large datasets.