YouTube faced backend complexity as its popularity grew, managed and scaled MySQL using Vitess, a custom solution.Vitess acts as a layer on top of MySQL, enabling horizontal scaling and traffic management for more intelligent backend operations.Challenges YouTube faced included slow queries, downtime issues, data loss risks, and latency problems for global users.To handle load distribution, YouTube used replicas for read queries, reducing primary database load and improving performance.YouTube balanced consistency and availability, implementing various read strategies based on data freshness requirements.Prime Cache was introduced to address replication lag issues in high write load scenarios, improving synchronization with the primary database.Vertical splitting and sharding were adopted by YouTube to efficiently manage large database sizes and distribute operations across multiple servers.Vitess automated sharding and featured query routing components VTTablet and VTGate to manage queries and connections efficiently.Vitess also automated tasks like reparenting and backups to simplify database management at scale for YouTube.Core Vitess features such as connection pooling, query safety, result reuse, row cache, and fail-safes helped YouTube scale effectively.