# Scaling Memcache at Facebook * **Enhancements to Memcached**: * **Lease Mechanism**: To mitigate the "thundering herd" problem, a lease mechanism is used. When a cache miss occurs, a lease token is assigned to one server, preventing multiple servers from simultaneously querying the backend database. * **Adaptive Slab Allocator**: This helps in better memory management by reducing fragmentation and optimizing memory allocation for different sizes of data​. * **Handling Consistency and Stale Data**: * **McSqueal and Mcrouter**: These tools are used to ensure data consistency across different regions by broadcasting invalidations whenever the data is updated in the database​. * **Replication and Invalidation**: SQL statements modifying data include the required cache keys, ensuring that updates are propagated to all relevant caches​. * **Scaling Strategies**: * **Regional Clusters**: Splitting servers into regional clusters reduces latency and improves reliability by keeping data closer to users and mitigating the effects of large-scale outages​. * **Different Pools for Different Data Types**: Assigning different pools to different types of data based on access patterns and memory requirements prevents negative interactions and optimizes performance​. * **Load Balancing and Failure Handling**: * **Gutter Servers**: These are used as fallback when primary memcached servers fail, preventing cascading failures that can overload the system​​. * **Distributed Read Requests**: Read requests are distributed across multiple replicas to balance the load and improve performance​. * **Performance Optimization**: * **Eviction Policies**: Utilizing Least Recently Used (LRU) eviction policies within each slab class ensures efficient memory use by discarding the least used items first​. * **High Availability**: The design incorporates mechanisms to handle server failures gracefully, ensuring high availability and reliability of the caching system​​. Ref: