Multicaches

Multicaches are caching systems that deploy more than a single cache instance or level to store frequently accessed data, aiming to reduce latency, balance load, and improve resilience. They are used across hardware and software domains, from processor architectures to web applications and distributed services.

In hardware, multicache typically refers to the cache hierarchy inside a CPU or between processor cores. Common configurations include L1, L2, and L3 caches, which may be private to a core or shared among cores. Caches can be inclusive (a data item present in higher levels is also in lower levels) or exclusive (data items occupy only one level). Coherence protocols, such as MESI, ensure that updates remain consistent across caches when multiple cores access the same memory region.

In software, multicaches describe tiered or multi-node caches. An application may maintain an in-process cache for speed, plus an external cache such as Memcached or Redis, and optionally a CDN or edge cache for geographic proximity. Cache policies include eviction (LRU, LFU), TTL-based expiration, and write strategies such as write-through, write-behind, or write-around. The cache-aside pattern is common: the application checks the cache, loads from a backing store on miss, and then updates the cache.

In distributed systems, multicaches spread across multiple nodes to scale and tolerate failures. Keys are partitioned (sharding) and often replicated for availability. Consistency is typically eventual rather than strict; invalidation messages, versioning, TTLs, and consistent hashing help coordinate updates and eviction across nodes. Architectures may use a combination of local caches and distributed caches to balance latency and coherence requirements.

Common challenges include maintaining data freshness, avoiding cache stampede, handling node failures, and monitoring hit rates and latency. Multicaches offer significant performance benefits for read-heavy workloads but introduce complexity in cache invalidation and consistency.