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EVCache

EVCache is Netflix's distributed in-memory caching solution, a wrapper around Memcached with Netflix-specific enhancements for high availability, global replication, client-side sharding, and multi-region operation. Open-source at github.com/Netflix/EVCache.

Within this wiki, EVCache is a stub — named as a canonical Netflix caching layer paired with persistent stores in KV DAL namespaces, but not yet deep-dived. Expansion is expected when a dedicated EVCache architecture post is ingested.

Role in the KV DAL story

Namespace configs in KV DAL can layer EVCache as a CACHE tier on top of a durable primary. Example from the post:

CACHE:
  physical_storage:
    type: CACHE
    cluster: evcache_kv_ngsegment
  config:
    default_cache_ttl: 180s

The combination — e.g. Cassandra as PRIMARY_STORAGE + EVCache as CACHE — gives "highly durable persistent storage AND lower- latency point reads" inside one logical namespace, without the calling microservice having to wire cache + DB coherency by hand.

Role in the Counter Abstraction

The Distributed Counter Abstraction uses EVCache in two distinct roles:

1. Best-Effort counter backing — a thin wrapper over EVCache incr/decr delivers a regional approximate counter with high throughput + low latency, at the cost of no cross-region replication on increments + no idempotency + no consistency guarantees. Code skeleton from the post:

return delta > 0
    ? cache.incr(counterCacheKey, delta, TTL)
    : cache.decr(counterCacheKey, Math.abs(delta), TTL);

ClearCount maps to cache.delete(key, ReplicaPolicy.ALL). (Source: sources/2024-11-13-netflix-netflixs-distributed-counter-abstraction)

1. Rollup Cache for Eventually-Consistent counters — caches the combined (lastRollupCount, lastRollupTs) as a single cached value per counter (combined to prevent count / timestamp mismatch). GetCount serves a point-read from this cache, accepting a few seconds of staleness. See patterns/sliding-window-rollup-aggregation.

Seen in

• sources/2024-09-19-netflix-netflixs-key-value-data-abstraction-layer — named as one of the backing stores KV DAL routes to, exemplified in the ngsegment namespace as a 180-second-TTL cache tier on top of a Cassandra primary.
• sources/2024-11-13-netflix-netflixs-distributed-counter-abstraction — dual role: Best-Effort counter backing + Rollup Cache for Eventually-Consistent counters. Canonical example of EVCache's native incr/decr as the substrate for a high-throughput regional approximate counter.

Related

• systems/netflix-kv-dal
• systems/netflix-distributed-counter — Best-Effort counter backing + Eventually-Consistent rollup cache.
• systems/apache-cassandra — commonly paired durable primary underneath an EVCache cache tier.
• concepts/best-effort-vs-eventually-consistent-counter
• companies/netflix