Multi Region Caching, Invalidation, and Trade Offs Between Consistency and Latency

Multi region distributed caching introduces significant complexity around consistency, latency, and cost. The most common production pattern is independent regional caches with no cross region coherence: each region (for example, us east, eu west, asia pacific) has its own cache cluster, and the database or origin storage is the single source of truth. Writes go directly to the origin, and cache invalidations or updates propagate to each regional cache independently via asynchronous event streams or change data capture. This design minimizes latency because reads are served from in region memory in under 1 millisecond, and avoids expensive cross region network transfers (which add 50 to 150 milliseconds and cost orders of magnitude more per gigabyte). However, it accepts eventual consistency: a write in us east might not invalidate the eu west cache for seconds to tens of seconds, so users in Europe could see stale data during that window. For globally shared hot data (for example, popular content metadata or configuration), some systems maintain a small global cache layer with active replication. Writes to the global cache synchronously or asynchronously replicate to all regions, providing stronger consistency at the cost of increased write latency (must wait for cross region replication) and complexity (conflict resolution, replication lag monitoring). Netflix has discussed using a small globally replicated cache for critical metadata while keeping the bulk of personalization data in regional caches with eventual consistency. Amazon CloudFront and other content delivery networks use push invalidation: when content changes at origin, an invalidation or purge message is sent to all edge caches globally, with propagation completing in seconds to minutes. This trades off invalidation complexity and cost for improved global consistency. The fundamental trade off is latency and cost versus staleness. Cross region cache reads add 50 to 150 milliseconds at p50 and can spike to 200 plus milliseconds at p99 due to network variability, making them unacceptable for latency sensitive paths. Cross region data transfer costs are typically 0.01 to 0.02 dollars per gigabyte, so a cache serving 1 terabyte per second of cross region traffic would incur 36,000 to 72,000 dollars per hour in transfer fees alone. In contrast, same region reads complete in under 1 millisecond and incur minimal cost. Production systems therefore strongly prefer region local reads, using multi region replication only for high value, low volume data or accepting staleness windows measured in seconds to minutes for the bulk of cached data. Version based invalidation can help: include a monotonic version number or logical sequence number in cached values, and reject stale entries when a higher version is known, limiting the impact of delayed invalidations.

A global social network writes user profile updates to a database in us east. The us east cache invalidates immediately (under 1 ms staleness), but eu west and asia pacific caches receive invalidation events asynchronously after 2 to 5 seconds due to event stream lag. European users may see stale profiles for 5 seconds after an update.

Netflix EVCache regional independence: a recommendation service in us east serves personalized video lists from local cache in under 1 ms. The same service in eu west serves from its own independent cache. A new video added to the catalog in us east propagates to eu west via Kafka change stream in 3 to 8 seconds, during which eu west caches do not include the new video.

Amazon CloudFront content invalidation: an e-commerce site updates a product image at origin. CloudFront sends purge requests to hundreds of edge locations globally. Most edges purge within 10 to 30 seconds, but some remote locations take up to 60 seconds. During this window, users may see the old image from edge caches.