Data Consistency in Multi-Region Deployments#

When you replicate data across regions, you are forced to choose between consistency, latency, and availability. You cannot have all three. Every multi-region system makes this tradeoff explicitly or, more dangerously, implicitly by ignoring it until production exposes the consequences.

The Fundamental Tension#

Strong consistency means every read sees the most recent write, regardless of which region it comes from. This requires cross-region coordination on every write (30-100ms per round trip). Eventual consistency means reads might see stale data, but replicas converge given enough time – usually milliseconds to seconds, but during partitions it can be minutes.

Rate Limiting Implementation Patterns#

Rate limiting controls how many requests a client can make within a time period. It protects services from overload, ensures fair usage across clients, prevents abuse, and provides a mechanism for graceful degradation under load. Every production API needs rate limiting at some layer.

Algorithm Comparison#

Fixed Window#

The simplest algorithm. Divide time into fixed windows (e.g., 1-minute intervals) and count requests per window. When the count exceeds the limit, reject requests until the next window starts.