Geo Replication & Multi-Region

Why this module exists

Geo replication improves user latency and regional resilience, but it introduces hard consistency-routing decisions.

In multi-region systems, every read/write path is a policy tradeoff:

• local and fast,
• global and fresh,
• or unavailable during failures.

Core decision surfaces

A geo request planner must decide:

1. which healthy regions to contact,
2. whether read and write quorums can be satisfied,
3. whether chosen quorum sizes are intersection-safe for current replica set.

Low-latency routing without quorum safety creates stale-read incidents.

Latency-aware quorum routing

Given per-region latency and replica counts, route by ascending latency. The observed latency is usually bounded by slowest selected region.

Tie-breaking must be deterministic; otherwise clients can disagree on route and create unstable tail latency.

Health-aware planning

Only healthy regions can contribute replicas in real-time planning.

Failure mode to avoid:

• planner counts unhealthy replicas toward quorum,
• request starts but cannot actually commit.

Healthy-capacity checks must happen before marking route feasible.

Quorum safety math

Let N be total healthy replicas available for key range. Classic quorum safety conditions:

• read/write intersection: R + W > N
• write/write intersection: W > N/2

If these are not true, routes may be fast but consistency is not guaranteed.

Typical geo policies

• Latency-first reads with bounded-staleness fallback.
• Write quorum routing that prioritizes local+near regions.
• Safety gating: reject unsafe quorum configurations instead of silently serving inconsistent mode.

These policies should be explicit in planner output.

Common implementation mistakes

• selecting regions by count instead of available replicas,
• ignoring health state in capacity checks,
• assuming route feasibility implies consistency safety,
• non-deterministic region order under latency ties,
• treating read and write route failures as the same condition.

What you will build

Geo Quorum Route Planner (capstone)

• choose read and write region sets by latency among healthy regions,
• compute resulting read/write latency,
• report whether quorum sizes are intersection-safe for healthy replica total.

By the end, you should be able to separate latency feasibility from correctness safety in geo routing decisions.