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docs/features/checked/router/region-aware-routing-algorithm.md

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+# Region-Aware Routing Algorithm
+
+## Module
+Router
+
+## Status
+IMPLEMENTED
+
+## Description
+Full routing algorithm with 3-tier region preference (Tier 0: same region, Tier 1: same continent, Tier 2: cross-continent), ping-based latency selection within tiers, heartbeat recency weighting, and round-robin/random tie-breaking for deterministic load distribution.
+
+## Implementation Details
+- **Modules**: `src/Router/__Libraries/StellaOps.Router.Gateway/Routing/`, `src/Router/__Libraries/StellaOps.Router.Gateway/Services/`
+- **Key Classes**:
+  - `DefaultRoutingPlugin` (`src/Router/__Libraries/StellaOps.Router.Gateway/Routing/DefaultRoutingPlugin.cs`) - implements 3-tier region preference with latency-based selection
+  - `PingTracker` (`src/Router/__Libraries/StellaOps.Router.Gateway/Services/PingTracker.cs`) - tracks ping-based latency measurements per instance
+  - `InMemoryRoutingState` (`src/Router/__Libraries/StellaOps.Router.Gateway/State/InMemoryRoutingState.cs`) - routing state with instance region metadata
+- **Interfaces**: `IRoutingPlugin`, `IGlobalRoutingState`, `IRegionProvider`
+- **Source**: batch_51/file_24.md
+
+## E2E Test Plan
+- [ ] Register instances in the same region, same continent, and cross-continent, then route a request and verify Tier 0 preference
+- [ ] Verify latency-based selection: within the same tier, confirm the lowest-latency instance is preferred
+- [ ] Verify heartbeat recency weighting: stale instances are deprioritized
+- [ ] Verify round-robin tie-breaking: when latency is equal, confirm requests distribute evenly
+- [ ] Remove all Tier 0 instances and verify fallback to Tier 1, then Tier 2