Federated queries: cross-instance and external source proxy

[mikemcdougall]

Context

Organizations run multiple Honua instances and still have legacy Esri services. A federation layer lets a single query span multiple data sources — making Honua the integration layer, not just a replacement.

Scope

• Cross-instance queries: Honua → Honua (gRPC transport)
• External source proxy: Honua → Esri REST, OGC WFS (HTTP transport)
• Single query can join local PostGIS data with remote sources
• Federation config via metadata resources (ADR-0023)
• Query planning: push filters to remote, join locally
• Timeout and circuit-breaker for remote sources

Acceptance Criteria

• Query that joins local layer with remote Esri feature service
• Federation sources configurable via Admin UI
• Latency and error metrics per federated source
• Circuit breaker prevents cascade failures

References

• ADR-0024: Enterprise tier feature