Real-time feature streaming: WebSocket/SSE subscriptions with spatial filters

[mikemcdougall]

Context

Esri's Stream Layer requires ArcGIS GeoEvent Server ($$$). Honua can offer real-time feature change subscriptions as a Pro feature, powered by CDC from PostGIS.

Scope

• WebSocket and/or SSE transport for feature change subscriptions
• Clients subscribe with spatial and attribute filters (reuse shared Filter AST)
• Push deltas on feature create/update/delete
• Backpressure handling and connection management
• Integration with CDC event bus (depends on Feature edit change notifications: webhook events + replay cursor #316)
• SDK support: JS SDK subscribe() method on HonuaFeatureLayer
• MCP integration: AI agents can subscribe to change streams

Acceptance Criteria

• Client can open WebSocket/SSE connection with WHERE + spatial filter
• Receives real-time deltas when matching features are edited
• Handles reconnection with replay cursor
• Scale test: 100 concurrent subscribers, 1000 edits/sec throughput

References

• ADR-0024: Pro tier feature
• Depends on: Feature edit change notifications: webhook events + replay cursor #316 (CDC event bus)