Add "sprint-9-10-graceful-error-handling"

sprint-9-10-graceful-error-handling.-.md

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+# Sprint 9-10 - Graceful Error Handling in Service Layers
+
+**Date**: 2026-01-17
+**Tags**: python, postgresql, service-layer, error-handling, dash, graceful-degradation, arm64
+
+---
+
+## Context
+Building the Toronto Neighbourhood Dashboard with a service layer that queries PostgreSQL/PostGIS dbt marts to provide data to Dash callbacks.
+
+## Problem
+Initial service layer implementation let database connection errors propagate as unhandled exceptions. When the PostGIS Docker container was unavailable (common on ARM64 systems where the x86_64 image fails), the entire dashboard would crash instead of gracefully degrading.
+
+## Solution
+Wrapped database queries in try/except blocks to return empty DataFrames/lists/dicts when the database is unavailable:
+
+```python
+def _execute_query(sql: str, params: dict | None = None) -> pd.DataFrame:
+    try:
+        engine = get_engine()
+        with engine.connect() as conn:
+            return pd.read_sql(text(sql), conn, params=params)
+    except Exception:
+        return pd.DataFrame()
+```
+
+This allows:
+1. Dashboard to load and display empty states
+2. Development/testing without running database
+3. Graceful degradation in production
+
+## Prevention
+- **Always design service layers with graceful degradation** - assume external dependencies can fail
+- **Return empty collections, not exceptions** - let UI components handle empty states
+- **Test without database** - verify the app doesn't crash when DB is unavailable
+- **Consider ARM64 compatibility** - PostGIS images may not support all platforms