Jun 26 MT-1 phase

app/tenancy/engine_cache.py

@@ -0,0 +1,59 @@
+"""
+app/tenancy/engine_cache.py
+---------------------------
+Process-local cache of per-tenant SQLAlchemy engines, keyed by tenant id.
+
+Each tenant has its own database, hence its own engine + connection pool.
+Engines are created lazily on first use and reused across requests. Total
+backend connections ≈ workers × cached-tenants × pool_size, so pool sizing is
+a real scaling lever (see MULTI_TENANT_PLAN.md §4); tune via config, or set a
+small pool / switch to NullPool when the tenant count grows large.
+
+`invalidate(tenant_id)` drops a cached engine (e.g. after credential rotation
+or tenant suspension); the next request rebuilds it.
+"""
+
+import threading
+
+from flask import current_app
+from sqlalchemy import create_engine
+
+_engines = {}
+_lock = threading.Lock()
+
+
+def get_tenant_engine(tenant):
+    """Return (building if needed) the cached engine for a TenantContext."""
+    engine = _engines.get(tenant.id)
+    if engine is not None:
+        return engine
+    with _lock:
+        engine = _engines.get(tenant.id)
+        if engine is None:
+            engine = create_engine(
+                tenant.db_uri,
+                pool_pre_ping=True,
+                pool_size=current_app.config.get('TENANT_ENGINE_POOL_SIZE', 5),
+                max_overflow=current_app.config.get('TENANT_ENGINE_MAX_OVERFLOW', 5),
+                pool_recycle=current_app.config.get('TENANT_ENGINE_POOL_RECYCLE', 1800),
+                future=True,
+            )
+            _engines[tenant.id] = engine
+    return engine
+
+
+def invalidate(tenant_id):
+    """Drop and dispose a cached tenant engine, if present."""
+    with _lock:
+        engine = _engines.pop(tenant_id, None)
+    if engine is not None:
+        engine.dispose()
+
+
+def clear():
+    """Dispose and drop all cached engines (test/teardown helper)."""
+    with _lock:
+        engines = list(_engines.values())
+        _engines.clear()
+    for engine in engines:
+        engine.dispose()