Introduce parameters to control the connection pool for engine creation (#14)

Author: danceb

README.md

@@ -24,6 +24,10 @@ Environment variables
 | `TENANT_HEADER`              | `<empty>`                   | The name of the HTTP header which contains the tenant name for multi-tenant setups. |
 | `TENANT_PATH_PREFIX`         | `@service_prefix@/@tenant@` | URL path prefix for all QWC services for multi-tenant setups.                       |
 | `TENANT_ACCESS_COOKIE_PATH`  | `<tenant_path_prefix>`      | Path for which the access cookie is valid for multi-tenant setups.                  |
+| `POOL_SIZE`                  | `5`                         | Maximum number of possible data base connections.                                   |
+| `MAX_OVERFLOW`               | `10`                        | Additional connections beyond pool_size during peak load.                           |
+| `POOL_TIMEOUT`               | `30`                        | Time (in seconds) to wait for a connection to become available.                     |
+| `POOL_RECYCLE`               | `-1`                        | Time (in seconds) after idle connections will be resetted.                          |
 
 Development
 ===========

qwc_services_core/database.py

@@ -1,7 +1,7 @@
 import os
 
 from sqlalchemy import create_engine
-
+from sqlalchemy.pool import QueuePool
 
 class DatabaseEngine():
     """Helper for database connections using SQLAlchemy engines"""
@@ -15,12 +15,24 @@ def db_engine(self, conn_str):
 
         :param str conn_str: DB connection string for SQLAlchemy engine
 
-        see http://docs.sqlalchemy.org/en/latest/core/engines.html#postgresql
+        see https://docs.sqlalchemy.org/en/latest/core/engines.html#postgresql
         """
+
+        db_pool_size = os.environ.get('POOL_SIZE', 5)
+        db_max_overflow = os.environ.get('MAX_OVERFLOW', 10)
+        db_pool_timeout = os.environ.get('POOL_TIMEOUT', 30)
+        db_pool_recycle = os.environ.get('POOL_RECYCLE', -1)
+
         engine = self.engines.get(conn_str)
         if not engine:
             engine = create_engine(
-                conn_str, pool_pre_ping=True, echo=False)
+                conn_str, 
+                poolclass=QueuePool,
+                pool_size=db_pool_size,
+                max_overflow=db_max_overflow, 
+                pool_timeout=db_pool_timeout,
+                pool_recycle=db_pool_recycle,
+                pool_pre_ping=True, echo=False)
             self.engines[conn_str] = engine
         return engine