Add max_connections configuration option

Allow the user to change the number of connections to the database
cluster units to their needs.
The default value is the same as without the configuration option.

Closes-Bug: #1929826
Change-Id: I10556e9d373cd8505c4e2e96d2b6031fe4ee9166

src/config.yaml

@@ -44,3 +44,11 @@ options:
         Time (in seconds) until the cache becomes invalid if not refreshed.
         Defaults to -1 (infinite). The value must be larger than
         auth_cache_refresh_interval else Router won't start.
+  max_connections:
+    type: int
+    default: 512
+    description: |
+        Maximum number of connections to allow in direction to each MySQL
+        routing sessions, with one for the RW unit and another one for the RO
+        units of the MySQL innodb cluster. Defaults to 512. A valid range is
+        between 1 and 65535.

src/lib/charm/openstack/mysql_router.py

@@ -673,6 +673,7 @@ class MySQLRouterCharm(charms_openstack.charm.OpenStackCharm):
             },
             "DEFAULT": {
                 "pid_file": self.mysqlrouter_pid_file,
+                "max_connections": str(self.options.max_connections),
             }
         }
 

unit_tests/test_lib_charm_openstack_mysql_router.py

@@ -695,6 +695,7 @@ class TestMySQLRouterCharm(test_utils.PatchHelper):
             "ttl": '5',
             "auth_cache_ttl": '10',
             "auth_cache_refresh_interval": '7',
+            "max_connections": '1000',
         }
 
         def _fake_config(key=None):
@@ -711,10 +712,13 @@ class TestMySQLRouterCharm(test_utils.PatchHelper):
         mrc.name = 'foobar'
         mrc.update_config_parameters = _mock_update_config_parameters
 
+        _metadata_config = _config_data.copy()
+        _metadata_config.pop('max_connections')
         _params = {
-            'metadata_cache:jujuCluster': _config_data,
+            'metadata_cache:jujuCluster': _metadata_config,
             'DEFAULT': {
                 'client_ssl_mode': "PASSTHROUGH",
+                'max_connections': _config_data['max_connections'],
                 'pid_file': '/run/mysql/mysqlrouter-foobar.pid'
             },
         }