port experimental max_connections (#425)

config.yaml

@@ -36,3 +36,11 @@ options:
   mysql-root-interface-database:
     description: "The database name for the legacy 'mysql' interface (root level access)"
     type: "string"
+# Experimental features
+  experimental-max-connections:
+    type: int
+    description: |
+      Maximum number of connections allowed to the MySQL server.
+      When set max-connections value take precedence over the memory utilizations
+      againts innodb_buffer_pool_size.
+      This is an experimental feature and may be removed in future releases.

lib/charms/mysql/v0/mysql.py

@@ -130,7 +130,7 @@ def wait_until_mysql_connection(self) -> None:
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 59
+LIBPATCH = 60
 
 UNIT_TEARDOWN_LOCKNAME = "unit-teardown"
 UNIT_ADD_LOCKNAME = "unit-add"
@@ -141,7 +141,8 @@ def wait_until_mysql_connection(self) -> None:
 BYTES_1MiB = 1048576  # 1 mebibyte
 RECOVERY_CHECK_TIME = 10  # seconds
 GET_MEMBER_STATE_TIME = 10  # seconds
-MIN_MAX_CONNECTIONS = 100
+MAX_CONNECTIONS_FLOOR = 10
+MIM_MEM_BUFFERS = 200 * BYTES_1MiB
 
 SECRET_INTERNAL_LABEL = "secret-id"
 SECRET_DELETED_LABEL = "None"
@@ -868,36 +869,60 @@ def render_mysqld_configuration(
         *,
         profile: str,
         memory_limit: Optional[int] = None,
+        experimental_max_connections: Optional[int] = None,
         snap_common: str = "",
     ) -> tuple[str, dict]:
         """Render mysqld ini configuration file.
 
         Args:
             profile: profile to use for the configuration (testing, production)
             memory_limit: memory limit to use for the configuration in bytes
+            experimental_max_connections: explicit max connections to use for the configuration
             snap_common: snap common directory (for log files locations in vm)
 
         Returns: a tuple with mysqld ini file string content and a the config dict
         """
+        max_connections = None
         performance_schema_instrument = ""
         if profile == "testing":
             innodb_buffer_pool_size = 20 * BYTES_1MiB
             innodb_buffer_pool_chunk_size = 1 * BYTES_1MiB
             group_replication_message_cache_size = 128 * BYTES_1MiB
-            max_connections = MIN_MAX_CONNECTIONS
+            max_connections = 100
             performance_schema_instrument = "'memory/%=OFF'"
         else:
             available_memory = self.get_available_memory()
             if memory_limit:
                 # when memory limit is set, we need to use the minimum
                 # between the available memory and the limit
                 available_memory = min(available_memory, memory_limit)
+
+            if experimental_max_connections:
+                # when set, we use the experimental max connections
+                # and it takes precedence over buffers usage
+                max_connections = experimental_max_connections
+                # we reserve 200MiB for memory buffers
+                # even when there's some overcommittment
+                available_memory = max(
+                    available_memory - max_connections * 12 * BYTES_1MiB, 200 * BYTES_1MiB
+                )
+
             (
                 innodb_buffer_pool_size,
                 innodb_buffer_pool_chunk_size,
                 group_replication_message_cache_size,
             ) = self.get_innodb_buffer_pool_parameters(available_memory)
-            max_connections = max(self.get_max_connections(available_memory), MIN_MAX_CONNECTIONS)
+
+            # constrain max_connections based on the available memory
+            # after innodb_buffer_pool_size calculation
+            available_memory -= innodb_buffer_pool_size + (
+                group_replication_message_cache_size or 0
+            )
+            if not max_connections:
+                max_connections = max(
+                    self.get_max_connections(available_memory), MAX_CONNECTIONS_FLOOR
+                )
+
             if available_memory < 2 * BYTES_1GiB:
                 # disable memory instruments if we have less than 2GiB of RAM
                 performance_schema_instrument = "'memory/%=OFF'"

src/charm.py

@@ -373,7 +373,9 @@ def _restart(self, event: EventBase) -> None:
             restart_states = {
                 self.restart_peers.data[unit].get("state", "unset") for unit in self.peers.units
             }
-            if restart_states != {"release"}:
+            if restart_states == {"unset"}:
+                logger.info("Restarting primary")
+            elif restart_states != {"release"}:
                 # Wait other units restart first to minimize primary switchover
                 message = "Primary restart deferred after other units"
                 logger.info(message)
@@ -384,6 +386,8 @@ def _restart(self, event: EventBase) -> None:
         container = self.unit.get_container(CONTAINER_NAME)
         if container.can_connect():
             container.restart(MYSQLD_SAFE_SERVICE)
+            sleep(10)
+            self._on_update_status(None)
 
     # =========================================================================
     # Charm event handlers
@@ -440,6 +444,7 @@ def _on_config_changed(self, event: EventBase) -> None:
         new_config_content, new_config_dict = self._mysql.render_mysqld_configuration(
             profile=self.config.profile,
             memory_limit=memory_limit_bytes,
+            experimental_max_connections=self.config.experimental_max_connections,
         )
 
         changed_config = compare_dictionaries(previous_config_dict, new_config_dict)
@@ -506,6 +511,7 @@ def _write_mysqld_configuration(self):
         new_config_content, _ = self._mysql.render_mysqld_configuration(
             profile=self.config.profile,
             memory_limit=memory_limit_bytes,
+            experimental_max_connections=self.config.experimental_max_connections,
         )
         self._mysql.write_content_to_file(path=MYSQLD_CONFIG_FILE, content=new_config_content)
 

src/config.py

@@ -9,6 +9,7 @@
 from typing import Optional
 
 from charms.data_platform_libs.v0.data_models import BaseConfigModel
+from charms.mysql.v0.mysql import MAX_CONNECTIONS_FLOOR
 from pydantic import validator
 
 logger = logging.getLogger(__name__)
@@ -53,6 +54,7 @@ class CharmConfig(BaseConfigModel):
     mysql_interface_database: Optional[str]
     mysql_root_interface_user: Optional[str]
     mysql_root_interface_database: Optional[str]
+    experimental_max_connections: Optional[int]
 
     @validator("profile")
     @classmethod
@@ -116,3 +118,14 @@ def database_name_validator(cls, value: str) -> Optional[str]:
             )
 
         return value
+
+    @validator("experimental_max_connections")
+    @classmethod
+    def experimental_max_connections_validator(cls, value: int) -> Optional[int]:
+        """Check experimental max connections."""
+        if value < MAX_CONNECTIONS_FLOOR:
+            raise ValueError(
+                f"experimental-max-connections must be greater than {MAX_CONNECTIONS_FLOOR}"
+            )
+
+        return value