host_monitoring_plugin.py

#  Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
#
#  Licensed under the Apache License, Version 2.0 (the "License").
#  You may not use this file except in compliance with the License.
#  You may obtain a copy of the License at
#
#  http://www.apache.org/licenses/LICENSE-2.0
#
#  Unless required by applicable law or agreed to in writing, software
#  distributed under the License is distributed on an "AS IS" BASIS,
#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
#  See the License for the specific language governing permissions and
#  limitations under the License.

from __future__ import annotations

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from aws_advanced_python_wrapper.driver_dialect import DriverDialect
    from aws_advanced_python_wrapper.hostinfo import HostInfo
    from aws_advanced_python_wrapper.pep249 import Connection
    from aws_advanced_python_wrapper.plugin_service import PluginService

from concurrent.futures import Future, TimeoutError
from dataclasses import dataclass
from queue import Queue
from threading import Event, Lock, RLock
from time import perf_counter_ns
from typing import Any, Callable, ClassVar, Dict, FrozenSet, Optional, Set

from _weakref import ReferenceType, ref

from aws_advanced_python_wrapper.errors import AwsWrapperError
from aws_advanced_python_wrapper.host_availability import HostAvailability
from aws_advanced_python_wrapper.pep249_methods import DbApiMethod
from aws_advanced_python_wrapper.plugin import (CanReleaseResources, Plugin,
                                                PluginFactory)
from aws_advanced_python_wrapper.thread_pool_container import \
    ThreadPoolContainer
from aws_advanced_python_wrapper.utils.concurrent import ConcurrentDict
from aws_advanced_python_wrapper.utils.log import Logger
from aws_advanced_python_wrapper.utils.messages import Messages
from aws_advanced_python_wrapper.utils.notifications import (
    ConnectionEvent, HostEvent, OldConnectionSuggestedAction)
from aws_advanced_python_wrapper.utils.properties import (Properties,
                                                          PropertiesUtils,
                                                          WrapperProperties)
from aws_advanced_python_wrapper.utils.rdsutils import RdsUtils
from aws_advanced_python_wrapper.utils.telemetry.telemetry import (
    TelemetryCounter, TelemetryTraceLevel)
from aws_advanced_python_wrapper.utils.utils import QueueUtils

logger = Logger(__name__)


class HostMonitoringPluginFactory(PluginFactory):
    @staticmethod
    def get_instance(plugin_service: PluginService, props: Properties) -> Plugin:
        return HostMonitoringPlugin(plugin_service, props)


class HostMonitoringPlugin(Plugin, CanReleaseResources):
    _SUBSCRIBED_METHODS: Set[str] = {
        DbApiMethod.CONNECT.method_name,
        DbApiMethod.NOTIFY_HOST_LIST_CHANGED.method_name,
        DbApiMethod.NOTIFY_CONNECTION_CHANGED.method_name
    }

    def __init__(self, plugin_service, props):
        dialect: DriverDialect = plugin_service.driver_dialect
        if not dialect.supports_abort_connection():
            raise AwsWrapperError(Messages.get_formatted(
                "HostMonitoringPlugin.ConfigurationNotSupported", type(dialect).__name__))

        self._props: Properties = props
        self._plugin_service: PluginService = plugin_service
        self._is_connection_initialized = False
        self._monitoring_host_info: Optional[HostInfo] = None
        self._rds_utils: RdsUtils = RdsUtils()
        self._monitor_service: MonitorService = MonitorService(plugin_service)
        self._lock: Lock = Lock()
        self._is_enabled = WrapperProperties.FAILURE_DETECTION_ENABLED.get_bool(self._props)
        self._failure_detection_time_ms = WrapperProperties.FAILURE_DETECTION_TIME_MS.get_int(self._props)
        self._failure_detection_interval = WrapperProperties.FAILURE_DETECTION_INTERVAL_MS.get_int(self._props)
        self._failure_detection_count = WrapperProperties.FAILURE_DETECTION_COUNT.get_int(self._props)
        HostMonitoringPlugin._SUBSCRIBED_METHODS.update(self._plugin_service.network_bound_methods)

    @property
    def subscribed_methods(self) -> Set[str]:
        return HostMonitoringPlugin._SUBSCRIBED_METHODS

    def connect(
            self,
            target_driver_func: Callable,
            driver_dialect: DriverDialect,
            host_info: HostInfo,
            props: Properties,
            is_initial_connection: bool,
            connect_func: Callable) -> Connection:
        conn = connect_func()
        if conn:
            rds_type = self._rds_utils.identify_rds_type(host_info.host)
            if rds_type.is_rds_cluster:
                host_info.reset_aliases()
                self._plugin_service.fill_aliases(conn, host_info)
        return conn

    def execute(self, target: object, method_name: str, execute_func: Callable, *args: Any, **kwargs: Any) -> Any:
        connection = self._plugin_service.current_connection
        if connection is None:
            raise AwsWrapperError(Messages.get_formatted("HostMonitoringPlugin.ConnectionNone", method_name))

        host_info = self._plugin_service.current_host_info
        if host_info is None:
            raise AwsWrapperError(Messages.get_formatted("HostMonitoringPlugin.HostInfoNoneForMethod", method_name))

        if not self._is_enabled or not self._plugin_service.is_network_bound_method(method_name):
            return execute_func()

        monitor_context = None
        result = None

        try:
            logger.debug("HostMonitoringPlugin.ActivatedMonitoring", method_name)
            monitor_context = self._monitor_service.start_monitoring(
                connection,
                self._get_monitoring_host_info().all_aliases,
                self._get_monitoring_host_info(),
                self._props,
                self._failure_detection_time_ms,
                self._failure_detection_interval,
                self._failure_detection_count
            )
            result = execute_func()
        finally:
            if monitor_context:
                with self._lock:
                    self._monitor_service.stop_monitoring(monitor_context)
                    if monitor_context.is_host_unavailable():
                        self._plugin_service.set_availability(
                            self._get_monitoring_host_info().all_aliases, HostAvailability.UNAVAILABLE)

                        driver_dialect = self._plugin_service.driver_dialect
                        if driver_dialect is not None and not driver_dialect.is_closed(connection):
                            try:
                                connection.close()
                            except Exception:
                                pass
                            raise AwsWrapperError(
                                Messages.get_formatted("HostMonitoringPlugin.UnavailableHost", host_info.as_alias()))
                logger.debug("HostMonitoringPlugin.MonitoringDeactivated", method_name)

        return result

    def notify_connection_changed(self, changes: Set[ConnectionEvent]) -> OldConnectionSuggestedAction:
        self._monitoring_host_info = None
        if ConnectionEvent.INITIAL_CONNECTION in changes:
            self._is_connection_initialized = True

        return OldConnectionSuggestedAction.NO_OPINION

    def notify_host_list_changed(self, changes: Dict[str, Set[HostEvent]]):
        if not self._is_connection_initialized:
            return

        for host, events in changes.items():
            if HostEvent.WENT_DOWN not in events and HostEvent.HOST_DELETED not in events:
                return

            monitoring_aliases = self._get_monitoring_host_info().all_aliases
            if len(monitoring_aliases) == 0:
                return

            if host in monitoring_aliases:
                self._monitor_service.stop_monitoring_host(monitoring_aliases)

    def _get_monitoring_host_info(self) -> HostInfo:
        if self._monitoring_host_info is None:
            current_host_info = self._plugin_service.current_host_info
            if current_host_info is None:
                raise AwsWrapperError("HostMonitoringPlugin.HostInfoNone")
            self._monitoring_host_info = current_host_info
            rds_type = self._rds_utils.identify_rds_type(self._monitoring_host_info.host)

            try:
                if rds_type.is_rds_cluster:
                    logger.debug("HostMonitoringPlugin.ClusterEndpointHostInfo")
                    self._monitoring_host_info = self._plugin_service.identify_connection()
                    if self._monitoring_host_info is None:
                        raise AwsWrapperError(
                            Messages.get_formatted(
                                "HostMonitoringPlugin.UnableToIdentifyConnection",
                                current_host_info.host,
                                self._plugin_service.host_list_provider))
                    self._plugin_service.fill_aliases(host_info=self._monitoring_host_info)
            except Exception as e:
                message = "HostMonitoringPlugin.ErrorIdentifyingConnection"
                logger.debug(message, e)
                raise AwsWrapperError(Messages.get_formatted(message, e)) from e
        return self._monitoring_host_info

    def release_resources(self):
        if self._monitor_service is not None:
            self._monitor_service.release_resources()

        self._monitor_service = None


class MonitoringContext:
    """
    Monitoring context for each connection.
    This contains each connection's criteria for whether a server should be considered unhealthy.
    The context is shared between the main thread and the monitor thread.
    """
    def __init__(
            self,
            monitor: Monitor,
            connection: Connection,
            target_dialect: DriverDialect,
            failure_detection_time_ms: int,
            failure_detection_interval_ms: int,
            failure_detection_count: int,
            aborted_connections_counter: TelemetryCounter | None):
        self._monitor: Monitor = monitor
        self._connection: Connection = connection
        self._target_dialect: DriverDialect = target_dialect
        self._failure_detection_time_ms: int = failure_detection_time_ms
        self._failure_detection_interval_ms: int = failure_detection_interval_ms
        self._failure_detection_count: int = failure_detection_count
        self._aborted_connections_counter = aborted_connections_counter

        self._monitor_start_time_ns: int = 0  # Time of monitor context submission
        self._active_monitoring_start_time_ns: int = 0  # Time when the monitor should start checking the connection
        self._unavailable_host_start_time_ns: int = 0
        self._current_failure_count: int = 0
        self._is_host_unavailable: bool = False
        self._is_active: bool = True

    @property
    def failure_detection_interval_ms(self) -> int:
        return self._failure_detection_interval_ms

    @property
    def failure_detection_count(self) -> int:
        return self._failure_detection_count

    @property
    def active_monitoring_start_time_ns(self) -> int:
        return self._active_monitoring_start_time_ns

    @property
    def monitor(self) -> Monitor:
        return self._monitor

    @property
    def is_active(self) -> bool:
        return self._is_active

    @is_active.setter
    def is_active(self, is_active: bool):
        self._is_active = is_active

    def is_host_unavailable(self) -> bool:
        return self._is_host_unavailable

    def set_monitor_start_time_ns(self, start_time_ns: int):
        self._monitor_start_time_ns = start_time_ns
        self._active_monitoring_start_time_ns = start_time_ns + self._failure_detection_time_ms * 1_000_000

    def _abort_connection(self):
        if self._connection is None or not self._is_active:
            return
        try:
            if self._target_dialect.is_closed(self._connection):
                return
            self._target_dialect.abort_connection(self._connection)
        except Exception as e:
            # log and ignore
            logger.debug("MonitorContext.ExceptionAbortingConnection", e)

    def update_host_status(
            self, host: str, status_check_start_time_ns: int, status_check_end_time_ns: int, is_available: bool):
        """
        Update whether the connection is still valid if the total elapsed time has passed the grace period.

        :param host: the host for logging purposes.
        :param status_check_start_time_ns: the time when connection status check started in nanoseconds.
        :param status_check_end_time_ns: the time when connection status check ended in nanoseconds.
        :param is_available: whether the connection is valid.
        :return:
        """
        if not self._is_active:
            return
        total_elapsed_time_ns = status_check_end_time_ns - self._monitor_start_time_ns
        if total_elapsed_time_ns > (self._failure_detection_time_ms * 1_000_000):
            self._set_host_availability(host, is_available, status_check_start_time_ns, status_check_end_time_ns)

    def _set_host_availability(
            self, host: str, is_available: bool, status_check_start_time_ns: int, status_check_end_time_ns: int):
        """
        Set whether the connection to the server is still available based on the monitoring settings set in connection
        properties. The monitoring settings include:
        - failure_detection_time_ms
        - failure_detection_interval_ms
        - failure_detection_count

        :param host: the host for logging purposes.
        :param is_available: whether the connection is still available.
        :param status_check_start_time_ns: the time when connection status check started in nanoseconds.
        :param status_check_end_time_ns: the time when connection status check ended in nanoseconds.
        """
        if is_available:
            self._current_failure_count = 0
            self._unavailable_host_start_time_ns = 0
            self._is_host_unavailable = False
            return

        self._current_failure_count += 1
        if self._unavailable_host_start_time_ns <= 0:
            self._unavailable_host_start_time_ns = status_check_start_time_ns
        unavailable_host_duration_ns = status_check_end_time_ns - self._unavailable_host_start_time_ns
        max_unavailable_host_duration_ms = \
            self._failure_detection_interval_ms * max(0, self._failure_detection_count)

        if unavailable_host_duration_ns > (max_unavailable_host_duration_ms * 1_000_000):
            logger.debug("MonitorContext.HostUnavailable", host)
            self._is_host_unavailable = True
            self._abort_connection()
            if self._aborted_connections_counter is not None:
                self._aborted_connections_counter.inc()
            return

        logger.debug("MonitorContext.HostNotResponding", host, self._current_failure_count)
        return


class Monitor:
    """
     This class uses a background thread to monitor a particular server with one or more active :py:class:`Connection`
     objects.
    """
    _DEFAULT_CONNECT_TIMEOUT_SEC = 10
    _INACTIVE_SLEEP_MS = 100
    _MIN_HOST_CHECK_TIMEOUT_MS = 3000
    _QUERY = "SELECT 1"

    def __init__(
            self,
            plugin_service: PluginService,
            host_info: HostInfo,
            props: Properties,
            monitor_container: MonitoringThreadContainer):
        self._plugin_service: PluginService = plugin_service
        self._host_info: HostInfo = host_info
        self._props: Properties = props
        self._monitor_container: MonitoringThreadContainer = monitor_container
        self._telemetry_factory = self._plugin_service.get_telemetry_factory()

        self._lock: Lock = Lock()
        self._active_contexts: Queue[MonitoringContext] = Queue()
        self._new_contexts: Queue[MonitoringContext] = Queue()
        self._monitoring_conn: Optional[Connection] = None
        self._is_stopped: Event = Event()
        self._monitor_disposal_time_ms: int = WrapperProperties.MONITOR_DISPOSAL_TIME_MS.get_int(props)
        self._context_last_used_ns: int = perf_counter_ns()
        self._host_check_timeout_ms: int = Monitor._MIN_HOST_CHECK_TIMEOUT_MS

        host_id = self._host_info.host_id if self._host_info.host_id is not None else self._host_info.host
        self._host_invalid_counter = self._telemetry_factory.create_counter(
            f"host_monitoring.host_unhealthy.count.{host_id}")

    @dataclass
    class HostStatus:
        is_available: bool
        elapsed_time_ns: int

    @property
    def is_stopped(self):
        return self._is_stopped.is_set()

    def stop(self):
        self._is_stopped.set()

    def start_monitoring(self, context: MonitoringContext):
        current_time_ns = perf_counter_ns()
        context.set_monitor_start_time_ns(current_time_ns)
        self._context_last_used_ns = current_time_ns
        self._new_contexts.put(context)

    def stop_monitoring(self, context: MonitoringContext):
        if context is None:
            logger.warning("Monitor.ContextNone")
            return

        context.is_active = False
        self._context_last_used_ns = perf_counter_ns()

    def clear_contexts(self):
        QueueUtils.clear(self._new_contexts)
        QueueUtils.clear(self._active_contexts)

    def run(self):
        try:
            self._is_stopped.clear()

            while not self.is_stopped:
                try:
                    current_time_ns = perf_counter_ns()
                    first_added_new_context = None

                    # Process new contexts
                    while (new_monitor_context := QueueUtils.get(self._new_contexts)) is not None:
                        if first_added_new_context == new_monitor_context:
                            # This context has already been processed.
                            # Add it back to the queue and process it in the next round.
                            self._new_contexts.put(new_monitor_context)
                            break

                        if not new_monitor_context.is_active:
                            # Discard inactive contexts
                            continue

                        if current_time_ns >= new_monitor_context.active_monitoring_start_time_ns:
                            # Submit the context for active monitoring
                            self._active_contexts.put(new_monitor_context)
                            continue

                        # The active monitoring start time has not been hit yet.
                        # Add the context back to the queue and check it later.
                        self._new_contexts.put(new_monitor_context)
                        if first_added_new_context is None:
                            first_added_new_context = new_monitor_context

                    if self._active_contexts.empty():
                        if (perf_counter_ns() - self._context_last_used_ns) >= self._monitor_disposal_time_ms * 1_000_000:
                            self._monitor_container.release_monitor(self)
                            break

                        self.sleep(Monitor._INACTIVE_SLEEP_MS / 1000)
                        continue

                    status_check_start_time_ns = perf_counter_ns()
                    self._context_last_used_ns = status_check_start_time_ns
                    status = self._check_host_status(self._host_check_timeout_ms)
                    delay_ms = -1
                    first_added_new_context = None

                    monitor_context: MonitoringContext
                    while (monitor_context := QueueUtils.get(self._active_contexts)) is not None:
                        with self._lock:
                            if not monitor_context.is_active:
                                # Discard inactive contexts
                                continue

                            if first_added_new_context == monitor_context:
                                # This context has already been processed by this loop.
                                # Add it back to the queue and exit the loop.
                                self._active_contexts.put(monitor_context)
                                break

                            # Process the context
                            monitor_context.update_host_status(
                                self._host_info.url,
                                status_check_start_time_ns,
                                status_check_start_time_ns + status.elapsed_time_ns,
                                status.is_available)

                            if not monitor_context.is_active or monitor_context.is_host_unavailable():
                                continue

                            # The context is still active and the host is still available. Continue monitoring the context.
                            self._active_contexts.put(monitor_context)
                            if first_added_new_context is None:
                                first_added_new_context = monitor_context

                            if delay_ms == -1 or delay_ms > monitor_context.failure_detection_interval_ms:
                                delay_ms = monitor_context.failure_detection_interval_ms

                    if delay_ms == -1:
                        delay_ms = Monitor._INACTIVE_SLEEP_MS
                    else:
                        # Subtract the time taken for the status check from the delay
                        delay_ms -= (status.elapsed_time_ns / 1_000_000)
                        if delay_ms <= 0:
                            delay_ms = Monitor._MIN_HOST_CHECK_TIMEOUT_MS
                        # Use this delay for all active contexts
                        self._host_check_timeout_ms = delay_ms

                    self.sleep(delay_ms / 1000)
                except InterruptedError as e:
                    raise e
                except Exception as e:
                    logger.debug("Monitor.ExceptionInMonitorLoop", self._host_info.host)
                    logger.debug(e, exc_info=True)
        except InterruptedError:
            logger.warning("Monitor.InterruptedException", self._host_info.host)
        except Exception as e:
            logger.debug("Monitor.StoppingMonitorUnhandledException", self._host_info.host)
            logger.debug(e, exc_info=True)
        finally:
            self._monitor_container.release_monitor(self)
            self.stop()
            if self._monitoring_conn is not None:
                try:
                    self._monitoring_conn.close()
                except Exception:
                    # Do nothing
                    pass
            self.stop()

    def _check_host_status(self, host_check_timeout_ms: int) -> HostStatus:
        context = self._telemetry_factory.open_telemetry_context(
            "connection status check", TelemetryTraceLevel.FORCE_TOP_LEVEL)

        if context is not None:
            context.set_attribute("url", self._host_info.url)

        start_ns = perf_counter_ns()
        try:
            driver_dialect = self._plugin_service.driver_dialect
            if self._monitoring_conn is None or driver_dialect.is_closed(self._monitoring_conn):
                monitoring_properties: Properties = PropertiesUtils.create_monitoring_properties(self._props)

                # Set a default connect timeout if the user hasn't configured one
                if monitoring_properties.get(WrapperProperties.CONNECT_TIMEOUT_SEC.name, None) is None:
                    monitoring_properties[WrapperProperties.CONNECT_TIMEOUT_SEC.name] = Monitor._DEFAULT_CONNECT_TIMEOUT_SEC

                logger.debug("Monitor.OpeningMonitorConnection", self._host_info.url)
                start_ns = perf_counter_ns()
                self._monitoring_conn = self._plugin_service.force_connect(self._host_info, monitoring_properties, None)
                logger.debug("Monitor.OpenedMonitorConnection", self._host_info.url)
                return Monitor.HostStatus(True, perf_counter_ns() - start_ns)

            start_ns = perf_counter_ns()
            is_available = self._is_host_available(self._monitoring_conn, host_check_timeout_ms / 1000)
            if not is_available:
                if self._host_invalid_counter is not None:
                    self._host_invalid_counter.inc()
            return Monitor.HostStatus(is_available, perf_counter_ns() - start_ns)
        except Exception:
            if self._host_invalid_counter is not None:
                self._host_invalid_counter.inc()
            return Monitor.HostStatus(False, perf_counter_ns() - start_ns)
        finally:
            if context is not None:
                context.close_context()

    def _is_host_available(self, conn: Connection, timeout_sec: float) -> bool:
        try:
            self._execute_conn_check(conn, timeout_sec)
            return True
        except TimeoutError:
            return False

    def _execute_conn_check(self, conn: Connection, timeout_sec: float):
        driver_dialect = self._plugin_service.driver_dialect
        with conn.cursor() as cursor:
            query = Monitor._QUERY
            driver_dialect.execute(DbApiMethod.CURSOR_EXECUTE.method_name, lambda: cursor.execute(query), query, exec_timeout=timeout_sec)
            cursor.fetchone()

    def sleep(self, duration: int):
        self._is_stopped.wait(duration)


class MonitoringThreadContainer:
    """
    This singleton class keeps track of all the monitoring threads and handles the creation and clean up of each
    monitoring thread.
    """

    _instance: ClassVar[Optional[MonitoringThreadContainer]] = None
    _lock: ClassVar[RLock] = RLock()
    _monitor_lock: ClassVar[RLock] = RLock()

    _monitor_map: ConcurrentDict[str, Monitor] = ConcurrentDict()
    _tasks_map: ConcurrentDict[Monitor, Future] = ConcurrentDict()
    _executor_name: ClassVar[str] = "MonitoringThreadContainerExecutor"

    def __init__(self):
        self._thread_pool = ThreadPoolContainer.get_thread_pool(self._executor_name)

    # This logic ensures that this class is a Singleton
    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            with cls._lock:
                if not cls._instance:
                    cls._instance = super().__new__(cls, *args, **kwargs)
        return cls._instance

    def get_or_create_monitor(self, host_aliases: FrozenSet[str], monitor_supplier: Callable) -> Monitor:
        if not host_aliases:
            raise AwsWrapperError(Messages.get("MonitoringThreadContainer.EmptyHostKeys"))

        with self._monitor_lock:
            monitor = None
            any_alias = next(iter(host_aliases))
            for host_alias in host_aliases:
                monitor = self._monitor_map.get(host_alias)
                any_alias = host_alias
                if monitor is not None:
                    break

            def _get_or_create_monitor(_) -> Monitor:
                supplied_monitor = monitor_supplier()
                if supplied_monitor is None:
                    raise AwsWrapperError(Messages.get("MonitoringThreadContainer.SupplierMonitorNone"))
                self._tasks_map.compute_if_absent(
                    supplied_monitor,
                    lambda _: self._thread_pool.submit(supplied_monitor.run))
                return supplied_monitor

            if monitor is None:
                monitor = self._monitor_map.compute_if_absent(any_alias, _get_or_create_monitor)
                if monitor is None:
                    raise AwsWrapperError(
                        Messages.get_formatted("MonitoringThreadContainer.ErrorGettingMonitor", host_aliases))

            for host_alias in host_aliases:
                self._monitor_map.put_if_absent(host_alias, monitor)

            return monitor

    @staticmethod
    def _cancel(monitor, future: Future) -> None:
        future.cancel()
        monitor.stop()
        return None

    def get_monitor(self, alias: str) -> Optional[Monitor]:
        return self._monitor_map.get(alias)

    def release_monitor(self, monitor: Monitor):
        with self._monitor_lock:
            self._monitor_map.remove_matching_values([monitor])
            self._tasks_map.compute_if_present(monitor, MonitoringThreadContainer._cancel)

    @staticmethod
    def clean_up():
        """Clean up any dangling monitoring threads created by the host monitoring plugin.
        This method should be called at the end of the application.
        The Host Monitoring Plugin creates monitoring threads in the background to monitor all connections established to each of cluster instances.
        The threads will terminate if there are no connections to the cluster instance the thread is monitoring for over a period of time,
        specified by the `monitor_disposal_time_ms`. Client applications can also manually call this method to clean up any dangling resources.
        This method should be called right before application termination.
        """
        if MonitoringThreadContainer._instance is None:
            return

        with MonitoringThreadContainer._lock:
            if MonitoringThreadContainer._instance is not None:
                MonitoringThreadContainer._instance._release_resources()
                MonitoringThreadContainer._instance = None

    def _release_resources(self):
        with self._monitor_lock:
            self._monitor_map.clear()
            self._tasks_map.apply_if(
                lambda monitor, future: not future.done() and not future.cancelled(),
                lambda monitor, future: future.cancel())

            for monitor, _ in self._tasks_map.items():
                monitor.stop()

            ThreadPoolContainer.release_pool(MonitoringThreadContainer._executor_name, wait=False)
            self._tasks_map.clear()


class MonitorService:
    def __init__(self, plugin_service: PluginService):
        self._plugin_service: PluginService = plugin_service
        self._monitor_container: MonitoringThreadContainer = MonitoringThreadContainer()
        self._cached_monitor_aliases: Optional[FrozenSet[str]] = None
        self._cached_monitor: Optional[ReferenceType[Monitor]] = None

        telemetry_factory = self._plugin_service.get_telemetry_factory()
        self._aborted_connections_counter = telemetry_factory.create_counter("host_monitoring.connections.aborted")

    def start_monitoring(self,
                         conn: Connection,
                         host_aliases: FrozenSet[str],
                         host_info: HostInfo,
                         props: Properties,
                         failure_detection_time_ms: int,
                         failure_detection_interval_ms: int,
                         failure_detection_count: int) -> MonitoringContext:
        if not host_aliases:
            raise AwsWrapperError(Messages.get_formatted("MonitorService.EmptyAliasSet", host_info))

        monitor: Optional[Monitor] = None if self._cached_monitor is None else self._cached_monitor()
        if monitor is None \
                or monitor.is_stopped \
                or self._cached_monitor_aliases is None \
                or self._cached_monitor_aliases != host_aliases:
            monitor = self._monitor_container.get_or_create_monitor(
                host_aliases, lambda: self._create_monitor(host_info, props, self._monitor_container))
            self._cached_monitor = ref(monitor)
            self._cached_monitor_aliases = host_aliases

        context = MonitoringContext(
            monitor, conn, self._plugin_service.driver_dialect, failure_detection_time_ms,
            failure_detection_interval_ms, failure_detection_count, self._aborted_connections_counter)
        monitor.start_monitoring(context)
        return context

    def _create_monitor(self, host_info: HostInfo, props: Properties, monitor_container: MonitoringThreadContainer):
        return Monitor(self._plugin_service, host_info, props, monitor_container)

    @staticmethod
    def stop_monitoring(context: MonitoringContext):
        monitor = context.monitor
        monitor.stop_monitoring(context)

    def stop_monitoring_host(self, host_aliases: FrozenSet):
        for alias in host_aliases:
            monitor = self._monitor_container.get_monitor(alias)
            if monitor is not None:
                monitor.clear_contexts()
                return

    def release_resources(self):
        self._monitor_container = None