path: root/kafka/coordinator/base.py

from __future__ import absolute_import, division

import abc
import copy
import logging
import time
import weakref

from kafka.vendor import six

from .heartbeat import Heartbeat
from .. import errors as Errors
from ..future import Future
from ..metrics import AnonMeasurable
from ..metrics.stats import Avg, Count, Max, Rate
from ..protocol.commit import GroupCoordinatorRequest, OffsetCommitRequest
from ..protocol.group import (HeartbeatRequest, JoinGroupRequest,
                            LeaveGroupRequest, SyncGroupRequest)

log = logging.getLogger('kafka.coordinator')


class BaseCoordinator(object):
    """
    BaseCoordinator implements group management for a single group member
    by interacting with a designated Kafka broker (the coordinator). Group
    semantics are provided by extending this class.  See ConsumerCoordinator
    for example usage.

    From a high level, Kafka's group management protocol consists of the
    following sequence of actions:

    1. Group Registration: Group members register with the coordinator providing
       their own metadata (such as the set of topics they are interested in).

    2. Group/Leader Selection: The coordinator select the members of the group
       and chooses one member as the leader.

    3. State Assignment: The leader collects the metadata from all the members
       of the group and assigns state.

    4. Group Stabilization: Each member receives the state assigned by the
       leader and begins processing.

    To leverage this protocol, an implementation must define the format of
    metadata provided by each member for group registration in
    :meth:`.group_protocols` and the format of the state assignment provided by
    the leader in :meth:`._perform_assignment` and which becomes available to
    members in :meth:`._on_join_complete`.
    """

    DEFAULT_CONFIG = {
        'group_id': 'kafka-python-default-group',
        'session_timeout_ms': 30000,
        'heartbeat_interval_ms': 3000,
        'retry_backoff_ms': 100,
        'api_version': (0, 9),
        'metric_group_prefix': '',
    }

    def __init__(self, client, metrics, **configs):
        """
        Keyword Arguments:
            group_id (str): name of the consumer group to join for dynamic
                partition assignment (if enabled), and to use for fetching and
                committing offsets. Default: 'kafka-python-default-group'
            session_timeout_ms (int): The timeout used to detect failures when
                using Kafka's group managementment facilities. Default: 30000
            heartbeat_interval_ms (int): The expected time in milliseconds
                between heartbeats to the consumer coordinator when using
                Kafka's group management feature. Heartbeats are used to ensure
                that the consumer's session stays active and to facilitate
                rebalancing when new consumers join or leave the group. The
                value must be set lower than session_timeout_ms, but typically
                should be set no higher than 1/3 of that value. It can be
                adjusted even lower to control the expected time for normal
                rebalances. Default: 3000
            retry_backoff_ms (int): Milliseconds to backoff when retrying on
                errors. Default: 100.
        """
        self.config = copy.copy(self.DEFAULT_CONFIG)
        for key in self.config:
            if key in configs:
                self.config[key] = configs[key]

        self._client = client
        self.generation = OffsetCommitRequest[2].DEFAULT_GENERATION_ID
        self.member_id = JoinGroupRequest[0].UNKNOWN_MEMBER_ID
        self.group_id = self.config['group_id']
        self.coordinator_id = None
        self.rejoin_needed = True
        self.rejoining = False
        self.heartbeat = Heartbeat(**self.config)
        self.heartbeat_task = HeartbeatTask(weakref.proxy(self))
        self.sensors = GroupCoordinatorMetrics(self.heartbeat, metrics,
                                               self.config['metric_group_prefix'])

    def __del__(self):
        if hasattr(self, 'heartbeat_task') and self.heartbeat_task:
            self.heartbeat_task.disable()

    @abc.abstractmethod
    def protocol_type(self):
        """
        Unique identifier for the class of protocols implements
        (e.g. "consumer" or "connect").

        Returns:
            str: protocol type name
        """
        pass

    @abc.abstractmethod
    def group_protocols(self):
        """Return the list of supported group protocols and metadata.

        This list is submitted by each group member via a JoinGroupRequest.
        The order of the protocols in the list indicates the preference of the
        protocol (the first entry is the most preferred). The coordinator takes
        this preference into account when selecting the generation protocol
        (generally more preferred protocols will be selected as long as all
        members support them and there is no disagreement on the preference).

        Note: metadata must be type bytes or support an encode() method

        Returns:
            list: [(protocol, metadata), ...]
        """
        pass

    @abc.abstractmethod
    def _on_join_prepare(self, generation, member_id):
        """Invoked prior to each group join or rejoin.

        This is typically used to perform any cleanup from the previous
        generation (such as committing offsets for the consumer)

        Arguments:
            generation (int): The previous generation or -1 if there was none
            member_id (str): The identifier of this member in the previous group
                or '' if there was none
        """
        pass

    @abc.abstractmethod
    def _perform_assignment(self, leader_id, protocol, members):
        """Perform assignment for the group.

        This is used by the leader to push state to all the members of the group
        (e.g. to push partition assignments in the case of the new consumer)

        Arguments:
            leader_id (str): The id of the leader (which is this member)
            protocol (str): the chosen group protocol (assignment strategy)
            members (list): [(member_id, metadata_bytes)] from
                JoinGroupResponse. metadata_bytes are associated with the chosen
                group protocol, and the Coordinator subclass is responsible for
                decoding metadata_bytes based on that protocol.

        Returns:
            dict: {member_id: assignment}; assignment must either be bytes
                or have an encode() method to convert to bytes
        """
        pass

    @abc.abstractmethod
    def _on_join_complete(self, generation, member_id, protocol,
                          member_assignment_bytes):
        """Invoked when a group member has successfully joined a group.

        Arguments:
            generation (int): the generation that was joined
            member_id (str): the identifier for the local member in the group
            protocol (str): the protocol selected by the coordinator
            member_assignment_bytes (bytes): the protocol-encoded assignment
                propagated from the group leader. The Coordinator instance is
                responsible for decoding based on the chosen protocol.
        """
        pass

    def coordinator_unknown(self):
        """Check if we know who the coordinator is and have an active connection

        Side-effect: reset coordinator_id to None if connection failed

        Returns:
            bool: True if the coordinator is unknown
        """
        if self.coordinator_id is None:
            return True

        if self._client.is_disconnected(self.coordinator_id):
            self.coordinator_dead('Node Disconnected')
            return True

        return False

    def ensure_coordinator_known(self):
        """Block until the coordinator for this group is known
        (and we have an active connection -- java client uses unsent queue).
        """
        while self.coordinator_unknown():

            # Prior to 0.8.2 there was no group coordinator
            # so we will just pick a node at random and treat
            # it as the "coordinator"
            if self.config['api_version'] < (0, 8, 2):
                self.coordinator_id = self._client.least_loaded_node()
                if self.coordinator_id is not None:
                    self._client.ready(self.coordinator_id)
                continue

            future = self._send_group_coordinator_request()
            self._client.poll(future=future)

            if future.failed():
                if future.retriable():
                    if getattr(future.exception, 'invalid_metadata', False):
                        log.debug('Requesting metadata for group coordinator request: %s', future.exception)
                        metadata_update = self._client.cluster.request_update()
                        self._client.poll(future=metadata_update)
                else:
                    raise future.exception  # pylint: disable-msg=raising-bad-type

    def need_rejoin(self):
        """Check whether the group should be rejoined (e.g. if metadata changes)

        Returns:
            bool: True if it should, False otherwise
        """
        return self.rejoin_needed

    def ensure_active_group(self):
        """Ensure that the group is active (i.e. joined and synced)"""
        if not self.need_rejoin():
            return

        if not self.rejoining:
            self._on_join_prepare(self.generation, self.member_id)
            self.rejoining = True

        while self.need_rejoin():
            self.ensure_coordinator_known()

            # ensure that there are no pending requests to the coordinator.
            # This is important in particular to avoid resending a pending
            # JoinGroup request.
            while not self.coordinator_unknown():
                if not self._client.in_flight_request_count(self.coordinator_id):
                    break
                self._client.poll(delayed_tasks=False)
            else:
                continue

            future = self._send_join_group_request()
            self._client.poll(future=future)

            if future.succeeded():
                member_assignment_bytes = future.value
                self._on_join_complete(self.generation, self.member_id,
                                       self.protocol, member_assignment_bytes)
                self.rejoining = False
                self.heartbeat_task.reset()
            else:
                assert future.failed()
                exception = future.exception
                if isinstance(exception, (Errors.UnknownMemberIdError,
                                          Errors.RebalanceInProgressError,
                                          Errors.IllegalGenerationError)):
                    continue
                elif not future.retriable():
                    raise exception  # pylint: disable-msg=raising-bad-type
                time.sleep(self.config['retry_backoff_ms'] / 1000)

    def _send_join_group_request(self):
        """Join the group and return the assignment for the next generation.

        This function handles both JoinGroup and SyncGroup, delegating to
        :meth:`._perform_assignment` if elected leader by the coordinator.

        Returns:
            Future: resolves to the encoded-bytes assignment returned from the
                group leader
        """
        if self.coordinator_unknown():
            e = Errors.GroupCoordinatorNotAvailableError(self.coordinator_id)
            return Future().failure(e)

        # send a join group request to the coordinator
        log.info("(Re-)joining group %s", self.group_id)
        request = JoinGroupRequest[0](
            self.group_id,
            self.config['session_timeout_ms'],
            self.member_id,
            self.protocol_type(),
            [(protocol,
              metadata if isinstance(metadata, bytes) else metadata.encode())
             for protocol, metadata in self.group_protocols()])

        # create the request for the coordinator
        log.debug("Sending JoinGroup (%s) to coordinator %s", request, self.coordinator_id)
        future = Future()
        _f = self._client.send(self.coordinator_id, request)
        _f.add_callback(self._handle_join_group_response, future, time.time())
        _f.add_errback(self._failed_request, self.coordinator_id,
                       request, future)
        return future

    def _failed_request(self, node_id, request, future, error):
        log.error('Error sending %s to node %s [%s]',
                  request.__class__.__name__, node_id, error)
        # Marking coordinator dead
        # unless the error is caused by internal client pipelining
        if not isinstance(error, (Errors.NodeNotReadyError,
                                  Errors.TooManyInFlightRequests)):
            self.coordinator_dead(error)
        future.failure(error)

    def _handle_join_group_response(self, future, send_time, response):
        error_type = Errors.for_code(response.error_code)
        if error_type is Errors.NoError:
            log.debug("Received successful JoinGroup response for group %s: %s",
                      self.group_id, response)
            self.member_id = response.member_id
            self.generation = response.generation_id
            self.rejoin_needed = False
            self.protocol = response.group_protocol
            log.info("Joined group '%s' (generation %s) with member_id %s",
                     self.group_id, self.generation, self.member_id)
            self.sensors.join_latency.record((time.time() - send_time) * 1000)
            if response.leader_id == response.member_id:
                log.info("Elected group leader -- performing partition"
                         " assignments using %s", self.protocol)
                self._on_join_leader(response).chain(future)
            else:
                self._on_join_follower().chain(future)

        elif error_type is Errors.GroupLoadInProgressError:
            log.debug("Attempt to join group %s rejected since coordinator %s"
                      " is loading the group.", self.group_id, self.coordinator_id)
            # backoff and retry
            future.failure(error_type(response))
        elif error_type is Errors.UnknownMemberIdError:
            # reset the member id and retry immediately
            error = error_type(self.member_id)
            self.member_id = JoinGroupRequest[0].UNKNOWN_MEMBER_ID
            log.debug("Attempt to join group %s failed due to unknown member id",
                      self.group_id)
            future.failure(error)
        elif error_type in (Errors.GroupCoordinatorNotAvailableError,
                            Errors.NotCoordinatorForGroupError):
            # re-discover the coordinator and retry with backoff
            self.coordinator_dead(error_type())
            log.debug("Attempt to join group %s failed due to obsolete "
                      "coordinator information: %s", self.group_id,
                      error_type.__name__)
            future.failure(error_type())
        elif error_type in (Errors.InconsistentGroupProtocolError,
                            Errors.InvalidSessionTimeoutError,
                            Errors.InvalidGroupIdError):
            # log the error and re-throw the exception
            error = error_type(response)
            log.error("Attempt to join group %s failed due to fatal error: %s",
                      self.group_id, error)
            future.failure(error)
        elif error_type is Errors.GroupAuthorizationFailedError:
            future.failure(error_type(self.group_id))
        else:
            # unexpected error, throw the exception
            error = error_type()
            log.error("Unexpected error in join group response: %s", error)
            future.failure(error)

    def _on_join_follower(self):
        # send follower's sync group with an empty assignment
        request = SyncGroupRequest[0](
            self.group_id,
            self.generation,
            self.member_id,
            {})
        log.debug("Sending follower SyncGroup for group %s to coordinator %s: %s",
                  self.group_id, self.coordinator_id, request)
        return self._send_sync_group_request(request)

    def _on_join_leader(self, response):
        """
        Perform leader synchronization and send back the assignment
        for the group via SyncGroupRequest

        Arguments:
            response (JoinResponse): broker response to parse

        Returns:
            Future: resolves to member assignment encoded-bytes
        """
        try:
            group_assignment = self._perform_assignment(response.leader_id,
                                                        response.group_protocol,
                                                        response.members)
        except Exception as e:
            return Future().failure(e)

        request = SyncGroupRequest[0](
            self.group_id,
            self.generation,
            self.member_id,
            [(member_id,
              assignment if isinstance(assignment, bytes) else assignment.encode())
             for member_id, assignment in six.iteritems(group_assignment)])

        log.debug("Sending leader SyncGroup for group %s to coordinator %s: %s",
                  self.group_id, self.coordinator_id, request)
        return self._send_sync_group_request(request)

    def _send_sync_group_request(self, request):
        if self.coordinator_unknown():
            e = Errors.GroupCoordinatorNotAvailableError(self.coordinator_id)
            return Future().failure(e)
        future = Future()
        _f = self._client.send(self.coordinator_id, request)
        _f.add_callback(self._handle_sync_group_response, future, time.time())
        _f.add_errback(self._failed_request, self.coordinator_id,
                       request, future)
        return future

    def _handle_sync_group_response(self, future, send_time, response):
        error_type = Errors.for_code(response.error_code)
        if error_type is Errors.NoError:
            log.info("Successfully joined group %s with generation %s",
                    self.group_id, self.generation)
            self.sensors.sync_latency.record((time.time() - send_time) * 1000)
            future.success(response.member_assignment)
            return

        # Always rejoin on error
        self.rejoin_needed = True
        if error_type is Errors.GroupAuthorizationFailedError:
            future.failure(error_type(self.group_id))
        elif error_type is Errors.RebalanceInProgressError:
            log.debug("SyncGroup for group %s failed due to coordinator"
                      " rebalance", self.group_id)
            future.failure(error_type(self.group_id))
        elif error_type in (Errors.UnknownMemberIdError,
                            Errors.IllegalGenerationError):
            error = error_type()
            log.debug("SyncGroup for group %s failed due to %s", self.group_id, error)
            self.member_id = JoinGroupRequest[0].UNKNOWN_MEMBER_ID
            future.failure(error)
        elif error_type in (Errors.GroupCoordinatorNotAvailableError,
                            Errors.NotCoordinatorForGroupError):
            error = error_type()
            log.debug("SyncGroup for group %s failed due to %s", self.group_id, error)
            self.coordinator_dead(error)
            future.failure(error)
        else:
            error = error_type()
            log.error("Unexpected error from SyncGroup: %s", error)
            future.failure(error)

    def _send_group_coordinator_request(self):
        """Discover the current coordinator for the group.

        Returns:
            Future: resolves to the node id of the coordinator
        """
        node_id = self._client.least_loaded_node()
        if node_id is None:
            return Future().failure(Errors.NoBrokersAvailable())

        log.debug("Sending group coordinator request for group %s to broker %s",
                  self.group_id, node_id)
        request = GroupCoordinatorRequest[0](self.group_id)
        future = Future()
        _f = self._client.send(node_id, request)
        _f.add_callback(self._handle_group_coordinator_response, future)
        _f.add_errback(self._failed_request, node_id, request, future)
        return future

    def _handle_group_coordinator_response(self, future, response):
        log.debug("Received group coordinator response %s", response)
        if not self.coordinator_unknown():
            # We already found the coordinator, so ignore the request
            log.debug("Coordinator already known -- ignoring metadata response")
            future.success(self.coordinator_id)
            return

        error_type = Errors.for_code(response.error_code)
        if error_type is Errors.NoError:
            ok = self._client.cluster.add_group_coordinator(self.group_id, response)
            if not ok:
                # This could happen if coordinator metadata is different
                # than broker metadata
                future.failure(Errors.IllegalStateError())
                return

            self.coordinator_id = response.coordinator_id
            log.info("Discovered coordinator %s for group %s",
                     self.coordinator_id, self.group_id)
            self._client.ready(self.coordinator_id)

            # start sending heartbeats only if we have a valid generation
            if self.generation > 0:
                self.heartbeat_task.reset()
            future.success(self.coordinator_id)
        elif error_type is Errors.GroupCoordinatorNotAvailableError:
            log.debug("Group Coordinator Not Available; retry")
            future.failure(error_type())
        elif error_type is Errors.GroupAuthorizationFailedError:
            error = error_type(self.group_id)
            log.error("Group Coordinator Request failed: %s", error)
            future.failure(error)
        else:
            error = error_type()
            log.error("Unrecognized failure in Group Coordinator Request: %s",
                      error)
            future.failure(error)

    def coordinator_dead(self, error):
        """Mark the current coordinator as dead."""
        if self.coordinator_id is not None:
            log.warning("Marking the coordinator dead (node %s) for group %s: %s.",
                        self.coordinator_id, self.group_id, error)
            self.coordinator_id = None

    def close(self):
        """Close the coordinator, leave the current group
        and reset local generation/memberId."""
        try:
            self._client.unschedule(self.heartbeat_task)
        except KeyError:
            pass

        if not self.coordinator_unknown() and self.generation > 0:
            # this is a minimal effort attempt to leave the group. we do not
            # attempt any resending if the request fails or times out.
            log.info('Leaving consumer group (%s).', self.group_id)
            request = LeaveGroupRequest[0](self.group_id, self.member_id)
            future = self._client.send(self.coordinator_id, request)
            future.add_callback(self._handle_leave_group_response)
            future.add_errback(log.error, "LeaveGroup request failed: %s")
            self._client.poll(future=future)

        self.generation = OffsetCommitRequest[2].DEFAULT_GENERATION_ID
        self.member_id = JoinGroupRequest[0].UNKNOWN_MEMBER_ID
        self.rejoin_needed = True

    def _handle_leave_group_response(self, response):
        error_type = Errors.for_code(response.error_code)
        if error_type is Errors.NoError:
            log.info("LeaveGroup request succeeded")
        else:
            log.error("LeaveGroup request failed: %s", error_type())

    def _send_heartbeat_request(self):
        """Send a heartbeat request"""
        request = HeartbeatRequest[0](self.group_id, self.generation, self.member_id)
        log.debug("Heartbeat: %s[%s] %s", request.group, request.generation_id, request.member_id)  # pylint: disable-msg=no-member
        future = Future()
        _f = self._client.send(self.coordinator_id, request)
        _f.add_callback(self._handle_heartbeat_response, future, time.time())
        _f.add_errback(self._failed_request, self.coordinator_id,
                       request, future)
        return future

    def _handle_heartbeat_response(self, future, send_time, response):
        self.sensors.heartbeat_latency.record((time.time() - send_time) * 1000)
        error_type = Errors.for_code(response.error_code)
        if error_type is Errors.NoError:
            log.debug("Received successful heartbeat response for group %s",
                      self.group_id)
            future.success(None)
        elif error_type in (Errors.GroupCoordinatorNotAvailableError,
                            Errors.NotCoordinatorForGroupError):
            log.warning("Heartbeat failed for group %s: coordinator (node %s)"
                        " is either not started or not valid", self.group_id,
                        self.coordinator_id)
            self.coordinator_dead(error_type())
            future.failure(error_type())
        elif error_type is Errors.RebalanceInProgressError:
            log.warning("Heartbeat failed for group %s because it is"
                        " rebalancing", self.group_id)
            self.rejoin_needed = True
            future.failure(error_type())
        elif error_type is Errors.IllegalGenerationError:
            log.warning("Heartbeat failed for group %s: generation id is not "
                        " current.", self.group_id)
            self.rejoin_needed = True
            future.failure(error_type())
        elif error_type is Errors.UnknownMemberIdError:
            log.warning("Heartbeat: local member_id was not recognized;"
                        " this consumer needs to re-join")
            self.member_id = JoinGroupRequest[0].UNKNOWN_MEMBER_ID
            self.rejoin_needed = True
            future.failure(error_type)
        elif error_type is Errors.GroupAuthorizationFailedError:
            error = error_type(self.group_id)
            log.error("Heartbeat failed: authorization error: %s", error)
            future.failure(error)
        else:
            error = error_type()
            log.error("Heartbeat failed: Unhandled error: %s", error)
            future.failure(error)


class HeartbeatTask(object):
    def __init__(self, coordinator):
        self._coordinator = coordinator
        self._heartbeat = coordinator.heartbeat
        self._client = coordinator._client
        self._request_in_flight = False

    def disable(self):
        try:
            self._client.unschedule(self)
        except KeyError:
            pass

    def reset(self):
        # start or restart the heartbeat task to be executed at the next chance
        self._heartbeat.reset_session_timeout()
        try:
            self._client.unschedule(self)
        except KeyError:
            pass
        if not self._request_in_flight:
            self._client.schedule(self, time.time())

    def __call__(self):
        if (self._coordinator.generation < 0 or
                self._coordinator.need_rejoin()):
            # no need to send the heartbeat we're not using auto-assignment
            # or if we are awaiting a rebalance
            log.info("Skipping heartbeat: no auto-assignment"
                     " or waiting on rebalance")
            return

        if self._coordinator.coordinator_unknown():
            log.warning("Coordinator unknown during heartbeat -- will retry")
            self._handle_heartbeat_failure(Errors.GroupCoordinatorNotAvailableError())
            return

        if self._heartbeat.session_expired():
            # we haven't received a successful heartbeat in one session interval
            # so mark the coordinator dead
            log.error("Heartbeat session expired - marking coordinator dead")
            self._coordinator.coordinator_dead('Heartbeat session expired')
            return

        if not self._heartbeat.should_heartbeat():
            # we don't need to heartbeat now, so reschedule for when we do
            ttl = self._heartbeat.ttl()
            log.debug("Heartbeat task unneeded now, retrying in %s", ttl)
            self._client.schedule(self, time.time() + ttl)
        else:
            self._heartbeat.sent_heartbeat()
            self._request_in_flight = True
            future = self._coordinator._send_heartbeat_request()
            future.add_callback(self._handle_heartbeat_success)
            future.add_errback(self._handle_heartbeat_failure)

    def _handle_heartbeat_success(self, v):
        log.debug("Received successful heartbeat")
        self._request_in_flight = False
        self._heartbeat.received_heartbeat()
        ttl = self._heartbeat.ttl()
        self._client.schedule(self, time.time() + ttl)

    def _handle_heartbeat_failure(self, e):
        log.warning("Heartbeat failed (%s); retrying", e)
        self._request_in_flight = False
        etd = time.time() + self._coordinator.config['retry_backoff_ms'] / 1000
        self._client.schedule(self, etd)


class GroupCoordinatorMetrics(object):
    def __init__(self, heartbeat, metrics, prefix, tags=None):
        self.heartbeat = heartbeat
        self.metrics = metrics
        self.metric_group_name = prefix + "-coordinator-metrics"

        self.heartbeat_latency = metrics.sensor('heartbeat-latency')
        self.heartbeat_latency.add(metrics.metric_name(
            'heartbeat-response-time-max', self.metric_group_name,
            'The max time taken to receive a response to a heartbeat request',
            tags), Max())
        self.heartbeat_latency.add(metrics.metric_name(
            'heartbeat-rate', self.metric_group_name,
            'The average number of heartbeats per second',
            tags), Rate(sampled_stat=Count()))

        self.join_latency = metrics.sensor('join-latency')
        self.join_latency.add(metrics.metric_name(
            'join-time-avg', self.metric_group_name,
            'The average time taken for a group rejoin',
            tags), Avg())
        self.join_latency.add(metrics.metric_name(
            'join-time-max', self.metric_group_name,
            'The max time taken for a group rejoin',
            tags), Avg())
        self.join_latency.add(metrics.metric_name(
            'join-rate', self.metric_group_name,
            'The number of group joins per second',
            tags), Rate(sampled_stat=Count()))

        self.sync_latency = metrics.sensor('sync-latency')
        self.sync_latency.add(metrics.metric_name(
            'sync-time-avg', self.metric_group_name,
            'The average time taken for a group sync',
            tags), Avg())
        self.sync_latency.add(metrics.metric_name(
            'sync-time-max', self.metric_group_name,
            'The max time taken for a group sync',
            tags), Avg())
        self.sync_latency.add(metrics.metric_name(
            'sync-rate', self.metric_group_name,
            'The number of group syncs per second',
            tags), Rate(sampled_stat=Count()))

        metrics.add_metric(metrics.metric_name(
            'last-heartbeat-seconds-ago', self.metric_group_name,
            'The number of seconds since the last controller heartbeat',
            tags), AnonMeasurable(
                lambda _, now: (now / 1000) - self.heartbeat.last_send))