synapse-product/synapse/federation/sender/__init__.py

# Copyright 2019 New Vector Ltd
#
# 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.

import abc
import logging
from typing import (
    TYPE_CHECKING,
    Collection,
    Dict,
    Hashable,
    Iterable,
    List,
    Optional,
    Set,
    Tuple,
)

from prometheus_client import Counter

import synapse.metrics
from synapse.api.presence import UserPresenceState
from synapse.events import EventBase
from synapse.federation.sender.per_destination_queue import PerDestinationQueue
from synapse.federation.sender.transaction_manager import TransactionManager
from synapse.federation.units import Edu
from synapse.metrics import (
    LaterGauge,
    event_processing_loop_counter,
    event_processing_loop_room_count,
    events_processed_counter,
)
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.types import JsonDict, ReadReceipt, RoomStreamToken
from synapse.util.metrics import Measure

if TYPE_CHECKING:
    from synapse.events.presence_router import PresenceRouter
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)

sent_pdus_destination_dist_count = Counter(
    "synapse_federation_client_sent_pdu_destinations:count",
    "Number of PDUs queued for sending to one or more destinations",
)

sent_pdus_destination_dist_total = Counter(
    "synapse_federation_client_sent_pdu_destinations:total",
    "Total number of PDUs queued for sending across all destinations",
)

# Time (in s) after Synapse's startup that we will begin to wake up destinations
# that have catch-up outstanding.
CATCH_UP_STARTUP_DELAY_SEC = 15

# Time (in s) to wait in between waking up each destination, i.e. one destination
# will be woken up every <x> seconds after Synapse's startup until we have woken
# every destination has outstanding catch-up.
CATCH_UP_STARTUP_INTERVAL_SEC = 5


class AbstractFederationSender(metaclass=abc.ABCMeta):
    @abc.abstractmethod
    def notify_new_events(self, max_token: RoomStreamToken) -> None:
        """This gets called when we have some new events we might want to
        send out to other servers.
        """
        raise NotImplementedError()

    @abc.abstractmethod
    async def send_read_receipt(self, receipt: ReadReceipt) -> None:
        """Send a RR to any other servers in the room

        Args:
            receipt: receipt to be sent
        """
        raise NotImplementedError()

    @abc.abstractmethod
    def send_presence_to_destinations(
        self, states: Iterable[UserPresenceState], destinations: Iterable[str]
    ) -> None:
        """Send the given presence states to the given destinations.

        Args:
            destinations:
        """
        raise NotImplementedError()

    @abc.abstractmethod
    def build_and_send_edu(
        self,
        destination: str,
        edu_type: str,
        content: JsonDict,
        key: Optional[Hashable] = None,
    ) -> None:
        """Construct an Edu object, and queue it for sending

        Args:
            destination: name of server to send to
            edu_type: type of EDU to send
            content: content of EDU
            key: clobbering key for this edu
        """
        raise NotImplementedError()

    @abc.abstractmethod
    def send_device_messages(self, destination: str) -> None:
        raise NotImplementedError()

    @abc.abstractmethod
    def wake_destination(self, destination: str) -> None:
        """Called when we want to retry sending transactions to a remote.

        This is mainly useful if the remote server has been down and we think it
        might have come back.
        """
        raise NotImplementedError()

    @abc.abstractmethod
    def get_current_token(self) -> int:
        raise NotImplementedError()

    @abc.abstractmethod
    def federation_ack(self, instance_name: str, token: int) -> None:
        raise NotImplementedError()

    @abc.abstractmethod
    async def get_replication_rows(
        self, instance_name: str, from_token: int, to_token: int, target_row_count: int
    ) -> Tuple[List[Tuple[int, Tuple]], int, bool]:
        raise NotImplementedError()


class FederationSender(AbstractFederationSender):
    def __init__(self, hs: "HomeServer"):
        self.hs = hs
        self.server_name = hs.hostname

        self.store = hs.get_datastore()
        self.state = hs.get_state_handler()

        self.clock = hs.get_clock()
        self.is_mine_id = hs.is_mine_id

        self._presence_router = None  # type: Optional[PresenceRouter]
        self._transaction_manager = TransactionManager(hs)

        self._instance_name = hs.get_instance_name()
        self._federation_shard_config = hs.config.worker.federation_shard_config

        # map from destination to PerDestinationQueue
        self._per_destination_queues = {}  # type: Dict[str, PerDestinationQueue]

        LaterGauge(
            "synapse_federation_transaction_queue_pending_destinations",
            "",
            [],
            lambda: sum(
                1
                for d in self._per_destination_queues.values()
                if d.transmission_loop_running
            ),
        )

        LaterGauge(
            "synapse_federation_transaction_queue_pending_pdus",
            "",
            [],
            lambda: sum(
                d.pending_pdu_count() for d in self._per_destination_queues.values()
            ),
        )
        LaterGauge(
            "synapse_federation_transaction_queue_pending_edus",
            "",
            [],
            lambda: sum(
                d.pending_edu_count() for d in self._per_destination_queues.values()
            ),
        )

        self._is_processing = False
        self._last_poked_id = -1

        # map from room_id to a set of PerDestinationQueues which we believe are
        # awaiting a call to flush_read_receipts_for_room. The presence of an entry
        # here for a given room means that we are rate-limiting RR flushes to that room,
        # and that there is a pending call to _flush_rrs_for_room in the system.
        self._queues_awaiting_rr_flush_by_room = (
            {}
        )  # type: Dict[str, Set[PerDestinationQueue]]

        self._rr_txn_interval_per_room_ms = (
            1000.0 / hs.config.federation_rr_transactions_per_room_per_second
        )

        # wake up destinations that have outstanding PDUs to be caught up
        self._catchup_after_startup_timer = self.clock.call_later(
            CATCH_UP_STARTUP_DELAY_SEC,
            run_as_background_process,
            "wake_destinations_needing_catchup",
            self._wake_destinations_needing_catchup,
        )

        self._external_cache = hs.get_external_cache()

    def _get_per_destination_queue(self, destination: str) -> PerDestinationQueue:
        """Get or create a PerDestinationQueue for the given destination

        Args:
            destination: server_name of remote server
        """
        queue = self._per_destination_queues.get(destination)
        if not queue:
            queue = PerDestinationQueue(self.hs, self._transaction_manager, destination)
            self._per_destination_queues[destination] = queue
        return queue

    def notify_new_events(self, max_token: RoomStreamToken) -> None:
        """This gets called when we have some new events we might want to
        send out to other servers.
        """
        # We just use the minimum stream ordering and ignore the vector clock
        # component. This is safe to do as long as we *always* ignore the vector
        # clock components.
        current_id = max_token.stream

        self._last_poked_id = max(current_id, self._last_poked_id)

        if self._is_processing:
            return

        # fire off a processing loop in the background
        run_as_background_process(
            "process_event_queue_for_federation", self._process_event_queue_loop
        )

    async def _process_event_queue_loop(self) -> None:
        try:
            self._is_processing = True
            while True:
                last_token = await self.store.get_federation_out_pos("events")
                next_token, events = await self.store.get_all_new_events_stream(
                    last_token, self._last_poked_id, limit=100
                )

                logger.debug("Handling %s -> %s", last_token, next_token)

                if not events and next_token >= self._last_poked_id:
                    break

                async def get_destinations_for_event(
                    event: EventBase,
                ) -> Collection[str]:
                    """Computes the destinations to which this event must be sent.

                    This returns an empty tuple when there are no destinations to send to,
                    or if this event is not from this homeserver and it is not sending
                    it on behalf of another server.

                    Will also filter out destinations which this sender is not responsible for,
                    if multiple federation senders exist.
                    """

                    # Only send events for this server.
                    send_on_behalf_of = event.internal_metadata.get_send_on_behalf_of()
                    is_mine = self.is_mine_id(event.sender)
                    if not is_mine and send_on_behalf_of is None:
                        return ()

                    if not event.internal_metadata.should_proactively_send():
                        return ()

                    destinations = None  # type: Optional[Set[str]]
                    if not event.prev_event_ids():
                        # If there are no prev event IDs then the state is empty
                        # and so no remote servers in the room
                        destinations = set()
                    else:
                        # We check the external cache for the destinations, which is
                        # stored per state group.

                        sg = await self._external_cache.get(
                            "event_to_prev_state_group", event.event_id
                        )
                        if sg:
                            destinations = await self._external_cache.get(
                                "get_joined_hosts", str(sg)
                            )

                    if destinations is None:
                        try:
                            # Get the state from before the event.
                            # We need to make sure that this is the state from before
                            # the event and not from after it.
                            # Otherwise if the last member on a server in a room is
                            # banned then it won't receive the event because it won't
                            # be in the room after the ban.
                            destinations = await self.state.get_hosts_in_room_at_events(
                                event.room_id, event_ids=event.prev_event_ids()
                            )
                        except Exception:
                            logger.exception(
                                "Failed to calculate hosts in room for event: %s",
                                event.event_id,
                            )
                            return ()

                    destinations = {
                        d
                        for d in destinations
                        if self._federation_shard_config.should_handle(
                            self._instance_name, d
                        )
                    }

                    destinations.discard(self.server_name)

                    if send_on_behalf_of is not None:
                        # If we are sending the event on behalf of another server
                        # then it already has the event and there is no reason to
                        # send the event to it.
                        destinations.discard(send_on_behalf_of)

                    if destinations:
                        now = self.clock.time_msec()
                        ts = await self.store.get_received_ts(event.event_id)

                        synapse.metrics.event_processing_lag_by_event.labels(
                            "federation_sender"
                        ).observe((now - ts) / 1000)

                        return destinations
                    return ()

                async def get_federatable_events_and_destinations(
                    events: Iterable[EventBase],
                ) -> List[Tuple[EventBase, Collection[str]]]:
                    with Measure(self.clock, "get_destinations_for_events"):
                        # Fetch federation destinations per event,
                        # skip if get_destinations_for_event returns an empty collection,
                        # return list of event->destinations pairs.
                        return [
                            (event, dests)
                            for (event, dests) in [
                                (event, await get_destinations_for_event(event))
                                for event in events
                            ]
                            if dests
                        ]

                events_and_dests = await get_federatable_events_and_destinations(events)

                # Send corresponding events to each destination queue
                await self._distribute_events(events_and_dests)

                await self.store.update_federation_out_pos("events", next_token)

                if events:
                    now = self.clock.time_msec()
                    ts = await self.store.get_received_ts(events[-1].event_id)

                    synapse.metrics.event_processing_lag.labels(
                        "federation_sender"
                    ).set(now - ts)
                    synapse.metrics.event_processing_last_ts.labels(
                        "federation_sender"
                    ).set(ts)

                    events_processed_counter.inc(len(events))

                    event_processing_loop_room_count.labels("federation_sender").inc(
                        len({event.room_id for event in events})
                    )

                event_processing_loop_counter.labels("federation_sender").inc()

                synapse.metrics.event_processing_positions.labels(
                    "federation_sender"
                ).set(next_token)

        finally:
            self._is_processing = False

    async def _distribute_events(
        self,
        events_and_dests: Iterable[Tuple[EventBase, Collection[str]]],
    ) -> None:
        """Distribute events to the respective per_destination queues.

        Also persists last-seen per-room stream_ordering to 'destination_rooms'.

        Args:
            events_and_dests: A list of tuples, which are (event: EventBase, destinations: Collection[str]).
                              Every event is paired with its intended destinations (in federation).
        """
        # Tuples of room_id + destination to their max-seen stream_ordering
        room_with_dest_stream_ordering = {}  # type: Dict[Tuple[str, str], int]

        # List of events to send to each destination
        events_by_dest = {}  # type: Dict[str, List[EventBase]]

        # For each event-destinations pair...
        for event, destinations in events_and_dests:

            # (we got this from the database, it's filled)
            assert event.internal_metadata.stream_ordering

            sent_pdus_destination_dist_total.inc(len(destinations))
            sent_pdus_destination_dist_count.inc()

            # ...iterate over those destinations..
            for destination in destinations:
                # ...update their stream-ordering...
                room_with_dest_stream_ordering[(event.room_id, destination)] = max(
                    event.internal_metadata.stream_ordering,
                    room_with_dest_stream_ordering.get((event.room_id, destination), 0),
                )

                # ...and add the event to each destination queue.
                events_by_dest.setdefault(destination, []).append(event)

        # Bulk-store destination_rooms stream_ids
        await self.store.bulk_store_destination_rooms_entries(
            room_with_dest_stream_ordering
        )

        for destination, pdus in events_by_dest.items():
            logger.debug("Sending %d pdus to %s", len(pdus), destination)

            self._get_per_destination_queue(destination).send_pdus(pdus)

    async def send_read_receipt(self, receipt: ReadReceipt) -> None:
        """Send a RR to any other servers in the room

        Args:
            receipt: receipt to be sent
        """

        # Some background on the rate-limiting going on here.
        #
        # It turns out that if we attempt to send out RRs as soon as we get them from
        # a client, then we end up trying to do several hundred Hz of federation
        # transactions. (The number of transactions scales as O(N^2) on the size of a
        # room, since in a large room we have both more RRs coming in, and more servers
        # to send them to.)
        #
        # This leads to a lot of CPU load, and we end up getting behind. The solution
        # currently adopted is as follows:
        #
        # The first receipt in a given room is sent out immediately, at time T0. Any
        # further receipts are, in theory, batched up for N seconds, where N is calculated
        # based on the number of servers in the room to achieve a transaction frequency
        # of around 50Hz. So, for example, if there were 100 servers in the room, then
        # N would be 100 / 50Hz = 2 seconds.
        #
        # Then, after T+N, we flush out any receipts that have accumulated, and restart
        # the timer to flush out more receipts at T+2N, etc. If no receipts accumulate,
        # we stop the cycle and go back to the start.
        #
        # However, in practice, it is often possible to flush out receipts earlier: in
        # particular, if we are sending a transaction to a given server anyway (for
        # example, because we have a PDU or a RR in another room to send), then we may
        # as well send out all of the pending RRs for that server. So it may be that
        # by the time we get to T+N, we don't actually have any RRs left to send out.
        # Nevertheless we continue to buffer up RRs for the room in question until we
        # reach the point that no RRs arrive between timer ticks.
        #
        # For even more background, see https://github.com/matrix-org/synapse/issues/4730.

        room_id = receipt.room_id

        # Work out which remote servers should be poked and poke them.
        domains_set = await self.state.get_current_hosts_in_room(room_id)
        domains = [
            d
            for d in domains_set
            if d != self.server_name
            and self._federation_shard_config.should_handle(self._instance_name, d)
        ]
        if not domains:
            return

        queues_pending_flush = self._queues_awaiting_rr_flush_by_room.get(room_id)

        # if there is no flush yet scheduled, we will send out these receipts with
        # immediate flushes, and schedule the next flush for this room.
        if queues_pending_flush is not None:
            logger.debug("Queuing receipt for: %r", domains)
        else:
            logger.debug("Sending receipt to: %r", domains)
            self._schedule_rr_flush_for_room(room_id, len(domains))

        for domain in domains:
            queue = self._get_per_destination_queue(domain)
            queue.queue_read_receipt(receipt)

            # if there is already a RR flush pending for this room, then make sure this
            # destination is registered for the flush
            if queues_pending_flush is not None:
                queues_pending_flush.add(queue)
            else:
                queue.flush_read_receipts_for_room(room_id)

    def _schedule_rr_flush_for_room(self, room_id: str, n_domains: int) -> None:
        # that is going to cause approximately len(domains) transactions, so now back
        # off for that multiplied by RR_TXN_INTERVAL_PER_ROOM
        backoff_ms = self._rr_txn_interval_per_room_ms * n_domains

        logger.debug("Scheduling RR flush in %s in %d ms", room_id, backoff_ms)
        self.clock.call_later(backoff_ms, self._flush_rrs_for_room, room_id)
        self._queues_awaiting_rr_flush_by_room[room_id] = set()

    def _flush_rrs_for_room(self, room_id: str) -> None:
        queues = self._queues_awaiting_rr_flush_by_room.pop(room_id)
        logger.debug("Flushing RRs in %s to %s", room_id, queues)

        if not queues:
            # no more RRs arrived for this room; we are done.
            return

        # schedule the next flush
        self._schedule_rr_flush_for_room(room_id, len(queues))

        for queue in queues:
            queue.flush_read_receipts_for_room(room_id)

    def send_presence_to_destinations(
        self, states: Iterable[UserPresenceState], destinations: Iterable[str]
    ) -> None:
        """Send the given presence states to the given destinations.
        destinations (list[str])
        """

        if not states or not self.hs.config.use_presence:
            # No-op if presence is disabled.
            return

        # Ensure we only send out presence states for local users.
        for state in states:
            assert self.is_mine_id(state.user_id)

        for destination in destinations:
            if destination == self.server_name:
                continue
            if not self._federation_shard_config.should_handle(
                self._instance_name, destination
            ):
                continue
            self._get_per_destination_queue(destination).send_presence(states)

    def build_and_send_edu(
        self,
        destination: str,
        edu_type: str,
        content: JsonDict,
        key: Optional[Hashable] = None,
    ) -> None:
        """Construct an Edu object, and queue it for sending

        Args:
            destination: name of server to send to
            edu_type: type of EDU to send
            content: content of EDU
            key: clobbering key for this edu
        """
        if destination == self.server_name:
            logger.info("Not sending EDU to ourselves")
            return

        if not self._federation_shard_config.should_handle(
            self._instance_name, destination
        ):
            return

        edu = Edu(
            origin=self.server_name,
            destination=destination,
            edu_type=edu_type,
            content=content,
        )

        self.send_edu(edu, key)

    def send_edu(self, edu: Edu, key: Optional[Hashable]) -> None:
        """Queue an EDU for sending

        Args:
            edu: edu to send
            key: clobbering key for this edu
        """
        if not self._federation_shard_config.should_handle(
            self._instance_name, edu.destination
        ):
            return

        queue = self._get_per_destination_queue(edu.destination)
        if key:
            queue.send_keyed_edu(edu, key)
        else:
            queue.send_edu(edu)

    def send_device_messages(self, destination: str) -> None:
        if destination == self.server_name:
            logger.warning("Not sending device update to ourselves")
            return

        if not self._federation_shard_config.should_handle(
            self._instance_name, destination
        ):
            return

        self._get_per_destination_queue(destination).attempt_new_transaction()

    def wake_destination(self, destination: str) -> None:
        """Called when we want to retry sending transactions to a remote.

        This is mainly useful if the remote server has been down and we think it
        might have come back.
        """

        if destination == self.server_name:
            logger.warning("Not waking up ourselves")
            return

        if not self._federation_shard_config.should_handle(
            self._instance_name, destination
        ):
            return

        self._get_per_destination_queue(destination).attempt_new_transaction()

    @staticmethod
    def get_current_token() -> int:
        # Dummy implementation for case where federation sender isn't offloaded
        # to a worker.
        return 0

    def federation_ack(self, instance_name: str, token: int) -> None:
        # It is not expected that this gets called on FederationSender.
        raise NotImplementedError()

    @staticmethod
    async def get_replication_rows(
        instance_name: str, from_token: int, to_token: int, target_row_count: int
    ) -> Tuple[List[Tuple[int, Tuple]], int, bool]:
        # Dummy implementation for case where federation sender isn't offloaded
        # to a worker.
        return [], 0, False

    async def _wake_destinations_needing_catchup(self) -> None:
        """
        Wakes up destinations that need catch-up and are not currently being
        backed off from.

        In order to reduce load spikes, adds a delay between each destination.
        """

        last_processed = None  # type: Optional[str]

        while True:
            destinations_to_wake = (
                await self.store.get_catch_up_outstanding_destinations(last_processed)
            )

            if not destinations_to_wake:
                # finished waking all destinations!
                self._catchup_after_startup_timer = None
                break

            last_processed = destinations_to_wake[-1]

            destinations_to_wake = [
                d
                for d in destinations_to_wake
                if self._federation_shard_config.should_handle(self._instance_name, d)
            ]

            for destination in destinations_to_wake:
                logger.info(
                    "Destination %s has outstanding catch-up, waking up.",
                    last_processed,
                )
                self.wake_destination(destination)
                await self.clock.sleep(CATCH_UP_STARTUP_INTERVAL_SEC)