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Make event persisters periodically announce position over replication. (#8499) 

Currently background proccesses stream the events stream use the "minimum persisted position" (i.e. `get_current_token()`) rather than the vector clock style tokens. This is broadly fine as it doesn't matter if the background processes lag a small amount. However, in extreme cases (i.e. SyTests) where we only write to one event persister the background processes will never make progress.

This PR changes it so that the `MultiWriterIDGenerator` keeps the current position of a given instance as up to date as possible (i.e using the latest token it sees if its not in the process of persisting anything), and then periodically announces that over replication. This then allows the "minimum persisted position" to advance, albeit with a small lag.
This commit is contained in:
Erik Johnston 2020-10-12 15:51:41 +01:00 committed by GitHub
parent 6905f5751a
commit 8de3703d21
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9 changed files with 128 additions and 34 deletions
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1
changelog.d/8499.misc Normal file
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@ -0,0 +1 @@
Allow events to be sent to clients sooner when using sharded event persisters.

13
docs/tcp_replication.md
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@ -15,7 +15,7 @@ example flow would be (where '>' indicates master to worker and
> SERVER example.com
< REPLICATE
> POSITION events master 53
> POSITION events master 53 53
> RDATA events master 54 ["$foo1:bar.com", ...]
> RDATA events master 55 ["$foo4:bar.com", ...]
@ -138,9 +138,9 @@ the wire:
< NAME synapse.app.appservice
< PING 1490197665618
< REPLICATE
> POSITION events master 1
> POSITION backfill master 1
> POSITION caches master 1
> POSITION events master 1 1
> POSITION backfill master 1 1
> POSITION caches master 1 1
> RDATA caches master 2 ["get_user_by_id",["@01register-user:localhost:8823"],1490197670513]
> RDATA events master 14 ["$149019767112vOHxz:localhost:8823",
"!AFDCvgApUmpdfVjIXm:localhost:8823","m.room.guest_access","",null]
@ -185,6 +185,11 @@ client (C):
updates via HTTP API, rather than via the DB, then processes should make the
request to the appropriate process.
Two positions are included, the "new" position and the last position sent respectively.
This allows servers to tell instances that the positions have advanced but no
data has been written, without clients needlessly checking to see if they
have missed any updates.
#### ERROR (S, C)
There was an error

4
synapse/replication/tcp/client.py
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@ -191,6 +191,10 @@ class ReplicationDataHandler:
async def on_position(self, stream_name: str, instance_name: str, token: int):
self.store.process_replication_rows(stream_name, instance_name, token, [])
# We poke the generic "replication" notifier to wake anything up that
# may be streaming.
self.notifier.notify_replication()
def on_remote_server_up(self, server: str):
"""Called when get a new REMOTE_SERVER_UP command."""

36
synapse/replication/tcp/commands.py
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@ -141,15 +141,23 @@ class RdataCommand(Command):
class PositionCommand(Command):
"""Sent by the server to tell the client the stream position without
needing to send an RDATA.
"""Sent by an instance to tell others the stream position without needing to
send an RDATA.
Two tokens are sent, the new position and the last position sent by the
instance (in an RDATA or other POSITION). The tokens are chosen so that *no*
rows were written by the instance between the `prev_token` and `new_token`.
(If an instance hasn't sent a position before then the new position can be
used for both.)
Format::
POSITION <stream_name> <instance_name> <token>
POSITION <stream_name> <instance_name> <prev_token> <new_token>
On receipt of a POSITION command clients should check if they have missed
any updates, and if so then fetch them out of band.
On receipt of a POSITION command instances should check if they have missed
any updates, and if so then fetch them out of band. Instances can check this
by comparing their view of the current token for the sending instance with
the included `prev_token`.
The `<instance_name>` is the process that sent the command and is the source
of the stream.
@ -157,18 +165,26 @@ class PositionCommand(Command):
NAME = "POSITION"
def __init__(self, stream_name, instance_name, token):
def __init__(self, stream_name, instance_name, prev_token, new_token):
self.stream_name = stream_name
self.instance_name = instance_name
self.token = token
self.prev_token = prev_token
self.new_token = new_token
@classmethod
def from_line(cls, line):
stream_name, instance_name, token = line.split(" ", 2)
return cls(stream_name, instance_name, int(token))
stream_name, instance_name, prev_token, new_token = line.split(" ", 3)
return cls(stream_name, instance_name, int(prev_token), int(new_token))
def to_line(self):
return " ".join((self.stream_name, self.instance_name, str(self.token)))
return " ".join(
(
self.stream_name,
self.instance_name,
str(self.prev_token),
str(self.new_token),
)
)
class ErrorCommand(_SimpleCommand):

24
synapse/replication/tcp/handler.py
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@ -101,8 +101,9 @@ class ReplicationCommandHandler:
self._streams_to_replicate = [] # type: List[Stream]
for stream in self._streams.values():
if stream.NAME == CachesStream.NAME:
# All workers can write to the cache invalidation stream.
if hs.config.redis.redis_enabled and stream.NAME == CachesStream.NAME:
# All workers can write to the cache invalidation stream when
# using redis.
self._streams_to_replicate.append(stream)
continue
@ -313,11 +314,14 @@ class ReplicationCommandHandler:
# We respond with current position of all streams this instance
# replicates.
for stream in self.get_streams_to_replicate():
# Note that we use the current token as the prev token here (rather
# than stream.last_token), as we can't be sure that there have been
# no rows written between last token and the current token (since we
# might be racing with the replication sending bg process).
current_token = stream.current_token(self._instance_name)
self.send_command(
PositionCommand(
stream.NAME,
self._instance_name,
stream.current_token(self._instance_name),
stream.NAME, self._instance_name, current_token, current_token,
)
)
@ -511,16 +515,16 @@ class ReplicationCommandHandler:
# If the position token matches our current token then we're up to
# date and there's nothing to do. Otherwise, fetch all updates
# between then and now.
missing_updates = cmd.token != current_token
missing_updates = cmd.prev_token != current_token
while missing_updates:
logger.info(
"Fetching replication rows for '%s' between %i and %i",
stream_name,
current_token,
cmd.token,
cmd.new_token,
)
(updates, current_token, missing_updates) = await stream.get_updates_since(
cmd.instance_name, current_token, cmd.token
cmd.instance_name, current_token, cmd.new_token
)
# TODO: add some tests for this
@ -536,11 +540,11 @@ class ReplicationCommandHandler:
[stream.parse_row(row) for row in rows],
)
logger.info("Caught up with stream '%s' to %i", stream_name, cmd.token)
logger.info("Caught up with stream '%s' to %i", stream_name, cmd.new_token)
# We've now caught up to position sent to us, notify handler.
await self._replication_data_handler.on_position(
cmd.stream_name, cmd.instance_name, cmd.token
cmd.stream_name, cmd.instance_name, cmd.new_token
)
self._streams_by_connection.setdefault(conn, set()).add(stream_name)

47
synapse/replication/tcp/resource.py
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@ -23,7 +23,9 @@ from prometheus_client import Counter
from twisted.internet.protocol import Factory
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.replication.tcp.commands import PositionCommand
from synapse.replication.tcp.protocol import ServerReplicationStreamProtocol
from synapse.replication.tcp.streams import EventsStream
from synapse.util.metrics import Measure
stream_updates_counter = Counter(
@ -84,6 +86,23 @@ class ReplicationStreamer:
# Set of streams to replicate.
self.streams = self.command_handler.get_streams_to_replicate()
# If we have streams then we must have redis enabled or on master
assert (
not self.streams
or hs.config.redis.redis_enabled
or not hs.config.worker.worker_app
)
# If we are replicating an event stream we want to periodically check if
# we should send updated POSITIONs. We do this as a looping call rather
# explicitly poking when the position advances (without new data to
# replicate) to reduce replication traffic (otherwise each writer would
# likely send a POSITION for each new event received over replication).
#
# Note that if the position hasn't advanced then we won't send anything.
if any(EventsStream.NAME == s.NAME for s in self.streams):
self.clock.looping_call(self.on_notifier_poke, 1000)
def on_notifier_poke(self):
"""Checks if there is actually any new data and sends it to the
connections if there are.
@ -91,7 +110,7 @@ class ReplicationStreamer:
This should get called each time new data is available, even if it
is currently being executed, so that nothing gets missed
"""
if not self.command_handler.connected():
if not self.command_handler.connected() or not self.streams:
# Don't bother if nothing is listening. We still need to advance
# the stream tokens otherwise they'll fall behind forever
for stream in self.streams:
@ -136,6 +155,8 @@ class ReplicationStreamer:
self._replication_torture_level / 1000.0
)
last_token = stream.last_token
logger.debug(
"Getting stream: %s: %s -> %s",
stream.NAME,
@ -159,6 +180,30 @@ class ReplicationStreamer:
)
stream_updates_counter.labels(stream.NAME).inc(len(updates))
else:
# The token has advanced but there is no data to
# send, so we send a `POSITION` to inform other
# workers of the updated position.
if stream.NAME == EventsStream.NAME:
# XXX: We only do this for the EventStream as it
# turns out that e.g. account data streams share
# their "current token" with each other, meaning
# that it is *not* safe to send a POSITION.
logger.info(
"Sending position: %s -> %s",
stream.NAME,
current_token,
)
self.command_handler.send_command(
PositionCommand(
stream.NAME,
self._instance_name,
last_token,
current_token,
)
)
continue
# Some streams return multiple rows with the same stream IDs,
# we need to make sure they get sent out in batches. We do
# this by setting the current token to all but the last of

10
synapse/storage/util/id_generators.py
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@ -524,6 +524,16 @@ class MultiWriterIdGenerator:
heapq.heappush(self._known_persisted_positions, new_id)
# If we're a writer and we don't have any active writes we update our
# current position to the latest position seen. This allows the instance
# to report a recent position when asked, rather than a potentially old
# one (if this instance hasn't written anything for a while).
our_current_position = self._current_positions.get(self._instance_name)
if our_current_position and not self._unfinished_ids:
self._current_positions[self._instance_name] = max(
our_current_position, new_id
)
# We move the current min position up if the minimum current positions
# of all instances is higher (since by definition all positions less
# that that have been persisted).

2
synapse/storage/util/sequence.py
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@ -126,6 +126,8 @@ class PostgresSequenceGenerator(SequenceGenerator):
if max_stream_id > last_value:
logger.warning(
"Postgres sequence %s is behind table %s: %d < %d",
self._sequence_name,
table,
last_value,
max_stream_id,
)

25
tests/storage/test_id_generators.py
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@ -199,10 +199,17 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
first_id_gen = self._create_id_generator("first", writers=["first", "second"])
second_id_gen = self._create_id_generator("second", writers=["first", "second"])
self.assertEqual(first_id_gen.get_positions(), {"first": 3, "second": 7})
self.assertEqual(first_id_gen.get_current_token_for_writer("first"), 3)
# The first ID gen will notice that it can advance its token to 7 as it
# has no in progress writes...
self.assertEqual(first_id_gen.get_positions(), {"first": 7, "second": 7})
self.assertEqual(first_id_gen.get_current_token_for_writer("first"), 7)
self.assertEqual(first_id_gen.get_current_token_for_writer("second"), 7)
# ... but the second ID gen doesn't know that.
self.assertEqual(second_id_gen.get_positions(), {"first": 3, "second": 7})
self.assertEqual(second_id_gen.get_current_token_for_writer("first"), 3)
self.assertEqual(second_id_gen.get_current_token_for_writer("second"), 7)
# Try allocating a new ID gen and check that we only see position
# advanced after we leave the context manager.
@ -211,7 +218,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
self.assertEqual(stream_id, 8)
self.assertEqual(
first_id_gen.get_positions(), {"first": 3, "second": 7}
first_id_gen.get_positions(), {"first": 7, "second": 7}
)
self.get_success(_get_next_async())
@ -279,7 +286,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
self._insert_row_with_id("first", 3)
self._insert_row_with_id("second", 5)
id_gen = self._create_id_generator("first", writers=["first", "second"])
id_gen = self._create_id_generator("worker", writers=["first", "second"])
self.assertEqual(id_gen.get_positions(), {"first": 3, "second": 5})
@ -319,14 +326,14 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
id_gen = self._create_id_generator("first", writers=["first", "second"])
self.assertEqual(id_gen.get_positions(), {"first": 3, "second": 5})
self.assertEqual(id_gen.get_positions(), {"first": 5, "second": 5})
self.assertEqual(id_gen.get_persisted_upto_position(), 3)
self.assertEqual(id_gen.get_persisted_upto_position(), 5)
async def _get_next_async():
async with id_gen.get_next() as stream_id:
self.assertEqual(stream_id, 6)
self.assertEqual(id_gen.get_persisted_upto_position(), 3)
self.assertEqual(id_gen.get_persisted_upto_position(), 5)
self.get_success(_get_next_async())
@ -388,7 +395,7 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
self._insert_row_with_id("second", 5)
# Initial config has two writers
id_gen = self._create_id_generator("first", writers=["first", "second"])
id_gen = self._create_id_generator("worker", writers=["first", "second"])
self.assertEqual(id_gen.get_persisted_upto_position(), 3)
self.assertEqual(id_gen.get_current_token_for_writer("first"), 3)
self.assertEqual(id_gen.get_current_token_for_writer("second"), 5)
@ -568,7 +575,7 @@ class BackwardsMultiWriterIdGeneratorTestCase(HomeserverTestCase):
self.get_success(_get_next_async2())
self.assertEqual(id_gen_1.get_positions(), {"first": -1, "second": -2})
self.assertEqual(id_gen_1.get_positions(), {"first": -2, "second": -2})
self.assertEqual(id_gen_2.get_positions(), {"first": -1, "second": -2})
self.assertEqual(id_gen_1.get_persisted_upto_position(), -2)
self.assertEqual(id_gen_2.get_persisted_upto_position(), -2)