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- # -*- coding: utf-8 -*-
- # Copyright 2014-2016 OpenMarket 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 heapq
- import logging
- import threading
- from collections import deque
- from contextlib import contextmanager
- from typing import Dict, List, Optional, Set, Union
- import attr
- from typing_extensions import Deque
- from synapse.metrics.background_process_metrics import run_as_background_process
- from synapse.storage.database import DatabasePool, LoggingTransaction
- from synapse.storage.types import Cursor
- from synapse.storage.util.sequence import PostgresSequenceGenerator
- logger = logging.getLogger(__name__)
- class IdGenerator:
- def __init__(self, db_conn, table, column):
- self._lock = threading.Lock()
- self._next_id = _load_current_id(db_conn, table, column)
- def get_next(self):
- with self._lock:
- self._next_id += 1
- return self._next_id
- def _load_current_id(db_conn, table, column, step=1):
- """
- Args:
- db_conn (object):
- table (str):
- column (str):
- step (int):
- Returns:
- int
- """
- # debug logging for https://github.com/matrix-org/synapse/issues/7968
- logger.info("initialising stream generator for %s(%s)", table, column)
- cur = db_conn.cursor(txn_name="_load_current_id")
- if step == 1:
- cur.execute("SELECT MAX(%s) FROM %s" % (column, table))
- else:
- cur.execute("SELECT MIN(%s) FROM %s" % (column, table))
- (val,) = cur.fetchone()
- cur.close()
- current_id = int(val) if val else step
- return (max if step > 0 else min)(current_id, step)
- class StreamIdGenerator:
- """Used to generate new stream ids when persisting events while keeping
- track of which transactions have been completed.
- This allows us to get the "current" stream id, i.e. the stream id such that
- all ids less than or equal to it have completed. This handles the fact that
- persistence of events can complete out of order.
- Args:
- db_conn(connection): A database connection to use to fetch the
- initial value of the generator from.
- table(str): A database table to read the initial value of the id
- generator from.
- column(str): The column of the database table to read the initial
- value from the id generator from.
- extra_tables(list): List of pairs of database tables and columns to
- use to source the initial value of the generator from. The value
- with the largest magnitude is used.
- step(int): which direction the stream ids grow in. +1 to grow
- upwards, -1 to grow downwards.
- Usage:
- async with stream_id_gen.get_next() as stream_id:
- # ... persist event ...
- """
- def __init__(self, db_conn, table, column, extra_tables=[], step=1):
- assert step != 0
- self._lock = threading.Lock()
- self._step = step
- self._current = _load_current_id(db_conn, table, column, step)
- for table, column in extra_tables:
- self._current = (max if step > 0 else min)(
- self._current, _load_current_id(db_conn, table, column, step)
- )
- self._unfinished_ids = deque() # type: Deque[int]
- def get_next(self):
- """
- Usage:
- async with stream_id_gen.get_next() as stream_id:
- # ... persist event ...
- """
- with self._lock:
- self._current += self._step
- next_id = self._current
- self._unfinished_ids.append(next_id)
- @contextmanager
- def manager():
- try:
- yield next_id
- finally:
- with self._lock:
- self._unfinished_ids.remove(next_id)
- return _AsyncCtxManagerWrapper(manager())
- def get_next_mult(self, n):
- """
- Usage:
- async with stream_id_gen.get_next(n) as stream_ids:
- # ... persist events ...
- """
- with self._lock:
- next_ids = range(
- self._current + self._step,
- self._current + self._step * (n + 1),
- self._step,
- )
- self._current += n * self._step
- for next_id in next_ids:
- self._unfinished_ids.append(next_id)
- @contextmanager
- def manager():
- try:
- yield next_ids
- finally:
- with self._lock:
- for next_id in next_ids:
- self._unfinished_ids.remove(next_id)
- return _AsyncCtxManagerWrapper(manager())
- def get_current_token(self) -> int:
- """Returns the maximum stream id such that all stream ids less than or
- equal to it have been successfully persisted.
- Returns:
- The maximum stream id.
- """
- with self._lock:
- if self._unfinished_ids:
- return self._unfinished_ids[0] - self._step
- return self._current
- def get_current_token_for_writer(self, instance_name: str) -> int:
- """Returns the position of the given writer.
- For streams with single writers this is equivalent to
- `get_current_token`.
- """
- return self.get_current_token()
- class MultiWriterIdGenerator:
- """An ID generator that tracks a stream that can have multiple writers.
- Uses a Postgres sequence to coordinate ID assignment, but positions of other
- writers will only get updated when `advance` is called (by replication).
- Note: Only works with Postgres.
- Args:
- db_conn
- db
- stream_name: A name for the stream.
- instance_name: The name of this instance.
- table: Database table associated with stream.
- instance_column: Column that stores the row's writer's instance name
- id_column: Column that stores the stream ID.
- sequence_name: The name of the postgres sequence used to generate new
- IDs.
- writers: A list of known writers to use to populate current positions
- on startup. Can be empty if nothing uses `get_current_token` or
- `get_positions` (e.g. caches stream).
- positive: Whether the IDs are positive (true) or negative (false).
- When using negative IDs we go backwards from -1 to -2, -3, etc.
- """
- def __init__(
- self,
- db_conn,
- db: DatabasePool,
- stream_name: str,
- instance_name: str,
- table: str,
- instance_column: str,
- id_column: str,
- sequence_name: str,
- writers: List[str],
- positive: bool = True,
- ):
- self._db = db
- self._stream_name = stream_name
- self._instance_name = instance_name
- self._positive = positive
- self._writers = writers
- self._return_factor = 1 if positive else -1
- # We lock as some functions may be called from DB threads.
- self._lock = threading.Lock()
- # Note: If we are a negative stream then we still store all the IDs as
- # positive to make life easier for us, and simply negate the IDs when we
- # return them.
- self._current_positions = {} # type: Dict[str, int]
- # Set of local IDs that we're still processing. The current position
- # should be less than the minimum of this set (if not empty).
- self._unfinished_ids = set() # type: Set[int]
- # Set of local IDs that we've processed that are larger than the current
- # position, due to there being smaller unpersisted IDs.
- self._finished_ids = set() # type: Set[int]
- # We track the max position where we know everything before has been
- # persisted. This is done by a) looking at the min across all instances
- # and b) noting that if we have seen a run of persisted positions
- # without gaps (e.g. 5, 6, 7) then we can skip forward (e.g. to 7).
- #
- # Note: There is no guarentee that the IDs generated by the sequence
- # will be gapless; gaps can form when e.g. a transaction was rolled
- # back. This means that sometimes we won't be able to skip forward the
- # position even though everything has been persisted. However, since
- # gaps should be relatively rare it's still worth doing the book keeping
- # that allows us to skip forwards when there are gapless runs of
- # positions.
- #
- # We start at 1 here as a) the first generated stream ID will be 2, and
- # b) other parts of the code assume that stream IDs are strictly greater
- # than 0.
- self._persisted_upto_position = (
- min(self._current_positions.values()) if self._current_positions else 1
- )
- self._known_persisted_positions = [] # type: List[int]
- self._sequence_gen = PostgresSequenceGenerator(sequence_name)
- # We check that the table and sequence haven't diverged.
- self._sequence_gen.check_consistency(
- db_conn, table=table, id_column=id_column, positive=positive
- )
- # This goes and fills out the above state from the database.
- self._load_current_ids(db_conn, table, instance_column, id_column)
- def _load_current_ids(
- self, db_conn, table: str, instance_column: str, id_column: str
- ):
- cur = db_conn.cursor(txn_name="_load_current_ids")
- # Load the current positions of all writers for the stream.
- if self._writers:
- # We delete any stale entries in the positions table. This is
- # important if we add back a writer after a long time; we want to
- # consider that a "new" writer, rather than using the old stale
- # entry here.
- sql = """
- DELETE FROM stream_positions
- WHERE
- stream_name = ?
- AND instance_name != ALL(?)
- """
- cur.execute(sql, (self._stream_name, self._writers))
- sql = """
- SELECT instance_name, stream_id FROM stream_positions
- WHERE stream_name = ?
- """
- cur.execute(sql, (self._stream_name,))
- self._current_positions = {
- instance: stream_id * self._return_factor
- for instance, stream_id in cur
- if instance in self._writers
- }
- # We set the `_persisted_upto_position` to be the minimum of all current
- # positions. If empty we use the max stream ID from the DB table.
- min_stream_id = min(self._current_positions.values(), default=None)
- if min_stream_id is None:
- # We add a GREATEST here to ensure that the result is always
- # positive. (This can be a problem for e.g. backfill streams where
- # the server has never backfilled).
- sql = """
- SELECT GREATEST(COALESCE(%(agg)s(%(id)s), 1), 1)
- FROM %(table)s
- """ % {
- "id": id_column,
- "table": table,
- "agg": "MAX" if self._positive else "-MIN",
- }
- cur.execute(sql)
- (stream_id,) = cur.fetchone()
- self._persisted_upto_position = stream_id
- else:
- # If we have a min_stream_id then we pull out everything greater
- # than it from the DB so that we can prefill
- # `_known_persisted_positions` and get a more accurate
- # `_persisted_upto_position`.
- #
- # We also check if any of the later rows are from this instance, in
- # which case we use that for this instance's current position. This
- # is to handle the case where we didn't finish persisting to the
- # stream positions table before restart (or the stream position
- # table otherwise got out of date).
- sql = """
- SELECT %(instance)s, %(id)s FROM %(table)s
- WHERE ? %(cmp)s %(id)s
- """ % {
- "id": id_column,
- "table": table,
- "instance": instance_column,
- "cmp": "<=" if self._positive else ">=",
- }
- cur.execute(sql, (min_stream_id * self._return_factor,))
- self._persisted_upto_position = min_stream_id
- with self._lock:
- for (instance, stream_id,) in cur:
- stream_id = self._return_factor * stream_id
- self._add_persisted_position(stream_id)
- if instance == self._instance_name:
- self._current_positions[instance] = stream_id
- cur.close()
- def _load_next_id_txn(self, txn) -> int:
- return self._sequence_gen.get_next_id_txn(txn)
- def _load_next_mult_id_txn(self, txn, n: int) -> List[int]:
- return self._sequence_gen.get_next_mult_txn(txn, n)
- def get_next(self):
- """
- Usage:
- async with stream_id_gen.get_next() as stream_id:
- # ... persist event ...
- """
- return _MultiWriterCtxManager(self)
- def get_next_mult(self, n: int):
- """
- Usage:
- async with stream_id_gen.get_next_mult(5) as stream_ids:
- # ... persist events ...
- """
- return _MultiWriterCtxManager(self, n)
- def get_next_txn(self, txn: LoggingTransaction):
- """
- Usage:
- stream_id = stream_id_gen.get_next(txn)
- # ... persist event ...
- """
- next_id = self._load_next_id_txn(txn)
- with self._lock:
- self._unfinished_ids.add(next_id)
- txn.call_after(self._mark_id_as_finished, next_id)
- txn.call_on_exception(self._mark_id_as_finished, next_id)
- # Update the `stream_positions` table with newly updated stream
- # ID (unless self._writers is not set in which case we don't
- # bother, as nothing will read it).
- #
- # We only do this on the success path so that the persisted current
- # position points to a persited row with the correct instance name.
- if self._writers:
- txn.call_after(
- run_as_background_process,
- "MultiWriterIdGenerator._update_table",
- self._db.runInteraction,
- "MultiWriterIdGenerator._update_table",
- self._update_stream_positions_table_txn,
- )
- return self._return_factor * next_id
- def _mark_id_as_finished(self, next_id: int):
- """The ID has finished being processed so we should advance the
- current position if possible.
- """
- with self._lock:
- self._unfinished_ids.discard(next_id)
- self._finished_ids.add(next_id)
- new_cur = None # type: Optional[int]
- if self._unfinished_ids:
- # If there are unfinished IDs then the new position will be the
- # largest finished ID less than the minimum unfinished ID.
- finished = set()
- min_unfinshed = min(self._unfinished_ids)
- for s in self._finished_ids:
- if s < min_unfinshed:
- if new_cur is None or new_cur < s:
- new_cur = s
- else:
- finished.add(s)
- # We clear these out since they're now all less than the new
- # position.
- self._finished_ids = finished
- else:
- # There are no unfinished IDs so the new position is simply the
- # largest finished one.
- new_cur = max(self._finished_ids)
- # We clear these out since they're now all less than the new
- # position.
- self._finished_ids.clear()
- if new_cur:
- curr = self._current_positions.get(self._instance_name, 0)
- self._current_positions[self._instance_name] = max(curr, new_cur)
- self._add_persisted_position(next_id)
- def get_current_token(self) -> int:
- """Returns the maximum stream id such that all stream ids less than or
- equal to it have been successfully persisted.
- """
- return self.get_persisted_upto_position()
- def get_current_token_for_writer(self, instance_name: str) -> int:
- """Returns the position of the given writer.
- """
- # If we don't have an entry for the given instance name, we assume it's a
- # new writer.
- #
- # For new writers we assume their initial position to be the current
- # persisted up to position. This stops Synapse from doing a full table
- # scan when a new writer announces itself over replication.
- with self._lock:
- return self._return_factor * self._current_positions.get(
- instance_name, self._persisted_upto_position
- )
- def get_positions(self) -> Dict[str, int]:
- """Get a copy of the current positon map.
- Note that this won't necessarily include all configured writers if some
- writers haven't written anything yet.
- """
- with self._lock:
- return {
- name: self._return_factor * i
- for name, i in self._current_positions.items()
- }
- def advance(self, instance_name: str, new_id: int):
- """Advance the postion of the named writer to the given ID, if greater
- than existing entry.
- """
- new_id *= self._return_factor
- with self._lock:
- self._current_positions[instance_name] = max(
- new_id, self._current_positions.get(instance_name, 0)
- )
- self._add_persisted_position(new_id)
- def get_persisted_upto_position(self) -> int:
- """Get the max position where all previous positions have been
- persisted.
- Note: In the worst case scenario this will be equal to the minimum
- position across writers. This means that the returned position here can
- lag if one writer doesn't write very often.
- """
- with self._lock:
- return self._return_factor * self._persisted_upto_position
- def _add_persisted_position(self, new_id: int):
- """Record that we have persisted a position.
- This is used to keep the `_current_positions` up to date.
- """
- # We require that the lock is locked by caller
- assert self._lock.locked()
- 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).
- min_curr = min(self._current_positions.values(), default=0)
- self._persisted_upto_position = max(min_curr, self._persisted_upto_position)
- # We now iterate through the seen positions, discarding those that are
- # less than the current min positions, and incrementing the min position
- # if its exactly one greater.
- #
- # This is also where we discard items from `_known_persisted_positions`
- # (to ensure the list doesn't infinitely grow).
- while self._known_persisted_positions:
- if self._known_persisted_positions[0] <= self._persisted_upto_position:
- heapq.heappop(self._known_persisted_positions)
- elif (
- self._known_persisted_positions[0] == self._persisted_upto_position + 1
- ):
- heapq.heappop(self._known_persisted_positions)
- self._persisted_upto_position += 1
- else:
- # There was a gap in seen positions, so there is nothing more to
- # do.
- break
- def _update_stream_positions_table_txn(self, txn: Cursor):
- """Update the `stream_positions` table with newly persisted position.
- """
- if not self._writers:
- return
- # We upsert the value, ensuring on conflict that we always increase the
- # value (or decrease if stream goes backwards).
- sql = """
- INSERT INTO stream_positions (stream_name, instance_name, stream_id)
- VALUES (?, ?, ?)
- ON CONFLICT (stream_name, instance_name)
- DO UPDATE SET
- stream_id = %(agg)s(stream_positions.stream_id, EXCLUDED.stream_id)
- """ % {
- "agg": "GREATEST" if self._positive else "LEAST",
- }
- pos = (self.get_current_token_for_writer(self._instance_name),)
- txn.execute(sql, (self._stream_name, self._instance_name, pos))
- @attr.s(slots=True)
- class _AsyncCtxManagerWrapper:
- """Helper class to convert a plain context manager to an async one.
- This is mainly useful if you have a plain context manager but the interface
- requires an async one.
- """
- inner = attr.ib()
- async def __aenter__(self):
- return self.inner.__enter__()
- async def __aexit__(self, exc_type, exc, tb):
- return self.inner.__exit__(exc_type, exc, tb)
- @attr.s(slots=True)
- class _MultiWriterCtxManager:
- """Async context manager returned by MultiWriterIdGenerator
- """
- id_gen = attr.ib(type=MultiWriterIdGenerator)
- multiple_ids = attr.ib(type=Optional[int], default=None)
- stream_ids = attr.ib(type=List[int], factory=list)
- async def __aenter__(self) -> Union[int, List[int]]:
- # It's safe to run this in autocommit mode as fetching values from a
- # sequence ignores transaction semantics anyway.
- self.stream_ids = await self.id_gen._db.runInteraction(
- "_load_next_mult_id",
- self.id_gen._load_next_mult_id_txn,
- self.multiple_ids or 1,
- db_autocommit=True,
- )
- with self.id_gen._lock:
- self.id_gen._unfinished_ids.update(self.stream_ids)
- if self.multiple_ids is None:
- return self.stream_ids[0] * self.id_gen._return_factor
- else:
- return [i * self.id_gen._return_factor for i in self.stream_ids]
- async def __aexit__(self, exc_type, exc, tb):
- for i in self.stream_ids:
- self.id_gen._mark_id_as_finished(i)
- if exc_type is not None:
- return False
- # Update the `stream_positions` table with newly updated stream
- # ID (unless self._writers is not set in which case we don't
- # bother, as nothing will read it).
- #
- # We only do this on the success path so that the persisted current
- # position points to a persisted row with the correct instance name.
- #
- # We do this in autocommit mode as a) the upsert works correctly outside
- # transactions and b) reduces the amount of time the rows are locked
- # for. If we don't do this then we'll often hit serialization errors due
- # to the fact we default to REPEATABLE READ isolation levels.
- if self.id_gen._writers:
- await self.id_gen._db.runInteraction(
- "MultiWriterIdGenerator._update_table",
- self.id_gen._update_stream_positions_table_txn,
- db_autocommit=True,
- )
- return False
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