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![Erik Johnston](/assets/img/avatar_default.png)
This allows us to have the logic on both master and workers, which is necessary to move event persistence off master. We also combine the instantiation of ID generators from DataStore and slave stores to the base worker stores. This allows us to select which process writes events independently of the master/worker splits.
377 lines
12 KiB
Python
377 lines
12 KiB
Python
# -*- coding: utf-8 -*-
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# Copyright 2014-2016 OpenMarket Ltd
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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import contextlib
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import threading
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from collections import deque
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from typing import Dict, Set, Tuple
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from typing_extensions import Deque
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from synapse.storage.database import Database, LoggingTransaction
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class IdGenerator(object):
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def __init__(self, db_conn, table, column):
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self._lock = threading.Lock()
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self._next_id = _load_current_id(db_conn, table, column)
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def get_next(self):
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with self._lock:
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self._next_id += 1
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return self._next_id
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def _load_current_id(db_conn, table, column, step=1):
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"""
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Args:
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db_conn (object):
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table (str):
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column (str):
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step (int):
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Returns:
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int
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"""
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cur = db_conn.cursor()
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if step == 1:
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cur.execute("SELECT MAX(%s) FROM %s" % (column, table))
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else:
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cur.execute("SELECT MIN(%s) FROM %s" % (column, table))
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(val,) = cur.fetchone()
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cur.close()
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current_id = int(val) if val else step
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return (max if step > 0 else min)(current_id, step)
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class StreamIdGenerator(object):
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"""Used to generate new stream ids when persisting events while keeping
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track of which transactions have been completed.
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This allows us to get the "current" stream id, i.e. the stream id such that
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all ids less than or equal to it have completed. This handles the fact that
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persistence of events can complete out of order.
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Args:
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db_conn(connection): A database connection to use to fetch the
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initial value of the generator from.
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table(str): A database table to read the initial value of the id
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generator from.
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column(str): The column of the database table to read the initial
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value from the id generator from.
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extra_tables(list): List of pairs of database tables and columns to
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use to source the initial value of the generator from. The value
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with the largest magnitude is used.
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step(int): which direction the stream ids grow in. +1 to grow
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upwards, -1 to grow downwards.
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Usage:
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with stream_id_gen.get_next() as stream_id:
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# ... persist event ...
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"""
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def __init__(self, db_conn, table, column, extra_tables=[], step=1):
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assert step != 0
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self._lock = threading.Lock()
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self._step = step
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self._current = _load_current_id(db_conn, table, column, step)
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for table, column in extra_tables:
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self._current = (max if step > 0 else min)(
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self._current, _load_current_id(db_conn, table, column, step)
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)
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self._unfinished_ids = deque() # type: Deque[int]
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def get_next(self):
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"""
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Usage:
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with stream_id_gen.get_next() as stream_id:
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# ... persist event ...
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"""
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with self._lock:
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self._current += self._step
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next_id = self._current
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self._unfinished_ids.append(next_id)
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@contextlib.contextmanager
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def manager():
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try:
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yield next_id
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finally:
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with self._lock:
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self._unfinished_ids.remove(next_id)
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return manager()
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def get_next_mult(self, n):
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"""
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Usage:
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with stream_id_gen.get_next(n) as stream_ids:
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# ... persist events ...
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"""
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with self._lock:
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next_ids = range(
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self._current + self._step,
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self._current + self._step * (n + 1),
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self._step,
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)
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self._current += n * self._step
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for next_id in next_ids:
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self._unfinished_ids.append(next_id)
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@contextlib.contextmanager
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def manager():
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try:
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yield next_ids
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finally:
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with self._lock:
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for next_id in next_ids:
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self._unfinished_ids.remove(next_id)
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return manager()
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def get_current_token(self):
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"""Returns the maximum stream id such that all stream ids less than or
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equal to it have been successfully persisted.
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Returns:
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int
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"""
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with self._lock:
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if self._unfinished_ids:
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return self._unfinished_ids[0] - self._step
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return self._current
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class ChainedIdGenerator(object):
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"""Used to generate new stream ids where the stream must be kept in sync
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with another stream. It generates pairs of IDs, the first element is an
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integer ID for this stream, the second element is the ID for the stream
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that this stream needs to be kept in sync with."""
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def __init__(self, chained_generator, db_conn, table, column):
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self.chained_generator = chained_generator
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self._table = table
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self._lock = threading.Lock()
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self._current_max = _load_current_id(db_conn, table, column)
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self._unfinished_ids = deque() # type: Deque[Tuple[int, int]]
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def get_next(self):
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"""
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Usage:
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with stream_id_gen.get_next() as (stream_id, chained_id):
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# ... persist event ...
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"""
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with self._lock:
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self._current_max += 1
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next_id = self._current_max
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chained_id = self.chained_generator.get_current_token()
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self._unfinished_ids.append((next_id, chained_id))
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@contextlib.contextmanager
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def manager():
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try:
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yield (next_id, chained_id)
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finally:
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with self._lock:
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self._unfinished_ids.remove((next_id, chained_id))
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return manager()
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def get_current_token(self):
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"""Returns the maximum stream id such that all stream ids less than or
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equal to it have been successfully persisted.
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"""
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with self._lock:
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if self._unfinished_ids:
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stream_id, chained_id = self._unfinished_ids[0]
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return stream_id - 1, chained_id
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return self._current_max, self.chained_generator.get_current_token()
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def advance(self, token: int):
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"""Stub implementation for advancing the token when receiving updates
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over replication; raises an exception as this instance should be the
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only source of updates.
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"""
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raise Exception(
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"Attempted to advance token on source for table %r", self._table
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)
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class MultiWriterIdGenerator:
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"""An ID generator that tracks a stream that can have multiple writers.
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Uses a Postgres sequence to coordinate ID assignment, but positions of other
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writers will only get updated when `advance` is called (by replication).
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Note: Only works with Postgres.
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Args:
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db_conn
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db
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instance_name: The name of this instance.
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table: Database table associated with stream.
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instance_column: Column that stores the row's writer's instance name
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id_column: Column that stores the stream ID.
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sequence_name: The name of the postgres sequence used to generate new
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IDs.
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"""
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def __init__(
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self,
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db_conn,
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db: Database,
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instance_name: str,
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table: str,
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instance_column: str,
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id_column: str,
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sequence_name: str,
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):
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self._db = db
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self._instance_name = instance_name
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self._sequence_name = sequence_name
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# We lock as some functions may be called from DB threads.
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self._lock = threading.Lock()
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self._current_positions = self._load_current_ids(
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db_conn, table, instance_column, id_column
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)
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# Set of local IDs that we're still processing. The current position
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# should be less than the minimum of this set (if not empty).
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self._unfinished_ids = set() # type: Set[int]
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def _load_current_ids(
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self, db_conn, table: str, instance_column: str, id_column: str
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) -> Dict[str, int]:
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sql = """
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SELECT %(instance)s, MAX(%(id)s) FROM %(table)s
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GROUP BY %(instance)s
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""" % {
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"instance": instance_column,
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"id": id_column,
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"table": table,
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}
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cur = db_conn.cursor()
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cur.execute(sql)
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# `cur` is an iterable over returned rows, which are 2-tuples.
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current_positions = dict(cur)
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cur.close()
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return current_positions
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def _load_next_id_txn(self, txn):
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txn.execute("SELECT nextval(?)", (self._sequence_name,))
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(next_id,) = txn.fetchone()
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return next_id
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async def get_next(self):
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"""
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Usage:
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with await stream_id_gen.get_next() as stream_id:
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# ... persist event ...
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"""
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next_id = await self._db.runInteraction("_load_next_id", self._load_next_id_txn)
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# Assert the fetched ID is actually greater than what we currently
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# believe the ID to be. If not, then the sequence and table have got
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# out of sync somehow.
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assert self.get_current_token() < next_id
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with self._lock:
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self._unfinished_ids.add(next_id)
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@contextlib.contextmanager
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def manager():
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try:
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yield next_id
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finally:
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self._mark_id_as_finished(next_id)
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return manager()
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def get_next_txn(self, txn: LoggingTransaction):
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"""
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Usage:
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stream_id = stream_id_gen.get_next(txn)
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# ... persist event ...
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"""
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next_id = self._load_next_id_txn(txn)
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with self._lock:
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self._unfinished_ids.add(next_id)
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txn.call_after(self._mark_id_as_finished, next_id)
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txn.call_on_exception(self._mark_id_as_finished, next_id)
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return next_id
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def _mark_id_as_finished(self, next_id: int):
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"""The ID has finished being processed so we should advance the
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current poistion if possible.
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"""
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with self._lock:
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self._unfinished_ids.discard(next_id)
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# Figure out if its safe to advance the position by checking there
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# aren't any lower allocated IDs that are yet to finish.
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if all(c > next_id for c in self._unfinished_ids):
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curr = self._current_positions.get(self._instance_name, 0)
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self._current_positions[self._instance_name] = max(curr, next_id)
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def get_current_token(self, instance_name: str = None) -> int:
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"""Gets the current position of a named writer (defaults to current
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instance).
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Returns 0 if we don't have a position for the named writer (likely due
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to it being a new writer).
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"""
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if instance_name is None:
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instance_name = self._instance_name
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with self._lock:
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return self._current_positions.get(instance_name, 0)
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def get_positions(self) -> Dict[str, int]:
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"""Get a copy of the current positon map.
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"""
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with self._lock:
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return dict(self._current_positions)
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def advance(self, instance_name: str, new_id: int):
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"""Advance the postion of the named writer to the given ID, if greater
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than existing entry.
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"""
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with self._lock:
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self._current_positions[instance_name] = max(
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new_id, self._current_positions.get(instance_name, 0)
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)
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