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# -*- coding: utf-8 -*-
# Copyright 2020 The Matrix.org Foundation C.I.C.
#
# 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 threading
from typing import Callable, Optional
from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine
from synapse.storage.types import Cursor
class SequenceGenerator(metaclass=abc.ABCMeta):
"""A class which generates a unique sequence of integers"""
@abc.abstractmethod
def get_next_id_txn(self, txn: Cursor) -> int:
"""Gets the next ID in the sequence"""
...
class PostgresSequenceGenerator(SequenceGenerator):
"""An implementation of SequenceGenerator which uses a postgres sequence"""
def __init__(self, sequence_name: str):
self._sequence_name = sequence_name
def get_next_id_txn(self, txn: Cursor) -> int:
txn.execute("SELECT nextval(?)", (self._sequence_name,))
return txn.fetchone()[0]
GetFirstCallbackType = Callable[[Cursor], int]
class LocalSequenceGenerator(SequenceGenerator):
"""An implementation of SequenceGenerator which uses local locking
This only works reliably if there are no other worker processes generating IDs at
the same time.
"""
def __init__(self, get_first_callback: GetFirstCallbackType):
"""
Args:
get_first_callback: a callback which is called on the first call to
get_next_id_txn; should return the curreent maximum id
"""
# the callback. this is cleared after it is called, so that it can be GCed.
self._callback = get_first_callback # type: Optional[GetFirstCallbackType]
# The current max value, or None if we haven't looked in the DB yet.
self._current_max_id = None # type: Optional[int]
self._lock = threading.Lock()
def get_next_id_txn(self, txn: Cursor) -> int:
# We do application locking here since if we're using sqlite then
# we are a single process synapse.
with self._lock:
if self._current_max_id is None:
assert self._callback is not None
self._current_max_id = self._callback(txn)
self._callback = None
self._current_max_id += 1
return self._current_max_id
def build_sequence_generator(
database_engine: BaseDatabaseEngine,
get_first_callback: GetFirstCallbackType,
sequence_name: str,
) -> SequenceGenerator:
"""Get the best impl of SequenceGenerator available
This uses PostgresSequenceGenerator on postgres, and a locally-locked impl on
sqlite.
Args:
database_engine: the database engine we are connected to
get_first_callback: a callback which gets the next sequence ID. Used if
we're on sqlite.
sequence_name: the name of a postgres sequence to use.
"""
if isinstance(database_engine, PostgresEngine):
return PostgresSequenceGenerator(sequence_name)
else:
return LocalSequenceGenerator(get_first_callback)
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