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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 contextlib
import threading
from collections import deque
class IdGenerator(object):
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
"""
cur = db_conn.cursor()
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(object):
"""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:
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()
def get_next(self):
"""
Usage:
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)
@contextlib.contextmanager
def manager():
try:
yield next_id
finally:
with self._lock:
self._unfinished_ids.remove(next_id)
return manager()
def get_next_mult(self, n):
"""
Usage:
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)
@contextlib.contextmanager
def manager():
try:
yield next_ids
finally:
with self._lock:
for next_id in next_ids:
self._unfinished_ids.remove(next_id)
return manager()
def get_current_token(self):
"""Returns the maximum stream id such that all stream ids less than or
equal to it have been successfully persisted.
Returns:
int
"""
with self._lock:
if self._unfinished_ids:
return self._unfinished_ids[0] - self._step
return self._current
class ChainedIdGenerator(object):
"""Used to generate new stream ids where the stream must be kept in sync
with another stream. It generates pairs of IDs, the first element is an
integer ID for this stream, the second element is the ID for the stream
that this stream needs to be kept in sync with."""
def __init__(self, chained_generator, db_conn, table, column):
self.chained_generator = chained_generator
self._lock = threading.Lock()
self._current_max = _load_current_id(db_conn, table, column)
self._unfinished_ids = deque()
def get_next(self):
"""
Usage:
with stream_id_gen.get_next() as (stream_id, chained_id):
# ... persist event ...
"""
with self._lock:
self._current_max += 1
next_id = self._current_max
chained_id = self.chained_generator.get_current_token()
self._unfinished_ids.append((next_id, chained_id))
@contextlib.contextmanager
def manager():
try:
yield (next_id, chained_id)
finally:
with self._lock:
self._unfinished_ids.remove((next_id, chained_id))
return manager()
def get_current_token(self):
"""Returns the maximum stream id such that all stream ids less than or
equal to it have been successfully persisted.
"""
with self._lock:
if self._unfinished_ids:
stream_id, chained_id = self._unfinished_ids[0]
return stream_id - 1, chained_id
return self._current_max, self.chained_generator.get_current_token()
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