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@@ -21,7 +21,7 @@ import threading
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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_max_id(db_conn, table, column)
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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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@@ -29,12 +29,16 @@ class IdGenerator(object):
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return self._next_id
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-def _load_max_id(db_conn, table, column):
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+def _load_current_id(db_conn, table, column, direction=1):
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cur = db_conn.cursor()
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- cur.execute("SELECT MAX(%s) FROM %s" % (column, table,))
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+ if direction == 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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- return int(val) if val else 1
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+ current_id = int(val) if val else direction
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+ return (max if direction == 1 else min)(current_id, direction)
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class StreamIdGenerator(object):
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@@ -45,17 +49,30 @@ class StreamIdGenerator(object):
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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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+ :param connection db_conn: A database connection to use to fetch the
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+ initial value of the generator from.
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+ :param str table: A database table to read the initial value of the id
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+ generator from.
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+ :param str column: The column of the database table to read the initial
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+ value from the id generator from.
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+ :param list extra_tables: 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 with
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+ the largest magnitude is used.
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+ :param int direction: which direction the stream ids grow in. +1 to grow
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+ upwards, -1 to grow downwards.
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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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- def __init__(self, db_conn, table, column, extra_tables=[]):
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+ def __init__(self, db_conn, table, column, extra_tables=[], direction=1):
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self._lock = threading.Lock()
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- self._current_max = _load_max_id(db_conn, table, column)
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+ self._direction = direction
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+ self._current = _load_current_id(db_conn, table, column, direction)
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for table, column in extra_tables:
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- self._current_max = max(
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- self._current_max,
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- _load_max_id(db_conn, table, column)
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+ self._current = (max if direction > 0 else min)(
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+ self._current,
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+ _load_current_id(db_conn, table, column, direction)
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)
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self._unfinished_ids = deque()
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@@ -66,8 +83,8 @@ class StreamIdGenerator(object):
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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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+ self._current += self._direction
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+ next_id = self._current
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self._unfinished_ids.append(next_id)
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@@ -88,8 +105,12 @@ class StreamIdGenerator(object):
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# ... persist events ...
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"""
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with self._lock:
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- next_ids = range(self._current_max + 1, self._current_max + n + 1)
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- self._current_max += n
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+ next_ids = range(
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+ self._current + self._direction,
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+ self._current + self._direction * (n + 1),
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+ self._direction
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+ )
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+ self._current += n
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for next_id in next_ids:
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self._unfinished_ids.append(next_id)
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@@ -105,15 +126,15 @@ class StreamIdGenerator(object):
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return manager()
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- def get_max_token(self):
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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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- return self._unfinished_ids[0] - 1
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+ return self._unfinished_ids[0] - self._direction
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- return self._current_max
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+ return self._current
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class ChainedIdGenerator(object):
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@@ -125,7 +146,7 @@ class ChainedIdGenerator(object):
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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._lock = threading.Lock()
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- self._current_max = _load_max_id(db_conn, table, column)
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+ self._current_max = _load_current_id(db_conn, table, column)
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self._unfinished_ids = deque()
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def get_next(self):
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@@ -137,7 +158,7 @@ class ChainedIdGenerator(object):
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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_max_token()
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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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@@ -151,7 +172,7 @@ class ChainedIdGenerator(object):
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return manager()
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- def get_max_token(self):
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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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@@ -160,4 +181,4 @@ class ChainedIdGenerator(object):
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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_max_token())
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+ return (self._current_max, self.chained_generator.get_current_token())
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