diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 9d851beaa5..86d04ea9ac 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -16,6 +16,11 @@
import contextlib
import threading
from collections import deque
+from typing import Dict, Set, Tuple
+
+from typing_extensions import Deque
+
+from synapse.storage.database import Database, LoggingTransaction
class IdGenerator(object):
@@ -87,7 +92,7 @@ class StreamIdGenerator(object):
self._current = (max if step > 0 else min)(
self._current, _load_current_id(db_conn, table, column, step)
)
- self._unfinished_ids = deque()
+ self._unfinished_ids = deque() # type: Deque[int]
def get_next(self):
"""
@@ -163,7 +168,7 @@ class ChainedIdGenerator(object):
self.chained_generator = chained_generator
self._lock = threading.Lock()
self._current_max = _load_current_id(db_conn, table, column)
- self._unfinished_ids = deque()
+ self._unfinished_ids = deque() # type: Deque[Tuple[int, int]]
def get_next(self):
"""
@@ -198,3 +203,163 @@ class ChainedIdGenerator(object):
return stream_id - 1, chained_id
return self._current_max, self.chained_generator.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
+ 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.
+ """
+
+ def __init__(
+ self,
+ db_conn,
+ db: Database,
+ instance_name: str,
+ table: str,
+ instance_column: str,
+ id_column: str,
+ sequence_name: str,
+ ):
+ self._db = db
+ self._instance_name = instance_name
+ self._sequence_name = sequence_name
+
+ # We lock as some functions may be called from DB threads.
+ self._lock = threading.Lock()
+
+ self._current_positions = self._load_current_ids(
+ db_conn, table, instance_column, id_column
+ )
+
+ # 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]
+
+ def _load_current_ids(
+ self, db_conn, table: str, instance_column: str, id_column: str
+ ) -> Dict[str, int]:
+ sql = """
+ SELECT %(instance)s, MAX(%(id)s) FROM %(table)s
+ GROUP BY %(instance)s
+ """ % {
+ "instance": instance_column,
+ "id": id_column,
+ "table": table,
+ }
+
+ cur = db_conn.cursor()
+ cur.execute(sql)
+
+ # `cur` is an iterable over returned rows, which are 2-tuples.
+ current_positions = dict(cur)
+
+ cur.close()
+
+ return current_positions
+
+ def _load_next_id_txn(self, txn):
+ txn.execute("SELECT nextval(?)", (self._sequence_name,))
+ (next_id,) = txn.fetchone()
+ return next_id
+
+ async def get_next(self):
+ """
+ Usage:
+ with await stream_id_gen.get_next() as stream_id:
+ # ... persist event ...
+ """
+ next_id = await self._db.runInteraction("_load_next_id", self._load_next_id_txn)
+
+ # Assert the fetched ID is actually greater than what we currently
+ # believe the ID to be. If not, then the sequence and table have got
+ # out of sync somehow.
+ assert self.get_current_token() < next_id
+
+ with self._lock:
+ self._unfinished_ids.add(next_id)
+
+ @contextlib.contextmanager
+ def manager():
+ try:
+ yield next_id
+ finally:
+ self._mark_id_as_finished(next_id)
+
+ return manager()
+
+ 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)
+
+ return next_id
+
+ def _mark_id_as_finished(self, next_id: int):
+ """The ID has finished being processed so we should advance the
+ current poistion if possible.
+ """
+
+ with self._lock:
+ self._unfinished_ids.discard(next_id)
+
+ # Figure out if its safe to advance the position by checking there
+ # aren't any lower allocated IDs that are yet to finish.
+ if all(c > next_id for c in self._unfinished_ids):
+ curr = self._current_positions.get(self._instance_name, 0)
+ self._current_positions[self._instance_name] = max(curr, next_id)
+
+ def get_current_token(self, instance_name: str = None) -> int:
+ """Gets the current position of a named writer (defaults to current
+ instance).
+
+ Returns 0 if we don't have a position for the named writer (likely due
+ to it being a new writer).
+ """
+
+ if instance_name is None:
+ instance_name = self._instance_name
+
+ with self._lock:
+ return self._current_positions.get(instance_name, 0)
+
+ def get_positions(self) -> Dict[str, int]:
+ """Get a copy of the current positon map.
+ """
+
+ with self._lock:
+ return dict(self._current_positions)
+
+ 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.
+ """
+
+ with self._lock:
+ self._current_positions[instance_name] = max(
+ new_id, self._current_positions.get(instance_name, 0)
+ )
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