Stream ordering and out of order insertions.

Handle the fact that events can be persisted out of order, and so to get
the "current max" stream token becomes non trivial - as we need to make
sure that *all* stream tokens less than the current max have also
successfully been persisted.

2 files changed, 140 insertions, 0 deletions

diff --git a/synapse/storage/util/__init__.py b/synapse/storage/util/__init__.py
new file mode 100644
index 0000000000..c488b10d3c
--- /dev/null
+++ b/synapse/storage/util/__init__.py
@@ -0,0 +1,14 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014, 2015 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.
diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
new file mode 100644
index 0000000000..8f419323a7
--- /dev/null
+++ b/synapse/storage/util/id_generators.py
@@ -0,0 +1,126 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014, 2015 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.
+
+from twisted.internet import defer
+
+from collections import deque
+import contextlib
+import threading
+
+
+class IdGenerator(object):
+    def __init__(self, table, column, store):
+        self.table = table
+        self.column = column
+        self.store = store
+        self._lock = threading.Lock()
+        self._next_id = None
+
+    @defer.inlineCallbacks
+    def get_next(self):
+        with self._lock:
+            if not self._next_id:
+                res = yield self.store._execute_and_decode(
+                    "IdGenerator_%s" % (self.table,),
+                    "SELECT MAX(%s) as mx FROM %s" % (self.column, self.table,)
+                )
+
+                self._next_id = (res and res[0] and res[0]["mx"]) or 1
+
+            i = self._next_id
+            self._next_id += 1
+            defer.returnValue(i)
+
+    def get_next_txn(self, txn):
+        with self._lock:
+            if self._next_id:
+                i = self._next_id
+                self._next_id += 1
+                return i
+            else:
+                txn.execute(
+                    "SELECT MAX(%s) FROM %s" % (self.column, self.table,)
+                )
+
+                val, = txn.fetchone()
+                self._next_id = val or 2
+
+                return 1
+
+
+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.
+
+    Usage:
+        with stream_id_gen.get_next_txn(txn) as stream_id:
+            # ... persist event ...
+    """
+    def __init__(self):
+        self._lock = threading.Lock()
+
+        self._current_max = None
+        self._unfinished_ids = deque()
+
+    def get_next_txn(self, txn):
+        """
+        Usage:
+            with stream_id_gen.get_next_txn(txn) as stream_id:
+                # ... persist event ...
+        """
+        with self._lock:
+            if not self._current_max:
+                self._compute_current_max(txn)
+
+            self._current_max += 1
+            next_id = self._current_max
+
+            self._unfinished_ids.append(next_id)
+
+        @contextlib.contextmanager
+        def manager():
+            yield next_id
+            with self._lock:
+                self._unfinished_ids.remove(next_id)
+
+        return manager()
+
+    def get_max_token(self, store):
+        """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:
+                return self._unfinished_ids[0] - 1
+
+            if not self._current_max:
+                return store.runInteraction(
+                    "_compute_current_max",
+                    self._compute_current_max,
+                )
+
+            return self._current_max
+
+    def _compute_current_max(self, txn):
+        txn.execute("SELECT MAX(stream_ordering) FROM events")
+        val, = txn.fetchone()
+
+        self._current_max = int(val) if val else 1
+
+        return self._current_max