Add storage module for tracking background updates.

The progress for each background update is stored as a JSON blob in the
database. Each background update is broken up into separate batches.
The batch size is automatically tuned to try avoid blocking single
threaded databases for too long.

synapse/storage/background_updates.py

@@ -0,0 +1,210 @@
+# -*- 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 ._base import SQLBaseStore
+
+from twisted.internet import defer
+
+import ujson as json
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+class BackgroundUpdatePerformance(object):
+    """Tracks the how long a background update is taking to update its items"""
+
+    def __init__(self, name):
+        self.name = name
+        self.total_item_count = 0
+        self.total_duration_ms = 0
+        self.avg_item_count = 0
+        self.avg_duration_ms = 0
+
+    def update(self, item_count, duration_ms):
+        """Update the stats after doing an update"""
+        self.total_item_count += item_count
+        self.total_duration_ms += duration_ms
+
+        # Exponential moving averages for the number of items updated and
+        # the duration.
+        self.avg_item_count += 0.1 * (item_count - self.avg_item_count)
+        self.avg_duration_ms += 0.1 * (duration_ms - self.avg_duration_ms)
+
+    def average_duration_ms_per_item(self):
+        """An estimate of how long it takes to do a single update.
+        Returns:
+            A duration in ms as a float
+        """
+        if self.total_item_count == 0:
+            return None
+        else:
+            # Use the exponential moving average so that we can adapt to
+            # changes in how long the update process takes.
+            return float(self.avg_duration_ms) / float(self.avg_item_count)
+
+
+class BackgroundUpdateStore(SQLBaseStore):
+    """ Background updates are updates to the database that run in the
+    background. Each update processes a batch of data at once. We attempt to
+    limit the impact of each update by monitoring how long each batch takes to
+    process and autotuning the batch size.
+    """
+
+    MINIMUM_BACKGROUND_BATCH_SIZE = 100
+    DEFAULT_BACKGROUND_BATCH_SIZE = 100
+
+    def __init__(self, hs):
+        super(BackgroundUpdateStore, self).__init__(hs)
+        self._background_update_performance = {}
+        self._background_update_queue = []
+        self._background_update_handlers = {}
+
+    @defer.inlineCallbacks
+    def do_background_update(self, desired_duration_ms):
+        """Does some amount of work on a background update
+        Args:
+            desired_duration_ms(float): How long we want to spend
+                updating.
+        Returns:
+            A deferred that completes once some amount of work is done.
+            The deferred will have a value of None if there is currently
+            no more work to do.
+        """
+        if not self._background_update_queue:
+            updates = yield self._simple_select_list(
+                "background_updates",
+                keyvalues=None,
+                retcols=("update_name",),
+            )
+            for update in updates:
+                self._background_update_queue.append(update['update_name'])
+
+        if not self._background_update_queue:
+            defer.returnValue(None)
+
+        update_name = self._background_update_queue.pop(0)
+        self._background_update_queue.append(update_name)
+
+        update_handler = self._background_update_handlers[update_name]
+
+        performance = self._background_update_performance.get(update_name)
+
+        if performance is None:
+            performance = BackgroundUpdatePerformance(update_name)
+            self._background_update_performance[update_name] = performance
+
+        duration_ms_per_item = performance.average_duration_ms_per_item()
+
+        if duration_ms_per_item is not None:
+            batch_size = int(desired_duration_ms / duration_ms_per_item)
+            # Clamp the batch size so that we always make progress
+            batch_size = max(batch_size, self.MINIMUM_BACKGROUND_BATCH_SIZE)
+        else:
+            batch_size = self.DEFAULT_BACKGROUND_BATCH_SIZE
+
+        progress_json = yield self._simple_select_one_onecol(
+            "background_updates",
+            keyvalues={"update_name": update_name},
+            retcol="progress_json"
+        )
+
+        progress = json.loads(progress_json)
+
+        time_start = self._clock.time_msec()
+        items_updated = yield update_handler(progress, batch_size)
+        time_stop = self._clock.time_msec()
+
+        duration_ms = time_stop - time_start
+
+        logger.info(
+            "Updating %. Updated %r items in %rms",
+            update_name, items_updated, duration_ms
+        )
+
+        performance.update(items_updated, duration_ms)
+
+        defer.returnValue(len(self._background_update_performance))
+
+    def register_background_update_handler(self, update_name, update_handler):
+        """Register a handler for doing a background update.
+
+        The handler should take two arguments:
+
+        * A dict of the current progress
+        * An integer count of the number of items to update in this batch.
+
+        The handler should return a deferred integer count of items updated.
+        The hander is responsible for updating the progress of the update.
+
+        Args:
+            update_name(str): The name of the update that this code handles.
+            update_handler(function): The function that does the update.
+        """
+        self._background_update_handlers[update_name] = update_handler
+
+    def start_background_update(self, update_name, progress):
+        """Starts a background update running.
+
+        Args:
+            update_name: The update to set running.
+            progress: The initial state of the progress of the update.
+
+        Returns:
+            A deferred that completes once the task has been added to the
+            queue.
+        """
+        # Clear the background update queue so that we will pick up the new
+        # task on the next iteration of do_background_update.
+        self._background_update_queue = []
+        progress_json = json.dumps(progress)
+
+        return self._simple_insert(
+            "background_updates",
+            {"update_name": update_name, "progress_json": progress_json}
+        )
+
+    def _end_background_update(self, update_name):
+        """Removes a completed background update task from the queue.
+
+        Args:
+            update_name(str): The name of the completed task to remove
+        Returns:
+            A deferred that completes once the task is removed.
+        """
+        self._background_update_queue = [
+            name for name in self._background_update_queue if name != update_name
+        ]
+        return self._simple_delete_one(
+            "background_updates", keyvalues={"update_name": update_name}
+        )
+
+    def _background_update_progress_txn(self, txn, update_name, progress):
+        """Update the progress of a background update
+
+        Args:
+            txn(cursor): The transaction.
+            update_name(str): The name of the background update task
+            progress(dict): The progress of the update.
+        """
+
+        progress_json = json.dumps(progress)
+
+        self._simple_update_one_txn(
+            txn,
+            "background_updates",
+            keyvalues={"update_name": update_name},
+            updatevalues={"progress_json": progress_json},
+        )

synapse/storage/schema/delta/25/00background_updates.sql

@@ -0,0 +1,21 @@
+/* Copyright 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.
+ */
+
+
+CREATE TABLE IF NOT EXISTS background_updates(
+    update_name TEXT NOT NULL, -- The name of the background update.
+    progress_json TEXT NOT NULL, -- The current progress of the update as JSON.
+    CONSTRAINT background_updates_uniqueness UNIQUE (update_name)
+);