synapse-product/synapse/storage/background_updates.py

# -*- 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},
        )