path: root/synapse/storage/_base.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.
import logging

from synapse.api.errors import StoreError
from synapse.events import FrozenEvent
from synapse.events.utils import prune_event
from synapse.util.logutils import log_function
from synapse.util.logcontext import preserve_context_over_fn, LoggingContext
from synapse.util.lrucache import LruCache
import synapse.metrics

from util.id_generators import IdGenerator, StreamIdGenerator

from twisted.internet import defer

from collections import namedtuple, OrderedDict
import functools
import itertools
import simplejson as json
import sys
import time
import threading

DEBUG_CACHES = False

logger = logging.getLogger(__name__)

sql_logger = logging.getLogger("synapse.storage.SQL")
transaction_logger = logging.getLogger("synapse.storage.txn")
perf_logger = logging.getLogger("synapse.storage.TIME")


metrics = synapse.metrics.get_metrics_for("synapse.storage")

sql_scheduling_timer = metrics.register_distribution("schedule_time")

sql_query_timer = metrics.register_distribution("query_time", labels=["verb"])
sql_txn_timer = metrics.register_distribution("transaction_time", labels=["desc"])
sql_getevents_timer = metrics.register_distribution("getEvents_time", labels=["desc"])

caches_by_name = {}
cache_counter = metrics.register_cache(
    "cache",
    lambda: {(name,): len(caches_by_name[name]) for name in caches_by_name.keys()},
    labels=["name"],
)


class Cache(object):

    def __init__(self, name, max_entries=1000, keylen=1, lru=False):
        if lru:
            self.cache = LruCache(max_size=max_entries)
            self.max_entries = None
        else:
            self.cache = OrderedDict()
            self.max_entries = max_entries

        self.name = name
        self.keylen = keylen
        self.sequence = 0
        self.thread = None
        caches_by_name[name] = self.cache

    def check_thread(self):
        expected_thread = self.thread
        if expected_thread is None:
            self.thread = threading.current_thread()
        else:
            if expected_thread is not threading.current_thread():
                raise ValueError(
                    "Cache objects can only be accessed from the main thread"
                )

    def get(self, *keyargs):
        if len(keyargs) != self.keylen:
            raise ValueError("Expected a key to have %d items", self.keylen)

        if keyargs in self.cache:
            cache_counter.inc_hits(self.name)
            return self.cache[keyargs]

        cache_counter.inc_misses(self.name)
        raise KeyError()

    def update(self, sequence, *args):
        self.check_thread()
        if self.sequence == sequence:
            # Only update the cache if the caches sequence number matches the
            # number that the cache had before the SELECT was started (SYN-369)
            self.prefill(*args)

    def prefill(self, *args):  # because I can't  *keyargs, value
        keyargs = args[:-1]
        value = args[-1]

        if len(keyargs) != self.keylen:
            raise ValueError("Expected a key to have %d items", self.keylen)

        if self.max_entries is not None:
            while len(self.cache) >= self.max_entries:
                self.cache.popitem(last=False)

        self.cache[keyargs] = value

    def invalidate(self, *keyargs):
        self.check_thread()
        if len(keyargs) != self.keylen:
            raise ValueError("Expected a key to have %d items", self.keylen)
        # Increment the sequence number so that any SELECT statements that
        # raced with the INSERT don't update the cache (SYN-369)
        self.sequence += 1
        self.cache.pop(keyargs, None)


def cached(max_entries=1000, num_args=1, lru=False):
    """ A method decorator that applies a memoizing cache around the function.

    The function is presumed to take zero or more arguments, which are used in
    a tuple as the key for the cache. Hits are served directly from the cache;
    misses use the function body to generate the value.

    The wrapped function has an additional member, a callable called
    "invalidate". This can be used to remove individual entries from the cache.

    The wrapped function has another additional callable, called "prefill",
    which can be used to insert values into the cache specifically, without
    calling the calculation function.
    """
    def wrap(orig):
        cache = Cache(
            name=orig.__name__,
            max_entries=max_entries,
            keylen=num_args,
            lru=lru,
        )

        @functools.wraps(orig)
        @defer.inlineCallbacks
        def wrapped(self, *keyargs):
            try:
                cached_result = cache.get(*keyargs)
                if DEBUG_CACHES:
                    actual_result = yield orig(self, *keyargs)
                    if actual_result != cached_result:
                        logger.error(
                            "Stale cache entry %s%r: cached: %r, actual %r",
                            orig.__name__, keyargs,
                            cached_result, actual_result,
                        )
                        raise ValueError("Stale cache entry")
                defer.returnValue(cached_result)
            except KeyError:
                # Get the sequence number of the cache before reading from the
                # database so that we can tell if the cache is invalidated
                # while the SELECT is executing (SYN-369)
                sequence = cache.sequence

                ret = yield orig(self, *keyargs)

                cache.update(sequence, *keyargs + (ret,))

                defer.returnValue(ret)

        wrapped.invalidate = cache.invalidate
        wrapped.prefill = cache.prefill
        return wrapped

    return wrap


class LoggingTransaction(object):
    """An object that almost-transparently proxies for the 'txn' object
    passed to the constructor. Adds logging and metrics to the .execute()
    method."""
    __slots__ = ["txn", "name", "database_engine", "after_callbacks"]

    def __init__(self, txn, name, database_engine, after_callbacks):
        object.__setattr__(self, "txn", txn)
        object.__setattr__(self, "name", name)
        object.__setattr__(self, "database_engine", database_engine)
        object.__setattr__(self, "after_callbacks", after_callbacks)

    def call_after(self, callback, *args):
        """Call the given callback on the main twisted thread after the
        transaction has finished. Used to invalidate the caches on the
        correct thread.
        """
        self.after_callbacks.append((callback, args))

    def __getattr__(self, name):
        return getattr(self.txn, name)

    def __setattr__(self, name, value):
        setattr(self.txn, name, value)

    def execute(self, sql, *args):
        self._do_execute(self.txn.execute, sql, *args)

    def executemany(self, sql, *args):
        self._do_execute(self.txn.executemany, sql, *args)

    def _do_execute(self, func, sql, *args):
        # TODO(paul): Maybe use 'info' and 'debug' for values?
        sql_logger.debug("[SQL] {%s} %s", self.name, sql)

        sql = self.database_engine.convert_param_style(sql)

        if args:
            try:
                sql_logger.debug(
                    "[SQL values] {%s} %r",
                    self.name, args[0]
                )
            except:
                # Don't let logging failures stop SQL from working
                pass

        start = time.time() * 1000

        try:
            return func(
                sql, *args
            )
        except Exception as e:
            logger.debug("[SQL FAIL] {%s} %s", self.name, e)
            raise
        finally:
            msecs = (time.time() * 1000) - start
            sql_logger.debug("[SQL time] {%s} %f", self.name, msecs)
            sql_query_timer.inc_by(msecs, sql.split()[0])


class PerformanceCounters(object):
    def __init__(self):
        self.current_counters = {}
        self.previous_counters = {}

    def update(self, key, start_time, end_time=None):
        if end_time is None:
            end_time = time.time() * 1000
        duration = end_time - start_time
        count, cum_time = self.current_counters.get(key, (0, 0))
        count += 1
        cum_time += duration
        self.current_counters[key] = (count, cum_time)
        return end_time

    def interval(self, interval_duration, limit=3):
        counters = []
        for name, (count, cum_time) in self.current_counters.items():
            prev_count, prev_time = self.previous_counters.get(name, (0, 0))
            counters.append((
                (cum_time - prev_time) / interval_duration,
                count - prev_count,
                name
            ))

        self.previous_counters = dict(self.current_counters)

        counters.sort(reverse=True)

        top_n_counters = ", ".join(
            "%s(%d): %.3f%%" % (name, count, 100 * ratio)
            for ratio, count, name in counters[:limit]
        )

        return top_n_counters


class SQLBaseStore(object):
    _TXN_ID = 0

    def __init__(self, hs):
        self.hs = hs
        self._db_pool = hs.get_db_pool()
        self._clock = hs.get_clock()

        self._previous_txn_total_time = 0
        self._current_txn_total_time = 0
        self._previous_loop_ts = 0

        # TODO(paul): These can eventually be removed once the metrics code
        #   is running in mainline, and we have some nice monitoring frontends
        #   to watch it
        self._txn_perf_counters = PerformanceCounters()
        self._get_event_counters = PerformanceCounters()

        self._get_event_cache = Cache("*getEvent*", keylen=3, lru=True,
                                      max_entries=hs.config.event_cache_size)

        self.database_engine = hs.database_engine

        self._stream_id_gen = StreamIdGenerator()
        self._transaction_id_gen = IdGenerator("sent_transactions", "id", self)
        self._state_groups_id_gen = IdGenerator("state_groups", "id", self)
        self._access_tokens_id_gen = IdGenerator("access_tokens", "id", self)
        self._pushers_id_gen = IdGenerator("pushers", "id", self)
        self._push_rule_id_gen = IdGenerator("push_rules", "id", self)

    def start_profiling(self):
        self._previous_loop_ts = self._clock.time_msec()

        def loop():
            curr = self._current_txn_total_time
            prev = self._previous_txn_total_time
            self._previous_txn_total_time = curr

            time_now = self._clock.time_msec()
            time_then = self._previous_loop_ts
            self._previous_loop_ts = time_now

            ratio = (curr - prev)/(time_now - time_then)

            top_three_counters = self._txn_perf_counters.interval(
                time_now - time_then, limit=3
            )

            top_3_event_counters = self._get_event_counters.interval(
                time_now - time_then, limit=3
            )

            perf_logger.info(
                "Total database time: %.3f%% {%s} {%s}",
                ratio * 100, top_three_counters, top_3_event_counters
            )

        self._clock.looping_call(loop, 10000)

    @defer.inlineCallbacks
    def runInteraction(self, desc, func, *args, **kwargs):
        """Wraps the .runInteraction() method on the underlying db_pool."""
        current_context = LoggingContext.current_context()

        start_time = time.time() * 1000

        after_callbacks = []

        def inner_func(conn, *args, **kwargs):
            with LoggingContext("runInteraction") as context:
                if self.database_engine.is_connection_closed(conn):
                    logger.debug("Reconnecting closed database connection")
                    conn.reconnect()

                current_context.copy_to(context)
                start = time.time() * 1000
                txn_id = self._TXN_ID

                # We don't really need these to be unique, so lets stop it from
                # growing really large.
                self._TXN_ID = (self._TXN_ID + 1) % (sys.maxint - 1)

                name = "%s-%x" % (desc, txn_id, )

                sql_scheduling_timer.inc_by(time.time() * 1000 - start_time)
                transaction_logger.debug("[TXN START] {%s}", name)
                try:
                    i = 0
                    N = 5
                    while True:
                        try:
                            txn = conn.cursor()
                            txn = LoggingTransaction(
                                txn, name, self.database_engine, after_callbacks
                            )
                            return func(txn, *args, **kwargs)
                        except self.database_engine.module.OperationalError as e:
                            # This can happen if the database disappears mid
                            # transaction.
                            logger.warn(
                                "[TXN OPERROR] {%s} %s %d/%d",
                                name, e, i, N
                            )
                            if i < N:
                                i += 1
                                try:
                                    conn.rollback()
                                except self.database_engine.module.Error as e1:
                                    logger.warn(
                                        "[TXN EROLL] {%s} %s",
                                        name, e1,
                                    )
                                continue
                        except self.database_engine.module.DatabaseError as e:
                            if self.database_engine.is_deadlock(e):
                                logger.warn("[TXN DEADLOCK] {%s} %d/%d", name, i, N)
                                if i < N:
                                    i += 1
                                    try:
                                        conn.rollback()
                                    except self.database_engine.module.Error as e1:
                                        logger.warn(
                                            "[TXN EROLL] {%s} %s",
                                            name, e1,
                                        )
                                    continue
                            raise
                except Exception as e:
                    logger.debug("[TXN FAIL] {%s} %s", name, e)
                    raise
                finally:
                    end = time.time() * 1000
                    duration = end - start

                    transaction_logger.debug("[TXN END] {%s} %f", name, duration)

                    self._current_txn_total_time += duration
                    self._txn_perf_counters.update(desc, start, end)
                    sql_txn_timer.inc_by(duration, desc)

        result = yield preserve_context_over_fn(
            self._db_pool.runWithConnection,
            inner_func, *args, **kwargs
        )

        for after_callback, after_args in after_callbacks:
            after_callback(*after_args)
        defer.returnValue(result)

    def cursor_to_dict(self, cursor):
        """Converts a SQL cursor into an list of dicts.

        Args:
            cursor : The DBAPI cursor which has executed a query.
        Returns:
            A list of dicts where the key is the column header.
        """
        col_headers = list(column[0] for column in cursor.description)
        results = list(
            dict(zip(col_headers, row)) for row in cursor.fetchall()
        )
        return results

    def _execute(self, desc, decoder, query, *args):
        """Runs a single query for a result set.

        Args:
            decoder - The function which can resolve the cursor results to
                something meaningful.
            query - The query string to execute
            *args - Query args.
        Returns:
            The result of decoder(results)
        """
        def interaction(txn):
            txn.execute(query, args)
            if decoder:
                return decoder(txn)
            else:
                return txn.fetchall()

        return self.runInteraction(desc, interaction)

    def _execute_and_decode(self, desc, query, *args):
        return self._execute(desc, self.cursor_to_dict, query, *args)

    # "Simple" SQL API methods that operate on a single table with no JOINs,
    # no complex WHERE clauses, just a dict of values for columns.

    @defer.inlineCallbacks
    def _simple_insert(self, table, values, or_ignore=False,
                       desc="_simple_insert"):
        """Executes an INSERT query on the named table.

        Args:
            table : string giving the table name
            values : dict of new column names and values for them
        """
        try:
            yield self.runInteraction(
                desc,
                self._simple_insert_txn, table, values,
            )
        except self.database_engine.module.IntegrityError:
            # We have to do or_ignore flag at this layer, since we can't reuse
            # a cursor after we receive an error from the db.
            if not or_ignore:
                raise

    @log_function
    def _simple_insert_txn(self, txn, table, values):
        keys, vals = zip(*values.items())

        sql = "INSERT INTO %s (%s) VALUES(%s)" % (
            table,
            ", ".join(k for k in keys),
            ", ".join("?" for _ in keys)
        )

        txn.execute(sql, vals)

    def _simple_insert_many_txn(self, txn, table, values):
        if not values:
            return

        # This is a *slight* abomination to get a list of tuples of key names
        # and a list of tuples of value names.
        #
        # i.e. [{"a": 1, "b": 2}, {"c": 3, "d": 4}]
        #         => [("a", "b",), ("c", "d",)] and [(1, 2,), (3, 4,)]
        #
        # The sort is to ensure that we don't rely on dictionary iteration
        # order.
        keys, vals = zip(*[
            zip(
                *(sorted(i.items(), key=lambda kv: kv[0]))
            )
            for i in values
            if i
        ])

        for k in keys:
            if k != keys[0]:
                raise RuntimeError(
                    "All items must have the same keys"
                )

        sql = "INSERT INTO %s (%s) VALUES(%s)" % (
            table,
            ", ".join(k for k in keys[0]),
            ", ".join("?" for _ in keys[0])
        )

        txn.executemany(sql, vals)

    def _simple_upsert(self, table, keyvalues, values,
                       insertion_values={}, desc="_simple_upsert", lock=True):
        """
        Args:
            table (str): The table to upsert into
            keyvalues (dict): The unique key tables and their new values
            values (dict): The nonunique columns and their new values
            insertion_values (dict): key/values to use when inserting
        Returns: A deferred
        """
        return self.runInteraction(
            desc,
            self._simple_upsert_txn, table, keyvalues, values, insertion_values,
            lock
        )

    def _simple_upsert_txn(self, txn, table, keyvalues, values, insertion_values={},
                           lock=True):
        # We need to lock the table :(, unless we're *really* careful
        if lock:
            self.database_engine.lock_table(txn, table)

        # Try to update
        sql = "UPDATE %s SET %s WHERE %s" % (
            table,
            ", ".join("%s = ?" % (k,) for k in values),
            " AND ".join("%s = ?" % (k,) for k in keyvalues)
        )
        sqlargs = values.values() + keyvalues.values()
        logger.debug(
            "[SQL] %s Args=%s",
            sql, sqlargs,
        )

        txn.execute(sql, sqlargs)
        if txn.rowcount == 0:
            # We didn't update and rows so insert a new one
            allvalues = {}
            allvalues.update(keyvalues)
            allvalues.update(values)
            allvalues.update(insertion_values)

            sql = "INSERT INTO %s (%s) VALUES (%s)" % (
                table,
                ", ".join(k for k in allvalues),
                ", ".join("?" for _ in allvalues)
            )
            logger.debug(
                "[SQL] %s Args=%s",
                sql, keyvalues.values(),
            )
            txn.execute(sql, allvalues.values())

    def _simple_select_one(self, table, keyvalues, retcols,
                           allow_none=False, desc="_simple_select_one"):
        """Executes a SELECT query on the named table, which is expected to
        return a single row, returning a single column from it.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
            retcols : list of strings giving the names of the columns to return

            allow_none : If true, return None instead of failing if the SELECT
              statement returns no rows
        """
        return self.runInteraction(
            desc,
            self._simple_select_one_txn,
            table, keyvalues, retcols, allow_none,
        )

    def _simple_select_one_onecol(self, table, keyvalues, retcol,
                                  allow_none=False,
                                  desc="_simple_select_one_onecol"):
        """Executes a SELECT query on the named table, which is expected to
        return a single row, returning a single column from it."

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
            retcol : string giving the name of the column to return
        """
        return self.runInteraction(
            desc,
            self._simple_select_one_onecol_txn,
            table, keyvalues, retcol, allow_none=allow_none,
        )

    def _simple_select_one_onecol_txn(self, txn, table, keyvalues, retcol,
                                      allow_none=False):
        ret = self._simple_select_onecol_txn(
            txn,
            table=table,
            keyvalues=keyvalues,
            retcol=retcol,
        )

        if ret:
            return ret[0]
        else:
            if allow_none:
                return None
            else:
                raise StoreError(404, "No row found")

    def _simple_select_onecol_txn(self, txn, table, keyvalues, retcol):
        sql = (
            "SELECT %(retcol)s FROM %(table)s WHERE %(where)s"
        ) % {
            "retcol": retcol,
            "table": table,
            "where": " AND ".join("%s = ?" % k for k in keyvalues.keys()),
        }

        txn.execute(sql, keyvalues.values())

        return [r[0] for r in txn.fetchall()]

    def _simple_select_onecol(self, table, keyvalues, retcol,
                              desc="_simple_select_onecol"):
        """Executes a SELECT query on the named table, which returns a list
        comprising of the values of the named column from the selected rows.

        Args:
            table (str): table name
            keyvalues (dict): column names and values to select the rows with
            retcol (str): column whos value we wish to retrieve.

        Returns:
            Deferred: Results in a list
        """
        return self.runInteraction(
            desc,
            self._simple_select_onecol_txn,
            table, keyvalues, retcol
        )

    def _simple_select_list(self, table, keyvalues, retcols,
                            desc="_simple_select_list"):
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the rows with,
            or None to not apply a WHERE clause.
            retcols : list of strings giving the names of the columns to return
        """
        return self.runInteraction(
            desc,
            self._simple_select_list_txn,
            table, keyvalues, retcols
        )

    def _simple_select_list_txn(self, txn, table, keyvalues, retcols):
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Args:
            txn : Transaction object
            table : string giving the table name
            keyvalues : dict of column names and values to select the rows with
            retcols : list of strings giving the names of the columns to return
        """
        if keyvalues:
            sql = "SELECT %s FROM %s WHERE %s" % (
                ", ".join(retcols),
                table,
                " AND ".join("%s = ?" % (k, ) for k in keyvalues)
            )
            txn.execute(sql, keyvalues.values())
        else:
            sql = "SELECT %s FROM %s" % (
                ", ".join(retcols),
                table
            )
            txn.execute(sql)

        return self.cursor_to_dict(txn)

    def _simple_update_one(self, table, keyvalues, updatevalues,
                           desc="_simple_update_one"):
        """Executes an UPDATE query on the named table, setting new values for
        columns in a row matching the key values.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
            updatevalues : dict giving column names and values to update
            retcols : optional list of column names to return

        If present, retcols gives a list of column names on which to perform
        a SELECT statement *before* performing the UPDATE statement. The values
        of these will be returned in a dict.

        These are performed within the same transaction, allowing an atomic
        get-and-set.  This can be used to implement compare-and-set by putting
        the update column in the 'keyvalues' dict as well.
        """
        return self.runInteraction(
            desc,
            self._simple_update_one_txn,
            table, keyvalues, updatevalues,
        )

    def _simple_update_one_txn(self, txn, table, keyvalues, updatevalues):
        update_sql = "UPDATE %s SET %s WHERE %s" % (
            table,
            ", ".join("%s = ?" % (k,) for k in updatevalues),
            " AND ".join("%s = ?" % (k,) for k in keyvalues)
        )

        txn.execute(
            update_sql,
            updatevalues.values() + keyvalues.values()
        )

        if txn.rowcount == 0:
            raise StoreError(404, "No row found")
        if txn.rowcount > 1:
            raise StoreError(500, "More than one row matched")

    def _simple_select_one_txn(self, txn, table, keyvalues, retcols,
                               allow_none=False):
        select_sql = "SELECT %s FROM %s WHERE %s" % (
            ", ".join(retcols),
            table,
            " AND ".join("%s = ?" % (k,) for k in keyvalues)
        )

        txn.execute(select_sql, keyvalues.values())

        row = txn.fetchone()
        if not row:
            if allow_none:
                return None
            raise StoreError(404, "No row found")
        if txn.rowcount > 1:
            raise StoreError(500, "More than one row matched")

        return dict(zip(retcols, row))

    def _simple_selectupdate_one(self, table, keyvalues, updatevalues=None,
                                 retcols=None, allow_none=False,
                                 desc="_simple_selectupdate_one"):
        """ Combined SELECT then UPDATE."""
        def func(txn):
            ret = None
            if retcols:
                ret = self._simple_select_one_txn(
                    txn,
                    table=table,
                    keyvalues=keyvalues,
                    retcols=retcols,
                    allow_none=allow_none,
                )

            if updatevalues:
                self._simple_update_one_txn(
                    txn,
                    table=table,
                    keyvalues=keyvalues,
                    updatevalues=updatevalues,
                )

                # if txn.rowcount == 0:
                #     raise StoreError(404, "No row found")
                if txn.rowcount > 1:
                    raise StoreError(500, "More than one row matched")

            return ret
        return self.runInteraction(desc, func)

    def _simple_delete_one(self, table, keyvalues, desc="_simple_delete_one"):
        """Executes a DELETE query on the named table, expecting to delete a
        single row.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
        """
        sql = "DELETE FROM %s WHERE %s" % (
            table,
            " AND ".join("%s = ?" % (k, ) for k in keyvalues)
        )

        def func(txn):
            txn.execute(sql, keyvalues.values())
            if txn.rowcount == 0:
                raise StoreError(404, "No row found")
            if txn.rowcount > 1:
                raise StoreError(500, "more than one row matched")
        return self.runInteraction(desc, func)

    def _simple_delete(self, table, keyvalues, desc="_simple_delete"):
        """Executes a DELETE query on the named table.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
        """

        return self.runInteraction(desc, self._simple_delete_txn)

    def _simple_delete_txn(self, txn, table, keyvalues):
        sql = "DELETE FROM %s WHERE %s" % (
            table,
            " AND ".join("%s = ?" % (k, ) for k in keyvalues)
        )

        return txn.execute(sql, keyvalues.values())

    def _simple_max_id(self, table):
        """Executes a SELECT query on the named table, expecting to return the
        max value for the column "id".

        Args:
            table : string giving the table name
        """
        sql = "SELECT MAX(id) AS id FROM %s" % table

        def func(txn):
            txn.execute(sql)
            max_id = self.cursor_to_dict(txn)[0]["id"]
            if max_id is None:
                return 0
            return max_id

        return self.runInteraction("_simple_max_id", func)

    def _get_events(self, event_ids, check_redacted=True,
                    get_prev_content=False, desc="_get_events"):
        return self.runInteraction(
            desc, self._get_events_txn, event_ids,
            check_redacted=check_redacted, get_prev_content=get_prev_content,
        )

    def _get_events_txn(self, txn, event_ids, check_redacted=True,
                        get_prev_content=False):
        N = 50  # Only fetch 100 events at a time.
        return list(itertools.chain(*[
            self._fetch_events_txn(
                txn, event_ids[i*N:(i+1)*N],
                check_redacted=check_redacted,
                get_prev_content=get_prev_content,
            )
            for i in range(1 + len(event_ids) / N)
        ]))

    def _invalidate_get_event_cache(self, event_id):
        for check_redacted in (False, True):
            for get_prev_content in (False, True):
                self._get_event_cache.invalidate(event_id, check_redacted,
                                                 get_prev_content)

    def _get_event_txn(self, txn, event_id, check_redacted=True,
                       get_prev_content=False, allow_rejected=False):

        start_time = time.time() * 1000

        def update_counter(desc, last_time):
            curr_time = self._get_event_counters.update(desc, last_time)
            sql_getevents_timer.inc_by(curr_time - last_time, desc)
            return curr_time

        try:
            ret = self._get_event_cache.get(event_id, check_redacted, get_prev_content)

            if allow_rejected or not ret.rejected_reason:
                return ret
            else:
                return None
        except KeyError:
            pass
        finally:
            start_time = update_counter("event_cache", start_time)

        sql = (
            "SELECT e.internal_metadata, e.json, r.redacts, rej.event_id "
            "FROM event_json as e "
            "LEFT JOIN rejections as rej USING (event_id) "
            "LEFT JOIN redactions as r ON e.event_id = r.redacts "
            "WHERE e.event_id = ? "
            "LIMIT 1 "
        )

        txn.execute(sql, (event_id,))

        res = txn.fetchone()

        if not res:
            return None

        internal_metadata, js, redacted, rejected_reason = res

        start_time = update_counter("select_event", start_time)

        result = self._get_event_from_row_txn(
            txn, internal_metadata, js, redacted,
            check_redacted=check_redacted,
            get_prev_content=get_prev_content,
            rejected_reason=rejected_reason,
        )
        self._get_event_cache.prefill(event_id, check_redacted, get_prev_content, result)

        if allow_rejected or not rejected_reason:
            return result
        else:
            return None

    def _fetch_events_txn(self, txn, events, check_redacted=True,
                          get_prev_content=False, allow_rejected=False):
        if not events:
            return []

        event_map = {}

        for event_id in events:
            try:
                ret = self._get_event_cache.get(event_id, check_redacted, get_prev_content)

                if allow_rejected or not ret.rejected_reason:
                    event_map[event_id] = ret
                else:
                    event_map[event_id] = None
            except KeyError:
                pass

        missing_events = [
            e for e in events
            if e not in event_map
        ]

        if missing_events:
            sql = (
                "SELECT e.internal_metadata, e.json, r.redacts, rej.event_id "
                " FROM event_json as e"
                " LEFT JOIN rejections as rej USING (event_id)"
                " LEFT JOIN redactions as r ON e.event_id = r.redacts"
                " WHERE e.event_id IN (%s)"
            ) % (",".join(["?"]*len(missing_events)),)

            txn.execute(sql, missing_events)
            rows = txn.fetchall()

            res = [
                self._get_event_from_row_txn(
                    txn, row[0], row[1], row[2],
                    check_redacted=check_redacted,
                    get_prev_content=get_prev_content,
                    rejected_reason=row[3],
                )
                for row in rows
            ]

            event_map.update({
                e.event_id: e
                for e in res if e
            })

            for e in res:
                self._get_event_cache.prefill(
                    e.event_id, check_redacted, get_prev_content, e
                )

        return [
            event_map[e_id] for e_id in events
            if e_id in event_map and event_map[e_id]
        ]

    def _get_event_from_row_txn(self, txn, internal_metadata, js, redacted,
                                check_redacted=True, get_prev_content=False,
                                rejected_reason=None):

        start_time = time.time() * 1000

        def update_counter(desc, last_time):
            curr_time = self._get_event_counters.update(desc, last_time)
            sql_getevents_timer.inc_by(curr_time - last_time, desc)
            return curr_time

        d = json.loads(js)
        start_time = update_counter("decode_json", start_time)

        internal_metadata = json.loads(internal_metadata)
        start_time = update_counter("decode_internal", start_time)

        if rejected_reason:
            rejected_reason = self._simple_select_one_onecol_txn(
                txn,
                table="rejections",
                keyvalues={"event_id": rejected_reason},
                retcol="reason",
            )

        ev = FrozenEvent(
            d,
            internal_metadata_dict=internal_metadata,
            rejected_reason=rejected_reason,
        )
        start_time = update_counter("build_frozen_event", start_time)

        if check_redacted and redacted:
            ev = prune_event(ev)

            redaction_id = self._simple_select_one_onecol_txn(
                txn,
                table="redactions",
                keyvalues={"redacts": ev.event_id},
                retcol="event_id",
            )

            ev.unsigned["redacted_by"] = redaction_id
            # Get the redaction event.

            because = self._get_event_txn(
                txn,
                redaction_id,
                check_redacted=False
            )

            if because:
                ev.unsigned["redacted_because"] = because
            start_time = update_counter("redact_event", start_time)

        if get_prev_content and "replaces_state" in ev.unsigned:
            prev = self._get_event_txn(
                txn,
                ev.unsigned["replaces_state"],
                get_prev_content=False,
            )
            if prev:
                ev.unsigned["prev_content"] = prev.get_dict()["content"]
            start_time = update_counter("get_prev_content", start_time)

        return ev

    def _parse_events(self, rows):
        return self.runInteraction(
            "_parse_events", self._parse_events_txn, rows
        )

    def _parse_events_txn(self, txn, rows):
        event_ids = [r["event_id"] for r in rows]

        return self._get_events_txn(txn, event_ids)

    def _has_been_redacted_txn(self, txn, event):
        sql = "SELECT event_id FROM redactions WHERE redacts = ?"
        txn.execute(sql, (event.event_id,))
        result = txn.fetchone()
        return result[0] if result else None

    def get_next_stream_id(self):
        with self._next_stream_id_lock:
            i = self._next_stream_id
            self._next_stream_id += 1
            return i


class _RollbackButIsFineException(Exception):
    """ This exception is used to rollback a transaction without implying
    something went wrong.
    """
    pass


class Table(object):
    """ A base class used to store information about a particular table.
    """

    table_name = None
    """ str: The name of the table """

    fields = None
    """ list: The field names """

    EntryType = None
    """ Type: A tuple type used to decode the results """

    _select_where_clause = "SELECT %s FROM %s WHERE %s"
    _select_clause = "SELECT %s FROM %s"
    _insert_clause = "REPLACE INTO %s (%s) VALUES (%s)"

    @classmethod
    def select_statement(cls, where_clause=None):
        """
        Args:
            where_clause (str): The WHERE clause to use.

        Returns:
            str: An SQL statement to select rows from the table with the given
            WHERE clause.
        """
        if where_clause:
            return cls._select_where_clause % (
                ", ".join(cls.fields),
                cls.table_name,
                where_clause
            )
        else:
            return cls._select_clause % (
                ", ".join(cls.fields),
                cls.table_name,
            )

    @classmethod
    def insert_statement(cls):
        return cls._insert_clause % (
            cls.table_name,
            ", ".join(cls.fields),
            ", ".join(["?"] * len(cls.fields)),
        )

    @classmethod
    def decode_single_result(cls, results):
        """ Given an iterable of tuples, return a single instance of
            `EntryType` or None if the iterable is empty
        Args:
            results (list): The results list to convert to `EntryType`
        Returns:
            EntryType: An instance of `EntryType`
        """
        results = list(results)
        if results:
            return cls.EntryType(*results[0])
        else:
            return None

    @classmethod
    def decode_results(cls, results):
        """ Given an iterable of tuples, return a list of `EntryType`
        Args:
            results (list): The results list to convert to `EntryType`

        Returns:
            list: A list of `EntryType`
        """
        return [cls.EntryType(*row) for row in results]

    @classmethod
    def get_fields_string(cls, prefix=None):
        if prefix:
            to_join = ("%s.%s" % (prefix, f) for f in cls.fields)
        else:
            to_join = cls.fields

        return ", ".join(to_join)


class JoinHelper(object):
    """ Used to help do joins on tables by looking at the tables' fields and
    creating a list of unique fields to use with SELECTs and a namedtuple
    to dump the results into.

    Attributes:
        tables (list): List of `Table` classes
        EntryType (type)
    """

    def __init__(self, *tables):
        self.tables = tables

        res = []
        for table in self.tables:
            res += [f for f in table.fields if f not in res]

        self.EntryType = namedtuple("JoinHelperEntry", res)

    def get_fields(self, **prefixes):
        """Get a string representing a list of fields for use in SELECT
        statements with the given prefixes applied to each.

        For example::

            JoinHelper(PdusTable, StateTable).get_fields(
                PdusTable="pdus",
                StateTable="state"
            )
        """
        res = []
        for field in self.EntryType._fields:
            for table in self.tables:
                if field in table.fields:
                    res.append("%s.%s" % (prefixes[table.__name__], field))
                    break

        return ", ".join(res)

    def decode_results(self, rows):
        return [self.EntryType(*row) for row in rows]