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-rw-r--r--synapse/config/_base.py10
-rw-r--r--synapse/config/database.py5
-rw-r--r--synapse/crypto/keyclient.py2
-rw-r--r--synapse/events/__init__.py6
-rw-r--r--synapse/federation/persistence.py7
-rw-r--r--synapse/federation/transport/server.py2
-rw-r--r--synapse/handlers/federation.py3
-rw-r--r--synapse/handlers/room.py2
-rw-r--r--synapse/http/client.py2
-rw-r--r--synapse/http/matrixfederationclient.py2
-rw-r--r--synapse/push/__init__.py2
-rw-r--r--synapse/push/pusherpool.py2
-rw-r--r--synapse/python_dependencies.py2
-rw-r--r--synapse/rest/client/v1/directory.py2
-rw-r--r--synapse/rest/client/v1/login.py2
-rw-r--r--synapse/rest/client/v1/presence.py2
-rw-r--r--synapse/rest/client/v1/profile.py2
-rw-r--r--synapse/rest/client/v1/push_rule.py2
-rw-r--r--synapse/rest/client/v1/pusher.py2
-rw-r--r--synapse/rest/client/v1/register.py2
-rw-r--r--synapse/rest/client/v1/room.py2
-rw-r--r--synapse/rest/client/v2_alpha/filter.py2
-rw-r--r--synapse/rest/media/v0/content_repository.py2
-rw-r--r--synapse/storage/__init__.py17
-rw-r--r--synapse/storage/_base.py26
-rw-r--r--synapse/storage/filtering.py2
-rw-r--r--synapse/storage/push_rule.py2
-rw-r--r--synapse/storage/roommember.py68
-rw-r--r--synapse/util/frozenutils.py8
-rw-r--r--synapse/util/lrucache.py110
30 files changed, 243 insertions, 57 deletions
diff --git a/synapse/config/_base.py b/synapse/config/_base.py
index 9b0f8c3c32..87cdbf1d30 100644
--- a/synapse/config/_base.py
+++ b/synapse/config/_base.py
@@ -28,6 +28,16 @@ class Config(object):
         pass
 
     @staticmethod
+    def parse_size(string):
+        sizes = {"K": 1024, "M": 1024 * 1024}
+        size = 1
+        suffix = string[-1]
+        if suffix in sizes:
+            string = string[:-1]
+            size = sizes[suffix]
+        return int(string) * size
+
+    @staticmethod
     def abspath(file_path):
         return os.path.abspath(file_path) if file_path else file_path
 
diff --git a/synapse/config/database.py b/synapse/config/database.py
index daa161c952..87efe54645 100644
--- a/synapse/config/database.py
+++ b/synapse/config/database.py
@@ -24,6 +24,7 @@ class DatabaseConfig(Config):
             self.database_path = ":memory:"
         else:
             self.database_path = self.abspath(args.database_path)
+        self.event_cache_size = self.parse_size(args.event_cache_size)
 
     @classmethod
     def add_arguments(cls, parser):
@@ -33,6 +34,10 @@ class DatabaseConfig(Config):
             "-d", "--database-path", default="homeserver.db",
             help="The database name."
         )
+        db_group.add_argument(
+            "--event-cache-size", default="100K",
+            help="Number of events to cache in memory."
+        )
 
     @classmethod
     def generate_config(cls, args, config_dir_path):
diff --git a/synapse/crypto/keyclient.py b/synapse/crypto/keyclient.py
index cd12349f67..74008347c3 100644
--- a/synapse/crypto/keyclient.py
+++ b/synapse/crypto/keyclient.py
@@ -19,7 +19,7 @@ from twisted.internet.protocol import Factory
 from twisted.internet import defer, reactor
 from synapse.http.endpoint import matrix_federation_endpoint
 from synapse.util.logcontext import PreserveLoggingContext
-import json
+import simplejson as json
 import logging
 
 
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index 8f0c6e959f..64e08223b0 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from synapse.util.frozenutils import freeze, unfreeze
+from synapse.util.frozenutils import freeze
 
 
 class _EventInternalMetadata(object):
@@ -140,10 +140,6 @@ class FrozenEvent(EventBase):
 
         return e
 
-    def get_dict(self):
-        # We need to unfreeze what we return
-        return unfreeze(super(FrozenEvent, self).get_dict())
-
     def __str__(self):
         return self.__repr__()
 
diff --git a/synapse/federation/persistence.py b/synapse/federation/persistence.py
index 85c82a4623..76a9dcd777 100644
--- a/synapse/federation/persistence.py
+++ b/synapse/federation/persistence.py
@@ -23,7 +23,8 @@ from twisted.internet import defer
 
 from synapse.util.logutils import log_function
 
-import json
+from syutil.jsonutil import encode_canonical_json
+
 import logging
 
 
@@ -70,7 +71,7 @@ class TransactionActions(object):
             transaction.transaction_id,
             transaction.origin,
             code,
-            json.dumps(response)
+            encode_canonical_json(response)
         )
 
     @defer.inlineCallbacks
@@ -100,5 +101,5 @@ class TransactionActions(object):
             transaction.transaction_id,
             transaction.destination,
             response_code,
-            json.dumps(response_dict)
+            encode_canonical_json(response_dict)
         )
diff --git a/synapse/federation/transport/server.py b/synapse/federation/transport/server.py
index 9c9f8d525b..2ffb37aa18 100644
--- a/synapse/federation/transport/server.py
+++ b/synapse/federation/transport/server.py
@@ -20,7 +20,7 @@ from synapse.api.errors import Codes, SynapseError
 from synapse.util.logutils import log_function
 
 import logging
-import json
+import simplejson as json
 import re
 
 
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 0f9c82fd06..77c81fe2da 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -23,6 +23,7 @@ from synapse.api.errors import (
 from synapse.api.constants import EventTypes, Membership, RejectedReason
 from synapse.util.logutils import log_function
 from synapse.util.async import run_on_reactor
+from synapse.util.frozenutils import unfreeze
 from synapse.crypto.event_signing import (
     compute_event_signature, add_hashes_and_signatures,
 )
@@ -311,7 +312,7 @@ class FederationHandler(BaseHandler):
         self.room_queues[room_id] = []
 
         builder = self.event_builder_factory.new(
-            event.get_pdu_json()
+            unfreeze(event.get_pdu_json())
         )
 
         handled_events = set()
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 0369b907a5..914742d913 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -462,7 +462,7 @@ class RoomMemberHandler(BaseHandler):
                 room_hosts,
                 room_id,
                 event.user_id,
-                event.get_dict()["content"],  # FIXME To get a non-frozen dict
+                event.content,  # FIXME To get a non-frozen dict
                 context
             )
         else:
diff --git a/synapse/http/client.py b/synapse/http/client.py
index 198f575cfa..d500e19c81 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -23,7 +23,7 @@ from twisted.web.http_headers import Headers
 
 from StringIO import StringIO
 
-import json
+import simplejson as json
 import logging
 import urllib
 
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index 056d446e42..406203acf2 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -33,7 +33,7 @@ from synapse.api.errors import (
 
 from syutil.crypto.jsonsign import sign_json
 
-import json
+import simplejson as json
 import logging
 import urllib
 import urlparse
diff --git a/synapse/push/__init__.py b/synapse/push/__init__.py
index 418a348a58..0659a1cb9b 100644
--- a/synapse/push/__init__.py
+++ b/synapse/push/__init__.py
@@ -22,7 +22,7 @@ import synapse.util.async
 import baserules
 
 import logging
-import json
+import simplejson as json
 import re
 
 logger = logging.getLogger(__name__)
diff --git a/synapse/push/pusherpool.py b/synapse/push/pusherpool.py
index 5a525befd7..7483d257bf 100644
--- a/synapse/push/pusherpool.py
+++ b/synapse/push/pusherpool.py
@@ -20,7 +20,7 @@ from httppusher import HttpPusher
 from synapse.push import PusherConfigException
 
 import logging
-import json
+import simplejson as json
 
 logger = logging.getLogger(__name__)
 
diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py
index e2a9d1f6a7..8164effeee 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -4,7 +4,7 @@ from distutils.version import LooseVersion
 logger = logging.getLogger(__name__)
 
 REQUIREMENTS = {
-    "syutil==0.0.2": ["syutil"],
+    "syutil>=0.0.3": ["syutil"],
     "matrix_angular_sdk>=0.6.1": ["syweb>=0.6.1"],
     "Twisted==14.0.2": ["twisted==14.0.2"],
     "service_identity>=1.0.0": ["service_identity>=1.0.0"],
diff --git a/synapse/rest/client/v1/directory.py b/synapse/rest/client/v1/directory.py
index 8ed7e2d669..420aa89f38 100644
--- a/synapse/rest/client/v1/directory.py
+++ b/synapse/rest/client/v1/directory.py
@@ -20,7 +20,7 @@ from synapse.api.errors import AuthError, SynapseError, Codes
 from synapse.types import RoomAlias
 from .base import ClientV1RestServlet, client_path_pattern
 
-import json
+import simplejson as json
 import logging
 
 
diff --git a/synapse/rest/client/v1/login.py b/synapse/rest/client/v1/login.py
index 7116ac98e8..b2257b749d 100644
--- a/synapse/rest/client/v1/login.py
+++ b/synapse/rest/client/v1/login.py
@@ -19,7 +19,7 @@ from synapse.api.errors import SynapseError
 from synapse.types import UserID
 from base import ClientV1RestServlet, client_path_pattern
 
-import json
+import simplejson as json
 
 
 class LoginRestServlet(ClientV1RestServlet):
diff --git a/synapse/rest/client/v1/presence.py b/synapse/rest/client/v1/presence.py
index 7feb4aadb1..78d4f2b128 100644
--- a/synapse/rest/client/v1/presence.py
+++ b/synapse/rest/client/v1/presence.py
@@ -21,7 +21,7 @@ from synapse.api.errors import SynapseError
 from synapse.types import UserID
 from .base import ClientV1RestServlet, client_path_pattern
 
-import json
+import simplejson as json
 import logging
 
 logger = logging.getLogger(__name__)
diff --git a/synapse/rest/client/v1/profile.py b/synapse/rest/client/v1/profile.py
index 15d6f3fc6c..1e77eb49cf 100644
--- a/synapse/rest/client/v1/profile.py
+++ b/synapse/rest/client/v1/profile.py
@@ -19,7 +19,7 @@ from twisted.internet import defer
 from .base import ClientV1RestServlet, client_path_pattern
 from synapse.types import UserID
 
-import json
+import simplejson as json
 
 
 class ProfileDisplaynameRestServlet(ClientV1RestServlet):
diff --git a/synapse/rest/client/v1/push_rule.py b/synapse/rest/client/v1/push_rule.py
index c4e7dfcf0e..b012f31084 100644
--- a/synapse/rest/client/v1/push_rule.py
+++ b/synapse/rest/client/v1/push_rule.py
@@ -27,7 +27,7 @@ from synapse.push.rulekinds import (
     PRIORITY_CLASS_MAP, PRIORITY_CLASS_INVERSE_MAP
 )
 
-import json
+import simplejson as json
 
 
 class PushRuleRestServlet(ClientV1RestServlet):
diff --git a/synapse/rest/client/v1/pusher.py b/synapse/rest/client/v1/pusher.py
index 80e9939b79..6045e86f34 100644
--- a/synapse/rest/client/v1/pusher.py
+++ b/synapse/rest/client/v1/pusher.py
@@ -19,7 +19,7 @@ from synapse.api.errors import SynapseError, Codes
 from synapse.push import PusherConfigException
 from .base import ClientV1RestServlet, client_path_pattern
 
-import json
+import simplejson as json
 
 
 class PusherRestServlet(ClientV1RestServlet):
diff --git a/synapse/rest/client/v1/register.py b/synapse/rest/client/v1/register.py
index c0423c2d45..d3399c446b 100644
--- a/synapse/rest/client/v1/register.py
+++ b/synapse/rest/client/v1/register.py
@@ -25,7 +25,7 @@ from synapse.util.async import run_on_reactor
 
 from hashlib import sha1
 import hmac
-import json
+import simplejson as json
 import logging
 import urllib
 
diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 410f19ccf6..0346afb1b4 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -23,7 +23,7 @@ from synapse.api.constants import EventTypes, Membership
 from synapse.types import UserID, RoomID, RoomAlias
 from synapse.events.utils import serialize_event
 
-import json
+import simplejson as json
 import logging
 import urllib
 
diff --git a/synapse/rest/client/v2_alpha/filter.py b/synapse/rest/client/v2_alpha/filter.py
index 6ddc495d23..703250cea8 100644
--- a/synapse/rest/client/v2_alpha/filter.py
+++ b/synapse/rest/client/v2_alpha/filter.py
@@ -21,7 +21,7 @@ from synapse.types import UserID
 
 from ._base import client_v2_pattern
 
-import json
+import simplejson as json
 import logging
 
 
diff --git a/synapse/rest/media/v0/content_repository.py b/synapse/rest/media/v0/content_repository.py
index 22e26e3cd5..e77a20fb2e 100644
--- a/synapse/rest/media/v0/content_repository.py
+++ b/synapse/rest/media/v0/content_repository.py
@@ -25,7 +25,7 @@ from twisted.web import server, resource
 from twisted.internet import defer
 
 import base64
-import json
+import simplejson as json
 import logging
 import os
 import re
diff --git a/synapse/storage/__init__.py b/synapse/storage/__init__.py
index a63c59a8a2..02b1f06854 100644
--- a/synapse/storage/__init__.py
+++ b/synapse/storage/__init__.py
@@ -44,7 +44,6 @@ from syutil.jsonutil import encode_canonical_json
 from synapse.crypto.event_signing import compute_event_reference_hash
 
 
-import json
 import logging
 import os
 
@@ -164,6 +163,9 @@ class DataStore(RoomMemberStore, RoomStore,
                            stream_ordering=None, is_new_state=True,
                            current_state=None):
 
+        # Remove the any existing cache entries for the event_id
+        self._get_event_cache.pop(event.event_id)
+
         # We purposefully do this first since if we include a `current_state`
         # key, we *want* to update the `current_state_events` table
         if current_state:
@@ -298,12 +300,16 @@ class DataStore(RoomMemberStore, RoomStore,
             or_replace=True,
         )
 
+        content = encode_canonical_json(
+            event.content
+        ).decode("UTF-8")
+
         vals = {
             "topological_ordering": event.depth,
             "event_id": event.event_id,
             "type": event.type,
             "room_id": event.room_id,
-            "content": json.dumps(event.get_dict()["content"]),
+            "content": content,
             "processed": True,
             "outlier": outlier,
             "depth": event.depth,
@@ -323,7 +329,10 @@ class DataStore(RoomMemberStore, RoomStore,
                 "prev_events",
             ]
         }
-        vals["unrecognized_keys"] = json.dumps(unrec)
+
+        vals["unrecognized_keys"] = encode_canonical_json(
+            unrec
+        ).decode("UTF-8")
 
         try:
             self._simple_insert_txn(
@@ -420,6 +429,8 @@ class DataStore(RoomMemberStore, RoomStore,
         )
 
     def _store_redaction(self, txn, event):
+        # invalidate the cache for the redacted event
+        self._get_event_cache.pop(event.redacts)
         txn.execute(
             "INSERT OR IGNORE INTO redactions "
             "(event_id, redacts) VALUES (?,?)",
diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 3e1ab0a159..be9934c66f 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -19,11 +19,12 @@ from synapse.events import FrozenEvent
 from synapse.events.utils import prune_event
 from synapse.util.logutils import log_function
 from synapse.util.logcontext import PreserveLoggingContext, LoggingContext
+from synapse.util.lrucache import LruCache
 
 from twisted.internet import defer
 
 import collections
-import json
+import simplejson as json
 import sys
 import time
 
@@ -128,6 +129,8 @@ class SQLBaseStore(object):
         self._txn_perf_counters = PerformanceCounters()
         self._get_event_counters = PerformanceCounters()
 
+        self._get_event_cache = LruCache(hs.config.event_cache_size)
+
     def start_profiling(self):
         self._previous_loop_ts = self._clock.time_msec()
 
@@ -579,6 +582,19 @@ class SQLBaseStore(object):
 
     def _get_event_txn(self, txn, event_id, check_redacted=True,
                        get_prev_content=False, allow_rejected=False):
+
+        start_time = time.time() * 1000
+        update_counter = self._get_event_counters.update
+
+        try:
+            cache = self._get_event_cache.setdefault(event_id, {})
+            # Separate cache entries for each way to invoke _get_event_txn
+            return cache[(check_redacted, get_prev_content, allow_rejected)]
+        except KeyError:
+            pass
+        finally:
+            start_time = update_counter("event_cache", start_time)
+
         sql = (
             "SELECT e.internal_metadata, e.json, r.event_id, rej.reason "
             "FROM event_json as e "
@@ -588,8 +604,6 @@ class SQLBaseStore(object):
             "LIMIT 1 "
         )
 
-        start_time = time.time() * 1000
-
         txn.execute(sql, (event_id,))
 
         res = txn.fetchone()
@@ -599,14 +613,16 @@ class SQLBaseStore(object):
 
         internal_metadata, js, redacted, rejected_reason = res
 
-        self._get_event_counters.update("select_event", start_time)
+        start_time = update_counter("select_event", start_time)
 
         if allow_rejected or not rejected_reason:
-            return self._get_event_from_row_txn(
+            result = self._get_event_from_row_txn(
                 txn, internal_metadata, js, redacted,
                 check_redacted=check_redacted,
                 get_prev_content=get_prev_content,
             )
+            cache[(check_redacted, get_prev_content, allow_rejected)] = result
+            return result
         else:
             return None
 
diff --git a/synapse/storage/filtering.py b/synapse/storage/filtering.py
index e86eeced45..457a11fd02 100644
--- a/synapse/storage/filtering.py
+++ b/synapse/storage/filtering.py
@@ -17,7 +17,7 @@ from twisted.internet import defer
 
 from ._base import SQLBaseStore
 
-import json
+import simplejson as json
 
 
 class FilteringStore(SQLBaseStore):
diff --git a/synapse/storage/push_rule.py b/synapse/storage/push_rule.py
index 620de71398..ae46b39cc1 100644
--- a/synapse/storage/push_rule.py
+++ b/synapse/storage/push_rule.py
@@ -20,7 +20,7 @@ from twisted.internet import defer
 
 import logging
 import copy
-import json
+import simplejson as json
 
 logger = logging.getLogger(__name__)
 
diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index c69dd995ce..779f9ce544 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -35,6 +35,11 @@ RoomsForUser = namedtuple(
 
 class RoomMemberStore(SQLBaseStore):
 
+    def __init__(self, *args, **kw):
+        super(RoomMemberStore, self).__init__(*args, **kw)
+
+        self._user_rooms_cache = {}
+
     def _store_room_member_txn(self, txn, event):
         """Store a room member in the database.
         """
@@ -98,6 +103,8 @@ class RoomMemberStore(SQLBaseStore):
 
                 txn.execute(sql, (event.room_id, domain))
 
+        self.invalidate_rooms_for_user(target_user_id)
+
     @defer.inlineCallbacks
     def get_room_member(self, user_id, room_id):
         """Retrieve the current state of a room member.
@@ -240,28 +247,53 @@ class RoomMemberStore(SQLBaseStore):
         results = self._parse_events_txn(txn, rows)
         return results
 
+    # TODO(paul): Create a nice @cached decorator to do this
+    #    @cached
+    #    def get_foo(...)
+    #        ...
+    #    invalidate_foo = get_foo.invalidator
+
+    @defer.inlineCallbacks
+    def get_rooms_for_user(self, user_id):
+        # TODO(paul): put some performance counters in here so we can easily
+        #   track what impact this cache is having
+        if user_id in self._user_rooms_cache:
+            defer.returnValue(self._user_rooms_cache[user_id])
+
+        rooms = yield self.get_rooms_for_user_where_membership_is(
+            user_id, membership_list=[Membership.JOIN],
+        )
+
+        # TODO(paul): Consider applying a maximum size; just evict things at
+        #   random, or consider LRU?
+
+        self._user_rooms_cache[user_id] = rooms
+        defer.returnValue(rooms)
+
+    def invalidate_rooms_for_user(self, user_id):
+        if user_id in self._user_rooms_cache:
+            del self._user_rooms_cache[user_id]
+
+    @defer.inlineCallbacks
     def user_rooms_intersect(self, user_id_list):
         """ Checks whether all the users whose IDs are given in a list share a
         room.
+
+        This is a "hot path" function that's called a lot, e.g. by presence for
+        generating the event stream. As such, it is implemented locally by
+        wrapping logic around heavily-cached database queries.
         """
-        def interaction(txn):
-            user_list_clause = " OR ".join(["m.user_id = ?"] * len(user_id_list))
-            sql = (
-                "SELECT m.room_id FROM room_memberships as m "
-                "INNER JOIN current_state_events as c "
-                "ON m.event_id = c.event_id "
-                "WHERE m.membership = 'join' "
-                "AND (%(clause)s) "
-                # TODO(paul): We've got duplicate rows in the database somewhere
-                #   so we have to DISTINCT m.user_id here
-                "GROUP BY m.room_id HAVING COUNT(DISTINCT m.user_id) = ?"
-            ) % {"clause": user_list_clause}
-
-            args = list(user_id_list)
-            args.append(len(user_id_list))
+        if len(user_id_list) < 2:
+            defer.returnValue(True)
 
-            txn.execute(sql, args)
+        deferreds = [self.get_rooms_for_user(u) for u in user_id_list]
+
+        results = yield defer.DeferredList(deferreds)
+
+        # A list of sets of strings giving room IDs for each user
+        room_id_lists = [set([r.room_id for r in result[1]]) for result in results]
 
-            return len(txn.fetchall()) > 0
+        # There isn't a setintersection(*list_of_sets)
+        ret = len(room_id_lists.pop(0).intersection(*room_id_lists)) > 0
 
-        return self.runInteraction("user_rooms_intersect", interaction)
+        defer.returnValue(ret)
diff --git a/synapse/util/frozenutils.py b/synapse/util/frozenutils.py
index a13a2015e4..9e10d37aec 100644
--- a/synapse/util/frozenutils.py
+++ b/synapse/util/frozenutils.py
@@ -21,6 +21,9 @@ def freeze(o):
     if t is dict:
         return frozendict({k: freeze(v) for k, v in o.items()})
 
+    if t is frozendict:
+        return o
+
     if t is str or t is unicode:
         return o
 
@@ -33,10 +36,11 @@ def freeze(o):
 
 
 def unfreeze(o):
-    if isinstance(o, frozendict) or isinstance(o, dict):
+    t = type(o)
+    if t is dict or t is frozendict:
         return dict({k: unfreeze(v) for k, v in o.items()})
 
-    if isinstance(o, basestring):
+    if t is str or t is unicode:
         return o
 
     try:
diff --git a/synapse/util/lrucache.py b/synapse/util/lrucache.py
new file mode 100644
index 0000000000..a45c673d32
--- /dev/null
+++ b/synapse/util/lrucache.py
@@ -0,0 +1,110 @@
+# -*- coding: utf-8 -*-
+# 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.
+
+
+class LruCache(object):
+    """Least-recently-used cache."""
+    # TODO(mjark) Add hit/miss counters
+    # TODO(mjark) Add mutex for linked list for thread safety.
+    def __init__(self, max_size):
+        cache = {}
+        list_root = []
+        list_root[:] = [list_root, list_root, None, None]
+
+        PREV, NEXT, KEY, VALUE = 0, 1, 2, 3
+
+        def add_node(key, value):
+            prev_node = list_root
+            next_node = prev_node[NEXT]
+            node = [prev_node, next_node, key, value]
+            prev_node[NEXT] = node
+            next_node[PREV] = node
+            cache[key] = node
+
+        def move_node_to_front(node):
+            prev_node = node[PREV]
+            next_node = node[NEXT]
+            prev_node[NEXT] = next_node
+            next_node[PREV] = prev_node
+            prev_node = list_root
+            next_node = prev_node[NEXT]
+            node[PREV] = prev_node
+            node[NEXT] = next_node
+            prev_node[NEXT] = node
+            next_node[PREV] = node
+
+        def delete_node(node):
+            prev_node = node[PREV]
+            next_node = node[NEXT]
+            prev_node[NEXT] = next_node
+            next_node[PREV] = prev_node
+            cache.pop(node[KEY], None)
+
+        def cache_get(key, default=None):
+            node = cache.get(key, None)
+            if node is not None:
+                move_node_to_front(node)
+                return node[VALUE]
+            else:
+                return default
+
+        def cache_set(key, value):
+            node = cache.get(key, None)
+            if node is not None:
+                move_node_to_front(node)
+                node[VALUE] = value
+            else:
+                add_node(key, value)
+                if len(cache) > max_size:
+                    delete_node(list_root[PREV])
+
+        def cache_set_default(key, value):
+            node = cache.get(key, None)
+            if node is not None:
+                return node[VALUE]
+            else:
+                add_node(key, value)
+                if len(cache) > max_size:
+                    delete_node(list_root[PREV])
+                return value
+
+        def cache_pop(key, default=None):
+            node = cache.get(key, None)
+            if node:
+                delete_node(node)
+                return node[VALUE]
+            else:
+                return default
+
+        self.sentinel = object()
+        self.get = cache_get
+        self.set = cache_set
+        self.setdefault = cache_set_default
+        self.pop = cache_pop
+
+    def __getitem__(self, key):
+        result = self.get(key, self.sentinel)
+        if result is self.sentinel:
+            raise KeyError()
+        else:
+            return result
+
+    def __setitem__(self, key, value):
+        self.set(key, value)
+
+    def __delitem__(self, key, value):
+        result = self.pop(key, self.sentinel)
+        if result is self.sentinel:
+            raise KeyError()