Open Source Monorail

appengine/monorail/services/caches.py

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is govered by a BSD-style
+# license that can be found in the LICENSE file or at
+# https://developers.google.com/open-source/licenses/bsd
+
+"""Classes to manage cached values.
+
+Monorail makes full use of the RAM of GAE frontends to reduce latency
+and load on the database.
+"""
+
+import logging
+
+from protorpc import protobuf
+
+from google.appengine.api import memcache
+
+from framework import framework_constants
+
+
+INVALIDATE_KIND_VALUES = ['user', 'project', 'issue', 'issue_id']
+DEFAULT_MAX_SIZE = 10000
+
+
+class RamCache(object):
+  """An in-RAM cache with distributed invalidation."""
+
+  def __init__(self, cache_manager, kind, max_size=None):
+    assert kind in INVALIDATE_KIND_VALUES
+    self.cache_manager = cache_manager
+    self.kind = kind
+    self.cache = {}
+    self.max_size = max_size or DEFAULT_MAX_SIZE
+
+  def CacheItem(self, key, item):
+    """Store item at key in this cache, discarding a random item if needed."""
+    if len(self.cache) >= self.max_size:
+      self.cache.popitem()
+
+    self.cache[key] = item
+
+  def CacheAll(self, new_item_dict):
+    """Cache all items in the given dict, dropping old items if needed."""
+    if len(new_item_dict) >= self.max_size:
+      logging.warn('Dumping the entire cache! %s', self.kind)
+      self.cache = {}
+    else:
+      while len(self.cache) + len(new_item_dict) > self.max_size:
+        self.cache.popitem()
+
+    self.cache.update(new_item_dict)
+
+  def GetItem(self, key):
+    """Return the cached item if present, otherwise None."""
+    return self.cache.get(key)
+
+  def HasItem(self, key):
+    """Return True if there is a value cached at the given key."""
+    return key in self.cache
+
+  def GetAll(self, keys):
+    """Look up the given keys.
+
+    Args:
+      keys: a list of cache keys to look up.
+
+    Returns:
+      A pair: (hits_dict, misses_list) where hits_dict is a dictionary of
+      all the given keys and the values that were found in the cache, and
+      misses_list is a list of given keys that were not in the cache.
+    """
+    hits, misses = {}, []
+    for key in keys:
+      try:
+        hits[key] = self.cache[key]
+      except KeyError:
+        misses.append(key)
+
+    return hits, misses
+
+  def LocalInvalidate(self, key):
+    """Drop the given key from this cache, without distributed notification."""
+    # logging.info('Locally invalidating %r in kind=%r', key, self.kind)
+    self.cache.pop(key, None)
+
+  def Invalidate(self, cnxn, key):
+    """Drop key locally, and append it to the Invalidate DB table."""
+    self.InvalidateKeys(cnxn, [key])
+
+  def InvalidateKeys(self, cnxn, keys):
+    """Drop keys locally, and append them to the Invalidate DB table."""
+    for key in keys:
+      self.LocalInvalidate(key)
+    if self.cache_manager:
+      self.cache_manager.StoreInvalidateRows(cnxn, self.kind, keys)
+
+  def LocalInvalidateAll(self):
+    """Invalidate all keys locally: just start over with an empty dict."""
+    logging.info('Locally invalidating all in kind=%r', self.kind)
+    self.cache = {}
+
+  def InvalidateAll(self, cnxn):
+    """Invalidate all keys in this cache."""
+    self.LocalInvalidateAll()
+    if self.cache_manager:
+      self.cache_manager.StoreInvalidateAll(cnxn, self.kind)
+
+
+class ValueCentricRamCache(RamCache):
+  """Specialized version of RamCache that stores values in InvalidateTable.
+
+  This is useful for caches that have non integer keys.
+  """
+
+  def LocalInvalidate(self, value):
+    """Use the specified value to drop entries from the local cache."""
+    keys_to_drop = []
+    # Loop through and collect all keys with the specified value.
+    for k, v in self.cache.iteritems():
+      if v == value:
+        keys_to_drop.append(k)
+    for k in keys_to_drop:
+      self.cache.pop(k, None)
+
+  def InvalidateKeys(self, cnxn, keys):
+    """Drop keys locally, and append their values to the Invalidate DB table."""
+    # Find values to invalidate.
+    values = [self.cache[key] for key in keys if self.cache.has_key(key)]
+    if len(values) == len(keys):
+      for value in values:
+        self.LocalInvalidate(value)
+      if self.cache_manager:
+        self.cache_manager.StoreInvalidateRows(cnxn, self.kind, values)
+    else:
+      # If a value is not found in the cache then invalidate the whole cache.
+      # This is done to ensure that we are not in an inconsistent state or in a
+      # race condition.
+      self.InvalidateAll(cnxn)
+
+
+class AbstractTwoLevelCache(object):
+  """A class to manage both RAM and memcache to retrieve objects.
+
+  Subclasses must implement the FetchItems() method to get objects from
+  the database when both caches miss.
+  """
+
+  # When loading a huge number of issues from the database, do it in chunks
+  # so as to avoid timeouts.
+  _FETCH_BATCH_SIZE = 10000
+
+  def __init__(
+      self, cache_manager, kind, memcache_prefix, pb_class, max_size=None,
+      use_value_centric_cache=False):
+    self.cache = cache_manager.MakeCache(
+        kind, max_size=max_size,
+        use_value_centric_cache=use_value_centric_cache)
+    self.memcache_prefix = memcache_prefix
+    self.pb_class = pb_class
+
+  def CacheItem(self, key, value):
+    """Add the given key-value pair to RAM and memcache."""
+    self.cache.CacheItem(key, value)
+    self._WriteToMemcache({key: value})
+
+  def HasItem(self, key):
+    """Return True if the given key is in the RAM cache."""
+    return self.cache.HasItem(key)
+
+  def GetAnyOnHandItem(self, keys, start=None, end=None):
+    """Try to find one of the specified items in RAM."""
+    if start is None:
+      start = 0
+    if end is None:
+      end = len(keys)
+    for i in xrange(start, end):
+      key = keys[i]
+      if self.cache.HasItem(key):
+        return self.cache.GetItem(key)
+
+    # Note: We could check memcache here too, but the round-trips to memcache
+    # are kind of slow.  And, getting too many hits from memcache actually
+    # fills our RAM cache too quickly and could lead to thrashing.
+
+    return None
+
+  def GetAll(self, cnxn, keys, use_cache=True, **kwargs):
+    """Get values for the given keys from RAM, memcache, or the DB.
+
+    Args:
+      cnxn: connection to the database.
+      keys: list of integer keys to look up.
+      use_cache: set to False to always hit the database.
+      **kwargs: any additional keywords are passed to FetchItems().
+
+    Returns:
+      A pair: hits, misses.  Where hits is {key: value} and misses is
+      a list of any keys that were not found anywhere.
+    """
+    if use_cache:
+      result_dict, missed_keys = self.cache.GetAll(keys)
+    else:
+      result_dict, missed_keys = {}, list(keys)
+
+    if missed_keys and use_cache:
+      memcache_hits, missed_keys = self._ReadFromMemcache(missed_keys)
+      result_dict.update(memcache_hits)
+
+    while missed_keys:
+      missed_batch = missed_keys[:self._FETCH_BATCH_SIZE]
+      missed_keys = missed_keys[self._FETCH_BATCH_SIZE:]
+      retrieved_dict = self.FetchItems(cnxn, missed_batch, **kwargs)
+      result_dict.update(retrieved_dict)
+      if use_cache:
+        self.cache.CacheAll(retrieved_dict)
+        self._WriteToMemcache(retrieved_dict)
+
+    still_missing_keys = [key for key in keys if key not in result_dict]
+    return result_dict, still_missing_keys
+
+  def _ReadFromMemcache(self, keys):
+    """Read the given keys from memcache, return {key: value}, missing_keys."""
+    memcache_hits = {}
+    cached_dict = memcache.get_multi(
+        [self._KeyToStr(key) for key in keys], key_prefix=self.memcache_prefix)
+
+    for key_str, serialized_value in cached_dict.iteritems():
+      value = self._StrToValue(serialized_value)
+      key = self._StrToKey(key_str)
+      memcache_hits[key] = value
+      self.cache.CacheItem(key, value)
+
+    still_missing_keys = [key for key in keys if key not in memcache_hits]
+    logging.info(
+        'decoded %d values from memcache %s, missing %d',
+        len(memcache_hits), self.memcache_prefix, len(still_missing_keys))
+    return memcache_hits, still_missing_keys
+
+  def _WriteToMemcache(self, retrieved_dict):
+    """Write entries for each key-value pair to memcache.  Encode PBs."""
+    strs_to_cache = {
+        self._KeyToStr(key): self._ValueToStr(value)
+        for key, value in retrieved_dict.iteritems()}
+    memcache.set_multi(
+        strs_to_cache, key_prefix=self.memcache_prefix,
+        time=framework_constants.MEMCACHE_EXPIRATION)
+    logging.info('cached batch of %d values in memcache %s',
+                 len(retrieved_dict), self.memcache_prefix)
+
+  def _KeyToStr(self, key):
+    """Convert our int IDs to strings for use as memcache keys."""
+    return str(key)
+
+  def _StrToKey(self, key_str):
+    """Convert memcache keys back to the ints that we use as IDs."""
+    return int(key_str)
+
+  def _ValueToStr(self, value):
+    """Serialize an application object so that it can be stored in memcache."""
+    if not self.pb_class:
+      return value
+    elif self.pb_class == int:
+      return str(value)
+    else:
+      return protobuf.encode_message(value)
+
+  def _StrToValue(self, serialized_value):
+    """Deserialize an application object that was stored in memcache."""
+    if not self.pb_class:
+      return serialized_value
+    elif self.pb_class == int:
+      return int(serialized_value)
+    else:
+      return protobuf.decode_message(self.pb_class, serialized_value)
+
+  def InvalidateKeys(self, cnxn, keys):
+    """Drop the given keys from both RAM and memcache."""
+    self.cache.InvalidateKeys(cnxn, keys)
+    memcache.delete_multi(
+        [self._KeyToStr(key) for key in keys], key_prefix=self.memcache_prefix)
+
+  def InvalidateAllKeys(self, cnxn, keys):
+    """Drop the given keys from memcache and invalidate all keys in RAM.
+
+    Useful for avoiding inserting many rows into the Invalidate table when
+    invalidating a large group of keys all at once. Only use when necessary.
+    """
+    self.cache.InvalidateAll(cnxn)
+    memcache.delete_multi(
+        [self._KeyToStr(key) for key in keys], key_prefix=self.memcache_prefix)
+
+  def GetAllAlreadyInRam(self, keys):
+    """Look only in RAM to return {key: values}, missed_keys."""
+    result_dict, missed_keys = self.cache.GetAll(keys)
+    return result_dict, missed_keys
+
+  def InvalidateAllRamEntries(self, cnxn):
+    """Drop all RAM cache entries. It will refill as needed from memcache."""
+    self.cache.InvalidateAll(cnxn)
+
+  def FetchItems(self, cnxn, keys, **kwargs):
+    """On RAM and memcache miss, hit the database."""
+    raise NotImplementedError()