samples/oauth/oauth_on_appengine/appengine_utilities/cache.py

# -*- coding: utf-8 -*-
"""
Copyright (c) 2008, appengine-utilities project
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
- Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright notice,
  this list of conditions and the following disclaimer in the documentation
  and/or other materials provided with the distribution.
- Neither the name of the appengine-utilities project nor the names of its
  contributors may be used to endorse or promote products derived from this
  software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
"""

# main python imports
import datetime
import pickle
import random
import __main__

# google appengine import
from google.appengine.ext import db
from google.appengine.api import memcache

# settings
DEFAULT_TIMEOUT = 3600 # cache expires after one hour (3600 sec)
CLEAN_CHECK_PERCENT = 50 # 15% of all requests will clean the database
MAX_HITS_TO_CLEAN = 100 # the maximum number of cache hits to clean on attempt


class _AppEngineUtilities_Cache(db.Model):
    # It's up to the application to determine the format of their keys
    cachekey = db.StringProperty()
    createTime = db.DateTimeProperty(auto_now_add=True)
    timeout = db.DateTimeProperty()
    value = db.BlobProperty()


class Cache(object):
    """
    Cache is used for storing pregenerated output and/or objects in the Big
    Table datastore to minimize the amount of queries needed for page
    displays. The idea is that complex queries that generate the same
    results really should only be run once. Cache can be used to store
    pregenerated value made from queries (or other calls such as
    urlFetch()), or the query objects themselves.
    """

    def __init__(self, clean_check_percent = CLEAN_CHECK_PERCENT,
      max_hits_to_clean = MAX_HITS_TO_CLEAN,
        default_timeout = DEFAULT_TIMEOUT):
        """
        Initializer

        Args:
            clean_check_percent: how often cache initialization should
                run the cache cleanup
            max_hits_to_clean: maximum number of stale hits to clean
            default_timeout: default length a cache item is good for
        """
        self.clean_check_percent = clean_check_percent
        self.max_hits_to_clean = max_hits_to_clean
        self.default_timeout = default_timeout

        if random.randint(1, 100) < self.clean_check_percent:
            self._clean_cache()

        if 'AEU_Events' in __main__.__dict__:
            __main__.AEU_Events.fire_event('cacheInitialized')

    def _clean_cache(self):
        """
        _clean_cache is a routine that is run to find and delete cache
        items that are old. This helps keep the size of your over all
        datastore down.
        """
        query = _AppEngineUtilities_Cache.all()
        query.filter('timeout < ', datetime.datetime.now())
        results = query.fetch(self.max_hits_to_clean)
        db.delete(results)
        #for result in results:
        #    result.delete()

    def _validate_key(self, key):
        if key == None:
            raise KeyError

    def _validate_value(self, value):
        if value == None:
            raise ValueError

    def _validate_timeout(self, timeout):
        if timeout == None:
            timeout = datetime.datetime.now() +\
            datetime.timedelta(seconds=DEFAULT_TIMEOUT)
        if type(timeout) == type(1):
            timeout = datetime.datetime.now() + \
                datetime.timedelta(seconds = timeout)
        if type(timeout) != datetime.datetime:
            raise TypeError
        if timeout < datetime.datetime.now():
            raise ValueError

        return timeout

    def add(self, key = None, value = None, timeout = None):
        """
        add adds an entry to the cache, if one does not already
        exist.
        """
        self._validate_key(key)
        self._validate_value(value)
        timeout = self._validate_timeout(timeout)

        if key in self:
            raise KeyError

        cacheEntry = _AppEngineUtilities_Cache()
        cacheEntry.cachekey = key
        cacheEntry.value = pickle.dumps(value)
        cacheEntry.timeout = timeout

        # try to put the entry, if it fails silently pass
        # failures may happen due to timeouts, the datastore being read
        # only for maintenance or other applications. However, cache
        # not being able to write to the datastore should not
        # break the application
        try:
            cacheEntry.put()
        except:
            pass

        memcache_timeout = timeout - datetime.datetime.now()
        memcache.set('cache-'+key, value, int(memcache_timeout.seconds))

        if 'AEU_Events' in __main__.__dict__:
            __main__.AEU_Events.fire_event('cacheAdded')

    def set(self, key = None, value = None, timeout = None):
        """
        add adds an entry to the cache, overwriting an existing value
        if one already exists.
        """
        self._validate_key(key)
        self._validate_value(value)
        timeout = self._validate_timeout(timeout)

        cacheEntry = self._read(key)
        if not cacheEntry:
            cacheEntry = _AppEngineUtilities_Cache()
            cacheEntry.cachekey = key
        cacheEntry.value = pickle.dumps(value)
        cacheEntry.timeout = timeout

        try:
            cacheEntry.put()
        except:
            pass

        memcache_timeout = timeout - datetime.datetime.now()
        memcache.set('cache-'+key, value, int(memcache_timeout.seconds))

        if 'AEU_Events' in __main__.__dict__:
            __main__.AEU_Events.fire_event('cacheSet')

    def _read(self, key = None):
        """
        _read returns a cache object determined by the key. It's set
        to private because it returns a db.Model object, and also
        does not handle the unpickling of objects making it not the
        best candidate for use. The special method __getitem__ is the
        preferred access method for cache data.
        """
        query = _AppEngineUtilities_Cache.all()
        query.filter('cachekey', key)
        query.filter('timeout > ', datetime.datetime.now())
        results = query.fetch(1)
        if len(results) is 0:
            return None
        return results[0]

        if 'AEU_Events' in __main__.__dict__:
            __main__.AEU_Events.fire_event('cacheReadFromDatastore')
        if 'AEU_Events' in __main__.__dict__:
            __main__.AEU_Events.fire_event('cacheRead')

    def delete(self, key = None):
        """
        Deletes a cache object determined by the key.
        """
        memcache.delete('cache-'+key)
        result = self._read(key)
        if result:
            if 'AEU_Events' in __main__.__dict__:
                __main__.AEU_Events.fire_event('cacheDeleted')
            result.delete()

    def get(self, key):
        """
        get is used to return the cache value associated with the key passed.
        """
        mc = memcache.get('cache-'+key)
        if mc:
            if 'AEU_Events' in __main__.__dict__:
                __main__.AEU_Events.fire_event('cacheReadFromMemcache')
            if 'AEU_Events' in __main__.__dict__:
                __main__.AEU_Events.fire_event('cacheRead')
            return mc
        result = self._read(key)
        if result:
            timeout = result.timeout - datetime.datetime.now()
            # print timeout.seconds
            memcache.set('cache-'+key, pickle.loads(result.value),
               int(timeout.seconds))
            return pickle.loads(result.value)
        else:
            raise KeyError

    def get_many(self, keys):
        """
        Returns a dict mapping each key in keys to its value. If the given
        key is missing, it will be missing from the response dict.
        """
        dict = {}
        for key in keys:
            value = self.get(key)
            if value is not None:
                dict[key] = val
        return dict

    def __getitem__(self, key):
        """
        __getitem__ is necessary for this object to emulate a container.
        """
        return self.get(key)

    def __setitem__(self, key, value):
        """
        __setitem__ is necessary for this object to emulate a container.
        """
        return self.set(key, value)

    def __delitem__(self, key):
        """
        Implement the 'del' keyword
        """
        return self.delete(key)

    def __contains__(self, key):
        """
        Implements "in" operator
        """
        try:
            r = self.__getitem__(key)
        except KeyError:
            return False
        return True

    def has_key(self, keyname):
        """
        Equivalent to k in a, use that form in new code
        """
        return self.__contains__(keyname)