118 lines
3.9 KiB
Python
118 lines
3.9 KiB
Python
# -*- coding: utf-8 -*-
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'''
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Database object cache.
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The shove psuedo-URL used for database object caches is the format used by
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SQLAlchemy:
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<driver>://<username>:<password>@<host>:<port>/<database>
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<driver> is the database engine. The engines currently supported SQLAlchemy are
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sqlite, mysql, postgres, oracle, mssql, and firebird.
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<username> is the database account user name
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<password> is the database accound password
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<host> is the database location
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<port> is the database port
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<database> is the name of the specific database
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For more information on specific databases see:
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http://www.sqlalchemy.org/docs/dbengine.myt#dbengine_supported
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'''
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import time
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import random
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from datetime import datetime
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try:
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from sqlalchemy import (
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MetaData, Table, Column, String, Binary, DateTime, select, update,
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insert, delete,
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)
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from shove import DbBase
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except ImportError:
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raise ImportError('Requires SQLAlchemy >= 0.4')
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__all__ = ['DbCache']
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class DbCache(DbBase):
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'''database cache backend'''
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def __init__(self, engine, **kw):
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super(DbCache, self).__init__(engine, **kw)
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# Get table name
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tablename = kw.get('tablename', 'cache')
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# Bind metadata
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self._metadata = MetaData(engine)
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# Make cache table
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self._store = Table(tablename, self._metadata,
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Column('key', String(60), primary_key=True, nullable=False),
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Column('value', Binary, nullable=False),
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Column('expires', DateTime, nullable=False),
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)
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# Create cache table if it does not exist
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if not self._store.exists():
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self._store.create()
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# Set maximum entries
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self._max_entries = kw.get('max_entries', 300)
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# Maximum number of entries to cull per call if cache is full
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self._maxcull = kw.get('maxcull', 10)
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# Set timeout
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self.timeout = kw.get('timeout', 300)
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def __getitem__(self, key):
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row = select(
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[self._store.c.value, self._store.c.expires],
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self._store.c.key == key
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).execute().fetchone()
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if row is not None:
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# Remove if item expired
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if row.expires < datetime.now().replace(microsecond=0):
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del self[key]
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raise KeyError(key)
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return self.loads(str(row.value))
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raise KeyError(key)
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def __setitem__(self, key, value):
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timeout, value, cache = self.timeout, self.dumps(value), self._store
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# Cull if too many items
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if len(self) >= self._max_entries:
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self._cull()
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# Generate expiration time
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expires = datetime.fromtimestamp(
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time.time() + timeout
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).replace(microsecond=0)
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# Update database if key already present
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if key in self:
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update(
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cache,
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cache.c.key == key,
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dict(value=value, expires=expires),
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).execute()
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# Insert new key if key not present
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else:
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insert(
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cache, dict(key=key, value=value, expires=expires)
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).execute()
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def _cull(self):
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'''Remove items in cache to make more room.'''
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cache, maxcull = self._store, self._maxcull
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# Remove items that have timed out
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now = datetime.now().replace(microsecond=0)
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delete(cache, cache.c.expires < now).execute()
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# Remove any items over the maximum allowed number in the cache
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if len(self) >= self._max_entries:
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# Upper limit for key query
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ul = maxcull * 2
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# Get list of keys
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keys = [
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i[0] for i in select(
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[cache.c.key], limit=ul
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).execute().fetchall()
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]
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# Get some keys at random
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delkeys = list(random.choice(keys) for i in xrange(maxcull))
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delete(cache, cache.c.key.in_(delkeys)).execute()
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