id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
515	[patch] additional set of cache backends	eugene@…	Jacob	"= New Backends =

Three more backends are introduced:

 * '''locmem:''' - simple thread-safe in-memory cache. Good for small web sites. 
   * Example: ''!locmem:///''.
 * '''file:''' - file-based multi-process/thread-safe cache.
   * Example: ''!file:///tmp/django.cache/lazutkin.com/'' - creates directory ''/tmp/django.cache/lazutkin.com'' and stores cached items as individual files.
 * '''sql:''' - sql-based multi-process/thread-safe cache.
   * Example: ''!sql://cache/'' --- uses table '''cache''' in current database to store cached items.

All three backends accept arguments described in [http://www.djangoproject.com/documentation/cache/ Django's cache framework]. Examples: ''!sql://cache/?timeout=60'', ''!locmem:///?max_entries=10&cull_frequency=2''.

Additionally '''simple:''' cache backend's bug is fixed.

Notes:
 * All backends use '''pickle''' ('''cPickle''') as means to save the object. I decided against '''marshal''' or custom solutions. For now.
 * '''locmem:'''
   * Requires reader-writer lock, which is implemented in '''synch.py''' (attached). This file should be placed into '''django/utils''' directory.
   * Implements simplified (a-la '''simple:''') cache culling strategy, when each '''cull_frequency''' item is removed to make room.
 * '''file:'''
   * Implements simplified (a-la '''simple:''') cache culling strategy, when each '''cull_frequency''' item is removed to make room.
 * '''sql:'''
   * When it is time to cull cache, '''sql:''' removes expired items first, then removes oldest items by access time to satisfy '''cull_frequency'''. This strategy has a drawback: every time cached item is accessed, its access time filed is updated.
   * It can be speeded up with stored procedures.

= Missing django-admin Support =

The missing part is '''django-admin''' support for:
 * creation of cache table for '''sql:''' backend
 * creation of cache indices for '''sql:'''
 * cleaning up '''sql:''' and '''file:''' caches

Pseudo model for cache table is:

{{{
class Cache(meta.Model):
    # id is assumed
    keyx       = meta.CharField(maxlength=255, unique=True),
    expiration = meta.DateTimeField(db_index=True),
    accessed   = meta.DateTimeField(db_index=True),
    valx       = meta.TextField(), # should be big enough to hold an item
}}}

For !TextField on MySQL see #489 --- when you create cache table manually make sure to use ''longtext'' instead of ''text''.

If you want to use MySQL, use patch from #463. Otherwise you may have strange random errors.

= Testing =

Notes:
 * I ran all new backends for several days monitoring cache entries.
 * '''sql:''' was tested locally using MySQL and SQLite. I use MySQL/MyISAM-based cache on my web site. I didn't try it with PostgreSQL nor MySQL/InnoDB.
"	defect	closed	Core (Cache system)		normal	fixed			Unreviewed	1	0	0	0	0	0