|Version 2 (modified by 10 years ago) (diff),|
This is the original Django GSoC proposal. There have been quite a few revisions since, but I'm posting this first for reference.
This addition to Django's ORM adds simple drop-in caching, compatible with
nearly all existing
QuerySet methods. It emphasizes
performance and compatibility, and providing configuration options with sane
defaults. All that is required for basic functionality is a suitable
CACHE_BACKEND setting and the addition of
.cache() to the appropriate
QuerySet chains. It also speeds up the lookup of related objects, and even
that of generic relations.
QuerySet class grows two new methods to add object caching:
cache(timeout=None, prefix='qscache:', smart=False)
timeoutdefaults to the amount specified in
prefixis in addition to
Cache keys are calculated with the content-type id and instance id, to accomodate generic relations.
QuerySetgrows some new attributes that affect how SQL is generated. When in effect, they cause the query to only retrieve primary keys of selected objects.
in_bulk()uses the cache directly, although cache misses will still require database hits, as usual. Methods such as
count()are largely unaffected by
cache(), but methods such as
distinct()are a more difficult case and will require some design decisions. Using
extra(select=...)is also a possibly unsolvable case.
values()has been used in the query,
cache()takes precedence and creates the values dictionary from cache. If a list of fields is specified in
cache()will still perform the equivalent of a
SELECT *. Perhaps another option could be added to allow retrieval of only the specified fields, which would break any regular cached lookup for that object.
select_related()is supported by the caching mechanism. The appropriate joins are still performed by the database; if joins were calculated with cached object foreign key values, cache misses could be very costly.
cache_generic(field, timeout=None, prefix='qscache:', smart=False)
fieldis the name of the generic foreign key field.
Without database-specific trickery it is non-trivial to perform SQL JOINs with generic relations. Currently, a database query is required for each generic foreign key relationship. The cache framework, while unable to reduce the initial number of database hits, greatly alleviates load when lists of generic objects are required. Using this method still loads generic foreign keys lazily, but more quickly, and also uses objects cached with
To achieve as much transparency as possible, the
QuerySet methods quietly
post_delete signal listeners the first time a
model is cached. Object deletion is trivial. On object creation or
modification, the preferred behavior is to create or update the cached key
rather than simply deleting the key and letting the cache regenerate it;
the rationale is that the object is most likely to be viewed immediately after
and caching it at
post_save is cheap. However, specific cases may not be
as accommodating. This is likely subject to debate or may need a global setting.
To reduce the number of cache misses, additional "smart" logic can be added. For example, the first time a model is registered to the cache signal listener, its model instances are expected to be uncached. In this case, rather than fetching only primary keys, the objects are retrieved as normal (and cached).
By storing the expiration time, this can also take effect whenever the
cached objects have likely timed out. All "smart" functionality is enabled
smart keyword argument.
- All caching code lives in a contrib app at first. A custom
QuerySetclass derives from the official class, overriding where appropriate. A
Managerclass with an overriden
get_query_set()is used for testing, and additional middleware, etc. are located in the same folder. Near or upon completion, the new code can be merged to trunk as Django proper. Hopefully the code will not be too invasive, but quite a few
QuerySetmethods will have to be hijacked.
- If the transaction middleware is enabled, it is desirable to have the cache
only update when the transaction succeeds. This is simple in implementation
but will couple the transaction middleware to the cache if not designed
properly. An additional middleware class can be created to handle this
case; however, it will have to stipulate placement immediately after the
TransactionMiddlewarein settings.py, and might be confused with the existing
- Write preliminary tests. Initial implementation of
cache()for single objects. Support almost all typical
- Devise a generic idiom for testing cache-related code. Work on agregates;
- Work on signal dispatching, cache coherency. Write more tests and preliminary documentation.
- Write "smart" cache logic. Explore other possible optimizations.
- Add transaction support. Design decision needed about extra middleware.
- Implement extra features if possible (
- Write up documentation, extensive tests, and example code. Possibly move from contrib into the main cache module.
- Refactor, especially if the new
QuerySethas been released. Continue merging with changes to trunk and testing.
- Allow for wiggle room,
QuerySetrefactoring work, cleanup, etc.