Redis, Memcached, and Database cache backends lack specialized many operation implementations

[Brian Atkinson]

The Redis cache backend has a specialized implementation of get_many and set_many which perform the get/set in one command to the backend. However the similar aget_many and aset_many methods are not specialized. This results in the performance of these async batch methods being significantly slower than they could be. We've seen significant performance improvements by adding the following methods to a sub-class of the built-in RedisCache class:

    async def aget_many(self, keys, version=None):
        return await sync_to_async(self.get_many)(keys, version=version)

    async def aset_many(self, data, timeout=DEFAULT_TIMEOUT, version=None):
        return await sync_to_async(self.set_many)(data, timeout=timeout, version=version)

These were the only ones we've overridden, but there might be a few others which could benefit from an improved implementation in a similar way. Happy to send a PR for a change similar to above for these missing specializations if desired.

[Simon Charette]

Memcached and database backends are also affected as well as delete_many

I wonder if we could potentially have BaseDatabaseCache.__init_subclass__ automatically add specialized aget_many, aset_many, adelete_many when one is declared by a subclass.

[H. White]

PR is updated. ​https://github.com/django/django/pull/19840

[H. White]

I have revised the PR based on feedback.
​https://github.com/django/django/pull/19840

[H. White]

PR is updated and ready for re-review.

[H. White]

Tests are updated, the PR is ready for re-review. ​https://github.com/django/django/pull/19840