15.2.98. camcops_server.cc_modules.cc_cache

camcops_server/cc_modules/cc_cache.py

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Copyright (C) 2012, University of Cambridge, Department of Psychiatry. Created by Rudolf Cardinal (rnc1001@cam.ac.uk).

This file is part of CamCOPS.

CamCOPS is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

CamCOPS is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with CamCOPS. If not, see <https://www.gnu.org/licenses/>.

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Cache functions.

1. The basic cache objects.

2. FIX FOR DOGPILE.CACHE FOR DECORATED FUNCTIONS, 2017-07-28 (PLUS SOME OTHER IMPROVEMENTS). SEE https://bitbucket.org/zzzeek/dogpile.cache/issues/96/error-in-python-35-with-use-of-deprecated

Crash using type-hinted functions under Python 3.5 with dogpile.cache==0.6.4:

Traceback (most recent call last):
  File "/usr/lib/python3.5/runpy.py", line 184, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.5/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/rudolf/Documents/code/camcops/server/camcops_server/cc_modules/cc_cache.py", line 64, in <module>
    unit_test_cache()
  File "/home/rudolf/Documents/code/camcops/server/camcops_server/cc_modules/cc_cache.py", line 50, in unit_test_cache
    def testfunc() -> str:
  File "/home/rudolf/dev/venvs/camcops/lib/python3.5/site-packages/dogpile/cache/region.py", line 1215, in decorator
    key_generator = function_key_generator(namespace, fn)
  File "/home/rudolf/dev/venvs/camcops/lib/python3.5/site-packages/dogpile/cache/util.py", line 31, in function_key_generator
    args = inspect.getargspec(fn)
  File "/usr/lib/python3.5/inspect.py", line 1045, in getargspec
    raise ValueError("Function has keyword-only arguments or annotations"
ValueError: Function has keyword-only arguments or annotations, use getfullargspec() API which can support them

3. CACHING NOTES

• We currently use ‘dogpile.cache.memory’ as the backend. This means that for single-process (single-thread or multithreaded) servers, the cache is unique, but that won’t work for multi-process (e.g. Gunicorn) servers.

• That means that in a multiprocess environment it’s fine to continue to use a memory cache for file-based stuff (e.g. configs, XML strings), but not for database-based stuff (e.g. which ID numbers are valid).

• Correct solutions WITH a cache for those database-based things include:

  • ignoring Python caching and relying on the MySQL query cache – but this is being removed because it’s not all that great:

    https://mysqlserverteam.com/mysql-8-0-retiring-support-for-the-query-cache/

  • using memcached (via dogpile.cache.pylibmc)

    https://www.ubergizmo.com/how-to/install-memcached-windows/

  • using redis (via dogpile.cache.redis and https://pypi.python.org/pypi/redis/ )

    https://stackoverflow.com/questions/10558465/memcached-vs-redis https://redis.io/ https://web.archive.org/web/20120118030804/http://simonwillison.net/static/2010/redis-tutorial/ http://oldblog.antirez.com/post/take-advantage-of-redis-adding-it-to-your-stack.html https://redis.io/topics/security

    redis unsupported under Windows:

    https://redis.io/download

• The other obvious alternative: don’t cache such stuff! This may all be premature optimization.

  https://msol.io/blog/tech/youre-probably-wrong-about-caching/

  The actual price is of the order of 0.6-1 ms per query, for the queries “find me all the ID number definitions” and “fetch the server settings”.

• The answer is probably:

  • continue to use dogpile.cache.memory for simple “fixed” stuff read from disk;

  • continue to use Pyramid’s per-request caching mechanism (@reify);

  • forget about database caching for now;

  • if it becomes a problem later, move to Redis

• Therefore:

  • there should be no calls to cache_region_static.delete

camcops_server.cc_modules.cc_cache.fkg(namespace: Optional[str], fn: Callable, to_str: Callable[[Any], str] = <built-in function repr>) → Callable[[Any], str]

As for fkg_allowing_type_hints(), but allowing keyword arguments.

For kwargs passed in, we will build a dict of all argname (key) to argvalue (values) pairs, including default args from the argspec, and then alphabetize the list before generating the key.

NOTE ALSO that once we have keyword arguments, we should be using repr(), because we need to distinguish

kwargs = {'p': 'another', 'q': 'thing'}
# ... which compat.string_type will make into
#         p=another q=thing
# ... from
kwargs = {'p': 'another q=thing'}

Also modified to make the cached function unique per INSTANCE for normal methods of a class.