django-lrucache-backend 4.0.0
A smarter local memory cache backend for Django
License: MIT
Requires Python: >=3.6.4
Keywords: django,cache,lru

django-lrucache-backend
=======================

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A global object cache backend for Django.

.. image:: https://github.com/kogan/django-lrucache-backend/blob/master/benchmarking/4.0.0/objects-get.png
   :alt: Cache Performance

`Set Performance <https://github.com/kogan/django-lrucache-backend/blob/master/benchmarking/4.0.0/objects-set.png>`_.

`Delete performance <https://github.com/kogan/django-lrucache-backend/blob/master/benchmarking/4.0.0/objects-delete.png>`_.

About
-----

`lrucache_backend` is an in-memory cache that has differing goals to the
`LocMemCache` backend that Django provides.

Originally its purpose was to improve the cache eviction strategy, using an LRU
algorithm over a random cull. However, Django 2.1 changed the LocMemCache to also
use an LRU strategy.

Now, `lrucache_backend` has two major differences from Django.

1. Eliminates key char validation, improving performance at the cost of not
   being portable with memcache backends.

2. Eliminates serialization (pickling), allowing instances to be shared across
   requests. Any mutations performed on cache objects are maintained.

`lrucache_backend` functions as a global object cache, and uses a familiar Django
interface to avoid poorly reimplementing local object caches in service layers:

.. code-block:: python

    def get_data_before(self):
        if not hasattr(self, '__data'):
            self.__data = self.expensive_query()
        return self.__data

    def get_data_after(self):
        lcache = caches['local']
        data = lcache.get('our_data')
        if not data:
            data = self.expensive_query()
            lcache.set('our_data', data, timeout=600)
        return data

The benefits (despite the longer method) include timeouts, sharing data between
requests, and avoiding network requests. This is especially useful when there
are hundreds or thousands of property accesses that would hit the cache where
network overhead would be prohibitive. The `Fat model` pattern can greatly
benefit from tiered caching.

Good for?
^^^^^^^^^

An in memory cache is good for small data that changes rarely. It's effectively
a global dictionary shared between requests in the same process. Small lookup
tables and database backed settings are good candidates.

A small number of keys should be used to avoid engaging the culling strategy
of the cache. Performance goes down fast as soon as the maximum number of keys
are reached, and keys start to evict.

This should **not** be used as your primary cache, but it makes for an
excellent secondary cache when you want to avoid the overhead of a network call.

Use for:

- Small lookup tables
- Settings
- Backing store for your service objects
- Remembering values for the duration of a request or celery task
- Small global template fragments like sidebars or footers
- Secondary cache

Bad for?
^^^^^^^^

An in memory cache is terrible for data that changes often. Because the cache
is process local, it's extremely difficult to coordinate cache invalidation
from external processes. For that reason, this library does nothing to support
cache invalidation.

The cache shares memory with the application, so it's extremely important to
avoid storing a lot of keys, or any large values.

Do **not** use for:

- Instance attributes/properties
- Full templates
- Tables with a large number of rows
- Large values
- Large lists
- Primary cache

Differences from LocMemCache
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

- Avoids pickling
- Avoids key name validation

Installation
------------

.. code-block:: bash

     pip install django-lrucache-backend

Requirements
^^^^^^^^^^^^

There are no longer any external dependencies. `lru-dict` used to be a dependency,
but as since been replaced with the built in `OrderedDict` as a consequence of
now deriving from the built in `LocMemCache`.


Usage
-----

Configure your `CACHES` Django setting appropriately:

.. code-block:: python

    CACHES = {
        'local': {
            'BACKEND': 'lrucache_backend.LRUObjectCache',
            'TIMEOUT': 600,
            'OPTIONS': {
                'MAX_ENTRIES': 100,
                'CULL_FREQUENCY: 100,
            },
            'NAME': 'optional-name'
        }
    }

Note:
    Set CULL_FREQUENCY == MAX_ENTRIES to only delete a single key per cull to
    only eliminate a single entry per cull. This maintains the LRU property most
    effectively, but can have performance implications.

And then use the cache as you would any other:

.. code-block:: python

    >>> from django.core.cache import caches

    >>> local = caches['local']
    >>> local.set('key', 123)
    >>> local.get('key')
    ... 123

If you're going to use this cache backend, then it's highly recommended to use
it as a non-default cache. That is, do not configure this cache under the
`default` name.

Local memory caches compete for memory with your application so it's in your
best interests to use it as sparingly and deliberately as possible.

Compatibility
-------------

Django 2.2+
Python 3.6+

Licence
-------

MIT

Authors
-------

`django-lrucache-backend` was written by `Josh Smeaton <josh.smeaton@gmail.com>`_.