deb-keystone/doc/source/devref/caching-layer.rst

Caching Layer

The caching layer is designed to be applied to any manager object within Keystone via the use of the on_arguments decorator provided in the keystone.common.cache module. This decorator leverages dogpile.cache caching system to provide a flexible caching backend.

It is recommended that each of the managers have an independent toggle within the config file to enable caching. The easiest method to utilize the toggle within the configuration file is to define a caching boolean option within that manager's configuration section (e.g. identity). Once that option is defined you can pass function to the on_arguments decorator with the named argument should_cache_fn. In the keystone.common.cache module, there is a function called should_cache_fn, which will provide a reference, to a function, that will consult the global cache enabled option as well as the specific manager's caching enable toggle.

Note

If a section-specific boolean option is not defined in the config section specified when calling should_cache_fn, the returned function reference will default to enabling caching for that manager.

Example use of cache and should_cache_fn (in this example, token is the manager):

from keystone.common import cache
SHOULD_CACHE = cache.should_cache_fn('token')

@cache.on_arguments(should_cache_fn=SHOULD_CACHE)
def cacheable_function(arg1, arg2, arg3):
    ...
    return some_value

With the above example, each call to the cacheable_function would check to see if the arguments passed to it matched a currently valid cached item. If the return value was cached, the caching layer would return the cached value; if the return value was not cached, the caching layer would call the function, pass the value to the SHOULD_CACHE function reference, which would then determine if caching was globally enabled and enabled for the token manager. If either caching toggle is disabled, the value is returned but not cached.

It is recommended that each of the managers have an independent configurable time-to-live (TTL). If a configurable TTL has been defined for the manager configuration section, it is possible to pass it to the cache.on_arguments decorator with the named-argument expiration_time. For consistency, it is recommended that this option be called cache_time and default to None. If the expiration_time argument passed to the decorator is set to None, the expiration time will be set to the global default (expiration_time option in the [cache] configuration section.

Example of using a section specific cache_time (in this example, identity is the manager):

from keystone.common import cache
SHOULD_CACHE = cache.should_cache_fn('identity')

@cache.on_arguments(should_cache_fn=SHOULD_CACHE,
                    expiration_time=CONF.identity.cache_time)
def cachable_function(arg1, arg2, arg3):
    ...
    return some_value

For cache invalidation, the on_arguments decorator will add an invalidate method (attribute) to your decorated function. To invalidate the cache, you pass the same arguments to the invalidate method as you would the normal function.

Example (using the above cacheable_function):

def invalidate_cache(arg1, arg2, arg3):
    cacheable_function.invalidate(arg1, arg2, arg3)

Warning

The on_arguments decorator does not accept keyword-arguments/named arguments. An exception will be raised if keyword arguments are passed to a caching-decorated function.

Note

In all cases methods work the same as functions except if you are attempting to invalidate the cache on a decorated bound-method, you need to pass self to the invalidate method as the first argument before the arguments.

dogpile.cache based MongoDB (NoSQL) backend

The dogpile.cache based MongoDB backend implementation allows for various MongoDB configurations, e.g., standalone, a replica set, sharded replicas, with or without SSL, use of TTL type collections, etc.

Example of typical configuration for MongoDB backend:

from dogpile.cache import region

arguments = {
    'db_hosts': 'localhost:27017',
    'db_name': 'ks_cache',
    'cache_collection': 'cache',
    'username': 'test_user',
    'password': 'test_password',

    # optional arguments
    'son_manipulator': 'my_son_manipulator_impl'
}

region.make_region().configure('keystone.cache.mongo',
                               arguments=arguments)

The optional son_manipulator is used to manipulate custom data type while its saved in or retrieved from MongoDB. If the dogpile cached values contain built-in data types and no custom classes, then the provided implementation class is sufficient. For further details, refer http://api.mongodb.org/python/current/examples/custom_type.html#automatic-encoding-and-decoding

Similar to other backends, this backend can be added via Keystone configuration in keystone.conf:

[cache]
# Global cache functionality toggle.
enabled = True

# Referring to specific cache backend
backend = keystone.cache.mongo

# Backend specific configuration arguments
backend_argument = db_hosts:localhost:27017
backend_argument = db_name:ks_cache
backend_argument = cache_collection:cache
backend_argument = username:test_user
backend_argument = password:test_password

This backend is registered in keystone.common.cache.core module. So, its usage is similar to other dogpile caching backends as it implements the same dogpile APIs.