Cache

The system provides a Python object that acts as a persistent datastore across multiple Python executions from within the same python-policy. One example use case is the following:

A DHCP script stores an option of a Discovery packet in cache, a RADIUS script retrieves it from the cache, and adds it in the RADIUS Accounting Request packet.

config>python>python-policy>cache needs to be configured before alc.cache can be used. See section Python cache support of Triple Play Service Delivery Architecture Guide for more information.

The alc.cache context-manager

The alc.cache object, which is an instance of class alc.Cache, is a context-manager that provides the actual cache object and a context in which this object can be used.

Note

Only the pre-provided alc.cache object should be used in script, the class alc.Cache should not be used directly.

To interact with the cache, a reference to the cache object needs to be retrieved using the “with” syntax:

>>> with alc.cache as cache:
...    # cache can be interacted with using the variable "cache"

Outside the scope created by the with-statement, the cache object does not exist and cannot be used.

The alc.Cache class

class alc.Cache

set(key, value, lifetime=600, /)

Associate the value with the key for a duration of the specified lifetime. If the entry already exists, the value and lifetime are updated.

Parameters

• key (bytes) – key

• value (bytes) – value

• lifetime (int) – time in seconds during which the entry remains valid in the cache. After this time, the entry is removed from the cache. The value must be in the range of the minimal to maximum value configured in python-policy.

Raises

ValueError – when the lifetime parameter is negative or zero

get(key, /)

Retrieves the value previously associated with the specified key. If the entry does not exist or has timed out, None is returned.

Parameters

key (bytes) – key

Return type

bytes or None

get_lifetime(key, /)

Retrieves the lifetime of the entry for the specified key. If the entry does not exist or has timed out, None` is returned.

Parameters

key (bytes) – key

Return type

int or None

set_lifetime(key, lifetime, /)

Updates the lifetime of the entry for the specified key. The value can both be increased or decreased, given it is in the range of the minimal to maximum value configured in python-policy. If the entry has timed out, no update is done.

Parameters

• key (bytes) – key

• lifetime (int) – time in seconds during which the entry remains valid in the cache. The value must be in the range of the minimal to maximum value configured in python-policy.

Raises

ValueError – when the lifetime parameter is negative or zero

clear(key, /)

Removes the entry for the specified key. If the entry does not exist (anymore), nothing is done.

Parameters

key (bytes) – key

Breaking changes

Context manager

Warning

The alc.cache object is now a context-manager that returns the actual cache object

Non-existing entries

Warning

The following actions on a non-existing entry don’t raise a KeyError exception:

• get() (returns None)

• clear()

• get_lifetime()

• set_lifetime()

Examples

Save

The following example saves a cache entry with as parameters:

• key: the User-Name attribute in the RADIUS message

• value: please remember this for me

• lifetime: 86400 seconds

import alc

# Perform any operations before needing the cache
username = alc.radius.attributes[1]
if username is not None:
    # using the "with" syntax, get a reference to the cache
    with alc.cache as cache:
        cache.set(username, b"please remember this for me", 86400)

    # other operations not needing the cache are best done outside of this scope
    print("entry saved in cache")

Get

The following example gets a cache entry with as key the User-Name attribute in the RADIUS message:

import alc

# Perform any operations before needing the cache
username = alc.radius.attributes[1]
if username is not None:
    # using the "with" syntax, get a reference to the cache and fetch the value
    with alc.cache as cache:
        value = cache.get(username)

    # other operations not needing the cache are best done outside of this scope
    if value is None:
        print("entry not found in cache")
    else:
        print("entry found in cache: {}".format(value))