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A library for caching function calls.

Project description

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A library for caching the outputs of functions based on the inputted parameters to reduce recomputing expensive computations and requesting frequently accessed content, and to improve performance.

@cache(StaticCache(),
       on_hit=lambda h: print('Hit %s time(s)' % h),
       on_miss=lambda m: print('Missed %s time(s)' % m))
def fibonacci(n):
    if n < 2:
        return n
    return fib(n-1) + fib(n-2)

Installation

The latest version of cache.me is available via pip:

pip install cache.me

Alternatively, you can download and install from source:

python setup.py install

Getting Started

The cache decorator contains the following signature:

@cache(algorithm, include_types=False, on_hit=None, on_miss=None, key_func=None)
def func(...)
    ...

It serves as both a function decorator, and a runnable wrapper and is configurable through it’s dynamic parameters.

In its simplest form, the decorator accepts different algorithms that can be used to cache the outputs of the function based on its inputs, otherwise known as memoization. Different algorithms are provided to perform this memoization for different access patterns, as described below.

The creation of the key is based on the arguments and keyword arguments passed in. The type information can also be used in the formation of the key by setting the include_types parameter to True. In addition, the arguments used for the function can be altered before use for the key creation via the key_func parameter. A common use case for this is to remove a parameter since as a timer or logging object from the key creation. The function should accept the same arguments as the calling function and return a tuple of the arguments and keyword arguments to use to create the key.

def key_changer(x, logger):
    return (x,), {}

@cache(RandomCache(size=30), key_func=key_changer)
def func(x, logger)
    ...

Two callback functions, on_hit and on_miss, can be passed into the function to be called when either a cache hit or cache miss occurs. For the most typical use cases, these are passed either the number of hits or number of misses occurred.

Bound Function Methods

In addition to caching the function, other methods are bound to the function for interacting with the cache. The following methods are added to the decorated function:

  • cache_info(): Returns a tuple of information about the cache containing the number of hits, the number of misses, the current size, and the maximum size.
  • cache_clear(): Clears the cache along with its statistics.
@cache(NMRUCache(30))
def func(...)
    ...

# Clear the cache
func.cache_clear()

# Grab cache information
hits, misses, current_size, max_size = func.cache_info()

Caching Algorithms

The following caching algorithms are provided by the library (although others could be extended from the BaseCache):

While each of these can be fed into the cache decorator, they can also be used on their own by simply creating an instance and calling the appropriate methods. Each implemented algorithm has an O(1) time complexity for accesses and insertions, and have the following methods and properties:

  • current_size: The current size of the cache.
  • hits: The number of cache hits.
  • max_size: The maximum size of the cache.
  • misses: The number of cache misses.
  • clear(): Clears the items in the cache.
  • get(key, sentinel): Gets an item in the cache.
  • put(key, value): Retrieves an item in the cache.
  • dynamic_methods(): Provides dynamic binding of methods for cache decorator.
  • create_key(...): Creates a cache key.

FIFO (First-in First-out)

The FIFOCache is a First-in First-out cache where keys are evicted in order of arrival when the cache is full. Accessing a key does not change the order of eviction.

@cache(FIFOCache(size=50))
def func(...)
    ...

LIFO (First-in First-out)

The LIFOCache is a Last-in First-out cache where keys are evicted in reverse order of arrival when the cache is full. Accessing a key does not change the order of eviction.

@cache(LIFOCache(size=50))
def func(...)
    ...

LFU (Least Frequently Used)

The LFUCache is a Least Frequently Used cache where keys which have been accessed the least number of times are evicted when the cache is full. The frequency list structure as described in “An O(1) algorithm for implementing the LFU cache eviction scheme” is implemented.

@cache(LFUCache(size=50))
def func(...)
    ...

LRU (Least Recently Used)

The LRUCache is a Least Recently Used cache where keys which have been accessed the least recently are evicted when the cache is full.

@cache(LRUCache(size=50))
def func(...)
    ...

MFU (Most Frequently Used)

The MFUCache is a Most Frequently Used cache where keys which have been accessed the most number of times are evicted when the cache is full. This uses the same frequency list structure as described in LFU.

@cache(MFUCache(size=50))
def func(...)
    ...

MQ (Multi-Queue)

The MQCache is a Multi-Queue cache in which multiple queues are used to hold levels of varying temperature (i.e. highly accessed and less accessed) along with a history buffer (similar to 2Q). This is implemented based on the paper “The Multi-Queue Replacement Algorithm for Second Level Buffer Caches” which uses a LRU queues for each level. The access count of each item is also recorded and used in determining which queue to promote the item to based on the queue_func parameter.

Items are also susceptible to being evicted over time. If an item isn’t accessed within a certain time it is bumped down to a lower queue and its expiration time is reset. This continues if the item isn’t accessed until it is eventually evicted to the lowest queue.

The history buffer is a FIFO queue that keeps track of items recently evicted from the queue. If an item is accessed while in the history buffer, it is placed in the appropriate queue based on it’s previous access frequency + 1.

@cache(MQCache(size=50, buffer_size=10, expire_time=100, num_queues=8,
       queue_func=lambda f: math.log(f, 2), access_based=True))
def func(...)
    ...

MRU (Most Recently Used)

The MRUCache is a Most Recently Used cache where keys which have been accessed the most recently are evicted when the cache is full.

@cache(MRUCache(size=50))
def func(...)
    ...

NMRU (Not Most Recently Used)

The NMRUCache is a Not Most Recently Used cache where keys which have not been access the most recently are evicted when the cache is full. When the cache is full, a random key other than the most recently inserted is removed.

@cache(NMRUCache(size=50))
def func(...)
    ...

RR (Random Replacement)

The RRCache is a Random Replacement cache where keys are evicted randomly, regardless of access of insertion order.

@cache(RRCache(size=50))
def func(...)
    ...

SLRU (Segmented Least Recently Used)

The SLRUCache is a Segmented Least Recently Used cache which is implemented with two queues, a LRU (the protected), and a FIFO (the probationary). Items are initially placed into the probationary queue when first placed into the cache. If this cache is full, items are evicted in the order of their arrival. If items are accessed while they are in the probationary queue, they are moved to the protected queue. They stay in this queue until it is full and the key which has been least recently used is moved back to the probationary queue.

Note that this cache implementation is very similar to the simple 2Q algorithm with the exception that items evicted from the protected cache are moved to the probationary (opposed to being immediately evicted).

@cache(SLRUCache(protected_size=50, probationary_size=20))
def func(...)
    ...

Static

The StaticCache is a simple cache with no key eviction. Keys are stored permanently, or at least until the cache is cleared.

@cache(StaticCache())
def func(...)
    ...

TLRU (Time-aware Least Recently Used)

The TLRUCache is a Time-aware Least Recently-Used cache where keys are prematurely evicted if their last access time is below a minimum limit, the expire_time. Time in this case is either a simple clock that is incremented each time the cache is accessed., or the actual time in seconds that has passed. This is determined by the access_based parameter. If reset_on_access is True, the expire_time is reset each time the item is accessed; otherwise it is expired from the time of initial insertion in the cache.

This is implemented with two LRU lists, one for LRU-based expiration and another for time-based expiration. This is required due to allowing reset_on_access to be False, thereby allowing items to be expired independent of how they are accessed.

@cache(TLRUCache(expire_time=100, size=50, access_based=False, reset_on_access=True))
def func(...)
    ...

TwoQ (simple 2Q or Two-Queue)

The TwoQCache is the simple 2Q cache described by the algorithm in “2Q: A Low Overhead High Performance Buffer Management Replacement Algorithm”. Two queues are used, an LRU (the primary), and a FIFO (the secondary). Items are initially placed into the secondary queue when placed into the cache. If the secondary queue is full, items are evicted in the order of their arrival. If items are accessed while they are in the secondary queue, they are moved to the primary queue. They stay in this queue until it is full and the key which has been least recently used is evicted.

@cache(TwoQ(primary_size=50, secondary_size=20))
def func(...)
    ...

TwoQFull (full 2Q or Two-Queue)

The TwoQFullCache is the full 2Q cache as described by the algorithm in “2Q: A Low Overhead High Performance Buffer Management Replacement Algorithm”. Three queues are used, an LRU (the primary), and two FIFO (the secondary in and out). Items are initially placed into the secondary “in” queue when first placed into the cache. If this queue is full, items are moved in the order of their arrival into the secondary “out” queue. If items are accessed while they are in the secondary “in” queue they stay in this queue in their current position.

If items are accessed while they are in the secondary “out” queue they are moved to the primary queue. If this cache is full, items are evicted in the order of their arrival. Items in the primary queue stay in this queue until it is full and the key which has been least recently used is evicted. This implementation yields O(1) access and insertion time.

@cache(TwoQFull(primary_size=50, secondary_in_size=20, secondary_out_size=20))
def func(...)
    ...

Advanced Usage

Instead of using as a decorator, cache can be used as an instead for wrapping an arbitrary number of function calls. This can be achieved via the run method.

def func_a():
    ...

def func_b():
    ...

cacher = cache(algorithm=...)

# Using same configured cache instance
cache.run(func_a, args, kwargs)
cache.run(func_b, args, kwargs)

Besides using the provided run method, like any decorator functions can be locally wrapped, passed around, and executed.

def func():
    ...

cacher = cache(algorithm=...)
cache_func = cacher(func)
cache_func(args, kwargs)

# Or as a one-off like so
cache(...)(func)(args, kwargs)

Both the on_hit and on_miss callback functions that can be passed into cache can actually be configured to accepts different number of parameters depending on the function. They can each either accept 0 parameters, the parameters that would be typically passed in, or the wrapped function’s args and kwargs in addition to the parameters typically given.

Optionally passing in the args and kwargs allows for building more complex callback functions. Each of the possible function variations are shown below.

def on_hit(): ...
def on_hit(error): ...
def on_hit(error, *args, **kwargs): ...

def on_miss(): ...
def on_miss(value): ...
def on_miss(value, *args, **kwargs): ...

In addition to the cache_info() and cache_clear() methods bound to the function, others can be dynamically bound to the function depending on the algorithm. None of the current basic implementations use this functionality, but this has a case for when creating one’s own or extending the existing algorithms. The dynamic methods are prefixed with “cache_<method-name>”.

import copy

class FancyCache(LRUCache):
    def __init__(self, size):
        super(size).__init__()

    def show_all(self):
        # Retrieve a copy of the current items in the cache
        return copy(self._map)

    def dynamic_methods(self):
        return ['show_all']

@cache(FancyCache())
def func(x, y)
    ...

func(1, 2)
func(3, 4)

# Dynamic methods are prefixed with 'cache_'
items = func.cache_show_all()

Contribution

Contributions or suggestions are welcome! Feel free to open an issue if a bug is found or an enhancement is desired, or even a pull request.

Changelog

All changes and versioning information can be found in the CHANGELOG.

License

Copyright (c) 2018 Jared Gillespie. See LICENSE for details.

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