Package buildbot :: Package util :: Module lru :: Class AsyncLRUCache

Class AsyncLRUCache

object --+
         |
        AsyncLRUCache

A least-recently-used cache, with a fixed maximum size. This cache is designed to control memory usage by minimizing duplication of objects, while avoiding unnecessary re-fetching of the same rows from the database.

Asynchronous locking is used to ensure that in the common case of multiple concurrent requests for the same key, only one fetch is performed.

All values are also stored in a weak valued dictionary, even after they have expired from the cache. This allows values that are used elsewhere in Buildbot to "stick" in the cache in case they are needed by another component. Weak references cannot be used for some types, so these types are not compatible with this class. Note that dictionaries can be weakly referenced if they are an instance of a subclass of dict.

If the result of the miss_fn is None, then the value is not cached; this is intended to avoid caching negative results.

This is based on Raymond Hettinger's implementation in http://code.activestate.com/recipes/498245-lru-and-lfu-cache-decorators/ licensed under the PSF license, which is GPL-compatiblie.

Instance Methods

__init__(self, miss_fn, max_size=50)
Constructor.

source code

get(self, key, **miss_fn_kwargs)
Fetch a value from the cache by key, invoking self.miss_fn(key) if the key is not in the cache. source code

put(self, key, value)
Update the cache with the given key and value, if the key is already in the cache.

source code

set_max_size(self, max_size)

source code

inv(self)
Check invariants and log if they are not met; used for debugging

source code

Inherited from object: __delattr__, __format__, __getattribute__, __hash__, __new__, __reduce__, __reduce_ex__, __repr__, __setattr__, __sizeof__, __str__, __subclasshook__

Class Variables
	sentinel = `object()`
	QUEUE_SIZE_FACTOR = `10`

Instance Variables
	hits cache hits so far
	max_size maximum allowed size of the cache
	misses cache misses leading to re-fetches, so far
	refhits cache misses found in the weak ref dictionary, so far

Properties
	cache
	concurrent
	max_queue
	miss_fn
	queue
	refcount
	weakrefs
Inherited from `object`: `__class__`

Method Details

init(self, miss_fn, max_size=50)
(Constructor)

source code

Constructor.

Parameters:

miss_fn - function to call, with key as parameter, for cache misses. This function must return a deferred.
max_size - maximum number of objects in the cache

Overrides: object.__init__

get(self, key, **miss_fn_kwargs)

source code

Fetch a value from the cache by key, invoking self.miss_fn(key) if the key is not in the cache.

Any additional keyword arguments are passed to the miss_fn as keyword arguments; these can supply additional information relating to the key. It is up to the caller to ensure that this information is functionally identical for each key value: if the key is already in the cache, the miss_fn will not be invoked, even if the keyword arguments differ.

Parameters:

key - cache key
**miss_fn_kwargs - keyword arguments to the miss_fn

Returns:

value via Deferred

put(self, key, value)

source code

Update the cache with the given key and value, if the key is already in the cache. This is intended to be used when updated values are available for an existing cached object, and does not record a reference to the key.

Parameters:

key - key to update
value - new value

Returns:

nothing

Class AsyncLRUCache

__init__(self, miss_fn, max_size=50) (Constructor)

get(self, key, **miss_fn_kwargs)

put(self, key, value)

init(self, miss_fn, max_size=50)
(Constructor)