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

source code

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.
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set_max_size(self, max_size) source code
 
inv(self)
Check invariants and log if they are not met; used for debugging
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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