A "dogpile" lock, one which allows a single thread to generate an expensive resource while other threads use the "old" value, until the "new" value is ready.
Dogpile is basically the locking code extracted from the Beaker package, for simple and generic usage.
A simple example:
from dogpile import Dogpile # store a reference to a "resource", some # object that is expensive to create. the_resource = [None] def some_creation_function(): # create the resource here the_resource = create_some_resource() def use_the_resource(): # some function that uses # the resource. Won't reach # here until some_creation_function() # has completed at least once. the_resource.do_something() # create Dogpile with 3600 second # expiry time dogpile = Dogpile(3600) with dogpile.acquire(some_creation_function): use_the_resource()
Above, some_creation_function() will be called when Dogpile.acquire() is first called. The remainder of the with block then proceeds. Concurrent threads which call Dogpile.acquire() during this initial period will be blocked until some_creation_function() completes.
Once the creation function has completed successfully the first time, new calls to Dogpile.acquire() will call some_creation_function() each time the "expiretime" has been reached, allowing only a single thread to call the function. Concurrent threads which call Dogpile.acquire() during this period will fall through, and not be blocked. It is expected that the "stale" version of the resource remain available at this time while the new one is generated.
Dogpile is at the core of the dogpile.cache package which provides for a basic cache API and sample backends based on the dogpile concept.
Please note Dogpile is new and has only had minimal production usage ! Comments and improvements are welcome. Since this is concurrency-oriented code, please review the source and let me know about potential issues. As always, use at your own risk!