# Dogpile

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.

## 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[0] = 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[0].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.

## Using a Value Function with a Memcached-like Cache

The dogpile lock includes a more intricate mode of usage to optimize the usage of a cache like Memcached. The difficulties Dogpile addresses in this mode are:

• Values can disappear from the cache at any time, before our expiration time is reached. Dogpile needs to be made aware of this and possibly call the creation function ahead of schedule.
• There's no function in a Memcached-like system to "check" for a key without actually retrieving it. If we need to "check" for a key each time, we'd like to use that value instead of calling it twice.
• If we did end up generating the value on this get, we should return that value instead of doing a cache round-trip.

To use this mode, the steps are as follows:

• Create the Dogpile lock with init=True, to skip the initial "force" of the creation function. This is assuming you'd like to rely upon the "check the value" function for the initial generation. Leave it at False if you'd like the application to regenerate the value unconditionally when the dogpile lock is first created (i.e. typically application startup).
• The "creation" function should return the value it creates.
• An additional "getter" function is passed to acquire() which should return the value to be passed to the context block. If the value isn't available, raise NeedRegenerationException.

Example:

from dogpile import Dogpile, NeedRegenerationException

def get_value_from_cache():
value = my_cache.get("some key")
if value is None:
raise NeedRegenerationException()
return value

def create_and_cache_value():
value = my_expensive_resource.create_value()
my_cache.put("some key", value)
return value

dogpile = Dogpile(3600, init=True)

with dogpile.acquire(create_and_cache_value, get_value_from_cache) as value:
return value


Note that get_value_from_cache() should not raise NeedRegenerationException a second time directly after create_and_cache_value() has been called.

## Locking the "write" phase against the "readers"

The dogpile lock can provide a mutex to the creation function itself, so that the creation function can perform certain tasks only after all "stale reader" threads have finished. The example of this is when the creation function has prepared a new datafile to replace the old one, and would like to switch in the "new" file only when other threads have finished using it.

To enable this feature, use SyncReaderDogpile(). SyncReaderDogpile.acquire_write_lock() then provides a safe-write lock for the critical section where readers should be blocked:

from dogpile import SyncReaderDogpile

def some_creation_function():
create_expensive_datafile()
with dogpile.acquire_write_lock():
replace_old_datafile_with_new()


## Using Dogpile for Caching

Dogpile is part of an effort to "break up" the Beaker package into smaller, simpler components (which also work better). Here, we illustrate how to replicate Beaker's "cache decoration" function, to decorate any function and store the value in Memcached:

import pylibmc

from dogpile import Dogpile, NeedRegenerationException

def cached(key, expiration_time):
"""A decorator that will cache the return value of a function
in memcached given a key."""

def get_value():
with mc_pool.reserve() as mc:
value = mc.get(key)
if value is None:
raise NeedRegenerationException()
return value

def gen_cached():
with mc_pool.reserve() as mc:
mc.put(key, value)

dogpile = Dogpile(expiration_time, init=True)

def decorate(fn):
def invoke():
with dogpile.acquire(gen_cached, get_value) as value:
return value
return invoke

return decorate


Above we can decorate any function as:

@cached("some key", 3600)
def generate_my_expensive_value():
return slow_database.lookup("stuff")


The Dogpile lock will ensure that only one thread at a time performs slow_database.lookup(), and only every 3600 seconds, unless Memcached has removed the value in which case it will be called again as needed.

In particular, Dogpile's system allows us to call the memcached get() function at most once per access, instead of Beaker's system which calls it twice, and doesn't make us call get() when we just created the value.

## Development Status

Please note Dogpile is new and has only had minimal production usage ! Comments and improvements are welcome but please review its workings and source code before use and use at your own risk !