# 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

dogpile = Dogpile(expiration_time, init=True)

def decorate(fn):
def gen_cached():
value = fn()
with mc_pool.reserve() as mc:
mc.put(key, value)
return value

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.

## Using Dogpile across lots of keys

The above patterns all feature the usage of Dogpile as an object held persistently for the lifespan of some value. Two more helpers can allow the dogpile to be created as needed and then disposed, while still maintaining that concurrent threads lock. Here's the memcached example again using that technique:

import pylibmc

from dogpile import Dogpile, NeedRegenerationException
import pickle
import time

def cache(expiration_time)
dogpile_registry = Dogpile.registry(expiration_time))

def get_or_create(key):

def get_value():
with mc_pool.reserve() as mc:
value = mc.get(key)
if value is None:
raise NeedRegenerationException()
# deserialize a tuple
# (value, createdtime)

dogpile = dogpile_registry.get(key)

def gen_cached():
value = fn()
with mc_pool.reserve() as mc:
# serialize a tuple
# (value, createdtime)
mc.put(key, pickle.dumps(value, time.time()))
return value

with dogpile.acquire(gen_cached, value_and_created_fn=get_value) as value:
return value

return get_or_create


Above, we use Dogpile.registry() to create a name-based "registry" of Dogpile objects. This object will provide to us a Dogpile object that's unique on a certain name. When all usages of that name are complete, the Dogpile object falls out of scope, so total number of keys used is not a memory issue. Then, tell Dogpile that we'll give it the "creation time" that we'll store in our cache - we do this using the value_and_created_fn argument, which assumes we'll be storing and loading the value as a tuple of (value, createdtime). The creation time should always be calculated via time.time(). The acquire() function returns just the first part of the tuple, the value, to us, and uses the createdtime portion to determine if the value is expired.

## Development Status

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!