proxy-pattern-pool

Proxy Pattern Pool

Generic Proxy and Pool classes for Python.

This module provides two classes:

• Proxy implements the proxy pattern, i.e. all calls to methods on the proxy are forwarded to an internally wrapped object. This allows to solve the classic chicken-and-egg importation and initialization possibly circular-dependency issue with Python modules:

  # File "database.py"
  db = Proxy()
  
  def init_app(config):
      db.set_obj(initialization from config)
  

  # File "app.py"
  import database
  from database import db  # db is a proxy to nothing
  …
  # delayed initialization
  database.init_app(config)
  
  # db is now a proxy to the initialized object
  

• Pool implements a thread-safe pool of things which can be used to store expensive-to-create objects such as database connections. The above proxy object creates a pool automatically depending on its parameters.

  Method _ret_obj must be called to return the object to the pool when done with it.

  This generic pool class can be used independently of the Proxy class.

Documentation

Proxy

Class Proxy manages accesses to one or more objects, possibly using a Pool, depending on the expected scope of said objects.

The Proxy constructors expects the following parameters:

• obj a single object SHARED between all threads.
• fun a function called for object creation, each time it is needed, for all other scopes.
• scope object scope as defined by Proxy.Scope:
  • SHARED one shared object (process level)
  • THREAD one object per thread (threading implementation)
  • WERKZEUG one object per greenlet (werkzeug implementation)
  • EVENTLET one object per greenlet (eventlet implementation)
  • GEVENT one object per greenlet (gevent implementation)
  • VERSATILE same as WERKZEUG default is SHARED or THREAD depending on whether an object of a function was passed for the object.
• set_name the name of a function to set the proxy contents, default is set. This parameter allows to avoid collisions with the proxied methods, if necessary. It is used as a prefix to have set_obj and set_fun functions which allow to reset the internal obj or fun.
• log_level set logging level, default None means no setting.
• max_size of pool, default None means no pooling.
• max_size and all other parameters are forwarded to Pool.

When max_size is not None, a Pool is created to store the created objects so as to reuse them. It is the responsability of the user to return the object when not needed anymore by calling _ret_obj explicitely. This is useful for code which keeps creating new threads, eg werkzeug. For a database connection, a good time to do that is just after a commit.

The proxy has a _has_obj method to test whether an object is available without extracting anything from the pool: this is useful to test whether returning the object is needed in some error handling pattern.

Pool

Class Pool manages a pool of objects in a thread-safe way. Its constructor expects the following parameters:

• fun how to create a new object; the function is passed the creation number.
• max_size maximum size of pool, 0 for unlimited (the default).
• min_size minimum size of pool, that many are created and maintained in advance.
• timeout maximum time to wait for something, only active under max_size.
• max_use after how many usage to discard an object.
• max_avail_delay when to discard an unused object.
• max_using_delay when to warn about object kept for a long time.
• max_using_delay_kill when to kill objects kept for a long time.
• health_freq run health check this every house keeper rounds.
• hk_delay force house keeping delay.
• log_level set logging level, default None means no setting.
• opener function to call when creating an object, default None means no call.
• getter function to call when getting an object, default None means no call.
• retter function to call when returning an object, default None means no call.
• closer function to call when discarding an object, default None means no call.
• stats function to call to generate a JSON-compatible structure for stats.
• health function to call to check for an available object health.
• tracer object debug helper, default None means less debug.

Objects are created on demand by calling fun when needed.

Proxy Example

Here is an example of a flask application with blueprints and a shared resource.

First, a shared module holds a proxy to a yet unknown object:

# file "Shared.py"
from ProxyPatternPool import Proxy
stuff = Proxy()
def init_app(s):
    stuff.set_obj(s)

This shared object is used by module with a blueprint:

# file "SubStuff.py"
from Flask import Blueprint
from Shared import stuff
sub = Blueprint(…)

@sub.get("/stuff")
def get_stuff():
    return str(stuff), 200

Then the application itself can load and initialize both modules in any order without risk of having some unitialized stuff imported:

# file "App.py"
from flask import Flask
app = Flask("stuff")

from SubStuff import sub
app.register_blueprint(sub, url_prefix="/sub")

import Shared
Shared.init_app("hello world!")

Notes

This module is rhetorical: because of the GIL Python is quite bad as a parallel language, so the point of creating threads which will mostly not really run in parallel is moot, thus the point of having a clever pool of stuff to be shared by these thread is even mooter! However, as the GIL is scheduled to go away in the coming years, starting from Python 3.13, it might start to make sense to have such a thing here!

In passing, it is interesting to note that the foremost driving motivation for getting read of the GIL is… data science. This tells something. In the past, people interested in parallelism, i.e. performance, say myself, would probably just turn away from this quite slow language. People from the networking www world would be satisfied with the adhoc asynchronous model, and/or just create many processes because in this context the need to communicate between active workers is limited. Now come the data scientist, who is not that interested in programming, is happy with Python and its ecosystem, in particular with the various ML libraries and the commodity of web-centric remote interfaces such as Jupyter. When confronted with a GIL-induced performance issue, they are more interested at fixing the problem than having to learn another language and port their stuff.

Shared object must be returned to the pool to avoid depleting resources. This may require some active cooperation from the infrastructure which may or may not be reliable. Consider monitoring your resources to detect unexpected status, eg database connections remaining idle in transaction and the like.

See Also:

• Psycopg Pool for pooling Postgres database connexions.
• Eventlet db_pool for pooling MySQL or Postgres database connexions.
• Discussion about database pool sizing (spoiler: small is beautiful).

License

This code is Public Domain.

All software has bug, this is software, hence… Beware that you may lose your hairs or your friends because of it. If you like it, feel free to send a postcard to the author.

Versions

Sources, documentation and issues are hosted on GitHub. Install package from PyPI. See version details.

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