Guild - pipelinable actors with late bindingMarch 07, 2014 at 11:51 PM | categories: open source, python, iot, bbc, actors, concurrency, kamaelia | View Comments
Guild is a python library for creating thread based applications.
Threads are represented using actors - objects with threadsafe methods. Calling a method puts a message on an inbound queue for execution within the thread. Guild actors can also have stub actor methods, representing output. These are stub methods which are expected to be rebound to actor methods on other actors. These stub methods are called late bind methods. This allows pipelines of Guild actors to be created in a similar way to Unix pipelines.
Additionally, Guild actors can be active or reactive. A reactive actor performs no actions until a message is received. An active guild actor can be active in two main ways: it can either repeatedly perform an action, or more complex behaviour can use a generator in a coroutine style. The use of a generator allows Guild actors to be stopped in a simpler fashion than traditional python threads. Finally, all Guild actors provide a default 'output' late-bindable method, to cover the common case of single input, single output.
Finally, Guild actors are just python objects and actors with additional functionality - it's designed to fit in with your code, not the other way round. This post covers some simple examples of usage of Guild, and how it differs (slightly) from traditional actors.
Getting and Installing
Installation is pretty simple:
$ git clone https://github.com/sparkslabs/guild $ cd guild $ sudo python setup.py install
If you'd prefer to build, install and use a debian package:
$ git clone https://github.com/sparkslabs/guild $ cd guild $ make deb $ sudo dpkg -i ../python-guild_1.0.0_all.deb
Example: viewing a webcam
This example shows the use of two actors - webcam capture, and image display. The thing to note here is that we could easily add other actors into the mix - for network serving, recording, analysis, etc. If we did, the examples below can be reused as is.
First of all the code, then a brief discussion.
import pygame, pygame.camera, time from guild.actor import * pygame.camera.init() class Camera(Actor): def gen_process(self): camera = pygame.camera.Camera(pygame.camera.list_cameras()) camera.start() while True: yield 1 frame = camera.get_image() self.output(frame) time.sleep(1.0/50) class Display(Actor): def __init__(self, size): super(Display, self).__init__() self.size = size def process_start(self): self.display = pygame.display.set_mode(self.size) @actor_method def show(self, frame): self.display.blit(frame, (0,0)) pygame.display.flip() input = show camera = Camera().go() display = Display( (800,600) ).go() pipeline(camera, display) time.sleep(30) stop(camera, display) wait_for(camera, display)
In this example, Camera is an active actor. That is it sits there, periodically grabbing frames from the webcam. To do this, it uses a generator as a main loop. This allows the fairly basic behaviour of grabbing frames for output to be clearly expressed. Note also this actor does use the normal blocking sleep function.
The Display Actor initialises by capturing the passed parameters. Once the actor has started, it's process_start method is called, enabling it to create a display, it then sits and waits for messages. These arrive when a caller calls the actor method 'show' our its alias 'input'. When that happens the upshot is that the show method is called, but in a threadsafe way - and it simply displays the image.
The setup/tear down code shows the following:
- Creation of, and starting of, the Camera actor
- Creation and start of the display
- Linking the output of the Camera to the Display
- The main thread then waits for 30 seconds - ie it allows the program to run for 30 seconds.
- The camera and display actors are then stopped
- And the main thread waits for the child threads to exit before exitting itself.
This could be simplified (and will be), but it shows that even though the actors had no specific shut down code, they shut down cleanly this way.
Example: following multiple log files looking for events
This example follows two log files, and grep/output lines matching a given pattern. In particular, it maps to this kind of command line:
$ (tail -f x.log & tail -f y.log) | grep pants
This example shows that there are still some areas that would benefit from additional syntactic sugar when it comes to wiring together pipelines. In particular, this example should be writable together like this:
Pipeline( Parallel( Follow("x.log"), Follow("y.log"), Grep("pants"), Printer() ).run()
However, I haven't implemented the necessary chassis yet (they will be).
Once again, first the code, then a discussion.
from guild.actor import * import re, sys, time class Follow(Actor): def __init__(self, filename): super(Follow, self).__init__() self.filename = filename self.f = None def gen_process(self): self.f = f = file(self.filename) f.seek(0,2) # seek to end while True: yield 1 line = f.readline() if not line: # no data, so wait time.sleep(0.1) else: self.output(line) def onStop(self): if self.f: self.f.close() class Grep(Actor): def __init__(self, pattern): super(Grep, self).__init__() self.regex = re.compile(pattern) @actor_method def input(self, line): if self.regex.search(line): self.output(line) class Printer(Actor): @actor_method def input(self, line): sys.stdout.write(line) sys.stdout.flush() follow1 = Follow("x.log").go() follow2 = Follow("y.log").go() grep = Grep("pants").go() printer = Printer().go() pipeline(follow1, grep, printer) pipeline(follow2, grep) wait_KeyboardInterrupt() stop(follow1, follow2, grep, printer) wait_for(follow1, follow2, grep, printer)
As you can see, like the bash example, we have two actors that tail/follow two different log files. These both feed into the same 'grep' actor that matches the given pattern, and these are finally passed to a Printer actor for display. Each actor shows slightly different aspects of Guild's model.
Follow is an active actor. It captures the filename to follow in the initialiser, and creates a placeholder for the associated file handle. The main loop them follows the file, calling its output method when it has a line. Finally, it will continue doing this until its .stop() method is called. When it is, the generator is killed (via a StopIteration exception being passed in), and the actor's onStop method is called allowing the actor to close the file.
Grep is a simple reactive actor with some setup. In particular, it takes the pattern provided, compiles a regex matcher using it. Then any actor call to its input method results in any matching lines to be passed through via its output method.
Printer is a simple reactive actor. Any actor call to it's input method results in the data passed in being sent to stdout.
Work in progress
It is worth noting that Guild at present is not a mature library yet, but is sufficiently useful for lots of tasks. In particular, one area Guild will improve on in - specifying coordination more compactly. For example, the Camera example could become:
Pipeline( Camera(), Display( (800,600) ) ).run()
That's a work in progress however, adding with other chassis, and other useful parts of kamaelia.
What are actors?
Actors are threads with a mailbox allowing them to receive and act upon messages. In the above webcam example, it has 2 threads, one for capturing images, and one for display. Images from the webcam end up in the mailbox for the display, which displays images it receives. Often actor libraries wrap up the action of sending a message to the mailbox of an actor via a method on the thread object.
The examples above demonstrate this above via the decorated methods:
- Display.show, Grep.input, Printer.input
All of these methods - when called by a client of the actor - take all the arguments passed in, along with their function and place on the actor's mailbox (a thread safe queue). The actor then has a main loop that checks this mailbox and executes the method within the thread.
How does Guild differ from the actor model?
In a traditional actor model, the code in the camera Actor might look like this:
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import pygame, pygame.camera, time from guild.actor import * pygame.camera.init() class Camera(Actor): def __init__(self, display): super(Camera, self).__init__() self.display = display def gen_process(self): camera = pygame.camera.Camera(pygame.camera.list_cameras()) camera.start() while True: yield 1 frame = camera.get_image() self.display.show(frame) time.sleep(1.0/50)
- NB: This is perfectly valid in Guild. If you don't want to use the idea of late bound methods or pipelining, then it can be used like any other actor library.
If you did this, the display code would not need any changes. The start-up code that links things together though would now need to look like this:
display = Display( (800,600) ).go() camera = Camera(display).go() # No pipeline line anymore time.sleep(30) stop(camera, display) wait_for(camera, display)
On the surface of things, this looks like a simplification, and on one level it is - we've removed one line from the program start-up code. Our camera object however now has its destination embedded at object initialisation and it's also become more complex, with zero increase in flexibility. In fact I'd argue you've lost flexibility, but I'll leave why for later.
For example, suppose we want to record the images to disk, we can do this by adding a third actor that can sit in the middle of others:
import time, os class FrameStore(Actor): def __init__(self, directory='Images', base='snap'): super(FrameStore, self).__init__() self.directory = directory self.base = base self.count = 0 def process_start(self): os.makedir(self.directory) try: os.makedirs("Images") except OSError, e: if e.errno != 17: raise @actor_method def input(self, frame): self.count += 1 now = time.strftime("%Y%m%d-%H%M%S",time.localtime()) filename = "%s/%s-%s-%05d.jpg" % (self.directory, self.base, now, self.count) pygame.image.save(frame, filename) self.output(frame)
This could then be used in a Guild pipeline system this way:
camera = Camera().go() framestore = FrameStore().go() display = Display( (800,600) ).go() pipeline(camera, framestore, display) time.sleep(30) stop(camera, framestore, display) wait_for(camera, framestore, display)
It's for this reason that Guild supports late bindable actor methods.
What's happening here is that the definition of Actor includes this:
class Actor(object): #... @late_bind_safe def output(self, *argv, **argd): pass
That means every actor has available "output" as a late bound actor method.
This pipeline called:
Essentially does this:
camera.bind("output", display, "input")
This transforms to a threadsafe version of this:
camera.output = display.input
As a result, it replaces the call camera.output with a call to display.input for us - meaning that it is as efficient to do camera.output as it is to do self.display.show in the example above - but significantly more flexible.
There are lots of fringe benefits of this - which are best discussed in later posts, but this does indicate best how Guild differs from the usual actor model.
Why write and release this?
About a year ago, I was working on a project with an aim of investigating various ideas relating to of the Internet of Things. (In particular, which definition of that really mattered to us, why, and what options it provided)
As part of that project, I wrote a small/just big though library suitable for testing some ideas I'd had regarding integrating some ideas in Kamaelia, with the syntactic sugar in the actor model. Essentially, to map Kamaelia's inboxes and messages to traditional actor methods, and maps outboxes to late bound actor methods. Use of standard names and/or aliases would allow pipelining.
Guild was the result, and it's proven itself useful in a couple out projects, hence its packaging as a standalone library. Like all such things, it's a work in progress, but it also has a cleaner to use version of Kamaelia's STM code, and includes some of the more useful components like pipelines and backplanes.
If you find it useful or spot a typo, please let me know.