99 Concurrent Bottles of Beer
99 bottles of beer on the wall, 99 bottles of beer. Take one down, pass it around, Take one down, pass it around, 97 bottles of beer on the wall, 97 bottles of beer 98 bottles of beer on the wall, 98 bottles of beer
The purpose of this page is to show solutions to common concurrent problems in different styles/toolkits. Inspired by 99 Bottles of Beer. It is not intended to demonstrate high-performance code, but rather to give potential users a sense of what typical code using the various libraries looks like.
These example are interesting, in that they provide an idea of clarity, how much boiler plate code is needed, how message passing looks, and how to yield to the operating system.
Include a brief description if you add to this page. Please make sure your source is well commented - concurrency is hard!
Contents
The Problem
Implement
#!/bin/sh tail -f /var/log/system.log |grep pants
in concurrent Python. On unix, you can send syslog messages via logger; filenames may vary.
Errata
Solutions using readline() will exhibit bugs if less than a full line is flushed to disk. If your input file is syslog, this shouldn't be a problem however.
Glyph makes the very valid point that these examples are in fact serial programs (ie, they don't do more than one thing at a time). A better example would be following multiple files simultaneously.
Solutions
Generator
Generators implement a "pull-style" approach to concurrency.
1 import time
2 import re
3
4 def follow(fname):
5 f = file(fname)
6 f.seek(0,2) # go to the end
7 while True:
8 l = f.readline()
9 if not l: # no data
10 time.sleep(.1)
11 else:
12 yield l
13
14 def grep(lines, pattern):
15 regex = re.compile(pattern)
16 for l in lines:
17 if regex.match(l):
18 yield l
19
20 def printer(lines):
21 for l in lines:
22 print l.strip()
23
24 f = follow('/var/log/system.log')
25 g = grep(f, ".*pants.*")
26 p = printer(g)
27
28 for i in p:
29 pass
Coroutines
The inversion of the generator example above, coroutines use a "push-style" approach to concurrency:
1 import time
2 import re
3 from functools import wraps
4
5
6 def coroutine(func):
7 @wraps(func)
8 def thing(*args, **kwargs):
9 gen = func(*args, **kwargs)
10 gen.next() # advance to the first yield
11 return gen
12 return thing
13
14 @coroutine
15 def follow(fname, next):
16 f = file(fname)
17 f.seek(0,2) # go to the end
18 while True:
19 l = f.readline()
20 if not l: # no data
21 time.sleep(.1)
22 else:
23 next.send(l)
24
25 @coroutine
26 def grep(pattern, next):
27 regex = re.compile(pattern)
28 while True:
29 l = yield
30 if regex.match(l):
31 next.send(l)
32
33 @coroutine
34 def printer():
35 while True:
36 l = yield
37 print l.strip()
38
39
40 p = printer()
41 g = grep('.*pants.*', p)
42 f = follow('/var/log/system.log', g)
Greenlets
Greenlets are similar to coroutines.
1 import greenlet
2 import time
3 import re
4
5 def follow(fname, next):
6 # setup
7 f = file(fname)
8 f.seek(0,2) # go to the end
9 # do stuff
10 while True:
11 l = f.readline()
12 if not l: # no data
13 time.sleep(.1)
14 else:
15 next.switch(l)
16
17 def grep(pattern, next):
18 # setup
19 regex = re.compile(pattern)
20
21 def do_stuff(l):
22 parent = greenlet.getcurrent().parent
23 while True:
24 if regex.match(l):
25 l = next.switch(l)
26 else:
27 l = parent.switch() # subtle!
28
29 return do_stuff
30
31 def printer(l):
32 # no setup
33 parent = greenlet.getcurrent().parent
34 # do stuff
35 while True:
36 print l.strip()
37 l = parent.switch()
38
39 p = greenlet.greenlet(printer)
40 g = greenlet.greenlet(grep(".*pants.*", p))
41 follow("/var/log/system.log", g)
Gevent
Gevent builds user-level threads on top of greenlets.
1 import re
2 import gevent
3 from gevent.queue import Queue
4
5 def follow(fname, dest):
6 # setup
7 f = file(fname)
8 f.seek(0,2) # go to the end
9 # do stuff
10 while True:
11 l = f.readline()
12 if not l: # no data
13 gevent.sleep(.1)
14 else:
15 dest.put(l)
16
17 def grep(pattern, source, dest):
18 # setup
19 regex = re.compile(pattern)
20
21 def do_stuff():
22 while True:
23 l = source.get()
24 if regex.match(l):
25 dest.put(l)
26
27 return do_stuff
28
29 def printer(source):
30 while True:
31 line = source.get()
32 print line.strip()
33
34 source_queue = Queue()
35 filtered_queue = Queue()
36
37 p = gevent.spawn(printer, filtered_queue)
38 g = gevent.spawn(grep(".*pants.*", source_queue, filtered_queue))
39 follow("/var/log/system.log", source_queue)
Kamaelia
1 import time
2 import re
3
4 import Axon
5 from Kamaelia.Chassis.Pipeline import Pipeline
6
7 # threaded due to the time.sleep() call
8 # No yield since a threaded component
9 class Follow(Axon.ThreadedComponent.threadedcomponent):
10 def __init__(self, fname, **argv):
11 self.fname = fname
12 super(Follow,self).__init__(**argv)
13 def main(self):
14 f = file(self.fname)
15 f.seek(0,2) # go to the end
16 while not self.dataReady("control"):
17 l = f.readline()
18 if not l: # no data
19 time.sleep(.1)
20 else:
21 self.send(l, "outbox")
22
23 self.send(self.recv("control"), "signal")
24
25 class Grep(Axon.Component.component):
26 # Default pattern, override in constructor with pattern="some pattern"
27 # See below
28 pattern = "."
29 def main(self):
30 regex = re.compile(self.pattern)
31 while not self.dataReady("control"):
32 for l in self.Inbox("inbox"):
33 if regex.match(l):
34 self.send(l, "outbox")
35 self.pause()
36 yield 1
37 self.send(self.recv("control"), "signal")
38
39 class Printer(Axon.Component.component):
40 def main(self):
41 while not self.dataReady("control"):
42 for l in self.Inbox("inbox"):
43 print l.strip()
44 self.pause()
45 yield 1
46 self.send(self.recv("control"), "signal")
47
48 Pipeline(
49 Follow('/var/log/system.log'),
50 Grep(".*pants.*"),
51 Printer(),
52 ).run()
Twisted
1 from twisted.protocols.basic import LineReceiver
2 from twisted.python import log
3
4 SLOW_INTERVAL = 1.0
5 FAST_INTERVAL = 0.001
6 SEEK_END = 2
7 BLOCKSIZE = 8192
8
9 class TailTransport(object):
10 def __init__(self, fileobj, protocol):
11 self.fileobj = fileobj
12 self.protocol = protocol
13 self.disconnecting = False
14
15 def start(self, clock):
16 self.clock = clock
17 self.fileobj.seek(0, SEEK_END)
18 self.protocol.makeConnection(self)
19 self.tick()
20
21 def tick(self):
22 anyData = self.fileobj.read(BLOCKSIZE)
23 try:
24 self.protocol.dataReceived(anyData)
25 except:
26 log.err()
27 if anyData:
28 interval = FAST_INTERVAL
29 else:
30 interval = SLOW_INTERVAL
31 self.clock.callLater(interval, self.tick)
32
33 class Grep(LineReceiver):
34 delimiter = '\n'
35 def __init__(self, term):
36 self.term = term
37
38 def lineReceived(self, line):
39 if self.term in line:
40 print line.rstrip("\n")
41
42 def main():
43 from twisted.internet import reactor
44 TailTransport(file("/var/log/syslog", "rb"),
45 Grep("pants")).start(reactor)
46 reactor.run()
47
48 main()
Fibra
1 import fibra
2 import re
3
4 def tail(f, output):
5 f.seek(0,2)
6 while True:
7 line = f.readline()
8 yield output.push(line) if line else 0.1 #push line, or sleep.
9
10 def grep(pattern, input, output):
11 regex = re.compile(pattern)
12 while True:
13 line = yield input.pop()
14 if regex.match(line):
15 yield output.push(line)
16
17 def printer(input):
18 while True:
19 line = yield input.pop()
20 print line.strip()
21
22 schedule = fibra.schedule()
23 schedule.install(tail(open("/var/log/syslog.log","r"), fibra.Tube("T2G")))
24 schedule.install(grep(".*pants.*", fibra.Tube("T2G"), fibra.Tube("G2P")))
25 schedule.install(printer(fibra.Tube("G2P")))
26 schedule.run()
Stackless
1 import stackless
2 import time
3 import re
4
5 @stackless.tasklet
6 def tail(f, output):
7 f.seek(0,2)
8 while True:
9 line = f.readline()
10 if line:
11 output.send(line)
12 else:
13 time.sleep(0.1)
14
15 @stackless.tasklet
16 def grep(pattern, input, output):
17 regex = re.compile(pattern)
18 while True:
19 line = input.receive()
20 if regex.match(line):
21 output.send(line)
22
23 @stackless.tasklet
24 def printer(input):
25 while True:
26 line = input.receive()
27 print line.strip()
28
29 T2G = stackless.channel()
30 G2P = stackless.channel()
31 tail(open("/var/log/syslog.log","r"), T2G)
32 grep(".*pants.*", T2G, G2P)
33 printer(G2P)
34 stackless.run()
circuits
1 import sys
2
3 from circuits.io import File
4 from circuits import Component
5 from circuits.net.protocols import LP
6
7 class Tail(Component):
8
9 def init(self, filename):
10 (File(filename, "r", autoclose=False) + LP()).register(self).seek(0, 2)
11
12 class Grep(Component):
13
14 def init(self, pattern):
15 self.pattern = pattern
16
17 def line(self, line):
18 if self.pattern in line:
19 print line
20
21 (Tail(sys.argv[1]) + Grep(sys.argv[2])).run()
pprocess
This example needs pprocess 0.5. The activity functions are similar to the generator (and other) solutions, and the differences lie in the use of the multigrep function, which is invoked to provide grep functionality for each pattern in a separate process, and in the way the multigrep function itself follows several files using the multifollow callable (the follow function invoked in a separate process). A channel is used in the follow function to communicate new lines which are then consumed via a queue in the grep function, which in turn communicates matching lines via a channel which are then consumed by the printer function.
1 import pprocess
2 import time
3 import re
4
5 def follow(ch, fname):
6 f = file(fname)
7 f.seek(0,2) # go to the end
8 while True:
9 l = f.readline()
10 if not l: # no data
11 time.sleep(.1)
12 else:
13 ch.send(l)
14
15 def grep(ch, lines, pattern):
16 regex = re.compile(pattern)
17 for l in lines:
18 if regex.match(l):
19 ch.send(l)
20
21 def printer(lines):
22 for l in lines:
23 print l.strip()
24
25 def multigrep(ch, pattern):
26 queue = pprocess.Queue(continuous=1)
27 multifollow = queue.manage(follow)
28
29 # Launch concurrent following activities.
30 multifollow('/var/log/system.log')
31 multifollow('/var/log/other.log')
32 multifollow('/var/log/another.log')
33
34 # Handle incoming lines using the specified pattern.
35 grep(ch, queue, pattern)
36
37 # Permit multiple simultaneous grep activities.
38 queue = pprocess.Queue(continuous=1)
39 multigrep = queue.manage(multigrep)
40
41 # Launch concurrent grep activities.
42 multigrep(".*pants.*")
43 multigrep(".*trousers.*")
44 multigrep(".*shorts.*")
45
46 # Print incoming lines.
47 p = printer(queue)
pypes
Here is a simple example based on the pypes framework. It should look similar to the Stackless example above. Pypes abstracts away the semantics of tasklets and channels and provides a model for looser coupling. This makes connecting components at runtime easier which is necessary since at the point in which the component is created, it has no idea what other components it might be interacting with.
1 # load the pypes framework
2 from pkg_resources import require
3 require('pypes')
4
5 import re
6 import time
7
8 from pypes.pipeline import Dataflow
9 from pypes.component import Component
10
11 class Tail(Component):
12 __metatype__ = 'ADAPTER'
13
14 def __init__(self, fp):
15 Component.__init__(self)
16 self.fp = fp
17
18 def run(self):
19 self.fp.seek(0,2)
20 while True:
21 self.receive('in')
22 line = self.fp.readline()
23 if line:
24 self.send('out', line.strip())
25 else:
26 self.yield_ctrl()
27
28 class Grep(Component):
29 __metatype__ = 'TRANSFORMER'
30
31 def __init__(self, pattern):
32 Component.__init__(self)
33 self.regex = re.compile(pattern)
34
35 def run(self):
36 while True:
37 for line in self.receive_all('in'):
38 if self.regex.match(line):
39 self.send('out', line)
40 self.yield_ctrl()
41
42 class Printer(Component):
43 __metatype__ = 'PUBLISHER'
44
45 def __init__(self):
46 Component.__init__(self)
47
48 def run(self):
49 while True:
50 for data in self.receive_all('in'):
51 print data
52 self.yield_ctrl()
53
54 tail = Tail(open('/var/log/system.log', 'r'))
55 grep = Grep('.*pants.*')
56 printer = Printer()
57
58 pipe = Dataflow({
59 tail: {grep:('out','in')},
60 grep: {printer:('out', 'in')}
61 })
62
63 while True:
64 pipe.send(None)
65 time.sleep(0.1)