Eventlet snippets

OpenStack services use the green threads model as described here. I found an excellent explanation of Eventlet in a video by Donovan Preston. I have typed in some of the snippets he uses here.

Spawning a green thread

	import eventlet
	def add(x, y):
	return x + y
	e = eventlet.spawn(add, 1, 2)
	print e
	res = e.wait()
	print '1 + 2 =', res

view raw spawn.py hosted with ❤ by GitHub

$ python spawn.py
<eventlet.greenthread.GreenThread object at 0xe77870>
1 + 2 = 3

Cooperative yielding

	import eventlet
	def func1():
	# suspend me and run something else
	# but switch back to me after 2 seconds (if you can)
	eventlet.sleep(2)
	print "func1"
	def func2():
	eventlet.sleep(1)
	print "func2"
	f1 = eventlet.spawn(func1)
	f2 = eventlet.spawn(func2)
	f1.wait()
	f2.wait()

view raw co-op.py hosted with ❤ by GitHub

$ python co-op.py 
func2
func1

	import eventlet
	def foo(name):
	for i in range(3):
	print "foo(%s): i=%d" % (name, i)
	eventlet.sleep()
	f1 = eventlet.spawn(foo, 'A')
	f2 = eventlet.spawn(foo, 'B')
	for i in range(3):
	print "main: i=%d" % i
	eventlet.sleep()
	f1.wait()
	f2.wait()

view raw interleave.py hosted with ❤ by GitHub

$ python interleave.py 
main:   i=0
foo(A): i=0
foo(B): i=0
main:   i=1
foo(A): i=1
foo(B): i=1
main:   i=2
foo(A): i=2
foo(B): i=2

Not so cooperative

	import eventlet
	def foo(name):
	for i in range(3):
	print "foo(%s): i=%d" % (name, i)
	f1 = eventlet.spawn(foo, 'A')
	f2 = eventlet.spawn(foo, 'B')
	for i in range(3):
	print "main: i=%d" % i
	f1.wait()
	f2.wait()

view raw un-co-op.py hosted with ❤ by GitHub

$ python un-co-op.py 

main: i=0

main: i=1

main: i=2

foo(A): i=0

foo(A): i=1

foo(A): i=2

foo(B): i=0

foo(B): i=1

foo(B): i=2

Synchronization queue

	import eventlet
	q = eventlet.Queue()
	def func1():
	print "func1", q.get()
	def func2():
	print "func2", q.get()
	waiton = (eventlet.spawn(func1), eventlet.spawn(func2))
	q.put("hello")
	q.put("world")
	for w in waiton:
	w.wait()

view raw queue.py hosted with ❤ by GitHub

$ python queue.py 

func1 hello

func2 world

Sychronization event

	import eventlet
	from eventlet import event
	evt = event.Event()
	def waiter():
	evt.wait()
	print "waiter"
	w = eventlet.spawn(waiter)
	print "sending"
	evt.send()
	print "sent"
	w.wait()

view raw waiter.py hosted with ❤ by GitHub

$ python waiter.py
sending
sent
waiter

	import eventlet
	from eventlet import event
	evt = event.Event()
	def waiter():
	print "waiter: before wait()"
	evt.wait()
	print "waiter: after wait()"
	w = eventlet.spawn(waiter)
	print "main: before send()"
	eventlet.sleep()
	evt.send()
	print "main: after send()"
	eventlet.sleep()
	print "main: at end"

view raw waiter2.py hosted with ❤ by GitHub

$ python waiter2.py
main: before send()
waiter: before wait()
main: after send()
waiter: after wait()
main: at end

Green pools

	import eventlet
	pool = eventlet.GreenPool(size=2)
	def printer(x):
	print x
	print "execute 1"
	pool.spawn(printer, 1)
	print "execute 2"
	pool.spawn(printer, 2)
	print "execute 3"
	pool.spawn(printer, 3)
	print "execute 4"
	pool.spawn(printer, 4)
	pool.waitall()

view raw green_pool.py hosted with ❤ by GitHub

$ python greenpool.py 
execute 1
execute 2
execute 3
1
2
execute 4
3
4

Green sockets

	import eventlet
	from eventlet.green import socket
	def handle_socket(reader, writer):
	print "client connected"
	while True:
	line = reader.readline()
	if not line:
	break
	writer.write(line)
	writer.flush()
	print "echoed", line.rstrip()
	print "disconnected"
	server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
	server.bind(('', 6000))
	server.listen(100)
	print "server listening on port 6000"
	while True:
	sock, addr = server.accept()
	eventlet.spawn(handle_socket, sock.makefile('r'), sock.makefile('w'))

view raw green_socket.py hosted with ❤ by GitHub

Start the echo server:
$ python greensocket.py
server listening on port 6000

then start another terminal and any lines typed are echoed: 

$ nc localhost 6000

something from client1

something from client1

Now the point is that the program is not blocked handling that socket. So start another terminal:

$ nc localhost 6000

something from client2

something from client2

Then press control-d in each nc to disconnect. The entire server output is:

$ python greensocket.py 

server listening on port 6000

client connected

echoed something from client1

client connected

echoed something from client2

disconnected

disconnected