Introducción

La estructura de este tutorial asume un nivel intermedio de conocimiento de Python pero no demasiado. Ningún conocimiento de concurrencia es esperado. El objetivo es darte las herramientas para que inicies con gevent, ayudarte a manejar tus problemas de concurrencias actuales y empezar a escribir aplicaciones asíncronas.

Contribudores

En orden cronológico de contribución: Stephen Diehl Jérémy Bethmont sww Bruno Bigras David Ripton Travis Cline Boris Feld youngsterxyf Eddie Hebert Alexis Metaireau Daniel Velkov

También un agradecimiento a Denis Bilenko por escribir gevent y orientar en la construcción de este tutorial.

Este es un documento colaborativo publicado bajo la Licencia MIT. Tienes algo que agregar? Viste un error?. Crear un fork y reporta un pull request Github. Todas las contribuciones son bienvenidas.

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Core

Greenlets

El patrón primario usado en gevent esGreenlet, una liviana corutina que Python brinda como un módulo de extensión de C. Greenlets todos corren dentro de procesos del SO para el programa principal pero son calendarizados cooperativamente.

Sólo un greenlet está siempre corriendo en un momento dado.

Esto difiere de cualquier construcción real de paralelismo proporcionada por librerías de multiprocesamiento o de hilos que hacen resuelven procesos e hilos POSIX que son programados por el sistema operativoy son realmente paralelos.

Ejecución Sincrona & Asíncrona

La idea principal de la concurrencia es que una larga tarea puede ser subdividida en una colección de subtareas que son calendarizadas para correr simultáneamente o asíncronamente, en lugar de una a la vez o sincrónicamente. Un conmutador entre dos tareas es conocido como cambio de contexto.

Un conmutador de contexto en gevent se realizá a través de yielding. En este ejemplo tenemos dos contextos que ceden a otro a través de invocar gevent.sleep(0).

import gevent

def foo():
    print('Running in foo')
    gevent.sleep(0)
    print('Explicit context switch to foo again')

def bar():
    print('Explicit context to bar')
    gevent.sleep(0)
    print('Implicit context switch back to bar')

gevent.joinall([
    gevent.spawn(foo),
    gevent.spawn(bar),
])

Running in foo
Explicit context to bar
Explicit context switch to foo again
Implicit context switch back to bar

It is illuminating to visualize the control flow of the program or walk through it with a debugger to see the context switches as they occur.

The real power of gevent comes when we use it for network and IO bound functions which can be cooperatively scheduled. Gevent has taken care of all the details to ensure that your network libraries will implicitly yield their greenlet contexts whenever possible. I cannot stress enough what a powerful idiom this is. But maybe an example will illustrate.

In this case the select() function is normally a blocking call that polls on various file descriptors.

import time
import gevent
from gevent import select

start = time.time()
tic = lambda: 'at %1.1f seconds' % (time.time() - start)

def gr1():
    # Busy waits for a second, but we don't want to stick around...
    print('Started Polling: ', tic())
    select.select([], [], [], 2)
    print('Ended Polling: ', tic())

def gr2():
    # Busy waits for a second, but we don't want to stick around...
    print('Started Polling: ', tic())
    select.select([], [], [], 2)
    print('Ended Polling: ', tic())

def gr3():
    print("Hey lets do some stuff while the greenlets poll, at", tic())
    gevent.sleep(1)

gevent.joinall([
    gevent.spawn(gr1),
    gevent.spawn(gr2),
    gevent.spawn(gr3),
])

Started Polling:  at 0.0 seconds
Started Polling:  at 0.0 seconds
Hey lets do some stuff while the greenlets poll, at at 0.0 seconds
Ended Polling:  at 2.0 seconds
Ended Polling:  at 2.0 seconds

Another somewhat synthetic example defines a task function which is non-deterministic (i.e. its output is not guaranteed to give the same result for the same inputs). In this case the side effect of running the function is that the task pauses its execution for a random number of seconds.

import gevent
import random

def task(pid):
    """
    Some non-deterministic task
    """
    gevent.sleep(random.randint(0,2)*0.001)
    print('Task', pid, 'done')

def synchronous():
    for i in range(1,10):
        task(i)

def asynchronous():
    threads = [gevent.spawn(task, i) for i in xrange(10)]
    gevent.joinall(threads)

print('Synchronous:')
synchronous()

print('Asynchronous:')
asynchronous()

Synchronous:
Task 1 done
Task 2 done
Task 3 done
Task 4 done
Task 5 done
Task 6 done
Task 7 done
Task 8 done
Task 9 done
Asynchronous:
Task 0 done
Task 2 done
Task 6 done
Task 7 done
Task 1 done
Task 4 done
Task 3 done
Task 5 done
Task 8 done
Task 9 done

In the synchronous case all the tasks are run sequentially, which results in the main programming blocking ( i.e. pausing the execution of the main program ) while each task executes.

The important parts of the program are the gevent.spawn which wraps up the given function inside of a Greenlet thread. The list of initialized greenlets are stored in the array threads which is passed to the gevent.joinall function which blocks the current program to run all the given greenlets. The execution will step forward only when all the greenlets terminate.

The important fact to notice is that the order of execution in the async case is essentially random and that the total execution time in the async case is much less than the sync case. In fact the maximum time for the synchronous case to complete is when each tasks pauses for 2 seconds resulting in a 20 seconds for the whole queue. In the async case the maximum runtime is roughly 2 seconds since none of the tasks block the execution of the others.

In a more common use case, asynchronously fetching data from a server, the runtime of fetch() will differ between requests, depending on the load on the remote server at the time of the request.

import gevent.monkey
gevent.monkey.patch_socket()

import gevent
import urllib2
import simplejson as json

def fetch(pid):
    response = urllib2.urlopen('http://json-time.appspot.com/time.json')
    result = response.read()
    json_result = json.loads(result)
    datetime = json_result['datetime']

    print 'Process ', pid, datetime
    return json_result['datetime']

def synchronous():
    for i in range(1,10):
        fetch(i)

def asynchronous():
    threads = []
    for i in range(1,10):
        threads.append(gevent.spawn(fetch, i))
    gevent.joinall(threads)

print 'Synchronous:'
synchronous()

print 'Asynchronous:'
asynchronous()

Determinism

As mentioned previously, greenlets are deterministic. Given the same configuration of greenlets and the same set of inputs, they always produce the same output. For example, let's spread a task across a multiprocessing pool and compare its results to the one of a gevent pool.

import time

def echo(i):
    time.sleep(0.001)
    return i

# Non Deterministic Process Pool

from multiprocessing.pool import Pool

p = Pool(10)
run1 = [a for a in p.imap_unordered(echo, xrange(10))]
run2 = [a for a in p.imap_unordered(echo, xrange(10))]
run3 = [a for a in p.imap_unordered(echo, xrange(10))]
run4 = [a for a in p.imap_unordered(echo, xrange(10))]

print( run1 == run2 == run3 == run4 )

# Deterministic Gevent Pool

from gevent.pool import Pool

p = Pool(10)
run1 = [a for a in p.imap_unordered(echo, xrange(10))]
run2 = [a for a in p.imap_unordered(echo, xrange(10))]
run3 = [a for a in p.imap_unordered(echo, xrange(10))]
run4 = [a for a in p.imap_unordered(echo, xrange(10))]

print( run1 == run2 == run3 == run4 )

False
True

Even though gevent is normally deterministic, sources of non-determinism can creep into your program when you begin to interact with outside services such as sockets and files. Thus even though green threads are a form of "deterministic concurrency", they still can experience some of the same problems that POSIX threads and processes experience.

The perennial problem involved with concurrency is known as a race condition. Simply put, a race condition occurs when two concurrent threads / processes depend on some shared resource but also attempt to modify this value. This results in resources which values become time-dependent on the execution order. This is a problem, and in general one should very much try to avoid race conditions since they result in a globally non-deterministic program behavior.

The best approach to this is to simply avoid all global state at all times. Global state and import-time side effects will always come back to bite you!

Spawning Greenlets

gevent provides a few wrappers around Greenlet initialization. Some of the most common patterns are:

import gevent
from gevent import Greenlet

def foo(message, n):
    """
    Each thread will be passed the message, and n arguments
    in its initialization.
    """
    gevent.sleep(n)
    print(message)

# Initialize a new Greenlet instance running the named function
# foo
thread1 = Greenlet.spawn(foo, "Hello", 1)

# Wrapper for creating and runing a new Greenlet from the named 
# function foo, with the passed arguments
thread2 = gevent.spawn(foo, "I live!", 2)

# Lambda expressions
thread3 = gevent.spawn(lambda x: (x+1), 2)

threads = [thread1, thread2, thread3]

# Block until all threads complete.
gevent.joinall(threads)

Hello
I live!

In addition to using the base Greenlet class, you may also subclass Greenlet class and override the _run method.

import gevent
from gevent import Greenlet

class MyGreenlet(Greenlet):

    def __init__(self, message, n):
        Greenlet.__init__(self)
        self.message = message
        self.n = n

    def _run(self):
        print(self.message)
        gevent.sleep(self.n)

g = MyGreenlet("Hi there!", 3)
g.start()
g.join()

Hi there!

Greenlet State

Like any other segment of code, Greenlets can fail in various ways. A greenlet may fail to throw an exception, fail to halt or consume too many system resources.

The internal state of a greenlet is generally a time-dependent parameter. There are a number of flags on greenlets which let you monitor the state of the thread:

• started -- Boolean, indicates whether the Greenlet has been started
• ready() -- Boolean, indicates whether the Greenlet has halted
• successful() -- Boolean, indicates whether the Greenlet has halted and not thrown an exception
• value -- arbitrary, the value returned by the Greenlet
• exception -- exception, uncaught exception instance thrown inside the greenlet

import gevent

def win():
    return 'You win!'

def fail():
    raise Exception('You fail at failing.')

winner = gevent.spawn(win)
loser = gevent.spawn(fail)

print(winner.started) # True
print(loser.started)  # True

# Exceptions raised in the Greenlet, stay inside the Greenlet.
try:
    gevent.joinall([winner, loser])
except Exception as e:
    print('This will never be reached')

print(winner.value) # 'You win!'
print(loser.value)  # None

print(winner.ready()) # True
print(loser.ready())  # True

print(winner.successful()) # True
print(loser.successful())  # False

# The exception raised in fail, will not propogate outside the
# greenlet. A stack trace will be printed to stdout but it
# will not unwind the stack of the parent.

print(loser.exception)

# It is possible though to raise the exception again outside
# raise loser.exception
# or with
# loser.get()

True
True
You win!
None
True
True
True
False
You fail at failing.

Program Shutdown

Greenlets that fail to yield when the main program receives a SIGQUIT may hold the program's execution longer than expected. This results in so called "zombie processes" which need to be killed from outside of the Python interpreter.

A common pattern is to listen SIGQUIT events on the main program and to invoke gevent.shutdown before exit.

import gevent
import signal

def run_forever():
    gevent.sleep(1000)

if __name__ == '__main__':
    gevent.signal(signal.SIGQUIT, gevent.shutdown)
    thread = gevent.spawn(run_forever)
    thread.join()

Timeouts

Timeouts are a constraint on the runtime of a block of code or a Greenlet.

import gevent
from gevent import Timeout

seconds = 10

timeout = Timeout(seconds)
timeout.start()

def wait():
    gevent.sleep(10)

try:
    gevent.spawn(wait).join()
except Timeout:
    print 'Could not complete'

They can also be used with a context manager, in a with statement.

import gevent
from gevent import Timeout

time_to_wait = 5 # seconds

class TooLong(Exception):
    pass

with Timeout(time_to_wait, TooLong):
    gevent.sleep(10)

In addition, gevent also provides timeout arguments for a variety of Greenlet and data stucture related calls. For example:

import gevent
from gevent import Timeout

def wait():
    gevent.sleep(2)

timer = Timeout(1).start()
thread1 = gevent.spawn(wait)

try:
    thread1.join(timeout=timer)
except Timeout:
    print('Thread 1 timed out')

# --

timer = Timeout.start_new(1)
thread2 = gevent.spawn(wait)

try:
    thread2.get(timeout=timer)
except Timeout:
    print('Thread 2 timed out')

# --

try:
    gevent.with_timeout(1, wait)
except Timeout:
    print('Thread 3 timed out')

Thread 1 timed out
Thread 2 timed out
Thread 3 timed out

Monkeypatching

Alas we come to dark corners of Gevent. I've avoided mentioning monkey patching up until now to try and motivate the powerful coroutine patterns but the time has come to discuss the dark arts of monkey-patching. If you noticed above we invoked the commnad monkey.patch_socket(). This is a purely side-effectful command to modify the standard library's socket library

import socket
print( socket.socket )

print "After monkey patch"
from gevent import monkey
monkey.patch_socket()
print( socket.socket )

import select
print select.select
monkey.patch_select()
print "After monkey patch"
print( select.select )

class 'socket.socket'
After monkey patch
class 'gevent.socket.socket'

built-in function select
After monkey patch
function select at 0x1924de8

Python's runtime allows for most objects to be modified at runtime including modules, classes, and even functions. This is generally an astoudingly bad idea since it creates an "implicit side-effect" that is most often extremely difficult to debug if problems occur, nevertheless in extreme situations where a library needs to alter the fundamental behavior of Python itself monkey patches can be used. In this case gevent is capable of patching most of the blocking system calls in the standard library including those in socket, ssl, threading and select modules to instead behave cooperatively.

For example, the Redis python bindings normally uses regular tcp sockets to communicate with the redis-server instance. Simply by invoking gevent.monkey.patch_all() we can make the redis bindings schedule requests cooperatively and work with the rest of our gevent stack.

This lets us integrate libraries that would not normally work with gevent without ever writing a single line of code. While monkey-patching is still evil, in this case it is a "useful evil".

Data Structures

Events

Events are a form of asynchronous communication between Greenlets.

import gevent
from gevent.event import AsyncResult

a = AsyncResult()

def setter():
    """
    After 3 seconds set wake all threads waiting on the value of
    a.
    """
    gevent.sleep(3)
    a.set()

def waiter():
    """
    After 3 seconds the get call will unblock.
    """
    a.get() # blocking
    print 'I live!'

gevent.joinall([
    gevent.spawn(setter),
    gevent.spawn(waiter),
])

A extension of the Event object is the AsyncResult which allows you to send a value along with the wakeup call. This is sometimes called a future or a deferred, since it holds a reference to a future value that can be set on an arbitrary time schedule.

import gevent
from gevent.event import AsyncResult
a = AsyncResult()

def setter():
    """
    After 3 seconds set the result of a.
    """
    gevent.sleep(3)
    a.set('Hello!')

def waiter():
    """
    After 3 seconds the get call will unblock after the setter
    puts a value into the AsyncResult.
    """
    print a.get()

gevent.joinall([
    gevent.spawn(setter),
    gevent.spawn(waiter),
])

Queues

Queues are ordered sets of data that have the usual put / get operations but are written in a way such that they can be safely manipulated across Greenlets.

For example if one Greenlet grabs an item off of the queue, the same item will not grabbed by another Greenlet executing simultaneously.

import gevent
from gevent.queue import Queue

tasks = Queue()

def worker(n):
    while not tasks.empty():
        task = tasks.get()
        print('Worker %s got task %s' % (n, task))
        gevent.sleep(0)

    print('Quitting time!')

def boss():
    for i in xrange(1,25):
        tasks.put_nowait(i)

gevent.spawn(boss).join()

gevent.joinall([
    gevent.spawn(worker, 'steve'),
    gevent.spawn(worker, 'john'),
    gevent.spawn(worker, 'nancy'),
])

Worker steve got task 1
Worker john got task 2
Worker nancy got task 3
Worker steve got task 4
Worker nancy got task 5
Worker john got task 6
Worker steve got task 7
Worker john got task 8
Worker nancy got task 9
Worker steve got task 10
Worker nancy got task 11
Worker john got task 12
Worker steve got task 13
Worker john got task 14
Worker nancy got task 15
Worker steve got task 16
Worker nancy got task 17
Worker john got task 18
Worker steve got task 19
Worker john got task 20
Worker nancy got task 21
Worker steve got task 22
Worker nancy got task 23
Worker john got task 24
Quitting time!
Quitting time!
Quitting time!

Queues can also block on either put or get as the need arises.

Each of the put and get operations has a non-blocking counterpart, put_nowait and get_nowait which will not block, but instead raise either gevent.queue.Empty or gevent.queue.Full in the operation is not possible.

In this example we have the boss running simultaneously to the workers and have a restriction on the Queue preventing it from containing more than three elements. This restriction means that the put operation will block until there is space on the queue. Conversely the get operation will block if there are no elements on the queue to fetch, it also takes a timeout argument to allow for the queue to exit with the exception gevent.queue.Empty if no work can found within the time frame of the Timeout.

import gevent
from gevent.queue import Queue, Empty

tasks = Queue(maxsize=3)

def worker(n):
    try:
        while True:
            task = tasks.get(timeout=1) # decrements queue size by 1
            print('Worker %s got task %s' % (n, task))
            gevent.sleep(0)
    except Empty:
        print('Quitting time!')

def boss():
    """
    Boss will wait to hand out work until a individual worker is
    free since the maxsize of the task queue is 3.
    """

    for i in xrange(1,10):
        tasks.put(i)
    print('Assigned all work in iteration 1')

    for i in xrange(10,20):
        tasks.put(i)
    print('Assigned all work in iteration 2')

gevent.joinall([
    gevent.spawn(boss),
    gevent.spawn(worker, 'steve'),
    gevent.spawn(worker, 'john'),
    gevent.spawn(worker, 'bob'),
])

Worker steve got task 1
Worker john got task 2
Worker bob got task 3
Worker steve got task 4
Worker bob got task 5
Worker john got task 6
Assigned all work in iteration 1
Worker steve got task 7
Worker john got task 8
Worker bob got task 9
Worker steve got task 10
Worker bob got task 11
Worker john got task 12
Worker steve got task 13
Worker john got task 14
Worker bob got task 15
Worker steve got task 16
Worker bob got task 17
Worker john got task 18
Assigned all work in iteration 2
Worker steve got task 19
Quitting time!
Quitting time!
Quitting time!

Groups and Pools

A group is a collection of running greenlets which are managed and scheduled together as group. It also doubles as parallel dispatcher that mirrors the Python multiprocessing library.

import gevent
from gevent.pool import Group

def talk(msg):
    for i in xrange(3):
        print(msg)

g1 = gevent.spawn(talk, 'bar')
g2 = gevent.spawn(talk, 'foo')
g3 = gevent.spawn(talk, 'fizz')

group = Group()
group.add(g1)
group.add(g2)
group.join()

group.add(g3)
group.join()

bar
bar
bar
foo
foo
foo
fizz
fizz
fizz

This is very useful for managing groups of asynchronous tasks.

As mentioned above, Group also provides an API for dispatching jobs to grouped greenlets and collecting their results in various ways.

import gevent
from gevent import getcurrent
from gevent.pool import Group

group = Group()

def hello_from(n):
    print('Size of group', len(group))
    print('Hello from Greenlet %s' % id(getcurrent()))

group.map(hello_from, xrange(3))

def intensive(n):
    gevent.sleep(3 - n)
    return 'task', n

print('Ordered')

ogroup = Group()
for i in ogroup.imap(intensive, xrange(3)):
    print(i)

print('Unordered')

igroup = Group()
for i in igroup.imap_unordered(intensive, xrange(3)):
    print(i)

Size of group 3
Hello from Greenlet 14533328
Size of group 3
Hello from Greenlet 14533808
Size of group 3
Hello from Greenlet 14534768
Ordered
('task', 0)
('task', 1)
('task', 2)
Unordered
('task', 2)
('task', 1)
('task', 0)

A pool is a structure designed for handling dynamic numbers of greenlets which need to be concurrency-limited. This is often desirable in cases where one wants to do many network or IO bound tasks in parallel.

import gevent
from gevent.pool import Pool

pool = Pool(2)

def hello_from(n):
    print('Size of pool', len(pool))

pool.map(hello_from, xrange(3))

Size of pool 2
Size of pool 2
Size of pool 1

Often when building gevent driven services one will center the entire service around a pool structure. An example might be a class which polls on various sockets.

from gevent.pool import Pool

class SocketPool(object):

    def __init__(self):
        self.pool = Pool(1000)
        self.pool.start()

    def listen(self, socket):
        while True:
            socket.recv()

    def add_handler(self, socket):
        if self.pool.full():
            raise Exception("At maximum pool size")
        else:
            self.pool.spawn(self.listen, socket)

    def shutdown(self):
        self.pool.kill()

Locks and Semaphores

A semaphore is a low level synchronization primitive that allows greenlets to coordinate and limit concurrent access or execution. A semaphore exposes two methods, acquire and release The difference between the number of times and a semaphore has been acquired and released is called the bound of the semaphore. If a semaphore bound reaches 0 it will block until another greenlet releases its acquisition.

from gevent import sleep
from gevent.pool import Pool
from gevent.coros import BoundedSemaphore

sem = BoundedSemaphore(2)

def worker1(n):
    sem.acquire()
    print('Worker %i acquired semaphore' % n)
    sleep(0)
    sem.release()
    print('Worker %i released semaphore' % n)

def worker2(n):
    with sem:
        print('Worker %i acquired semaphore' % n)
        sleep(0)
    print('Worker %i released semaphore' % n)

pool = Pool()
pool.map(worker1, xrange(0,2))
pool.map(worker2, xrange(3,6))

Worker 0 acquired semaphore
Worker 1 acquired semaphore
Worker 0 released semaphore
Worker 1 released semaphore
Worker 3 acquired semaphore
Worker 4 acquired semaphore
Worker 3 released semaphore
Worker 4 released semaphore
Worker 5 acquired semaphore
Worker 5 released semaphore

A semaphore with bound of 1 is known as a Lock. it provides exclusive execution to one greenlet. They are often used to ensure that resources are only in use at one time in the context of a program.

Thread Locals

Gevent also allows you to specify data which is local to the greenlet context. Internally, this is implemented as a global lookup which addresses a private namespace keyed by the greenlet's getcurrent() value.

import gevent
from gevent.local import local

stash = local()

def f1():
    stash.x = 1
    print(stash.x)

def f2():
    stash.y = 2
    print(stash.y)

    try:
        stash.x
    except AttributeError:
        print("x is not local to f2")

g1 = gevent.spawn(f1)
g2 = gevent.spawn(f2)

gevent.joinall([g1, g2])

1
2
x is not local to f2

Many web framework thats integrate with gevent store HTTP session objects inside of gevent thread locals. For example using the Werkzeug utility library and its proxy object we can create Flask style request objects.

from gevent.local import local
from werkzeug.local import LocalProxy
from werkzeug.wrappers import Request
from contextlib import contextmanager

from gevent.wsgi import WSGIServer

_requests = local()
request = LocalProxy(lambda: _requests.request)

@contextmanager
def sessionmanager(environ):
    _requests.request = Request(environ)
    yield
    _requests.request = None

def logic():
    return "Hello " + request.remote_addr

def application(environ, start_response):
    status = '200 OK'

    with sessionmanager(environ):
        body = logic()

    headers = [
        ('Content-Type', 'text/html')
    ]

    start_response(status, headers)
    return [body]

WSGIServer(('', 8000), application).serve_forever()

import gevent
from gevent.subprocess import Popen, PIPE

# Uses a green pipe which is cooperative
sub = Popen(['uname'], stdout=PIPE)
read_output = gevent.spawn(sub.stdout.read)

output = read_output.join()
print(output.value)

Linux

import gevent
from multiprocessing import Process, Pipe
from gevent.socket import wait_read, wait_write

# To Process
a, b = Pipe()

# From Process
c, d = Pipe()

def relay():
    for i in xrange(10):
        msg = b.recv()
        c.send(msg + " in " + str(i))

def put_msg():
    for i in xrange(10):
        wait_write(a.fileno())
        a.send('hi')

def get_msg():
    for i in xrange(10):
        wait_read(d.fileno())
        print(d.recv())

if __name__ == '__main__':
    proc = Process(target=relay)
    proc.start()

    g1 = gevent.spawn(get_msg)
    g2 = gevent.spawn(put_msg)
    gevent.joinall([g1, g2], timeout=1)

import gevent
from gevent.queue import Queue

class Actor(gevent.Greenlet):

    def __init__(self):
        self.inbox = Queue()
        Greenlet.__init__(self)

    def receive(self, message):
        """
        Define in your subclass.
        """
        raise NotImplemented()

    def _run(self):
        self.running = True

        while self.running:
            message = self.inbox.get()
            self.receive(message)

import gevent
from gevent.queue import Queue
from gevent import Greenlet

class Pinger(Actor):
    def receive(self, message):
        print message
        pong.inbox.put('ping')
        gevent.sleep(0)

class Ponger(Actor):
    def receive(self, message):
        print message
        ping.inbox.put('pong')
        gevent.sleep(0)

ping = Pinger()
pong = Ponger()

ping.start()
pong.start()

ping.inbox.put('start')
gevent.joinall([ping, pong])

# Note: Remember to ``pip install pyzmq gevent_zeromq``
import gevent
from gevent_zeromq import zmq

# Global Context
context = zmq.Context()

def server():
    server_socket = context.socket(zmq.REQ)
    server_socket.bind("tcp://127.0.0.1:5000")

    for request in range(1,10):
        server_socket.send("Hello")
        print('Switched to Server for ', request)
        # Implicit context switch occurs here
        server_socket.recv()

def client():
    client_socket = context.socket(zmq.REP)
    client_socket.connect("tcp://127.0.0.1:5000")

    for request in range(1,10):

        client_socket.recv()
        print('Switched to Client for ', request)
        # Implicit context switch occurs here
        client_socket.send("World")

publisher = gevent.spawn(server)
client    = gevent.spawn(client)

gevent.joinall([publisher, client])

Switched to Server for  1
Switched to Client for  1
Switched to Server for  2
Switched to Client for  2
Switched to Server for  3
Switched to Client for  3
Switched to Server for  4
Switched to Client for  4
Switched to Server for  5
Switched to Client for  5
Switched to Server for  6
Switched to Client for  6
Switched to Server for  7
Switched to Client for  7
Switched to Server for  8
Switched to Client for  8
Switched to Server for  9
Switched to Client for  9

# On Unix: Access with ``$ nc 127.0.0.1 5000`` 
# On Window: Access with ``$ telnet 127.0.0.1 5000``

from gevent.server import StreamServer

def handle(socket, address):
    socket.send("Hello from a telnet!\n")
    for i in range(5):
        socket.send(str(i) + '\n')
    socket.close()

server = StreamServer(('127.0.0.1', 5000), handle)
server.serve_forever()

HTTP/1.1 200 OK
Content-Type: text/plain
Transfer-Encoding: chunked

8
<p>Hello

9
World</p>

0

from gevent.wsgi import WSGIServer

def application(environ, start_response):
    status = '200 OK'
    body = '<p>Hello World</p>'

    headers = [
        ('Content-Type', 'text/html')
    ]

    start_response(status, headers)
    return [body]

WSGIServer(('', 8000), application).serve_forever()

from gevent.pywsgi import WSGIServer

def application(environ, start_response):
    status = '200 OK'

    headers = [
        ('Content-Type', 'text/html')
    ]

    start_response(status, headers)
    yield "<p>Hello"
    yield "World</p>"

WSGIServer(('', 8000), application).serve_forever()

$ ab -n 10000 -c 100 http://127.0.0.1:8000/

import gevent
from gevent.queue import Queue, Empty
from gevent.pywsgi import WSGIServer
import simplejson as json

data_source = Queue()

def producer():
    while True:
        data_source.put_nowait('Hello World')
        gevent.sleep(1)

def ajax_endpoint(environ, start_response):
    status = '200 OK'
    headers = [
        ('Content-Type', 'application/json')
    ]

    start_response(status, headers)

    while True:
        try:
            datum = data_source.get(timeout=5)
            yield json.dumps(datum) + '\n'
        except Empty:
            pass

gevent.spawn(producer)

WSGIServer(('', 8000), ajax_endpoint).serve_forever()

# Simple gevent-websocket server
import json
import random

from gevent import pywsgi, sleep
from geventwebsocket.handler import WebSocketHandler

class WebSocketApp(object):
    '''Send random data to the websocket'''

    def __call__(self, environ, start_response):
        ws = environ['wsgi.websocket']
        x = 0
        while True:
            data = json.dumps({'x': x, 'y': random.randint(1, 5)})
            ws.send(data)
            x += 1
            sleep(0.5)

server = pywsgi.WSGIServer(("", 10000), WebSocketApp(),
    handler_class=WebSocketHandler)
server.serve_forever()

<html>
    <head>
        <title>Minimal websocket application</title>
        <script type="text/javascript" src="jquery.min.js"></script>
        <script type="text/javascript">
        $(function() {
            // Open up a connection to our server
            var ws = new WebSocket("ws://localhost:10000/");

            // What do we do when we get a message?
            ws.onmessage = function(evt) {
                $("#placeholder").append('<p>' + evt.data + '</p>')
            }
            // Just update our conn_status field with the connection status
            ws.onopen = function(evt) {
                $('#conn_status').html('<b>Connected</b>');
            }
            ws.onerror = function(evt) {
                $('#conn_status').html('<b>Error</b>');
            }
            ws.onclose = function(evt) {
                $('#conn_status').html('<b>Closed</b>');
            }
        });
    </script>
    </head>
    <body>
        <h1>WebSocket Example</h1>
        <div id="conn_status">Not Connected</div>
        <div id="placeholder" style="width:600px;height:300px;"></div>
    </body>
</html>

# Micro gevent chatroom.
# ----------------------

from flask import Flask, render_template, request

from gevent import queue
from gevent.pywsgi import WSGIServer

import simplejson as json

app = Flask(__name__)
app.debug = True

rooms = {
    'topic1': Room(),
    'topic2': Room(),
}

users = {}

class Room(object):

    def __init__(self):
        self.users = set()
        self.messages = []

    def backlog(self, size=25):
        return self.messages[-size:]

    def subscribe(self, user):
        self.users.add(user)

    def add(self, message):
        for user in self.users:
            print user
            user.queue.put_nowait(message)
        self.messages.append(message)

class User(object):

    def __init__(self):
        self.queue = queue.Queue()

@app.route('/')
def choose_name():
    return render_template('choose.html')

@app.route('/<uid>')
def main(uid):
    return render_template('main.html',
        uid=uid,
        rooms=rooms.keys()
    )

@app.route('/<room>/<uid>')
def join(room, uid):
    user = users.get(uid, None)

    if not user:
        users[uid] = user = User()

    active_room = rooms[room]
    active_room.subscribe(user)
    print 'subscribe', active_room, user

    messages = active_room.backlog()

    return render_template('room.html',
        room=room, uid=uid, messages=messages)

@app.route("/put/<room>/<uid>", methods=["POST"])
def put(room, uid):
    user = users[uid]
    room = rooms[room]

    message = request.form['message']
    room.add(':'.join([uid, message]))

    return ''

@app.route("/poll/<uid>", methods=["POST"])
def poll(uid):
    try:
        msg = users[uid].queue.get(timeout=10)
    except queue.Empty:
        msg = []
    return json.dumps(msg)

if __name__ == "__main__":
    http = WSGIServer(('', 5000), app)
    http.serve_forever()