uWSGI Mules

Mules are worker processes living in the uWSGI stack but not reachable via socket connections, that can be used as a generic subsystem to offload tasks. You can see them as a more primitive spooler.

They can access the entire uWSGI API and can manage signals and be communicated with through a simple string-based message system.

To start a mule (you can start an unlimited number of them), use the mule option as many times as you need.

Mules have two modes,

  • Signal only mode (the default). In this mode the mules load your application as normal workers would. They can only respond to uWSGI signals.
  • Programmed mode. In this mode mules load a program separate from your application. See ProgrammedMules.

By default each mule starts in signal-only mode.

uwsgi --socket :3031 --mule --mule --mule --mule

Basic usage

import uwsgi
from uwsgidecorators import timer, signal, filemon

# run a timer in the first available mule
@timer(30, target='mule')
def hello(signum):
    print "Hi! I am responding to signal %d, running on mule %d" % (signum, uwsgi.mule_id())

# map signal 17 to mule 2
@signal(17, target='mule2')
def i_am_mule2(signum):
    print "Greetings! I am running in mule number two."

# monitor /tmp and arouse all of the mules on modifications
@filemon('/tmp', target='mules')
def tmp_modified(signum):
    print "/tmp has been modified. I am mule %d!" % uwsgi.mule_id()

Giving a brain to mules

As mentioned before, mules can be programmed. To give custom logic to a mule, give the mule option a path to a script (it must end in ”.py”) or a “package.module:callable” value.

uwsgi --socket :3031 --mule=somaro.py --mule=mypkg.myapp:run_mule --mule --mule

This will run 4 mules, 2 in signal-only mode, one running somaro.py and one running mypkg.myapp:run_mule.

# somaro.py
from threading import Thread
import time

def loop1():
    while True:
        print "loop1: Waiting for messages... yawn."
        message = uwsgi.mule_get_msg()
        print message

def loop2():
    print "Hi! I am loop2."
    while True:
        print "This is a thread!"

t = Thread(target=loop2)
t.daemon = True

if __name__ == '__main__':

So as you can see from the example, you can use mule_get_msg() to receive messages in a programmed mule. Multiple threads in the same programmed mule can wait for messages.

If you want to block a mule to wait on an uWSGI signal instead of a message you can use uwsgi.signal_wait().

Use uwsgi.mule_msg() to send a message to a programmed mule. Mule messages can be sent from anywhere in the uWSGI stack, including but not limited to workers, the spoolers, another mule.

# Send the string "ciuchino" to mule1.
# If you do not specify a mule ID, the message will be processed by the first available programmed mule.
uwsgi.mule_msg("ciuchino", 1)

As you can spawn an unlimited number of mules, you may need some form of synchronization – for example if you are developing a task management subsystem and do not want two mules to be able to start the same task simultaneously. You’re in luck – see Locks.