With a stateless Twisted app you can scale by adding more instances. Unless you are explicitly offloading to subprocesses you will often have spare cores on the same box as your existing instance. But to exploit them you end up running haproxy or faking a load balancer with iptables. Are there any other options?
With the SO_REUSEPORT socket flag multiple processes can listen on the same port, but this isn't available from twisted (yet). But with systemd and socket activation we can use it today.
As a proof of concept we'll make a 4 core HTTP static web service. In a fresh Ubuntu 16.04 VM install python-twisted-web.
In /etc/systemd/system/web@.socket:
[Unit]
Description=Socket for worker %i
[Socket]
ListenStream = 8080
ReusePort = yes
Service = www@%i.service
[Install]
WantedBy = sockets.target
And in /etc/systemd/system/web@.service
[Unit]
Description=Worker %i
Requires=www@%i.socket
[Service]
Type=simple
ExecStart=/usr/bin/twistd --nodaemon --logfile=- --pidfile= web --port systemd:domain=INET:index:0 --path /tmp
NonBlocking=yes
User=nobody
Group=nobody
Restart=always
Then to get 4 cores:
$ systemctl enable --now www@0.socket
$ systemctl enable --now www@1.socket
$ systemctl enable --now www@2.socket
$ systemctl enable --now www@3.socket
Lets test it. In a python shell:
import urllib
import time
while True:
urllib.urlopen('http://172.16.140.136:8080/').read()
time.sleep(1)
And in another terminal you can tail the logs with journalctl:
$ sudo journalctl -f -u www@*.service
Apr 26 02:43:51 ubuntu twistd[10441]: 2017-04-26 02:43:51-0700 [-] - - - [26/Apr/2017:09:43:51 +0000] "GET / HTTP/1.0" 200 2081 "-" "Python-urllib/1.17"
Apr 26 02:43:52 ubuntu twistd[10441]: 2017-04-26 02:43:52-0700 [-] - - - [26/Apr/2017:09:43:52 +0000] "GET / HTTP/1.0" 200 2081 "-" "Python-urllib/1.17"
Apr 26 02:43:53 ubuntu twistd[10444]: 2017-04-26 02:43:53-0700 [-] - - - [26/Apr/2017:09:43:53 +0000] "GET / HTTP/1.0" 200 2081 "-" "Python-urllib/1.17"
Apr 26 02:43:54 ubuntu twistd[10452]: 2017-04-26 02:43:54-0700 [-] - - - [26/Apr/2017:09:43:54 +0000] "GET / HTTP/1.0" 200 2081 "-" "Python-urllib/1.17"
Apr 26 02:43:55 ubuntu twistd[10452]: 2017-04-26 02:43:55-0700 [-] - - - [26/Apr/2017:09:43:55 +0000] "GET / HTTP/1.0" 200 2081 "-" "Python-urllib/1.17"
Apr 26 02:43:56 ubuntu twistd[10447]: 2017-04-26 02:43:56-0700 [-] - - - [26/Apr/2017:09:43:56 +0000] "GET / HTTP/1.0" 200 2081 "-" "Python-urllib/1.17"
Apr 26 02:43:57 ubuntu twistd[10450]: 2017-04-26 02:43:57-0700 [-] - - - [26/Apr/2017:09:43:57 +0000] "GET / HTTP/1.0" 200 2081 "-" "Python-urllib/1.17"
As you can see the twisted[pid] changes as different cores handle requests.
If you deploy new code you can systemctl restart www@*.service to restart all cores.
systemctl enable will mean the 4 cores are available on next boot, too.