For those unaware: systemd is a replacement for the traditional unix init process. It is the first process to be brought up by the kernel, and is responsible for all the user-space tasks in booting a system. Conveniently, it also has a --user mode switch that allows you to use it yourself as a session-level init service. As someone who hates being root and loves repeatable configuration, I’m tremendously pleased to be able to offload as much of my session management as I can to something that’s built for the task (and doesn’t rely on hard-coded, root-owned paths).

I’ve heard a lot of complaining from sysadmins who seem to prefer a tangled mess of shell scripts in /etc/init.d/, but I’m a big fan of systemd for its user-friendly features - like restarting stuff when it dies, listing running services, showing me logging output, and being able to reliably kill daemons. There are other ways to accomplish all of these, but systemd is easy to use, and works very well in my experience.

Socket activation

Recently I was wondering how to implement systemd socket activation in python. The idea is much like inetd, in which you tell systemd which port to listen on, and it opens up the port for you. When a request comes in, it immediately spawns your server and hands over the open socket to seamlessly allow your program to service the request. That way, services can be started only as necessary, making for a quicker boot and less resource usage for infrequently accessed services.

The question is, of course, how am I supposed to grab a socket from systemd? It turns out it’s not hard, but there are some tricks. For my app, I am using the BaseHTTPServer module. Specifically, the threaded variant that you can construct using multiple inheritance:

class ThreadedHTTPServer(ThreadingMixIn, HTTPServer):

I couldn’t find much existing information on how to do this in python, so I thought I’d write up what I found. One useful resource was this tutorial for ruby, since python and ruby are pretty similar.

Nuts and bolts

Obviously, systemd needs to tell you that it has a socket open for you. The way it does this is by placing your process ID into $LISTEN_PID. So to tell if you should try and use an existing socket, you can check:

if os.environ.get('LISTEN_PID', None) == str(os.getpid()):
	# inherit the socket
	# start the server normally

Given that you normally pass a host and port into the HTTPServer class’ constructor, how can you make it bind to a socket systemd gives you? It turns out to be fairly simple:

class SocketInheritingHTTPServer(ThreadedHTTPServer):
	"""A HttpServer subclass that takes over an inherited socket from systemd"""
	def __init__(self, address_info, handler, fd, bind_and_activate=True):
		ThreadedHTTPServer.__init__(self, address_info, handler, bind_and_activate=False)
		self.socket = socket.fromfd(fd, self.address_family, self.socket_type)
		if bind_and_activate:
			# NOTE: systemd provides ready-bound sockets, so we only need to activate:

You construct this just like you would the normal ThreadedHTTPServer, but with an extra fd keyword argument. It passes bind_and_activate=False to prevent the parent class from binding the socket, overrides the instance’s self.socket, and then activates the server.

The final piece of the puzzle is the somewhat-arbitrary knowledge that systemd passes you in sockets beginning at file descriptor #3. So you can just pass in fd=3 to the SocketInheritingHTTPServer. If you have a server that has multiple ports configured in your .socket file, you can check $LISTEN_FDS

And that’s it! I’ve only just learnt this myself, so I may be missing some detail. But it seems to work just fine. If you want to see the full details, you can have a look at the commit I just made to edit-server, which includes a simple script to simulate what systemd does when it hands you an open socket, for quick testing. You can also have a look at the service files to see how the .socket and .service systemd units are set up.