+
+
+s6
+Software
+skarnet.org
+
+
+ How do I perform socket activation with s6 ?
+
+
+ First, it's important to realize that you don't need
+socket activation. It's a marketing word used by systemd
+advocates that mixes a couple useful architecture concepts and several
+horrible ideas, for a very minor speed benefit. Read
+this mail
+and
+this
+post for details.
+
+
+
+ - s6 will not help you implement super-servers in process 1,
+because doing so is bad engineering.
+However, it will help you set up super-servers. The
+s6-ipcserver
+program, for Unix domain sockets, as well as the
+s6-tcpserver4 and
+s6-tcpserver6
+programs, for TCP INET domain sockets (available in the
+s6-networking
+package) are super-servers you can use to
+your heart's content. They are even wrappers around simpler programs,
+and you can use their components in the way you choose: bind sockets,
+drop privileges, accept connections from clients, it's all about what you
+write in your command line. Super-servers are a good thing; using process 1
+to act as a super-server is not. s6 provides you with the tools to get
+the good without the bad.
+ - s6 will not help you run all your services before their
+dependencies are met, because doing so is very bad engineering.
+However, it will provide you with:
+
+ - s6 will not help you centralize all your socket information
+in process 1, because doing so is contrary to modularity and independence
+of services. However, s6
+will provide you with a way to store your open sockets and
+retrieve them when you want, which it calls "fd holding":
+s6-fdholder-daemon.
+
+
+ So, how do I open all my sockets first, store them, and dispatch them
+to daemons later ?
+
+
+ Again, it's not necessary to do that: you'll be fine, and quite speedy,
+just starting your
+daemons in their good time. You will not reap any noticeable
+benefit from performing "socket activation". But if you really want to:
+
+
+
+ - Make sure you have an early supervision infrastructure running.
+Ideally, you would make s6-svscan your
+process 1.
+ - Start an early fd-holding
+service. Let's say the fd-holding daemon is listening on socket
+/service/fdholder/s.
+ - For every Unix domain socket /my/socket you need to open, run
+s6-ipcserver-socketbinder /my/socket s6-fdholder-store /service/fdholder/s
+unix:/my/socket. You can do the same with INET domain sockets.
+ - Proceed to your initialization.
+ - When you want to run a daemon myserverd that accepts clients
+connecting to /my/socket, run s6-fdholder-retrieve
+/service/fdholder/s unix:/my/socket myserverd. myserverd
+will be executed with /my/socket as its standard input.
+ - The descriptors remain safely stored in the fd-holding daemon and you
+can retrieve them again whenever you want, for instance when your service
+crashes and is restarted.
+
+
+
+ That is all there is to it. You don't have to use specific libraries
+or write complex unit files, you just need to understand how a command
+line works. This is Unix.
+
+
+
+