Post Syndicated from Lennart Poettering original https://0pointer.net/blog/projects/socket-activation.html
systemd
not only brings improvements for administrators and users, it also
brings a (small) number of new APIs with it. In this blog story (which might
become the first of a series) I hope to shed some light on one of the
most important new APIs in systemd:
Socket Activation
In the original blog
story about systemd I tried to explain why socket activation is a
wonderful technology to spawn services. Let’s reiterate the background
here a bit.
The basic idea of socket activation is not new. The inetd
superserver was a standard component of most Linux and Unix systems
since time began: instead of spawning all local Internet services
already at boot, the superserver would listen on behalf of the
services and whenever a connection would come in an instance of the
respective service would be spawned. This allowed relatively weak
machines with few resources to offer a big variety of services at the
same time. However it quickly got a reputation for being somewhat
slow: since daemons would be spawned for each incoming connection a
lot of time was spent on forking and initialization of the services
— once for each connection, instead of once for them all.
Spawning one instance per connection was how inetd was primarily
used, even though inetd actually understood another mode: on the first
incoming connection it would notice this via poll() (or
select()) and spawn a single instance for all future
connections. (This was controllable with the
wait/nowait options.) That way the first connection
would be slow to set up, but subsequent ones would be as fast as with
a standalone service. In this mode inetd would work in a true
on-demand mode: a service would be made available lazily when it was
required.
inetd’s focus was clearly on AF_INET (i.e. Internet) sockets. As
time progressed and Linux/Unix left the server niche and became
increasingly relevant on desktops, mobile and embedded environments
inetd was somehow lost in the troubles of time. Its reputation for
being slow, and the fact that Linux’ focus shifted away from only
Internet servers made a Linux machine running inetd (or one of its newer
implementations, like xinetd) the exception, not the rule.
When Apple engineers worked on optimizing the MacOS boot time they
found a new way to make use of the idea of socket activation: they
shifted the focus away from AF_INET sockets towards AF_UNIX
sockets. And they noticed that on-demand socket activation was only
part of the story: much more powerful is socket activation when used
for all local services including those which need to be started
anyway on boot. They implemented these ideas in launchd, a central building
block of modern MacOS X systems, and probably the main reason why
MacOS is so fast booting up.
But, before we continue, let’s have a closer look what the benefits
of socket activation for non-on-demand, non-Internet services in
detail are. Consider the four services Syslog, D-Bus, Avahi and the
Bluetooth daemon. D-Bus logs to Syslog, hence on traditional Linux
systems it would get started after Syslog. Similarly, Avahi requires
Syslog and D-Bus, hence would get started after both. Finally
Bluetooth is similar to Avahi and also requires Syslog and D-Bus but
does not interface at all with Avahi. Sinceoin a traditional
SysV-based system only one service can be in the process of getting
started at a time, the following serialization of startup would take
place: Syslog → D-Bus → Avahi → Bluetooth (Of course, Avahi and
Bluetooth could be started in the opposite order too, but we have to
pick one here, so let’s simply go alphabetically.). To illustrate
this, here’s a plot showing the order of startup beginning with system
startup (at the top).
Certain distributions tried to improve this strictly serialized
start-up: since Avahi and Bluetooth are independent from each other,
they can be started simultaneously. The parallelization is increased,
the overall startup time slightly smaller. (This is visualized in the
middle part of the plot.)
Socket activation makes it possible to start all four services
completely simultaneously, without any kind of ordering. Since the
creation of the listening sockets is moved outside of the daemons
themselves we can start them all at the same time, and they are able
to connect to each other’s sockets right-away. I.e. in a single step
the /dev/log and /run/dbus/system_bus_socket sockets
are created, and in the next step all four services are spawned
simultaneously. When D-Bus then wants to log to syslog, it just writes
its messages to /dev/log. As long as the socket buffer does
not run full it can go on immediately with what else it wants to do
for initialization. As soon as the syslog service catches up it will
process the queued messages. And if the socket buffer runs full then
the client logging will temporarily block until the socket is writable
again, and continue the moment it can write its log messages. That
means the scheduling of our services is entirely done by the kernel:
from the userspace perspective all services are run at the same time,
and when one service cannot keep up the others needing it will
temporarily block on their request but go on as soon as these
requests are dispatched. All of this is completely automatic and
invisible to userspace. Socket activation hence allows us to
drastically parallelize start-up, enabling simultaneous start-up of
services which previously were thought to strictly require
serialization. Most Linux services use sockets as communication
channel. Socket activation allows starting of clients and servers of
these channels at the same time.
But it’s not just about parallelization. It offers a number of
other benefits:
- We no longer need to configure dependencies explicitly. Since the
sockets are initialized before all services they are simply available,
and no userspace ordering of service start-up needs to take place
anymore. Socket activation hence drastically simplifies configuration
and development of services.
- If a service dies its listening socket stays around, not losing a
single message. After a restart of the crashed service it can continue
right where it left off.
- If a service is upgraded we can restart the service while keeping
around its sockets, thus ensuring the service is continously
responsive. Not a single connection is lost during the upgrade.
- We can even replace a service during runtime in a way that is
invisible to the client. For example, all systems running systemd
start up with a tiny syslog daemon at boot which passes all log
messages written to /dev/log on to the kernel message
buffer. That way we provide reliable userspace logging starting from
the first instant of boot-up. Then, when the actual rsyslog daemon is
ready to start we terminate the mini daemon and replace it with the
real daemon. And all that while keeping around the original logging
socket and sharing it between the two daemons and not losing a single
message. Since rsyslog flushes the kernel log buffer to disk after
start-up all log messages from the kernel, from early-boot and from
runtime end up on disk.
For another explanation of this idea consult the original blog
story about systemd.
Socket activation has been available in systemd since its
inception. On Fedora 15 a number of services have been modified to
implement socket activation, including Avahi, D-Bus and rsyslog (to continue with the example above).
systemd’s socket activation is quite comprehensive. Not only classic
sockets are support but related technologies as well:
- AF_UNIX sockets, in the flavours SOCK_DGRAM, SOCK_STREAM and SOCK_SEQPACKET; both in the filesystem and in the abstract namespace
- AF_INET sockets, i.e. TCP/IP and UDP/IP; both IPv4 and IPv6
- Unix named pipes/FIFOs in the filesystem
- AF_NETLINK sockets, to subscribe to certain kernel features. This
is currently used by udev, but could be useful for other
netlink-related services too, such as audit.
- Certain special files like /proc/kmsg or device nodes like /dev/input/*.
- POSIX Message Queues
A service capable of socket activation must be able to receive its
preinitialized sockets from systemd, instead of creating them
internally. For most services this requires (minimal)
patching. However, since systemd actually provides inetd compatibility
a service working with inetd will also work with systemd — which is
quite useful for services like sshd for example.
So much about the background of socket activation, let’s now have a
look how to patch a service to make it socket activatable. Let’s start
with a theoretic service foobard. (In a later blog post we’ll focus on
real-life example.)
Our little (theoretic) service includes code like the following for
creating sockets (most services include code like this in one way or
another):
/* Source Code Example #1: ORIGINAL, NOT SOCKET-ACTIVATABLE SERVICE */
...
union {
struct sockaddr sa;
struct sockaddr_un un;
} sa;
int fd;
fd = socket(AF_UNIX, SOCK_STREAM, 0);
if (fd < 0) {
fprintf(stderr, "socket(): %m\n");
exit(1);
}
memset(&sa, 0, sizeof(sa));
sa.un.sun_family = AF_UNIX;
strncpy(sa.un.sun_path, "/run/foobar.sk", sizeof(sa.un.sun_path));
if (bind(fd, &sa.sa, sizeof(sa)) < 0) {
fprintf(stderr, "bind(): %m\n");
exit(1);
}
if (listen(fd, SOMAXCONN) < 0) {
fprintf(stderr, "listen(): %m\n");
exit(1);
}
...
A socket activatable service may use the following code instead:
/* Source Code Example #2: UPDATED, SOCKET-ACTIVATABLE SERVICE */
...
#include "sd-daemon.h"
...
int fd;
if (sd_listen_fds(0) != 1) {
fprintf(stderr, "No or too many file descriptors received.\n");
exit(1);
}
fd = SD_LISTEN_FDS_START + 0;
...
systemd might pass you more than one socket (based on
configuration, see below). In this example we are interested in one
only. sd_listen_fds()
returns how many file descriptors are passed. We simply compare that
with 1, and fail if we got more or less. The file descriptors systemd
passes to us are inherited one after the other beginning with fd
#3. (SD_LISTEN_FDS_START is a macro defined to 3). Our code hence just
takes possession of fd #3.
As you can see this code is actually much shorter than the
original. This of course comes at the price that our little service
with this change will no longer work in a non-socket-activation
environment. With minimal changes we can adapt our example to work nicely
both with and without socket activation:
/* Source Code Example #3: UPDATED, SOCKET-ACTIVATABLE SERVICE WITH COMPATIBILITY */
...
#include "sd-daemon.h"
...
int fd, n;
n = sd_listen_fds(0);
if (n > 1) {
fprintf(stderr, "Too many file descriptors received.\n");
exit(1);
} else if (n == 1)
fd = SD_LISTEN_FDS_START + 0;
else {
union {
struct sockaddr sa;
struct sockaddr_un un;
} sa;
fd = socket(AF_UNIX, SOCK_STREAM, 0);
if (fd < 0) {
fprintf(stderr, "socket(): %m\n");
exit(1);
}
memset(&sa, 0, sizeof(sa));
sa.un.sun_family = AF_UNIX;
strncpy(sa.un.sun_path, "/run/foobar.sk", sizeof(sa.un.sun_path));
if (bind(fd, &sa.sa, sizeof(sa)) < 0) {
fprintf(stderr, "bind(): %m\n");
exit(1);
}
if (listen(fd, SOMAXCONN) < 0) {
fprintf(stderr, "listen(): %m\n");
exit(1);
}
}
...
With this simple change our service can now make use of socket
activation but still works unmodified in classic environments. Now,
let’s see how we can enable this service in systemd. For this we have
to write two systemd unit files: one describing the socket, the other
describing the service. First, here’s foobar.socket:
[Socket]
ListenStream=/run/foobar.sk
[Install]
WantedBy=sockets.target
And here’s the matching service file foobar.service:
[Service]
ExecStart=/usr/bin/foobard
If we place these two files in /etc/systemd/system we can
enable and start them:
# systemctl enable foobar.socket
# systemctl start foobar.socket
Now our little socket is listening, but our service not running
yet. If we now connect to /run/foobar.sk the service will be
automatically spawned, for on-demand service start-up. With a
modification of foobar.service we can start our service
already at startup, thus using socket activation only for
parallelization purposes, not for on-demand auto-spawning anymore:
[Service]
ExecStart=/usr/bin/foobard
[Install]
WantedBy=multi-user.target
And now let’s enable this too:
# systemctl enable foobar.service
# systemctl start foobar.service
Now our little daemon will be started at boot and on-demand,
whatever comes first. It can be started fully in parallel with its
clients, and when it dies it will be automatically restarted when it
is used the next time.
A single .socket file can include multiple ListenXXX stanzas, which
is useful for services that listen on more than one socket. In this
case all configured sockets will be passed to the service in the exact
order they are configured in the socket unit file. Also,
you may configure various socket settings in the .socket
files.
In real life it’s a good idea to include description strings in
these unit files, to keep things simple we’ll leave this out of our
example. Speaking of real-life: our next installment will cover an
actual real-life example. We’ll add socket activation to the CUPS
printing server.
The sd_listen_fds() function call is defined in sd-daemon.h
and sd-daemon.c. These
two files are currently drop-in .c sources which projects should
simply copy into their source tree. Eventually we plan to turn this
into a proper shared library, however using the drop-in files allows
you to compile your project in a way that is compatible with socket
activation even without any compile time dependencies on
systemd. sd-daemon.c is liberally licensed, should compile
fine on the most exotic Unixes and the algorithms are trivial enough
to be reimplemented with very little code if the license should
nonetheless be a problem for your project. sd-daemon.c
contains a couple of other API functions besides
sd_listen_fds() that are useful when implementing socket
activation in a project. For example, there’s sd_is_socket()
which can be used to distuingish and identify particular sockets when
a service gets passed more than one.
Let me point out that the interfaces used here are in no way bound
directly to systemd. They are generic enough to be implemented in
other systems as well. We deliberately designed them as simple and
minimal as possible to make it possible for others to adopt similar
schemes.
Stay tuned for the next installment. As mentioned, it will cover a
real-life example of turning an existing daemon into a
socket-activatable one: the CUPS printing service. However, I hope
this blog story might already be enough to get you started if you plan
to convert an existing service into a socket activatable one. We
invite everybody to convert upstream projects to this scheme. If you
have any questions join us on #systemd on freenode.
