Server::Starter
a superdaemon to hot-deploy server programs
Cybozu Labs, Inc.
Kazuho Oku

Hot deployment
 what is it?
 upgrading web application without restarting the
application server
 the goals
 no downtime
 no resource leaks
 fail-safe
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 2

Current techniques
 restart the interpreter (mod_perl)
 pros: graceful
 cons: XS may cause resource leaks, service-down
on deployment failure, cannot implement in pure-
perl
 bind to unix socket (FastCGI)
 pros: graceful, fail-safe
 cons: only useful for local-machine
communication
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 3

Current techniques (cont'd)
 exec(myself) (Net::Server)
 pros: graceful, pure-perl
 cons: file descriptor leaks, service-down on
deployment failure
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 4

Server::Starter
 a superdaemon for hot-deploying TCP servers
 superdaemon binds to TCP ports, then spawns
the application server
listen
accept
fork & exec
SIGTERM
app. logic
accept
spawn
app. servers
fork & exec
app. logic
SIGHUP
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 5

Reaching the Goals
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 6

No downtime
 listening socket shared by old and new
generation app. servers
 old app. servers receive SIGTERM after
new servers start
listen
accept
fork & exec
SIGTERM
accept
logic
app.
spawn
app. servers
fork & exec
app. logic
SIGHUP
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 7

No resource leaks
 no chance of resource leaks
 every generation of app. servers spawned from
superdaemon
listen
accept
fork & exec
SIGTERM
accept
logic
app.
spawn
app. servers
fork & exec
app. logic
SIGHUP
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 8

Fail-safe
 old app. server retired if and only if the
new app. server starts up successfully
 service continues even if the updated app. server
fails to start, in cases like missing modules, etc.
 a good practice is to do self-testing in the app.
server before starting to serve client connections
 is also an efficient way to preload modules
listen
accept
fork & exec
SIGTERM
accept
logic
app.
spawn
app. servers
fork & exec
app. logic
SIGHUP
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 9

Demo
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 10

Using Server::Starter
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 11

The Low-level Code
# from command line
% start_server --port=80 my_httpd
# in my_httpd
use Server::Starter qw(server_ports);
my $listen_sock = IO::Socket::INET->new(
Proto => 'tcp',
);
$listen_sock->fdopen((values %{server_ports()})[0], 'w')
or die "failed to bind to listening socket:$!";
while (1) {
if (my $conn = $listen_sock->accept) {
....
}
}
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 12

Net::Server::SS::PreFork
 subclass of Net::Server::PreFork
# from command line
% start_server --port=80 my_server.pl
# in my_server.pl
use base qw(Net::Server::SS::PreFork);
sub process_request {
#...code...
}
__PACKAGE__->run();
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 13

Using together with HTTP::Server::Simple
 HTTP::Server::Simple can use
Net::Server::SS::PreFork as a backend
 and many WAFs support HTTP::Server::Simple
package MyServer;
use base qw(HTTP::Server::Simple::CGI);
sub net_server { 'Net::Server::SS::PreFork' };
sub handle_request {
print "HTTP/1.0 200 HOKrnContent-Type: text/plainrnrnHello World";
}
1;
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 14

Using together with PSGI / Plack
 started writing Plack::Impl::SSPreFork
 on my github fork of Plack
 uid, etc. aren't configurable yet :-(
# from command line
% start_server --port=80 -- plackup -i SSPreFork MyApp.pm
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 15

Launching from daemontools
 daemontools
 a (better) alternative to init.d scripts, by DJB
 start_server script is designed to be run
under daemontools
 restart using –h (SIGHUP)
 all logs to STDERR
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 16

ToDo
 Support for FastCGI
 although ... (ry
 init.d-style startup mode
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 17

Conclusion
 with Server::Starter, it is easy to write
hot-deployable TCP servers
Sep 10 2009 Server::Starter - a superdaemon to hot-deploy server programs 18