Server::Starter is a superdaemon that allows for hot deployment of server programs without downtime or resource leaks. It works by binding to TCP ports and spawning application server processes to handle connections while signaling old processes to shutdown when new ones are launched. This allows servers to be upgraded without interrupting service and prevents issues like file descriptor leaks that can occur with other hot deployment techniques. The superdaemon approach ensures new processes launch successfully before retiring old ones, providing a fail-safe deployment mechanism.

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

2. Hot deployment
 what is it?
 upgrading web application without restarting the
application server
 the goals
 no downtime
 no resource leaks
 fail-safe
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3. 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
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4. Current techniques (cont'd)
 exec(myself) (Net::Server)
 pros: graceful, pure-perl
 cons: file descriptor leaks, service-down on
deployment failure
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5. 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
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6. Reaching the Goals
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7. 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
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8. 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
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9. 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
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10. Demo
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11. Using Server::Starter
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12. 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) {
....
}
}
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13. 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();
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14. 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;
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15. 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
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16. 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
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17. ToDo
 Support for FastCGI
 although ... (ry
 init.d-style startup mode
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18. Conclusion
 with Server::Starter, it is easy to write
hot-deployable TCP servers
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