+---
+## "Normal" [userspace] programs
+
+.big[
+1. Parse args.
+2. Read input.
+3. Do stuff.
+4. Write results as output.
+]
+
+---
+class: pizza
+background-image: url(img/pizza.jpg)
+background-size: 100%
+
+--
+```bash
+# orderpizza --crust-type=thin \
+ --toppings=cheese,pepperoni,mushrooms
+```
+
+--
+```perl
+# source code of the orderpizza program
+
+my $order = get_order_from_args(@ARGV);
+
+my $ingredients = gather_ingredients($order);
+
+my $pizza = prepare_and_bake($order, $ingredentials);
+
+print($pizza);
+```
+
+???
+But some programs are long-lived.
+
+---
+## Event-driven programs
+
+.big[
+1. Events happen.
+2. When events happen, some code runs and does something.
+]
+
+???
+At it's core, event-driven programming is this simple.
+
+But there are some complications and things to knows, which is why this talk exists.
+
+---
+class: center, middle
+
+![Help desk](img/helpdesk.jpg)
+
+.small[
+Image by Soniachat8.
+This image is licensed under the [Creative Commons Attribution-Share Alike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
+]
+
+---
+name: graph-eventloop
+class: center, middle
+
+## Event loop
+
+![Event loop](img/eventloop.svg)
+
+???
+We'll refine this model as we go.
+
+---
+class: ex-hwinterrupts
+
+## Hardware interrupts
+
+```bash
+# cat /proc/interrupts
+ CPU0 CPU1
+ 0: 51 0 IR-IO-APIC 2-edge timer
+ 1: 685006 5 IR-IO-APIC 1-edge i8042
+ 8: 0 0 IR-IO-APIC 8-edge rtc0
+ 9: 1724419 6314 IR-IO-APIC 9-fasteoi acpi
+ 12: 12300601 138 IR-IO-APIC 12-edge i8042
+ 16: 0 0 IR-IO-APIC 16-fasteoi i801_smbus
+ 120: 0 0 DMAR-MSI 0-edge dmar0
+ 121: 0 0 DMAR-MSI 1-edge dmar1
+ 122: 7009890 45112 IR-PCI-MSI 327680-edge xhci_hcd
+ 123: 44 3 IR-PCI-MSI 360448-edge mei_me
+ 124: 509 0 IR-PCI-MSI 1048576-edge rtsx_pci
+ 125: 80130 0 IR-PCI-MSI 2621440-edge nvme0q0, nvme0q1
+ 126: 121892439 2961357 IR-PCI-MSI 32768-edge i915
+ 127: 49 100 IR-PCI-MSI 514048-edge snd_hda_intel:card0
+ 128: 0 79412 IR-PCI-MSI 2621441-edge nvme0q2
+
+...
+```
+
+???
+When a HW interrupt happens, the appropriate code to run is locate in the interrupt service routine
+
+---
+class: ex-pressenter
+
+## Your first event-driven program
+
+.middle[
+```perl
+print "Press <Enter> to continue.";
+<STDIN>;
+
+handle_enter_keypress_event();
+```
+]
+
+---
+class: ex-pressenter
+
+## Your first event-driven program
+
+.middle[
+```perl
+print "Press <Enter> to continue.";
+*<STDIN>;
+
+handle_enter_keypress_event();
+```
+]
+
+.big[
+- Wait for an event
+]
+
+---
+class: ex-pressenter
+
+## Your first event-driven program
+
+.middle[
+```perl
+print "Press <Enter> to continue.";
+*<STDIN>;
+
+handle_enter_keypress_event();
+```
+]
+
+.big[
+- Wait for an event
+- Event source
+]
+
+---
+class: ex-pressenter
+
+## Your first event-driven program
+
+.middle[
+```perl
+print "Press <Enter> to continue.";
+<STDIN>;
+
+*handle_enter_keypress_event();
+```
+]
+
+.big[
+- Wait for an event
+- Event source
+- Event handler
+]
+
+---
+class: ex-signals
+
+## Signals
+
+```perl
+use POSIX;
+
+$SIG{USR1} = sub { handle_usr1_signal_event() };
+$SIG{ALRM} = sub { handle_timer_event() };
+$SIG{QUIT} = sub { exit() };
+
+alarm(3);
+
+while (1) { POSIX::pause() }
+```
+
+---
+class: ex-signals
+
+## Signals
+
+```perl
+use POSIX;
+
+*$SIG{USR1} = sub { handle_usr1_signal_event() };
+*$SIG{ALRM} = sub { handle_timer_event() };
+*$SIG{QUIT} = sub { exit() };
+
+alarm(3);
+
+while (1) { POSIX::pause() }
+```
+
+???
+Notice that this program is capable of handling more than just one event handler at a time.
+
+---
+class: ex-signals
+
+## Signals
+
+```perl
+use POSIX;
+
+$SIG{USR1} = sub { handle_usr1_signal_event() };
+$SIG{ALRM} = sub { handle_timer_event() };
+$SIG{QUIT} = sub { exit() };
+
+alarm(3);
+
+*while (1) { POSIX::pause() }
+```
+
+.big[
+- Program yields time to the kernel
+]
+
+???
+This example has a loop, but it's not really doing anything.
+
+But actually it is doing something very important: It's yielding its processing time to the kernel.
+
+And kernel informs the program of events.
+
+You might be able to use `sleep` as well, but some implementations of libc
+in the past have implemented `sleep` using `alarm`.
+
+Don't know how common that is nowadays, but old habbits...
+
+---
+class: ex-gui
+
+## Graphical user interface example
+
+```perl
+use Gtk3 '-init';
+
+my $window = Gtk3::Window->new;
+
+$window->signal_connect('key-press-event' => \&handle_keypress_event);
+$window->signal_connect('delete-event' => \&Gtk3::main_quit);
+
+$window->show_all;
+
+Gtk3::main();
+```
+
+???
+A user interface happens to be a good place to use evented code because humans are spontaneous and unpredictable.
+
+---
+class: ex-gui
+
+## Graphical user interface example
+
+```perl
+use Gtk3 '-init';
+
+my $window = Gtk3::Window->new;
+
+*$window->signal_connect('key-press-event' => \&handle_keypress_event);
+*$window->signal_connect('delete-event' => \&Gtk3::main_quit);
+
+$window->show_all;
+
+Gtk3::main();
+```
+
+.big[
+- Event handlers
+]
+
+---
+class: ex-gui
+
+## Graphical user interface example
+
+```perl
+use Gtk3 '-init';
+
+my $window = Gtk3::Window->new;
+
+$window->signal_connect('key-press-event' => \&handle_keypress_event);
+$window->signal_connect('delete-event' => \&Gtk3::main_quit);
+
+$window->show_all;
+
+*Gtk3::main();
+```
+
+.big[
+- Event handlers
+- Yield and wait for events
+]
+
+---
+class: event-types
+
+## Types of events
+
+- IO
+
+???
+Data available on a socket, or a new connection.
+
+Server or client, data across a wire cannot typically be relied upon to arrive in a predictable fashion, so an
+event-driven architect makes a lot of sense for network applications.
+
+--
+- Signals
+
+???
+As far as my program is concerned, it can receive a signal or message from another program at any time.
+
+--
+- Timer
+
+???
+If I need something to happen to happen in five minutes or at a specific absolute time, using the idea of an alarm clock
+is tempting. I can set an alarm and pretend that the clock itself is a source of events.
+
+--
+- User input
+
+???
+Human beings of course are masters of spontaneity.
+
+Are they going to press a button on the keyboard next, or move the mouse? If my program is connected to a microphone,
+maybe the human is going to start talking to the program. The program has to be ready for anything, so defining and
+accepting "events" for all the different ways that a human can interact with the program is a good way to go.
+
+--
+- Anything that can happen spontaneously in the real world.
+
+???
+Lots of other external systems (besides humans) can "generate" events.
+
+Lot of this requires kernel facilities. Speaking of which, how are these types of things implemented?
+
+---
+class: syscalls
+
+## How event-driven userspace code works
+
+### syscalls
+
+- [`pause`](http://man.he.net/man2/pause) - Sleeps until signal
+
+--
+- [`select`](http://man.he.net/man2/select), [`poll`](http://man.he.net/man2/poll), [`epoll`](http://man.he.net/man7/epoll), [`kqueue`](https://www.freebsd.org/cgi/man.cgi?format=ascii&sektion=2&query=kqueue) - Monitor multiple file descriptors
+
+--
+- [`clock_gettime`](http://man.he.net/man2/clock_gettime) - What time is it now?
+
+---
+class: center, middle
+
+## Reactor pattern
+
+---
+## Reactor pattern
+
+.big[
+- Queues events asynchronously.
+- Demultiplexes and dispatches synchronously.
+]
+
+---
+name: graph-reactor
+class: center, middle
+
+## Reactor pattern
+
+![Reactor](img/reactor.svg)
+
+---
+class: ex-basicreactor1
+
+## Using a reactor
+
+```perl
+my $reactor = My::Reactor->new;
+
+# Set up event handlers
+$reactor->slurp_file($filepath, \&handle_slurp_event);
+$reactor->timer(5, \&handle_timer_event);
+$reactor->listen($socket, \&handle_new_connect_event);
+...
+
+$reactor->run_loop;
+```
+
+---
+class: ex-basicreactor2
+
+## The basic reactor implementation
+
+```perl
+our $timers = [...];
+our $io_handles = [...];
+
+while (1) {
+ my $next_timer = find_next_timer($timers);
+
+ poll($io_handles, $next_timer->time_from_now);
+
+ handle_ready_io_handles($io_handles);
+ handle_expired_timers($timers);
+}
+```
+
+---
+class: ex-basicreactor2
+
+## The basic reactor implementation
+
+```perl
+our $timers = [...];
+our $io_handles = [...];
+
+while (1) {
+ my $next_timer = find_next_timer($timers);
+
+* poll($io_handles, $next_timer->time_from_now);
+
+ handle_ready_io_handles($io_handles);
+ handle_expired_timers($timers);
+}
+```
+
+---
+
+## Reactor examples on CPAN
+
+.big[
+- [`POE::Loop::IO_Poll`](https://metacpan.org/source/POE::Loop::IO_Poll)
+- [`POE::Loop::Select`](https://metacpan.org/source/POE::Loop::Select)
+- [`AnyEvent::Loop`](https://metacpan.org/source/AnyEvent::Loop)
+- [`IO::Async::Loop::Poll`](https://metacpan.org/source/IO::Async::Loop::Poll)
+- [`IO::Async::Loop::Select`](https://metacpan.org/source/IO::Async::Loop::Select)
+- [`Mojo::Reactor::Poll`](https://metacpan.org/source/Mojo::Reactor::Poll)
+]
+
+???
+These links, which will be available to you with the slides, link directly to the source code of these modules on
+metacpan so you can take a look at how they work.
+
+---
+name: not-all-roses
+class: center, middle
+
+![Thorns](img/thorn.jpg)
+
+## Watch out for the thorns...
+
+???
+There are some special considerations you need to take when writing event-driven code.
+
+---
+class: center, middle
+
+## Exceptions for error handling
+
+---
+class: center, middle
+
+### Problem: No exception handler up the call stack
+
+---
+class: ex-exceptions
+
+## Rule: Don't die/throw in event handlers.
+
+--
+### Error callback pattern
+
+```perl
+do_something_asynchronously(
+ callback => sub { ... },
+ on_error => sub { ... },
+);
+```
+
+---
+class: ex-exceptions
+
+## Rule: Don't die/throw in event handlers.
+
+### Use promises
+
+```perl
+my $promise = do_something_asynchronously();
+
+$promise->on_done(sub { ... });
+$promise->on_fail(sub { ... });
+```
+
+---
+class: center, middle
+
+## `SIGPIPE`
+
+---
+class: sigpipe
+## `SIGPIPE`
+
+- Sent to your program when it writes to a pipe that was closed.
+
+--
+- Default signal handler terminates the program.
+
+---
+class: ex-sigpipe
+
+## Solution: Ignore `SIGPIPE`
+
+```perl
+$SIG{PIPE} = 'IGNORE';
+```
+
+???
+Some event loops do this for you.
+
+--
+.big[
+Look for `EPIPE` from syscalls (like [`write`](http://man.he.net/man2/write)) instead.
+
+(You *are* checking return codes from your system calls... right?)
+]
+
+---
+class: center, middle
+
+## Use [`Future::AsyncAwait`](https://metacpan.org/pod/Future::AsyncAwait).
+
+???
+If you have used JavaScript recently, you may have used its "async/await" feature to clean up your non-blocking code.
+
+---
+class: center, middle
+
+### Yes, Perl can do it, too!
+
+---
+class: ex-asyncawait
+
+## Without async and await
+
+```perl
+use Future;
+
+sub do_two_things {
+ return do_first_thing()->then(sub {
+ my $first = shift;
+
+ return do_second_thing($first)->then(sub {
+ my $second = shift;
+
+ return Future->done([$first, $second]);
+ });
+ });
+}
+```
+
+---
+class: ex-asyncawait
+
+## With async and await
+
+```perl
+use Future::AsyncAwait;
+
+async sub do_two_things
+{
+ my $first = await do_first_thing();
+
+ my $second = await do_second_thing($first);
+
+ return [$first, $second];
+}
+```
+
+???
+There are caveats: Localized variable assignments don't work, nor anything that has implied local-like behavior.
+---
+
+## Events in the world
+
+
+