This document discusses event loops and how they work. Event loops allow non-blocking operations by listening for events like network data or user input in parallel. They achieve this through callbacks, select(), threads, or other asynchronous programming techniques. Common examples of event loops include browser JavaScript, game engines, servers, and other applications that perform non-blocking IO operations. Event loops can be complex to work with correctly due to issues like race conditions, so abstractions like promises are recommended.

1. EVENT LOOPS
Everything you wanted to know
but couldn't find on StackOverflow

2. ABOUT ME
• I've misunderstood event loops for over a decade
• 2–3 years ago I set out on a quest to really
understand them
• I failed
• but learned a few things

3. WE ALL START SOMEWHERE
<?php
echo "im in ur pagez, writin php!!1!"
?>
alert("Hi mom!");

4. WHY ISTHIS SO SLOW?…
for my $host (@hosts) {
say $host . " is " . (ping($host) ? "up" : "down");
}
for host in hosts:
print host + " is " + ("up" if ping(host) else "down")

5. …BECAUSE NETWORKS

6. FORKING

7. <?php
function ping($host) {
...
}
foreach ($hosts as $host) {
$pid = pcntl_fork();
if ($pid) {
pcntl_wait($status);
} else {
ping($host);
}
}
?>

8. THREADING

9. Some people, when confronted with a problem, think,
“I know, I'll use threads”—
now two they hav erpoblesm.

10. Knock knock.
Race condition.
Who's there?

11. –Apple, "Threaded Programming Guide:Thread Management"
“Another cost to consider when writing threaded
code is the production costs. Designing a threaded
application can sometimes require fundamental
changes to the way you organize your application’s
data structures. Making those changes might be
necessary to avoid the use of synchronization, which
can itself impose a tremendous performance penalty
on poorly designed applications. Designing those data
structures, and debugging problems in threaded code,
can increase the time it takes to develop a threaded
application.Avoiding those costs can create bigger
problems at runtime, however, if your threads spend
too much time waiting on locks or doing nothing.”

12. NON-BLOCKING IO

13. LET’S ORDER
BREAKFAST!
pancakes
soft-boiled egg
orange juice

14. BLOCKING CHEF
mix
pancakes
heat skillet
cook
pancakes
boil water cook eggs cut juice

15. NON-BLOCKING CHEF
mix
pancakes
heat skillet
cook
pancakes
boil water cook eggs
cut juice

16. for host in hosts:
clientsocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
clientsocket.setblocking(0)
clientsocket.connect((host, port))
while 1:
reads, writes, fails = select.select(in, out, [], wait)
# do something with these handles
...

17. EVENT LOOPS

19. while (!mExiting)
NS_ProcessNextEvent(thread);

20. DEVICES & GAMES

22. UNITY 3D GAME LOOP
• physics loop: runs until caught up to current frame
• event updates once per loop iteration ("tick")
• network events
• rendering

23. $.get("some/page.html", function (data) {
$("#response").html(data);
});
var req = http.request({path: 'some/page.html'}, function(res) {
res.on('data', function(data) {
console.log(data);
});
});
$ua->get('some/page.html', sub {
say pop->res->body;
});

24. http.get(url, function(response) {
print response.body
})
Loop.Run()
reactor (event sources)
http.get(url function(response) {
print response.body
}
Loop.Run()
message queue (callbacks)

25. <script type=“text/javascript">
$(document).ready(function() {
jQuery.ajax(“http://www.perl.org”,
{
success: function(data) {
alert("loaded!")
}
})
})
</script>
jQuery.ajax(“http://www.perl.org”,
{
success: function(data) {
alert("loaded!")
}
})
success: function(data) {
alert("loaded!")
}

26. CAVEAT EMPTOR
• event loops can be hard to work with
• don't use callbacks except for simple things
• use Promises, Futures and other abstractions

27. IMAGE CREDITS
• stopwatch, cloud, database, info by Austin Condiff
from the Noun Project
• server by aLf from the Noun Project
• browser by Cindy Hu from the Noun Project
• code by useiconic.com from the Noun Project