Don't Wait; Generate!

Ian Littman / @iansltx

ScotlandPHP Conference 2018

https://ian.im/gen18

function translate(string $texan) : string

switch ($texan) {
    case "y'all":
        return "some of you";
    case "all y'all":
        return "all of you";
    default:
        return $texan;
}

We'll Answer

• What's a Generator?
• What's a Coroutine?
• Why async/non-blocking I/O/event loops?
• Why not?
• What a Generator + Coroutine based app looks like

This is a generator

function gRange($min, $max, $step): \Generator
{
    for ($i = $min; $i < $max; $i += $step) {
        yield $i;
    }
}

foreach (gRange(12, 97, 5) as $n) {
    echo "$n\n"; // 12, 17, 22, etc.
}

What's going on here?

• Resumable function
• Uses yield rather than (or in addition to) return
• Incremental, iterable results
• Behaves a bit like an Iterator
• Values and Exceptions can be sent/thrown in
  (more on this in a moment)

<spooky>This is a generator</spooky>

function spooky(): \Generator
{
    while (true) {
        yield random_int(0, 1) ? 'O' : 'o';
    }
}

foreach (spooky() as $chr) {
    echo $chr; usleep(100000);
}

When can I use it?

• PHP 5.5+, but 7.0+ gets you...
  • return
  • yield from
  • Throwables
• Was on HHVM and predecessors before PHP
• ES2015 (aka ES6) via function*
• C# (.NET 4.5+)
• Python 2.2+

Before we look at our next example...

If you call a function that yields, you will not execute that function. You'll get back a generator. To execute the function, you (or the event loop) will interact with that generator.

Also, "return" means something different for a generator than for a normal function.

A Visual Example

Generator

Parent

$g = gen(1);

$a = $g->current();

2

$b = yield $arg1 + 1;

$c = $g->send($a + 1);

$d = yield $b + 2;

5

3

function gen($arg1)

$e = $g->send($c + 1);

6

return $d + 2;

null

echo $g->getReturn();

8

We just did a coroutine.

• Yield: stop execution until caller restarts it via send()
• Yield from: pass through execution until yielded-from object has nothing else to yield
• Return: use just like normal
• Cooperative multitasking!

Standard PHP-FPM Request Model

Client (or Load Balancer)

Web Server (nginx or Apache)

FastCGI Daemon (php-fpm) + Workers

HTTP

FastCGI

Pros

• Common
• Multicore
• Shared-nothing (safe)
• Fast for static resources
• Library support
  • Slim\Http Req\Res
  • Aura.Sql
  • ...basically anything else
• Don't worry (much) about blocking the thread

Cons

• No in-request parallelism*
• Blocking I/O
• Not memory-efficient
• Startup penalty on every req
• Not 12-factor
  • Process manager (runit)
  • nginx
  • php-fpm + workers

* Ignoring curl_multi and wrappers (e.g. Guzzle)

DEMO TIME: ngx + FPM + SLIM 3 APP

iansltx/raphple

Event Loops!

Mind the loop!

• Async I/O
• For compute-heavy operations...
  • Don't do in-process if you can avoid it
  • Don't do all at once if it must be in-process

Callbacks/Promises

• Very common (used in e.g. ReactPHP), but...
• Hard to follow execution flow
  • Error callback convention (vs. Exceptions)
  • Messages only at function borders
  • Callback Pyramid of Doom

Why a Generator?

• Easier to follow
• Cleaner error handling
• Pass control inside functions

Generators in an Event Loop

1. Run until blocking I/O
2. Yield promise representing blocking I/O
3. Event loop skips coroutine until promise is resolved (can use yield from inside a coroutine when you want to call another coroutine and wait for it to complete)
4. Event loop send()s promise result to coroutine
5. Repeat from 1 until coroutine is complete (return)

Use Node (or Hack)? This should look familar.

• async functions -> Generators
• await -> yield

Event Loop Extensions

• Not required, but highly recommended
• ev
• event
• php-uv

Amphp http-server Request Model

Client (or Load Balancer)

Application Server (AMPHP)

HTTP

var_dump(is_12_factor()); // bool(true)

Pros

• No per-request bootstrap time
• Fewer moving parts (12F app)
• Async execution
• Generator based (!pyramid)
• Async database access
• Lower memory use per request
• Fast!

Cons

• A bit fragile*
• Requires port match
• Single-threaded**
• Plenty to refactor

* Throwables to the rescue!
** amphp/cluster to the rescue!

Demo Time: Amp

Thanks! Questions?

• ian.im/gen18 - these slides
• joind.in/talk/39a75 - rate this talk!
• github.com/iansltx/raphple - raffle code
• twitter.com/iansltx - me
• github.com/iansltx - my code
• "Give Way" photo from Flickr
• Yield sign photo is mine