Alert

Alert provides event reactors for powering event-driven, non-blocking PHP applications.

Features

Alert adds the following functionality previously absent from the PHP non-blocking space:

• Pause/resume for individual event/signal/IO observers
• Multiple watchers for individual streams
• Cross-OS process signal handling (yes, even in Windows)

Dependencies

• PHP 5.4+

Optional PHP extensions may be used for great performance justice. An extension is also necessary if you need to watch for process signals in your application:

• (preferred) php-uv for libuv backends.
• pecl libevent for libevent backends. Windows libevent extension DLLs are available here. php-uv is preferred, but libevent is better than nothing.

Installation

Via composer:

$ php composer.phar require rdlowrey/alert:~0.11.x

The Guide

Event Reactor Concepts

• Reactor Implementations
• Reactor == Task Scheduler
• The Universal Reactor

Controlling the Reactor

• run()
• tick()
• stop()

Timer Watchers

• immediately()
• once()
• repeat()
• at()

Stream IO Watchers

• onReadable()
• onWritable()
• watchStream()

Pausing, Resuming and Cancelling Watchers

• disable()
• enable()
• cancel()

Process Signal Watchers

Addenda

• Callback Invocation Parameters
• Watcher Cancellation Safety
• An Important Note on Writability Watchers
• Process Signal Number Availability
• IO Performance

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Event Reactor Concepts

Reactor Implementations

It may surprise people to learn that the PHP standard library already has everything we need to write event-driven and non-blocking applications. We only reach the limits of native PHP's functionality in this area when we ask it to poll several hundred streams for read/write capability at the same time. Even in this case, though, the fault is not with PHP but the underlying system select() call which is linear in its performance degradation as load increases.

For performance that scales out to high volume we require more advanced capabilities currently found only in extensions. If you wish to, for example, service 10,000 simultaneous clients in an Alert-backed socket server you would definitely need to use one of the reactors based on a PHP extension. However, if you're using Alert in a strictly local program for non-blocking concurrency or you don't need to handle more than ~100 or so simultaneous clients in a server application the native PHP functionality is perfectly adequate.

Alert currently exposes three separate implementations for its standard Reactor interface. Each behaves exactly the same way from an external API perspective. The main differences have to do with underlying performance characteristics. The one capability that the extension-based reactors do offer that's unavailable with the native implementation is the ability to watch for process control signals. The current implementations are listed here:

Class	Extension
Alert\NativeReactor	n/a
Alert\UvReactor	php-uv
Alert\LibeventReactor	pecl/libevent

As mentioned, only UvReactor and LibeventReactor implement the Alert\SignalReactor interface to offer cross-operating system signal handling capabilities. At this time use of the UvReactor is recommended over LibeventReactor as the php-uv extension offers more in the way of tangentially related (but useful) functionality for robust non-blocking applications.

Reactor == Task Scheduler

The first thing we need to understand to program effectively using an event loop is this:

The event reactor is our task scheduler.

The reactor controls program flow as long as it runs. Once we tell the reactor to run it will control program flow until the application errors out, has nothing left to do, or is explicitly stopped. Consider this very simple example:

<?php // be sure to include the autoload.php file
echo "-before run()-\n";
Alert\run(function() {
    Alert\repeat(function() { echo "tick\n"; }, $msInterval = 1000);
    Alert\once(function() { Alert\stop(); }, $msDelay = 5000);
});
echo "-after stop()-\n";

Upon execution of the above example you should see output like this:

-before run()-
tick
tick
tick
tick
tick
-after stop()-

Hopefully this output demonstrates the concept that what happens inside the event reactor's run loop is like its own separate program. Your script will not continue past the point of Reactor::run() unless one of the previously mentioned conditions for stoppage is met.

While an application can and often does take place entirely inside the confines of the run loop, we can also use the reactor to do things like the following example which imposes a short-lived timeout for interactive console input:

<?php
$number = null;
$stdinWatcher = null;
stream_set_blocking(STDIN, false);

echo "Please input a random number: ";

Alert\run(function() use (&$stdinWatcher, &$number) {
    $stdinWatcher = Alert\onReadable(STDIN, function() use (&$number) {
        $number = fgets(STDIN);
        Alert\stop(); // <-- we got what we came for; exit the loop
    });
    Alert\once(function() {
        Alert\stop(); // <-- you took too long; exit the loop
    }, $msInterval = 5000);
});

if (is_null($number)) {
    echo "You took too long so we chose the number '4' by fair dice roll\n";
} else {
    echo "Your number is: ", (int) $number, "\n";
}

Alert\cancel($stdinWatcher); // <-- clean up after ourselves
stream_set_blocking(STDIN, true);

// Continue doing regular synchronous things here.

The details of what's happening in this example are unimportant and involve functionality that will be covered later. For now, the takeaway should simply be that it's possible tomove in and out of the event loop like a ninja.

The Universal Reactor

In the above example we use the reactor's procedural API to register stream IO and timere watchers. However, Alert also exposes an object API. Though it almost never makes sense to run multiple event loop instances in a single-threaded process, instantiating Reactor objects in your application can make things significantly more testable. Note that the function API uses a single static reactor instance for all operations (universal). Below you'll find the same example from above section rewritten to use the Alert\NativeReactor class .

<?php
$number = null;
$stdinWatcher = null;
stream_set_blocking(STDIN, false);

echo "Please input a random number: ";

$reactor = new Alert\NativeReactor;
$reactor->run(function($reactor) use (&$stdinWatcher, &$number) {
    $stdinWatcher = $reactor->onReadable(STDIN, function() use ($reactor, &$number) {
        $number = fgets(STDIN);
        $reactor->stop();
    });
    $reactor->once(function() {
        $reactor->stop();
    }, $msInterval = 5000);
});

if (is_null($number)) {
    echo "You took too long to respond, so we chose '4' by fair dice roll\n";
} else {
    echo "Your number is: ", (int) $number, "\n";
}

$reactor->cancel($stdinWatcher); // <-- clean up after ourselves
stream_set_blocking(STDIN, true);

Always remember: bugs arising from the existence of multiple reactor instances are exceedingly difficult to debug. The reason for this should be relatively clear. It's because running one event loop will block script execution and prevent others from executing at the same time. This sort of "loop starvation" results in events that inexplicably fail to trigger. You should endeavor to always use the same reactor instance in your application when you instantiate and use the object API. Because the event loop is often a truly global feature of an application the procedural API functions use a static instance to ensure the same Reactor is reused. Be careful about instantiating reactors manually and mixing in calls to the function API.

Controlling the Reactor

run()

The primary way an application interacts with the event reactor is to schedule events for execution and then simply let the program run. Once Reactor::run() is invoked the event loop will run indefinitely until there are no watchable timer events, IO streams or signals remaining to watch. Long-running programs generally execute entirely inside the confines of a single Reactor::run() call.

tick()

The event loop tick is the basic unit of flow control in a non-blocking application. This method will execute a single iteration of the event loop before returning. Reactor::tick() may be used inside a custom while loop to implement "wait" functionality in concurrency primitives such as futures and promises.

stop()

The event reactor loop can be stopped at any time while running. When Reactor::stop() is invoked the reactor loop will return control to the userland script at the end of the current iteration of the event loop. This method may be used to yield control from the reactor even if events or watchable IO streams are still pending.

Timer Watchers

Alert exposes several ways to schedule timer watchers. Let's look at some details for each method ...

immediately()

• Schedule a callback to execute in the next iteration of the event loop
• This method guarantees a clean call stack to avoid starvation of other events in the current iteration of the loop if called recursively. An "immediately" callback is always executed in the next tick of the event loop.
• After an "immediately" timer watcher executes it is automatically garbage collected by the reactor so there is no need for applications to manually cancel the associated watcher ID.
• Like all watchers, "immediately" timers may be disabled and reenabled. If you disable this watcher between the time you schedule it and the time that it actually runs the reactor will not be able to garbage collect it until it executes. Therefore you must manually cancel an immediately watcher yourself if it never actually executes to free any associated resources.

once()

• Schedule a callback to execute after a delay of n milliseconds
• A "once" watcher is also automatically garbage collected by the reactor after execution and applications should not manually cancel it unless they wish to discard the watcher entirely prior to execution.
• A "once" watcher that is disabled has its delay time reset so that the original delay time starts again from zero once reenabled.
• Like "immediately" watchers, a timer scheduled for one-time execution must be manually cancelled to free resources if it never runs due to being disabled by the application after creation.

repeat()

• Schedule a callback to repeatedly execute every n millisconds.
• Unlike one-time watchers, "repeat" timer resources must be explicitly cancelled to free the associated resources. Failure to free "repeat" watchers once their purpose is fulfilled will result in memory leaks in your application.
• Like all other watchers, "repeat" timers may be disabled/reenabled at any time.

at()

• Schedule a callback to execute at a specific time in the future. Future time may either be an integer unix timestamp or any string parsable by PHP's strtotime() function.
• In all other respects "at" watchers are the same as "immediately" and "once" timers.

Stream IO Watchers

Stream watchers are how we know that data exists to read or that write buffers are empty. These notifications are how we're able to actually create things like http servers and asynchronous database libraries using the event reactor. As such, stream IO watchers form the backbone of all non-blocking operations with Alert.

There are two classes of IO watcher:

• Readability watchers
• Writability watchers

onReadable()

Watchers registered via Reactor::onReadable() trigger their callbacks in the following situations:

• When data is available to read on the stream under observation
• When the stream is at EOF (for sockets, this means the connection is lost)

A common usage pattern for reacting to readable data looks something like this example:

<?php
define('IO_GRANULARITY', 32768);

function isStreamDead($socket) {
    return !is_resource($socket) || @feof($socket);
}

$client->watcherId = Alert\onReadable($client->socket, function() use ($client) {
    $newData = @fread($client->socket, IO_GRANULARITY);
    if ($newData != "") {
        // There was actually data and not an EOF notification. Let's consume it!
        parseIncrementalData($client, $newData);
    } elseif (isStreamDead($client->socket)) {
        // If the read data == "" we need to make sure the stream isn't dead
        closeClientAndClearAnyAssociatedResources($client);
    }
});

In the above example we've done a few very simple things:

• Register a readability watcher for a socket that will trigger our callback when there is data available to read.

• When we read data from the stream in our triggered callback we pass that to a stateful parser that does something domain-specific when certain conditions are met.

• If the fread() call indicates that the socket connection is dead we clean up any resources we've allocated for the storage of this stream. This process should always include calling Reactor::cancel() on any reactor watchers we registered in relation to the stream.

onWritable()

• Streams are essentially "always" writable. The only time they aren't is when their respective write buffers are full.

A common usage pattern for reacting to writability involves initializing a writability watcher without enabling it when a client first connects to a server. Once incomplete writes occur we're then able to "unpause" the write watcher using Reactor::enable() until data is fully sent without having to create and cancel new watcher resources on the same stream multiple times.

watchStream()

The Reactor::watchStream() functionality exposes both readability and writability watcher registration in a single function as a convenience for programmers who wish to use the same API for all IO watchers and specify flags to denote desired behavior.

The Alert\Reactor interface exposes the following flags for use with Reactor::watchStream:

• Reactor::WATCH_READ
• Reactor::WATCH_WRITE
• Reactor::WATCH_NOW

So if you wished to use the watchStream() API to register a readability watcher that was enabled immediately you would do so like this:

<?php
$flags = Alert\Reactor::WATCH_READ | Reactor::WATCH_NOW;
$readWatcherId = Alert\watchStream($stream, $myCallbackFunction, $flags);

IMPORTANT: The main difference between watchStream() and the explicity IO watcher registration functions is that watchStream() WILL NOT enable watchers by default. To enable a watcher at registration time via watchStream() you must pass the WATCH_NOW flag.

Pausing, Resuming and Cancelling Watchers

All watchers, regardless of type, can be temporarily disabled and enabled in addition to being cleared via Reactor::cancel(). This allows for advanced capabilities such as disabling the acceptance of new socket clients in server applications when simultaneity limits are reached. In general, the performance characteristics of watcher reuse via pause/resume are favorable by comparison to repeatedly cancelling and re-registering watchers.

disable()

A simple disable example:

<?php

$reactor = new Alert\NativeReactor;

// Register a watcher we'll disable
$watcherIdToDisable = $reactor->once(function() {
    echo "I'll never execute in one second because: disable()\n";
}, $msDelay = 1000);

// Register a watcher to perform the disable() operation
$reactor->once(function() use ($watcherIdToDisable, $reactor) {
    echo "Disabling WatcherId: ", $watcherIdToDisable, "\n";
    $reactor->disable($watcherIdToDisable);
}, $msDelay = 500);

$reactor->run();

After our second watcher callback executes the reactor loop exits because there are no longer any enabled watchers registered to process.

enable()

Using enable() is just as simple as the disable() example we just saw:

<?php

$reactor = new Alert\NativeReactor;

// Register a watcher
$myWatcherId = $reactor->repeat(function() {
    echo "tick\n";
}, $msDelay = 1000);

// Disable the watcher
$reactor->disable($myWatcherId);

// Remember, nothing happens until the reactor runs, so it doesn't matter that we
// previously created and disabled $myWatcherId
$reactor->run(function($reactor) use ($myWatcherId) {
    // Immediately enable the watcher when the reactor starts
    $reactor->enable($myWatcherId);
    // Now that it's enabled we'll see tick output in our console every 1000ms.
});

For a slightly more complex use case, let's look at a common scenario where a server might create a write watcher that is initially disabled but subsequently enabled as necessary:

<?php

class Server {
    private $reactor;
    private $clients = [];
    public function __construct(Alert\Reactor $reactor) {
        $this->reactor = $reactor;
    }

    public function startServer() {
        // ... server bind and accept logic would exist here
        $this->reactor->run();
    }

    private function onNewClient($sock) {
        $socketId = (int) $sock;
        $client = new ClientStruct;
        $client->socket = $sock;
        $readWatcher = $this->reactor->onReadable($sock, function() use ($client) {
            $this->onReadable($client);
        });
        $writeWatcher = $this->reactor->onReadable($sock, function() use ($client) {
            $this->doWrite($client);
        }, $enableNow = false); // <-- let's initialize the watcher as "disabled"

        $client->readWatcher = $readWatcher;
        $client->writeWatcher = $writeWatcher;

        $this->clients[$socketId] = $client;
    }

    // ... other class implementation details here ...

    private function writeToClient($client, $data) {
        $client->writeBuffer .= $data;
        $this->doWrite($client);
    }

    private function doWrite(ClientStruct $client) {
        $bytesToWrite = strlen($client->writeBuffer);
        $bytesWritten = @fwrite($client->socket, $client->writeBuffer);

        if ($bytesToWrite === $bytesWritten) {
            $this->reactor->disable($client->writeWatcher);
        } elseif ($bytesWritten >= 0) {
            $client->writeBuffer = substr($client->writeBuffer, $bytesWritten);
            $this->reactor->enable($client->writeWatcher);
        } elseif ($this->isSocketDead($client->socket)) {
            $this->unloadClient($client);
        }
    }

    // ... other class implementation details here ...
}

cancel()

It's important to always cancel persistent watchers once you're finished with them or you'll create memory leaks in your application. This functionality works in exactly the same way as the above enable/disable examples:

<?php
Alert\run(function() {
    $myWatcherId = Alert\repeat(function() {
        echo "tick\n";
    }, $msInterval = 1000);

    // Cancel $myWatcherId in five seconds and exit the reactor loop
    Alert\once(function() use ($myWatcherId) {
        Alert\cancel($myWatcherId);
    }, $msDelay = 5000);
});

Process Signal Watchers

The Alert\SignalReactor extends the base reactor interface to expose an API for handling process control signals in your application like any other event. Simply use a compatible event reactor implementation (UvReactor or LibeventReactor, preferably the former) and interact with its SignalReactor::onSignal() method. Consider:

<?php
(new Alert\UvReactor)->run(function($reactor) {
    // Let's tick off output once per second so we can see activity.
    $reactor->repeat(function() {
            echo "tick: ", date('c'), "\n";
    }, $msInterval = 1000);

    // What to do when a SIGINT signal is received
    $watcherId = $reactor->onSignal(UV::SIGINT, function() {
        echo "Caught SIGINT! exiting ...\n";
        exit;
    });
});

As should be clear from the above example, signal watchers may be enabled, disabled and cancelled like any other event.

Addenda

Callback Invocation Parameters

All watcher callbacks are invoked using the same standardized parameter order:

Watcher Type	Callback Signature
immediately()	function(Reactor $reactor, $watcherId)
once()	function(Reactor $reactor, $watcherId)
repeat()	function(Reactor $reactor, $watcherId)
at()	function(Reactor $reactor, $watcherId)
watchStream()	function(Reactor $reactor, $watcherId, $stream)
onReadable()	function(Reactor $reactor, $watcherId, $stream)
onWritable()	function(Reactor $reactor, $watcherId, $stream)
onSignal()	function(Reactor $reactor, $watcherId, $signo)

Watcher Cancellation Safety

It is always safe to cancel a watcher from within its own callback. For example:

<?php
$increment = 0;
Alert\repeat(function($reactor, $watcherId) use (&$increment) {
    echo "tick\n";
    if (++$increment >= 3) {
        $reactor->cancel($watcherId); // <-- cancel myself!
    }
}, $msDelay = 50);

An Important Note on Writability

Because streams are essentially "always" writable you should only enable writability watchers while you have data to send. If you leave these watchers enabled when your application doesn't have anything to write the watcher will trigger endlessly until disabled or cancelled. This will max out your CPU. If you're seeing inexplicably high CPU usage in your application it's a good bet you've got a writability watcher that you failed to disable or cancel after you were finished with it.

Process Signal Number Availability

Using the SignalReactor interface is relatively straightforward with the php-uv extension because it exposes UV::SIG* constants for watchable signals. Applications using the LibeventReactor to will need to manually specify the appropriate integer signal numbers when registering signal watchers.