Icicle is a PHP library for writing asynchronous code using synchronous coding techniques.

Icicle uses Coroutines built with Promises to facilitate writing asynchronous code using techniques normally used to write synchronous code, such as returning values and throwing exceptions, instead of using nested callbacks typically found in asynchronous code.

• Coroutines: Interruptible functions for building asynchronous code using synchronous coding patterns and error handling.
• Promises: Placeholders for future values of asynchronous operations. Callbacks registered with promises may return values and throw exceptions.
• Loop (event loop): Used to schedule functions, run timers, handle signals, and poll sockets for pending data or await for space to write.
• Streams: Common interface for reading and writing data.
• Sockets: Asynchronous stream sockets.

• HTTP: Asynchronous HTTP server and client (under development).
• DNS: Asynchronous DNS resolver and connector.
• React Adaptor: Adapts the event loop and promises of Icicle to interfaces compatible with components built for React.

• PHP 5.5+

The recommended way to install Icicle is with the Composer package manager. (See the Composer installation guide for information on installing and using Composer.)

Run the following command to use Icicle in your project:

composer require icicleio/icicle

You can also manually edit composer.json to add Icicle as a project requirement.

// composer.json
{
    "require": {
        "icicleio/icicle": "0.4.*"
    }
}

• openssl extension: Enables using SSL/TLS on sockets.
• pcntl extension: Enables custom signal handling.
• event extension: Allows for the most performant event loop implementation.
• libevent extension: Similar to the event extension, it allows for a more performant event loop implementation.

Icicle implements promises based on the Promises/A+ specification, adding support for cancellation.

Promises are objects that act as placeholders for the future value of an asynchronous operation. Pending promises may either be fulfilled with any value (including other promises, null, and exceptions) or rejected with any value (non-exceptions are encapsulated in an exception). Once a promise is fulfilled or rejected (resolved) with a value, the promise cannot becoming pending and the resolution value cannot change.

Callback functions are the primary way of accessing the resolution value of promises. Unlike other APIs that use callbacks, promises provide an execution context to callback functions, allowing callbacks to return values and throw exceptions.

All promise objects implement a common interface: Icicle\Promise\PromiseInterface. While the primary promise implementation is Icicle\Promise\Promise, several other classes also implement Icicle\Promise\PromiseInterface.

The Icicle\Promise\PromiseInterface::then(callable $onFulfilled = null, callable $onRejected = null) method is the primary way to register callbacks that receive either the value used to fulfill the promise or the exception used to reject the promise. A promise is returned by then(), which is resolved with the return value of a callback or rejected if a callback throws an exception.

The Icicle\Promise\PromiseInterface::done(callable $onFulfilled = null, callable $onRejected = null) method registers callbacks that should either consume promised values or handle errors. No value is returned from done(). Values returned by callbacks registered using done() are ignored and exceptions thrown from callbacks are re-thrown in an uncatchable way.

More on using callbacks to interact with promises...

use Icicle\Dns\Executor\Executor;
use Icicle\Dns\Resolver\Resolver;
use Icicle\Loop\Loop;
use Icicle\Socket\Client\ClientInterface;
use Icicle\Socket\Client\Connector;

$resolver = new Resolver(new Executor('8.8.8.8'));

$promise1 = $resolver->resolve('example.com'); // Method returning a promise.

$promise2 = $promise1->then(
    function (array $ips) { // Called if $promise1 is fulfilled.
        $connector = new Connector();
		return $connector->connect($ips[0], 80); // Method returning a promise.
		// $promise2 will adopt the state of the promise returned above.
    }
);

$promise2->done(
    function (ClientInterface $client) { // Called if $promise2 is fulfilled.
        echo "Asynchronously connected to example.com:80\n";
    },
    function (Exception $exception) { // Called if $promise1 or $promise2 is rejected.
        echo "Asynchronous task failed: {$exception->getMessage()}\n";
    }
);

Loop::run();

The example above uses the DNS component to resolve the IP address for a domain, then connect to the resolved IP address. The resolve() method of $resolver and the connect() method of $connector both return promises. $promise1 created by resolve() will either be fulfilled or rejected:

• If $promise1 is fulfilled, the callback function registered in the call to $promise1->then() is executed, using the fulfillment value of $promise1 as the argument to the function. The callback function then returns the promise from connect(). The resolution of $promise2 will then be determined by the resolution of this returned promise ($promise2 will adopt the state of the promise returned by connect()).
• If $promise1 is rejected, $promise2 is rejected since no $onRejected callback was registered in the call to $promise1->then()

More on promise resolution and propagation...

• Asynchronous resolution (callbacks are not called before then() or done() return).
• Convenience methods for registering special callbacks to handle promise resolution.
• Lazy execution of promise-creating functions.
• Operations on collections of promises to join, select, iterate, and map to other promises or values.
• Support for promise cancellation.
• Methods to convert synchronous functions or callback-based functions into functions accepting and returning promises.

Promise API documentation

Coroutines are interruptible functions implemented using Generators. A Generator usually uses the yield keyword to yield a value from a set to implement an iterator. Coroutines use the yield keyword to define interruption points. When a coroutine yields a value, execution of the coroutine is temporarily interrupted, allowing other tasks to be run, such as I/O, timers, or other coroutines.

When a coroutine yields a promise, execution of the coroutine is interrupted until the promise is resolved. If the promise is fulfilled with a value, the yield statement that yielded the promise will take on the resolved value. For example, $value = (yield Icicle\Promise\Promise::resolve(2.718)); will set $value to 2.718 when execution of the coroutine is resumed. If the promise is rejected, the exception used to reject the promise will be thrown into the function at the yield statement. For example, yield Icicle\Promise\Promise::reject(new Exception()); would behave identically to replacing the yield statement with throw new Exception();.

Note that no callbacks need to be registered with the promises yielded in a coroutine and errors are reported using thrown exceptions, which will bubble up to the calling context if uncaught in the same way exceptions bubble up in synchronous code.

The example below creates an Icicle\Coroutine\Coroutine instance from a function returning a Generator. (Icicle\Dns\Connector\Connector in the DNS component uses a coroutine structured similarly to the one below, except it attempts to connect to other IPs returned from the resolver if the first one fails.)

use Icicle\Coroutine\Coroutine;
use Icicle\Dns\Executor\Executor;
use Icicle\Dns\Resolver\Resolver;
use Icicle\Loop\Loop;
use Icicle\Socket\Client\Connector;

$generator = function () {
    try {
        $resolver = new Resolver(new Executor('8.8.8.8'));
        
        // Coroutine pauses until yielded promise is fulfilled or rejected.
        $ips = (yield $resolver->resolve('example.com'));
		
        $connector = new Connector();
        
        // Coroutine pauses again until yielded promise is fulfilled or rejected.
        $client = (yield $connector->connect($ips[0], 80));
		
        echo "Asynchronously connected to example.com:80\n";
    } catch (Exception $exception) {
        echo "Asynchronous task failed: {$exception->getMessage()}\n";
    }
};

$coroutine = new Coroutine($generator());

Loop::run();

The example above does the same thing as the example in the section on promises above, but instead uses a coroutine to structure asynchronous code like synchronous code. Fulfillment values of promises are accessed through simple variable assignments and exceptions used to reject promises are caught using a try/catch block, rather than creating and registering callback functions to each promise.

An Icicle\Coroutine\Coroutine object is also a promise, implementing Icicle\Promise\PromiseInterface. The promise is fulfilled with the last value yielded from the generator (or fulfillment value of the last yielded promise) or rejected if an exception is thrown from the generator. A coroutine may then yield other coroutines, suspending execution of the calling coroutine until the yielded coroutine has completed execution. If a coroutine yields a Generator, it will automatically be converted to a Icicle\Coroutine\Coroutine and handled in the same way as a yielded coroutine.

Coroutine API documentation

The event loop schedules functions, runs timers, handles signals, and polls sockets for pending reads and available writes. There are several event loop implementations available depending on what PHP extensions are available. The Icicle\Loop\SelectLoop class uses only core PHP functions, so it will work on any PHP installation, but is not as performant as some of the other available implementations. All event loops implement Icicle\Loop\LoopInterface and provide the same features.

The event loop should be accessed via the static methods of the Icicle\Loop\Loop facade class. The Icicle\Loop\Loop::init() method allows a specific or custom implementation of Icicle\Loop\LoopInterface to be used as the event loop.

The Icicle\Loop\Loop::run() method runs the event loop and will not return until the event loop is stopped or no events are pending in the loop.

The following code demonstrates how timers can be created to execute functions after a number of seconds elapses using the Icicle\Loop\Loop::timer() method.

use Icicle\Loop\Loop;

// Note that the Loop class is a facade to an instance of Icicle\Loop\LoopInterface (see description above).

Loop::timer(1, function () { // Executed after 1 second.
	echo "First.\n";
	Loop::timer(1.5, function () { // Executed after 1.5 seconds.
	    echo "Second.\n";
	});
	echo "Third.\n";
	Loop::timer(0.5, function () { // Executed after 0.5 seconds.
		echo "Fourth.\n";
	});
	echo "Fifth.\n";
});

echo "Starting event loop.\n";
Loop::run();

The above code will output:

Starting event loop.
First.
Third.
Fifth.
Fourth.
Second.

Loop API documentation

Streams represent a common promise-based API that may be implemented by classes that read or write sequences of binary data to facilitate interoperability. The stream component defines three interfaces, one of which should be used by all streams.

• Icicle\Stream\ReadableStreamInterface: Interface to be used by streams that are only readable.
• Icicle\Stream\WritableStreamInterface: Interface to be used by streams that are only writable.
• Icicle\Stream\DuplexStreamInterface: Interface to be used by streams that are readable and writable. Extends both Icicle\Stream\ReadableStreamInterface and Icicle\Stream\WritableStreamInterface.

Streams API documentation

The socket component implements network sockets as promise-based streams, server, and datagram. Creating a server and accepting connections is very simple, requiring only a few lines of code.

The example below implements HTTP server that responds to any request with Hello world! implemented using the promise-based server and client provided by the Socket component.

use Icicle\Loop\Loop;
use Icicle\Socket\Client\ClientInterface;
use Icicle\Socket\Server\ServerFactory;

$server = (new ServerFactory())->create('localhost', 60000);

$handler = function (ClientInterface $client) use (&$handler, &$error, $server) {
    $server->accept()->done($handler, $error);
    
    $response  = "HTTP/1.1 200 OK\r\n";
    $response .= "Content-Length: 13\r\n";
    $response .= "Connection: close\r\n";
    $response .= "\r\n";
    $response .= "Hello, world!";
    
    $client->end($response);
};

$error = function (Exception $e) {
    echo "Error: {$e->getMessage()}\n";
};

$server->accept()->done($handler, $error);

echo "Server listening on {$server->getAddress()}:{$server->getPort()}\n";

Loop::run();

The example below shows the same HTTP server as above, instead implemented using a coroutine.

use Icicle\Coroutine\Coroutine;
use Icicle\Loop\Loop;
use Icicle\Socket\Client\ClientInterface;
use Icicle\Socket\Server\ServerInterface;
use Icicle\Socket\Server\ServerFactory;

$server = (new ServerFactory())->create('localhost', 60000);

$generator = function (ServerInterface $server) {
    echo "Server listening on {$server->getAddress()}:{$server->getPort()}\n";
    
    $generator = function (ClientInterface $client) {
        $response  = "HTTP/1.1 200 OK\r\n";
        $response .= "Content-Length: 13\r\n";
        $response .= "Connection: close\r\n";
        $response .= "\r\n";
        $response .= "Hello, world!";
        
        yield $client->write($response);
        
        $client->close();
    };
    
    try {
        while ($server->isOpen()) {
            $coroutine = new Coroutine($generator(yield $server->accept()));
        }
    } catch (Exception $e) {
        echo "Error: {$e->getMessage()}\n";
    }
};

$coroutine = new Coroutine($generator($server));

Loop::run();

Sockets API documentation

The example below shows how the components outlined above can be combined to quickly create an asynchronous echo server, capable of simultaneously handling many clients.

use Icicle\Coroutine\Coroutine;
use Icicle\Loop\Loop;
use Icicle\Socket\Client\ClientInterface;
use Icicle\Socket\Server\ServerInterface;
use Icicle\Socket\Server\ServerFactory;

// Connect using `nc localhost 60000`.

$server = (new ServerFactory())->create('127.0.0.1', 60000);

$generator = function (ServerInterface $server) {
    $generator = function (ClientInterface $client) {
        try {
            yield $client->write("Want to play shadow? (Type 'exit' to quit)\n");
			
            while ($client->isReadable()) {
                $data = (yield $client->read());
                
                $data = trim($data, "\n");
                
                if ("exit" === $data) {
                    yield $client->end("Goodbye!\n");
                } else {
                    yield $client->write("Echo: {$data}\n");
                }
            }
        } catch (Exception $e) {
            echo "Client error: {$e->getMessage()}\n";
            $client->close();
        }
    };
    
    echo "Echo server running on {$server->getAddress()}:{$server->getPort()}\n";
    
    while ($server->isOpen()) {
        try {
            $coroutine = new Coroutine($generator(yield $server->accept()));
        } catch (Exception $e) {
            echo "Error accepting client: {$e->getMessage()}\n";
        }
    }
};

$coroutine = new Coroutine($generator($server));

Loop::run();