Considerations when using Swoole

Because Swoole uses an event loop, and because it is able to load your application exactly once, you must take several precautions when using it to serve your application.

Long-running processes

When using the Swoole HTTP server, your application runs within an event loop. One benefit of this is that you can then defer execution of code until the next tick of the loop. This can be used to delay long-running code from executing until after a response has been sent to the client, which can obviate the need for tools such as message queues.

The problem, however, is that when a worker does begin to handle the deferred functionality, it will run as long as needed until the work is done. This then means that the worker is blocked from handling new requests until that work is done.

If you have enough workers, or the number of such long-running processes if few and far-between, this may not be an issue for you. However, it is a commonly documented issue in other similar systems such as Node.js. The solution in these cases is the same as for general PHP applications: add a message queue to your systems infrastructure, and delegate such work to the message queue instead.

PDO Coroutine Support

Please be aware that enabling coroutine support with Swoole\Runtime::enableCoroutine() only decorates MySql PDO connections with coroutines; other drivers (e.g., pdo_pgsql) remain blocking as of Swoole 4.1.2. > For more details, visit the related bug report on the Swoole issue tracker.

Sessions

Because Swoole runs as a PHP CLI application, it generally does not play well with the PHP Session extension. However, there are alternatives.

zend-expressive-session-cache

zend-expressive-session-cache provides a persistence adapter for zend-expressive-session that uses a PSR-6 CacheItemPoolInterface implementation for storing and retrieving sessions. This approach requires that you setup a backend cache storage for your session data.

psr7-sessions/storageless

PSR7Session provides session middleware that uses JWT tokens within the session cookie to transmit session data between the server and client. This approach requires no central session storage, but does impose limits on the amount of information you can store in a session.

Stateless services

The typical PHP model is that the engine is fired up, runs your code, and then tears down again, for every single request. As such, PHP is said to have a "shared nothing architecture". This is a tremendous boon to developers, as they can ignore things found in lower level languages, such as garbage cleanup, memory management, and more.

This model also comes with a cost: every single request requires bootstrapping your application. Benchmarks we have performed show that bootstrapping is often the most expensive operation in applications, often accounting for 25-50% of total resource usage and execution time.

One reason technologies such as Swoole can provide a performance boost is due to the fact that they can bootstrap your application exactly once, often during startup. This alone can account for the performance boost of many applications.

However, it has a price: you now need to consider what changes may happen inside the various classes in your dependency injection container, and the impact those changes may have on later requests, or even other requests happening concurrently.

As one example: zend-expressive-template provides an interface, TemplateRendererInterface, that allows you to render a template. That interface also allows you to provide template paths, and default parameters to pass to every template, and these methods are often invoked within factories or delegators in order to configure the renderer implementation. However, we have also documented using addDefaultParam() for passing values discovered in the request to later handlers. This practice accumulates state in the renderer that can cause problems later:

  • Flash messages discovered in one request might then be pushed to templates renderered in subsequent requests — when they are no longer in scope.

  • User details from one request might persist to a template rendered for an unauthenticated user in another request, exposing information.

These are clearly problematic behaviors!

As such, you must guard against state in services you provide in your dependency injection container, as any state changes have ramifications for other requests. Write services to be stateless, and/or mark state-changing methods as @internal to prevent users from calling them in non-bootstrap code.

If the services are provided by a third party, you have a few options:

  • Decorating an existing service that implements an interface to make it stateless.

  • Extending a service to make state-changing methods no-ops.

  • Injecting factories that produce the stateful services, instead of the service itself.

We'll look at each in detail.

Decoration

If a service implements an interface, you can decorate the service to make it stateless. Well-written interfaces will be stateless by design, and not provide methods meant to internally change state. In these situations, you can create a proxy class that decorates the original service:

class ProxyService implements OriginalInterface
{
    /** @var OriginalInterface */
    private $proxy;

    public function __construct(OriginalInterface $proxy)
    {
        $this->proxy = $proxy;
    }

    public function someMethodDefinedInInterface(string $argument) : Result
    {
        return $this->proxy->someMethodDefinedInInterface($argument);
    }
}

You would then:

  • Map the factory for the original service to the implementation name.
  • Create a factory that consumes the original service, and produces the proxy.
  • Map the interface name to the factory that creates the proxy.
// in config/autoload/dependencies.global.php:

return [
    'dependencies' => [
        'factories' => [
            OriginalImplementation::class => OriginalImplementationFactory::class,
            OriginalInterface::class => ProxyServiceFactory::class,
        ],
    ],
];

If you were writing to the interface, and not the implementation, you can now guarantee that any non-interface methods that changed state can now no longer be called.

If the interface itself defines methods that modify state, we recommend writing a proxy that implements those methods as no-ops and/or that raises exceptions when those methods are invoked. (The latter approach ensures that you discover quickly when code is exercising those methods.) In each case, you would then use a delegator factory, to decorate the original instance in the proxy class:

function (ContainerInterface $container, string $name, callable $callback)
{
    return new ProxyService($callback());
}

(You can also use the delegator factory approach with the previous proxy service example.)

Extension

When a service does not implement an interface, but exposes methods that change internal state, you can extend the original class to make the methods that change state into no-ops, or have them raise exceptions. (The latter approach ensures that you discover quickly when code is exercising those methods.)

As an example, let's say you have a class DataMapper that defines a method setTable() in it, and that method would change the database table the mapper would query. This is a potentially bad situation!

We could extend the class as follows:

class StatelessDataMapper extends DataMapper
{
    public function setTable(string $table) : void
    {
        throw new \DomainException(sprintf(
            '%s should not be called in production code!',
            __METHOD__
        ));
    }
}

In your factory that creates an instance of DataMapper, have it instead return a StatelessDataMapper instance, and you're now safe.

Factories

Another approach is to modify your consuming code to accept a factory that will produce the service you'll consume, instead of the service itself. This approach ensures that the service is created only when needed, mitigating any state change issues.

As an example, consider the following middleware that currently consumes a template renderer:

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Zend\Diactoros\Response\HtmlResponse;
use Zend\Expressive\Template\TemplateRendererInterface;

class SomeHandler implements RequestHandlerInterface
{
    /** @var TemplateRendererInterface */
    private $renderer;

    public function __construct(TemplateRendererInterface $renderer)
    {
        $this->renderer = $renderer;
    }

    public function handle(ServerRequestInterface $request) : ResponseInterface
    {
        return new HtmlResponse($this->renderer->render(
            'app::some-handler',
            []
        ));
    }
}

What we will do is modify it to accept a callable to the constructor. We will then call that factory just before we need the renderer; we will not store the result in the handler, as we want to ensure we have a new instance each time.

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Zend\Diactoros\Response\HtmlResponse;
use Zend\Expressive\Template\TemplateRendererInterface;

class SomeHandler implements RequestHandlerInterface
{
    /** @var callable */
    private $rendererFactory;

    public function __construct(callable $rendererFactory)
    {
        $this->rendererFactory = $rendererFactory;
    }

    public function handle(ServerRequestInterface $request) : ResponseInterface
    {
        /** @var TemplateRendererInterface $renderer */
        $renderer = ($this->rendererFactory)();
        return new HtmlResponse($renderer->render(
            'app::some-handler',
            []
        ));
    }
}

From here, we create a factory for our dependency injection container that will return the factory we use here. As an example, if we are using the zend-view integration, we might do the following:


use Psr\Container\ContainerInterface;
use Zend\Expressive\Template\TemplateRendererInterface;
use Zend\Expressive\ZendView\ZendViewRendererFactory;

class ZendViewRendererFactoryFactory
{
    public function __invoke(ContainerInterface $container) : callable
    {
        $factory = new ZendViewRendererFactory();
        return function () use ($container, $factory) : TemplateRendererInterface {
            return $factory($container);
        };
    }
}

If we mapped this to the "service" Zend\Expressive\Template\TemplateRendererInterfaceFactory, our factory for the SomeHandler class would then look like:

use Zend\Expressive\Template\TemplateRendererInterfaceFactory;

function (ContainerInterface $container) : SomeHandler
{
    return new SomeHandler(
        $container->get(TemplateRendererInterfaceFactory::class)
    );
}

This approach ensures we get a new instance with known state at precisely the moment we wish to execute the functionality. By ensuring we do not store the instance in any way, we also ensure it is garbage collected when the instance goes out of scope (i.e., when the method ends).

Handling the template data problem

If we want our services to be stateless, how do we handle problems such as the documented addDefaultParam() issue referenced earlier?

In this case, the original problem was "how do we get common request data into templates?" The solution originally provided was to alter the state of the template renderer. Another solution, however, is one we've also documented previously: use server attributes to pass data between middleware.

In this particular case, the middleware documented in the original solution could be modified to provide data to a request attribute, instead of altering the state of the template renderer. It might then become:

namespace App\Middleware;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Zend\Expressive\Router\RouteResult;
use Zend\Expressive\Session\Authentication\UserInterface;
use Zend\Expressive\Session\Flash\FlashMessagesInterface;

class TemplateDefaultsMiddleware implements MiddlewareInterface
{
    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler) : ResponseInterface
    {
        $routeResult = $request->getAttribute(RouteResult::class);
        $flashMessages = $request->getAttribute(FlashMessagesInterface::class);

        $defaults = [
            // Inject the current user, or null if there isn't one.
            // This is named security so it will not interfere with your user admin pages
            'security' => $request->getAttribute(UserInterface::class),

            // Inject the currently matched route name.
            'matchedRouteName' => $routeResult ? $routeResult->getMatchedRouteName() : null,

            // Inject all flash messages
            'notifications' => $flashMessages ? $flashMessages->getFlashes() : [],
        ];

        return $handler->handle($request->withAttribute(__CLASS__, $defaults));
    }
}

Once that change is made, you would then change your handler to do the following:

  • Pull that attribute, providing a default [] value.
  • Merge the pulled value with any local values when rendering the template.

For example:

$defaultParams = $request->getAttribute(TemplateDefaultsMiddleware::class, []);
return new HtmlResponse($renderer->render(
    'some::template',
    array_merge($defaultParams, [
        // handler-specific parameters here
    ])
));

This approach, while it requires more work on the part of handler authors, ensures that the renderer state does not vary between requests, making it safer for usage with Swoole and other long-running processes.

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