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Discovery

The discovery service allows to find installed resources and auto-install missing ones.

Currently available discovery services:

  • PSR-17 Factory Discovery
  • PSR-18 HTTP Client Discovery
  • PSR-7 Message Factory Discovery (deprecated in favor of PSR-17)
  • PSR-7 URI Factory Discovery (deprecated in favor of PSR-17)
  • PSR-7 Stream Factory Discovery (deprecated in favor of PSR-17)
  • HTTP Async Client Discovery
  • HTTP Client Discovery (deprecated in favor of PSR-18)
  • Mock Client Discovery (not enabled by default)

The principle is always the same: you call the static find method on the discovery service. The discovery service will try to locate a suitable implementation. If no implementation is found, an Http\Discovery\Exception\NotFoundException is thrown.

Discovery is simply a convenience wrapper to statically access clients and factories for when Dependency Injection is not an option. Discovery is particularly useful in libraries that want to offer zero-configuration services relying on the virtual packages.

Using discovery in a shared library

The goal of the PSR standards is that libraries do not depend on specific implementations but only on the standard. The library should only require the PSR standards.

To run tests, you might still need an implementation. We recommend to explicitly require that, but only for development. To build a library that needs to send HTTP requests, you could do:

$ composer require --dev symfony/http-client
$ composer require --dev nyholm/psr7

Then, you can disable the Composer plugin provided by php-http/discovery because you just installed the dev dependencies you need for testing:

$ composer config allow-plugins.php-http/discovery false

Finally, you need to require php-http/discovery and the generic implementations that your library is going to need:

$ composer require php-http/discovery:^1.17
$ composer require psr/http-client-implementation:*
$ composer require psr/http-factory-implementation:*

Now, you're ready to make an HTTP request:

use Http\Discovery\Psr18Client;

$client = new Psr18Client();

$request = $client->createRequest('GET', 'https://example.com');
$response = $client->sendRequest($request);

.. versionadded:: 1.17
    The ``Psr18Client`` is available since v1.17.

Internally, this code will use whatever PSR-7, PSR-17 and PSR-18 implementations your users have installed.

It is best practice to allow the users of your library to optionally specify the ClientInterface instance and only fallback to discovery when no explicit client has been specified.

Auto-installation

.. versionadded:: 1.15
    Auto-installation of missing dependencies is available since v1.15.

Discovery embeds a composer plugin that can auto-install missing implementations when an application does not specify any specific implementation.

If a library requires both php-http/discovery and one of the supported virtual packages (see :doc:`httplug/library-developers`), but no implementation for the virtual package is already installed, the plugin will auto-install the best matching known implementation.

For example, if the project requires react/event-loop, the plugin will select php-http/react-adapter to meet a missing dependency on php-http/client-implementation.

The following abstractions are currently supported:

  • php-http/async-client-implementation
  • php-http/client-implementation
  • psr/http-client-implementation
  • psr/http-factory-implementation
  • psr/http-message-implementation

Note

Auto-installation is only done for libraries that directly require php-http/discovery to avoid unexpected dependency installation.

If you do not want auto-installation to happen, you can chose to not enable the composer plugin of the discovery component:

composer config allow-plugins.php-http/discovery false

Strategies

The package uses strategies to select an implementation.

The default strategy contains a list of preferences that looks for well-known implementations: Symfony, Guzzle, Diactoros and Slim Framework.

Once a strategy provided a candidate, the result is cached in memory and reused for further discovery calls in the same process.

To register a custom strategy, implement the Http\Discovery\Strategy\DiscoveryStrategy interface and register your strategy with the prependStrategy, appendStrategy or setStrategies method of the corresponding discovery type.

Implementation Pinning

.. versionadded:: 1.17
    Pinning the preferred implementation is available since v1.17.

In case there are several implementations available, the application can pin which implementation to prefer. You can specify the implementation for one of the standards:

$ composer config extra.discovery.psr/http-factory-implementation GuzzleHttp\Psr7\HttpFactory

This will update your composer.json file to add the following configuration:

"extra": {
    "discovery": {
        "psr/http-factory-implementation": "GuzzleHttp\\Psr7\\HttpFactory"
    }
}

You can also pin single interfaces, e.g. for the PSR-17 factories:

"extra": {
    "discovery": {
        "Psr\\Http\\Message\\RequestFactoryInterface": "Slim\\Psr7\\Factory\\RequestFactory"
    }
}

Don't forget to run composer install to apply the changes, and ensure that the composer plugin is enabled:

$ composer config allow-plugins.php-http/discovery true
$ composer install

Note

Implementation pinning only works if the composer plugin of discovery is allowed. If you disabled the plugin, you need to configure your own discovery if you need a specific implementation selection.

Installation

$ composer require php-http/discovery

Common Errors

Could not find resource using any discovery strategy

If you get an error saying "Could not find resource using any discovery strategy." it means that all the discovery :ref:`strategies <discovery-strategies>` have failed. Most likely, your project is missing the message factories and/or a PRS-7 implementation. See the :doc:`user documentation <httplug/users>`.

To resolve this you may run

$ composer require php-http/curl-client guzzlehttp/psr7 php-http/message

No factories found

The error "No message factories found. To use Guzzle, Diactoros or Slim Framework factories install php-http/message and the chosen message implementation." tells you that no discovery strategy could find an installed implementation of PSR-7 and/or factories for that implementation. You need to install those libraries. If you want to use Guzzle you may run:

$ composer require php-http/message guzzlehttp/psr7

No HTTPlug clients found

The error "No HTTPlug clients found. Make sure to install a package providing 'php-http/client-implementation'" says that we cannot find a client. See our :doc:`list of clients <clients>` and install one of them.

$ composer require php-http/curl-client

HTTP Client Discovery

This type of discovery finds an HTTP Client implementation:

use Http\Client\HttpClient;
use Http\Discovery\HttpClientDiscovery;

class MyClass
{
    /**
     * @var HttpClient
     */
    private $httpClient;

    /**
     * @param HttpClient|null $httpClient Client to do HTTP requests, if not set, auto discovery will be used to find a HTTP client.
     */
    public function __construct(HttpClient $httpClient = null)
    {
        $this->httpClient = $httpClient ?: HttpClientDiscovery::find();
    }
}

HTTP Asynchronous Client Discovery

This type of discovery finds a HTTP asynchronous Client implementation:

use Http\Client\HttpAsyncClient;
use Http\Discovery\HttpAsyncClientDiscovery;

class MyClass
{
    /**
     * @var HttpAsyncClient
     */
    private $httpAsyncClient;

    /**
     * @param HttpAsyncClient|null $httpAsyncClient Client to do HTTP requests, if not set, auto discovery will be used to find an asynchronous client.
     */
    public function __construct(HttpAsyncClient $httpAsyncClient = null)
    {
        $this->httpAsyncClient = $httpAsyncClient ?: HttpAsyncClientDiscovery::find();
    }
}

PSR-17 Factory Discovery

This type of discovery finds a factory for a PSR-17 implementation:

use Psr\Http\Message\RequestFactoryInterface;
use Psr\Http\Message\ResponseFactoryInterface;
use Http\Discovery\Psr17FactoryDiscovery;

class MyClass
{
    /**
     * @var RequestFactoryInterface
     */
    private $requestFactory;

    /**
     * @var ResponseFactoryInterface
     */
    private $responseFactory;

    /**
     * @var ServerRequestFactoryInterface
     */
    private $serverRequestFactory;

    /**
     * @var StreamFactoryInterface
     */
    private $streamFactory;

    /**
     * @var UploadedFileFactoryInterface
     */
    private $uploadedFileFactory;

    /**
     * @var UriFactoryInterface
     */
    private $uriFactory;

    public function __construct(
        RequestFactoryInterface $requestFactory = null,
        ResponseFactoryInterface $responseFactory = null,
        ServerRequestFactoryInterface $serverRequestFactory = null,
        StreamFactoryInterface $streamFactory = null,
        UploadedFileFactoryInterface $uploadedFileFactory = null,
        UriFactoryInterface = $uriFactoryInterface = null
    ) {
        $this->requestFactory = $requestFactory ?: Psr17FactoryDiscovery::findRequestFactory();
        $this->responseFactory = $responseFactory ?: Psr17FactoryDiscovery::findResponseFactory();
        $this->serverRequestFactory = $serverRequestFactory ?: Psr17FactoryDiscovery::findServerRequestFactory();
        $this->streamFactory = $streamFactory ?: Psr17FactoryDiscovery::findStreamFactory();
        $this->uploadedFileFactory = $uploadedFileFactory ?: Psr17FactoryDiscovery::findUploadedFileFactory();
        $this->uriFactory = $uriFactory ?: Psr17FactoryDiscovery::findUriFactory();
    }
}

PSR-17 Factory

The package also provides an Http\Discovery\Psr17Factory class that can be instantiated to get a generic PSR-17 factory:

use Http\Discovery\Psr17Factory;

$factory = new Psr17Factory();

// use any PSR-17 methods, e.g.
$request = $factory->createRequest();

Internally, this class relies on the concrete PSR-17 factories that are installed in your project and can use discovery to find implementations if you do not specify them in the constructor.

Psr17Factory provides two additional methods that allow creating server requests or URI objects from the PHP superglobals:

$serverRequest = $factory->createServerRequestFromGlobals();
$uri = $factory->createUriFromGlobals();

.. versionadded:: 1.15
   The ``Psr17Factory`` class is available since version 1.15.

PSR-18 Client Discovery

This type of discovery finds a PSR-18_ HTTP Client implementation:

use Psr\Http\Client\ClientInterface;
use Http\Discovery\Psr18ClientDiscovery;

class MyClass
{
    /**
     * @var ClientInterface
     */
    private $httpClient;

    public function __construct(ClientInterface $httpClient = null)
    {
        $this->httpClient = $httpClient ?: Psr18ClientDiscovery::find();
    }
}

PSR-7 Message Factory Discovery

.. versionadded:: 1.6
    This is deprecated and will be removed in 2.0. Consider using PSR-17 Factory Discovery.

This type of discovery finds a :ref:`message-factory` for a PSR-7_ Message implementation:

use Http\Message\MessageFactory;
use Http\Discovery\MessageFactoryDiscovery;

class MyClass
{
    /**
     * @var MessageFactory
     */
    private $messageFactory;

    /**
     * @param MessageFactory|null $messageFactory to create PSR-7 requests.
     */
    public function __construct(MessageFactory $messageFactory = null)
    {
        $this->messageFactory = $messageFactory ?: MessageFactoryDiscovery::find();
    }
}

PSR-7 URI Factory Discovery

.. versionadded:: 1.6
    This is deprecated and will be removed in 2.0. Consider using PSR-17 Factory Discovery.

This type of discovery finds a URI factory for a PSR-7_ URI implementation:

use Http\Message\UriFactory;
use Http\Discovery\UriFactoryDiscovery;

class MyClass
{
    /**
     * @var UriFactory
     */
    private $uriFactory;

    /**
     * @param UriFactory|null $uriFactory to create UriInterface instances from strings.
     */
    public function __construct(UriFactory $uriFactory = null)
    {
        $this->uriFactory = $uriFactory ?: UriFactoryDiscovery::find();
    }
}

Mock Client Discovery

You may find yourself testing parts of your application that are dependent on an HTTP Client using the Discovery Service, but do not necessarily need to perform the request nor contain any special configuration. In this case, the Http\Mock\Client from the php-http/mock-client package is typically used to fake requests and keep your tests nicely decoupled. However, for the best stability in a production environment, the mock client is not set to be found via the Discovery Service. Attempting to run a test which relies on discovery and uses a mock client will result in an Http\Discovery\Exception\NotFoundException. Thankfully, Discovery gives us a Mock Client strategy that can be added straight to the Discovery. Let's take a look:

use MyCustomService;
use Http\Mock\Client as MockClient;
use Http\Discovery\Psr18ClientDiscovery;
use Http\Discovery\Strategy\MockClientStrategy;

class MyCustomServiceTest extends TestCase
{
    public function setUp()
    {
        Psr18ClientDiscovery::prependStrategy(MockClientStrategy::class);

        $this->service = new MyCustomService;
    }

    public function testMyCustomServiceDoesSomething()
    {
        // Test...
    }
}

In the example of a test class above, we have our MyCustomService which relies on an HTTP Client implementation. We do not need to test that the actual request our custom service makes is successful in this test class, so it makes sense to use the Mock Client. However, we do want to make sure that our dependency injection using the Discovery service properly works, as this is a major feature of our service. By calling the HttpClientDiscovery's prependStrategy method and passing in the MockClientStrategy namespace, we have now added the ability to discover the mock client and our tests will work as desired.

It is important to note that you must explicitly enable the MockClientStrategy and that it is not used by the Discovery Service by default. It is simply provided as a convenient option when writing tests.