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Merge branch '7.2' into 7.3
* 7.2: [HttpClient] Update the concurrent requests section
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http_client.rst

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Concurrent Requests
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-------------------
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Thanks to responses being lazy, requests are always managed concurrently.
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On a fast enough network, the following code makes 379 requests in less than
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half a second when cURL is used::
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Symfony's HTTP client makes asynchronous HTTP requests by default. This means
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you don't need to configure anything special to send multiple requests in parallel
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and process them efficiently.
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Here's a practical example that fetches metadata about several Symfony
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components from the Packagist API in parallel::
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$packages = ['console', 'http-kernel', '...', 'routing', 'yaml'];
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$responses = [];
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for ($i = 0; $i < 379; ++$i) {
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$uri = "https://http2.akamai.com/demo/tile-$i.png";
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$responses[] = $client->request('GET', $uri);
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foreach ($packages as $package) {
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$uri = sprintf('https://repo.packagist.org/p2/symfony/%s.json', $package);
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// send all requests concurrently (they won't block until response content is read)
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$responses[$package] = $client->request('GET', $uri);
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}
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foreach ($responses as $response) {
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$content = $response->getContent();
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// ...
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$results = [];
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// iterate through the responses and read their content
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foreach ($responses as $package => $response) {
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// process response data somehow ...
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$results[$package] = $response->toArray();
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}
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As you can read in the first "for" loop, requests are issued but are not consumed
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yet. That's the trick when concurrency is desired: requests should be sent
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first and be read later on. This will allow the client to monitor all pending
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requests while your code waits for a specific one, as done in each iteration of
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the above "foreach" loop.
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As you can see, the requests are sent in the first loop, but their responses
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aren't consumed until the second one. This is the key to achieving parallel and
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concurrent execution: dispatch all requests first, and read them later.
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This allows the client to handle all pending responses efficiently while your
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code waits only when necessary.
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.. note::
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The maximum number of concurrent requests that you can perform depends on
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the resources of your machine (e.g. your operating system may limit the
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number of simultaneous reads of the file that stores the certificates
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file). Make your requests in batches to avoid these issues.
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The maximum number of concurrent requests depends on your system's resources
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(e.g. the operating system might limit the number of simultaneous connections
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or access to certificate files). To avoid hitting these limits, consider
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processing requests in batches.
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Multiplexing Responses
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~~~~~~~~~~~~~~~~~~~~~~
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If you look again at the snippet above, responses are read in requests' order.
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But maybe the 2nd response came back before the 1st? Fully asynchronous operations
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require being able to deal with the responses in whatever order they come back.
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In the previous example, responses are read in the same order as the requests
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were sent. However, it's possible that, for instance, the second response arrives
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before the first. To handle such cases efficiently, you need fully asynchronous
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processing, which allows responses to be handled in whatever order they arrive.
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In order to do so, the
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:method:`Symfony\\Contracts\\HttpClient\\HttpClientInterface::stream`
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accepts a list of responses to monitor. As mentioned
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To achieve this, the
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:method:`Symfony\\Contracts\\HttpClient\\HttpClientInterface::stream` method
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can be used to monitor a list of responses. As mentioned
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:ref:`previously <http-client-streaming-responses>`, this method yields response
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chunks as they arrive from the network. By replacing the "foreach" in the
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snippet with this one, the code becomes fully async::
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chunks as soon as they arrive over the network. Replacing the standard ``foreach``
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loop with the following version enables true asynchronous behavior::
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foreach ($client->stream($responses) as $response => $chunk) {
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if ($chunk->isFirst()) {
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// headers of $response just arrived
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// $response->getHeaders() is now a non-blocking call
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// the $response headers just arrived
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// $response->getHeaders() is now non-blocking
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} elseif ($chunk->isLast()) {
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// the full content of $response just completed
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// $response->getContent() is now a non-blocking call
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// the full $response body has been received
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// $response->getContent() is now non-blocking
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} else {
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// $chunk->getContent() will return a piece
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// of the response body that just arrived
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// $chunk->getContent() returns a piece of the body that just arrived
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}
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}
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.. tip::
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Use the ``user_data`` option combined with ``$response->getInfo('user_data')``
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to track the identity of the responses in your foreach loops.
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Use the ``user_data`` option along with ``$response->getInfo('user_data')``
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to identify each response during streaming.
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Dealing with Network Timeouts
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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