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Streaming responses let you process data as it arrives from the server rather than waiting for the entire response to buffer in memory. This is particularly valuable when working with LLM APIs that generate tokens incrementally, downloading large files, or consuming real-time event streams.

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Enabling Streaming

To receive a streaming response, set the stream option on the request: 
use Cognesy\Http\Data\HttpRequest;

$request = new HttpRequest(
    url: 'https://api.example.com/stream',
    method: 'GET',
    headers: ['Accept' => 'text/event-stream'],
    body: '',
    options: ['stream' => true],
);
// @doctest id="1f45"
You can also enable streaming on an existing request using withStreaming(): 
$request = $request->withStreaming(true);
// @doctest id="7c82"

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Consuming the Stream

Once you have a streaming request, call stream() on the pending response. This returns a PHP Generator that yields string chunks: 
foreach ($client->send($request)->stream() as $chunk) {
    echo $chunk;
    flush();
}
// @doctest id="09c9"
Each chunk is a raw string as received from the transport layer. The size of individual chunks depends on the driver and the streamChunkSize setting in HttpClientConfig (default: 256 bytes).
Note: You do not need to explicitly set stream => true on the request when using PendingHttpResponse::stream(). The pending response will force streaming mode automatically. However, setting it on the request is useful when middleware needs to know the intended mode before execution.

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Streaming LLM Responses

Streaming is essential for AI/LLM integrations where responses are generated token by token. Here is a typical pattern for streaming a chat completion: 
$request = new HttpRequest(
    url: 'https://api.openai.com/v1/chat/completions',
    method: 'POST',
    headers: [
        'Content-Type' => 'application/json',
        'Authorization' => 'Bearer ' . $apiKey,
    ],
    body: [
        'model' => 'gpt-4',
        'messages' => [
            ['role' => 'user', 'content' => 'Write a haiku about PHP.'],
        ],
        'stream' => true,
    ],
    options: ['stream' => true],
);

foreach ($client->send($request)->stream() as $chunk) {
    echo $chunk;
    flush();
}
// @doctest id="053b"
The raw chunks from the transport layer will contain server-sent event framing (e.g., data: {...}\n\n). To parse these into clean payloads, use the EventSourceMiddleware.

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Server-Sent Events with EventSourceMiddleware

The EventSourceMiddleware handles the SSE protocol for you. It strips the data: prefixes, buffers partial lines, and yields complete event payloads: 
use Cognesy\Http\Extras\Middleware\EventSource\EventSourceMiddleware;

$client = $client->withMiddleware(
    (new EventSourceMiddleware(true))
        ->withParser(fn(string $payload): string => $payload),
    'eventsource',
);
// @doctest id="3ca0"
The parser callback receives the raw payload string from each data: line and returns the value to yield. Return false to skip an event. This is useful for filtering out [DONE] markers or parsing JSON: 
$client = $client->withMiddleware(
    (new EventSourceMiddleware(true))
        ->withParser(function (string $payload): string|bool {
            if ($payload === '[DONE]') {
                return false; // skip
            }
            return $payload;
        }),
    'eventsource',
);
// @doctest id="6e3f"
You can also attach listeners for debugging or event dispatching: 
use Cognesy\Http\Extras\Support\EventSource\Listeners\PrintToConsole;
use Cognesy\Http\Config\DebugConfig;

$middleware = (new EventSourceMiddleware(true))
    ->withListeners(new PrintToConsole(new DebugConfig(httpEnabled: true)))
    ->withParser(fn(string $payload): string => $payload);
// @doctest id="ff0e"

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Downloading Large Files

Streaming is the right approach for downloading large files without exhausting memory: 
$request = new HttpRequest(
    url: 'https://example.com/large-dataset.csv',
    method: 'GET',
    headers: [],
    body: '',
    options: ['stream' => true],
);

$handle = fopen('dataset.csv', 'wb');

foreach ($client->send($request)->stream() as $chunk) {
    fwrite($handle, $chunk);
}

fclose($handle);
// @doctest id="7614"

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Considerations

When working with streaming responses, keep these points in mind:
  • Memory usage. Streaming avoids buffering the entire response, but be careful not to accumulate chunks in a variable unless you actually need the full content.
  • Connection stability. Streaming connections stay open longer and are more sensitive to network interruptions. Pair streaming with retry middleware for resilience.
  • Timeouts. The idleTimeout setting in HttpClientConfig controls how long the client waits between data packets. Set it to -1 to disable idle timeouts for long-lived streams.
  • Body access. Calling body() on a streamed HttpResponse throws a LogicException. Always use stream() for streamed responses.
  • Middleware order. Middleware that decorates the stream (like EventSourceMiddleware) should be registered before middleware that reads the final content.