How to use curl_multi() without blocking

by on January 26, 2009

You can find the latest version of this library on Github.

A more efficient implementation of curl_multi()
curl_multi is a great way to process multiple HTTP requests in parallel in PHP. curl_multi is particularly handy when working with large data sets (like fetching thousands of RSS feeds at one time). Unfortunately there is very little documentation on the best way to implement curl_multi. As a result, most of the examples around the web are either inefficient or fail entirely when asked to handle more than a few hundred requests.

The problem is that most implementations of curl_multi wait for each set of requests to complete before processing them. If there are too many requests to process at once, they usually get broken into groups that are then processed one at a time. The problem with this is that each group has to wait for the slowest request to download. In a group of 100 requests, all it takes is one slow one to delay the processing of 99 others. The larger the number of requests you are dealing with, the more noticeable this latency becomes.

The solution is to process each request as soon as it completes. This eliminates the wasted CPU cycles from busy waiting. I also created a queue of cURL requests to allow for maximum throughput. Each time a request is completed, I add a new one from the queue. By dynamically adding and removing links, we keep a constant number of links downloading at all times. This gives us a way to throttle the amount of simultaneous requests we are sending. The result is a faster and more efficient way of processing large quantities of cURL requests in parallel.

function rolling_curl($urls, $callback, $custom_options = null) {

    // make sure the rolling window isn't greater than the # of urls
    $rolling_window = 5;
    $rolling_window = (sizeof($urls) < $rolling_window) ? sizeof($urls) : $rolling_window;

    $master = curl_multi_init();
    $curl_arr = array();

    // add additional curl options here
    $std_options = array(CURLOPT_RETURNTRANSFER => true,
    $options = ($custom_options) ? ($std_options + $custom_options) : $std_options;

    // start the first batch of requests
    for ($i = 0; $i < $rolling_window; $i++) {
        $ch = curl_init();
        $options[CURLOPT_URL] = $urls[$i];
        curl_multi_add_handle($master, $ch);

    do {
        while(($execrun = curl_multi_exec($master, $running)) == CURLM_CALL_MULTI_PERFORM);
        if($execrun != CURLM_OK)
        // a request was just completed -- find out which one
        while($done = curl_multi_info_read($master)) {
            $info = curl_getinfo($done['handle']);
            if ($info['http_code'] == 200)  {
                $output = curl_multi_getcontent($done['handle']);

                // request successful.  process output using the callback function.

                // start a new request (it's important to do this before removing the old one)
                $ch = curl_init();
                $options[CURLOPT_URL] = $urls[$i++];  // increment i
                curl_multi_add_handle($master, $ch);

                // remove the curl handle that just completed
                curl_multi_remove_handle($master, $done['handle']);
            } else {
                // request failed.  add error handling.
    } while ($running);
    return true;

Note: I set my max number of parallel requests ($rolling_window) to 100 5. Be sure to update this value according to the bandwidth available on your server / servers you are curling. Be nice and read this first.

Updated 3/6/09: Fixed a missing semi-colon. Thanks to Steve Gricci for catching the typo.

Updated 4/2/09: Made some changes to increase reusability. rolling_curl now expects a $callback parameter for a function that will process each response. It also accepts an array called $options that let’s you add custom curl options such as authentication, custom headers, etc

Updated 4/8/09: Fixed a new bug that was introduced with the last update. Thanks to Damian Clement for alerting me to the problem.