Whenever I've needed a quick and easy solution for parallel batch processing, particularly where I need to pull a lot of data through external APIs ( like REST or SOAP for instance ), I've found php Curl extension's ability to run multiple http queries in parallel via curl_multi_select invaluable.
uk.php.net/manual/en/function.curl-
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While not fully asynchronous, data IS processed as it becomes available within the scope of any multi fetch query.