Asynchronous Operations: Concurrent

This functionality requires HHVM 4.6 or later.

concurrent concurrently awaits all awaits within a concurrent block and it works with await-as-an-expression as well!

Note: concurrent doesn't mean multithreading

Syntax

concurrent {
  $x = await x_async();
  await void_async();
  $sum = await y_async() + await z_async()();
}
$y = $x + $sum;

Order-of-execution

Each of the statements in a concurrent block should be thought of as running concurrently. This means there shouldn't be any dependencies between the statements in a concurrent block.

Similar to await-as-an-expression, concurrent blocks don't provide a guaranteed order-of-execution between expressions being evaluated for any statement in the concurrent block. We guarantee that modifications to locals will happen after all other expressions resolve and will happen in the order in which they would happen outside a concurrent block.

For this example, we provide no guarantee on the execution order of the calls to x(), y_async() and z_async(). The assignments into $result are guaranteed to be well ordered however.

$result = vec[];
concurrent {
  $result[] = x() + await y_async();
  $result[] = await z_async();
}

Exceptions

If any statement in a concurrent block throws, there are no guarantees about which (if any) awaited values were assigned or which exception will be propagated if more than one of them threw. For example if you have:

$x = 0;
try {
  concurrent {
    $x = await async { return 1; };
    await async { throw new Exception('foo'); }
    await async { throw new Exception('bar'); }
  }
} catch (Exception $e) {
  var_dump($x, $e->getMessage());
}

Then it is explicitly undefined whether var_dump will see $x === 0 or $x === 1 and whether the message will be 'foo' or 'bar'.

If you need granular exception handling, consider using nested try-catch blocks inside the concurrent block:

concurrent {
  $x = await async {
    try {
      ...
      return <success path>;
    } catch (...) {
      return <fallback>;
    }
  };
  $y = await async {
    // same here
  };
}