Async: Examples

Here some code examples representing a slew of possible async scenarios. Obviously this does not cover all possible situations, but they should give you an idea of how and where async can be used effectively. Some of these examples are found spread out through the rest of the async documentation; they are added here again for consolidation purposes.


This example shows the basic tenants of async, particularly the keywords used, etc.


namespace Hack\UserDocumentation\Async\Examples\Examples\Basic;

// async specifies a function will return an awaitable. Awaitable<string> means
// that the awaitable will ultimately return a string when complete
async function trivial(): Awaitable<string> {
  return "Hello";

async function call_trivial(): Awaitable<void> {
  // These first two lines could be combined into
  //     $result = await trivial();
  // but wanted to show the process

  // get awaitable that you can wait for completion
  $aw = trivial();
  // wait for the operation to complete and get the result
  $result = await $aw;
  echo $result; // "Hello"



To get the result of an awaitable in a non-async function, you can use join().


namespace Hack\UserDocumentation\Async\Guidelines\Examples\Join;

async function join_async(): Awaitable<string> {
  return "Hello";

// In an async function, you would await an awaitable.
// In a non-async function, or the global scope, you can
// use `join` to force the the awaitable to run to its completion.
$s = \HH\Asio\join(join_async());


You can have async closures, including using the shorter lambda syntax.


namespace Hack\UserDocumentation\Async\Examples\Examples\Closures;

async function closure_async(): Awaitable<void> {
  // closure
  $hello = async function(): Awaitable<string> {
    return 'Hello';
  // lambda
  $bye = async ($str) ==> $str;

  // The call style to either closure or lambda is the same
  $rh = await $hello();
  $rb = await $bye("bye");

  echo $rh . " " . $rb . \PHP_EOL;

Hello bye

Data Fetching

This shows you a way to organize your async functions in such a way that we have a nice clean data dependency graph.


namespace Hack\UserDocumentation\Async\Guidelines\Examples\DataDependencies;

// So we can use asio-utilities function vm()
require __DIR__ . "/../../../../vendor/hh_autoload.php";

class PostData {
  // using constructor argument promotion
  public function __construct(public string $text) {}

async function fetch_all_post_ids_for_author(int $author_id)
  : Awaitable<array<int>> {

  // Query database, etc., but for now, just return made up stuff
  return array(4, 53, 99);

async function fetch_post_data(int $post_id): Awaitable<PostData> {
  // Query database, etc. but for now, return something random
  return new PostData(\str_shuffle("ABCDEFGHIJKLMNOPQRSTUVWXYZ"));

async function fetch_comment_count(int $post_id): Awaitable<int> {
  // Query database, etc., but for now, return something random
  return \rand(0, 50);

async function fetch_page_data(int $author_id)
  : Awaitable<Vector<(PostData, int)>> {

  $all_post_ids = await fetch_all_post_ids_for_author($author_id);
  // An async closure that will turn a post ID into a tuple of
  // post data and comment count
  $post_fetcher = async function(int $post_id): Awaitable<(PostData, int)> {
    list($post_data, $comment_count) =
      await \HH\Asio\v(array(
    /* The problem is that v takes Traverable<Awaitable<T>> and returns
     * Awaitable<Vector<T>>, but there isn't a good value of T that represents
     * both ints and PostData, so they're currently almost a union type.
     * Now we need to tell the typechecker what's going on.
     * In the future, we plan to add HH\Asio\va() - VarArgs - to support this.
     * This will have a type signature that varies depending on the number of
     * arguments, for example:
     *  - va(Awaitable<T1>, Awaitable<T2>): Awaitable<(T1, T2)>
     *  - va(Awaitable<T1>,
     *       Awaitable<T2>,
     *       Awaitable<T3>): Awaitable<(T1, T2, T3)>
     * And so on, with no need for T1, T2, ... Tn to be related types.
    invariant($post_data instanceof PostData, "This is good");
    invariant(is_int($comment_count), "This is good");
    return tuple($post_data, $comment_count);

  // Transform the array of post IDs into an array of results,
  // using the vm() function from asio-utilities
  return await \HH\Asio\vm($all_post_ids, $post_fetcher);

async function generate_page(int $author_id): Awaitable<string> {
  $tuples = await fetch_page_data($author_id);
  $page = "";
  foreach ($tuples as $tuple) {
    list($post_data, $comment_count) = $tuple;
    // Normally render the data into HTML, but for now, just create a
    // normal string
    $page .= $post_data->text . " " . $comment_count . \PHP_EOL;
  return $page;

print \HH\Asio\join(generate_page(13324)); // just made up a user id

Accessing MySQL

Use the async mysql extension to perform database connection and queries.


namespace Hack\UserDocumentation\Async\Extensions\Examples\MySQL;

require __DIR__ . '/';

use \Hack\UserDocumentation\Async\Extensions\Examples\AsyncMysql\ConnectionInfo as CI;

async function get_connection(): Awaitable<\AsyncMysqlConnection> {
  // Get a connection pool with default options
  $pool = new \AsyncMysqlConnectionPool(array());
  // Change credentials to something that works in order to test this code
  return await $pool->connect(

async function fetch_user_name(\AsyncMysqlConnection $conn,
                               int $user_id) : Awaitable<string> {
  // Your table and column may differ, of course
  $result = await $conn->queryf(
    'SELECT name from test_table WHERE userID = %d',
  // There shouldn't be more than one row returned for one user id
  invariant($result->numRows() === 1, 'one row exactly');
  // A vector of vector objects holding the string values of each column
  // in the query
  $vector = $result->vectorRows();
  return $vector[0][0]; // We had one column in our query

async function get_user_info(\AsyncMysqlConnection $conn,
                             string $user): Awaitable<Vector<Map>> {
  $result = await $conn->queryf(
    'SELECT * from test_table WHERE name = %s',
  // A vector of map objects holding the string values of each column
  // in the query, and the keys being the column names
  $map = $result->mapRows();
  return $map;

async function async_mysql_tutorial(): Awaitable<void> {
  $conn = await get_connection();
  if ($conn !== null) {
    $result = await fetch_user_name($conn, 2);
    $info = await get_user_info($conn, 'Fred Emmott');
    \var_dump($info instanceof Vector);
    \var_dump($info[0] instanceof Map);

string(11) "Fred Emmott"


Use rescheduling (via HH\Asio\later()) to batch up operations to send multiple keys in a single request over a high latency network (for example purposes, the network isn't high latency, but just returns something random).


namespace Hack\UserDocumentation\Async\Guidelines\Examples\Batching;

// For asio-utilities function later(), etc.
require __DIR__ . "/../../../../vendor/hh_autoload.php";

async function b_one(string $key): Awaitable<string> {
  $subkey = await Batcher::lookup($key);
  return await Batcher::lookup($subkey);

async function b_two(string $key): Awaitable<string> {
  return await Batcher::lookup($key);

async function batching(): Awaitable<void> {
  $results = await \HH\Asio\v(array(b_one('hello'), b_two('world')));
  \printf("%s\n%s\n", $results[0], $results[1]);


class Batcher {
  private static array<string> $pendingKeys = array();
  private static ?Awaitable<array<string, string>> $aw = null;

  public static async function lookup(string $key): Awaitable<string> {
    // Add this key to the pending batch
    self::$pendingKeys[] = $key;
    // If there's no awaitable about to start, create a new one
    if (self::$aw === null) {
      self::$aw = self::go();
    // Wait for the batch to complete, and get our result from it
    $results = await self::$aw;
    return $results[$key];

  private static async function go(): Awaitable<array<string, string>> {
    // Let other awaitables get into this batch
    await \HH\Asio\later();
    // Now this batch has started; clear the shared state
    $keys = self::$pendingKeys;
    self::$pendingKeys = array();
    self::$aw = null;
    // Do the multi-key roundtrip
    return await multi_key_lookup($keys);

async function multi_key_lookup(array<string> $keys)
  : Awaitable<array<string, string>> {

  // lookup multiple keys, but, for now, return something random
  $r = array();
  foreach ($keys as $key) {
    $r[$key] = \str_shuffle("ABCDEF");
  return $r;


You can use rescheduling in a polling loop to allow other awaitables to run. You may need a polling loop in a case where a service does not have an async function to add to the scheduler.


namespace Hack\UserDocumentation\Async\Examples\Examples\Polling;

// For asio-utilities function later(), etc.
require __DIR__ . "/../../../../vendor/hh_autoload.php";

// Of course, this is all made up :)
class Polling {
  private int $count = 0;
  public function isReady(): bool {
    if ($this->count++ === 10) {
      return true;
    return false;
  public function getResult(): int {
    return 23;

async function do_polling(Polling $p): Awaitable<int> {
  echo "do polling 1" . \PHP_EOL;
  // No async function in Polling, so loop until we are ready, but let
  // other awaitables go via later()
  while (!$p->isReady()) {
    await \HH\Asio\later();
  echo "\ndo polling 2" . \PHP_EOL;
  return $p->getResult();

async function no_polling(): Awaitable<string> {
  echo '.';
  return \str_shuffle("ABCDEFGH");

async function polling_example(): Awaitable<void> {
  $handles = array(do_polling(new Polling()));
  // To make this semi-realistic, call no_polling a bunch of times to show
  // that do_polling is waiting.
  for ($i = 0; $i < 50; $i++) {
    $handles[] = no_polling();

  $results = await \HH\Asio\v($handles);

do polling 1
do polling 2