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Mojo::AsyncAwait - An Async/Await implementation for Mojolicious


  use Mojolicious::Lite -signatures;
  use Mojo::AsyncAwait;

  get '/' => async sub ($c) {

    my $mojo = await $c->ua->get_p('');
    my $cpan = await $c->ua->get_p('');

    $c->render(json => {
      mojo => $mojo->result->code,
      cpan => $cpan->result->code



Async/await is a language-independent pattern that allows nonblocking asynchronous code to be structured simliarly to blocking code. This is done by allowing execution to be suspended by the await keyword and returning once the promise passed to await has been fulfilled.

This pattern simplies the use of both promises and nonblocking code in general and is therefore a very exciting development for writing asynchronous systems.

If you are going to use this module to create async controllers actions in Mojolicious applications (as seen in the "SYNOPSIS") before Mojolicious version 8.28, you are highly encouraged to also use Mojolicious::Plugin::PromiseActions in order to properly handle exceptions in your action.


The primary goal of this module is to provide a useful Async/Await implementation for users of the Mojolicious ecosystem. It is for this reason that Mojo::Promise is used when new promises are created. Because this is the primary goal, the intention is for it to remain useful even as other goals are considered.

Secondarily, it is intended to be a testbed for early implementations of Async/Await in the Perl 5 language. It is for this reason that the implementation details are intended to be replaceable. The result should hopefully still be backwards compatible, mostly because the interface is so simple. After all, it is just two keywords.

Of course, I always intend as much as possible that Mojolicious-focused code is as useful as practically possible for the broader Perl 5 ecosystem. It is for this reason that while this module returns Mojo::Promises, it can accept any then-able (read: promise) which conforms enough to the Promises/A+ standard. The Promises/A+ standard is intended to increase the interoperability of promises, and while that line becomes more gray in Perl 5 where we don't have a single ioloop implementation, we try our best.

Finally the third goal is to improve the mobility of the knowledge of this pattern between languages. Users of Javascript probably are already familiar with this patthern; when coming to Perl 5 they will want to continue to use it. Likewise, as Perl 5 users take on new languages, if they are familiar with common patterns in their new language, they will have an easier time learning. Having a useable Async/Await library in Perl 5 is key to keeping Perl 5 relevent in moderning coding.


This module actually does very little on its own, it simply loads and imports backend implementations of Async/Await. The reason to use this module really would be to use current default implementation without regards to what that implementation is nor how it works.

When it is loaded, the MOJO_ASYNCAWAIT_BACKEND is checked, if not set then the current default is used.

  # From environment
  use Mojo::AsyncAwait;

  # Currently provided default
  use Mojo::AsyncAwait;

The backend is specified either as a fully qualified module name, e.g. Mojo::AsyncAwait::Backend::CoolBackend or using the + as a shortcut for Mojo::AsyncAwait::Backend::, e.g. +CoolBackend which would mean exactly the same as the former.


First and foremost, this is all a little bit crazy. Please consider carefully before using this code in production.

While many languages have async/await as a core language feature, currently in Perl we must rely on modules that provide the mechanism of suspending and resuming execution.

The default implementation relies on Coro which does some very magical things to the Perl interpreter. Other less magical implementations are in the works however none are available yet. As available implementations change or stabilize, that default may be changed. Backend implementations may be added or even be spun off. If your application depends on the backend implementation, you may import it manually or use the described mechanisms to load it. In that case you should be sure to add the backend to your dependency list in case it is spun off in the future.

Also note that while a Coro-based implementation need not rely on "await" being called directly from an "async" function, it is currently prohibitied because it is likely that other/future implementations will rely on that behavior and thus it should not be relied upon.


Regardless of backend, Mojo::AsyncAwait provides two keywords (i.e. functions), both exported by default. Depending on backend, their exact behavior might change slightly, however, implementers should attempt to follow the api described here as closely as possible.

Some backends may allow additional options to be passed to the keywords; those options should be kept minimal and if possible follow the conventions described in Mojo::AsyncAwait::Backend::Coro. This generic document will not describe those additional options.


  my $sub = async sub { ... };

The async keyword wraps a subroutine as an asynchronous subroutine which is able to be suspended via "await". The return value(s) of the subroutine, when called, will be wrapped in a Mojo::Promise.

The async keyword must be called with a subroutine reference, which will be the body of the async subroutine.

Note that the returned subroutine reference is not invoked for you. If you want to immediately invoke it, you need to so manually.

  my $promise = async(sub{ ... })->();

If called with a preceding name, the subroutine will be installed into the current package with that name.

  async installed_sub => sub { ... };

Unlike the case of an anonymous wrapped async subroutine reference described above, if the subroutine is installed, nothing is returned.


  my $tx = await Mojo::UserAgent->new->get_p('');
  my @results = await (async sub { ...; return @async_results })->();

The await keyword suspends execution of an async sub until a promise is fulfilled, returning the promise's results. In list context all promise results are returned. For ease of use, in scalar context the first promise result is returned and the remainder are discarded.

If the value passed to await is not a promise (defined as having a then method), it will be wrapped in a Mojo::Promise for consistency. This is mostly inconsequential to the user.

Note that await can only take one promise as an argument. If you wanted to await multiple promises you probably want "all" in Mojo::Promise or less likely "race" in Mojo::Promise.

  my $results = await Mojo::Promise->all(@promises);


Joel Berger <>

Marcus Ramberg <>


Sebastian Riedel <>


Matt S Trout (mst)

Paul Evans (LeoNerd)

John Susek


Copyright (C) 2018, "AUTHORS" and "CONTRIBUTORS".

This program is free software, you can redistribute it and/or modify it under the terms of the Artistic License version 2.0.





MDN Async/Await