The Mojolicious Core Team is delighted to announce the release and immediate availability of version 9.0 of the Mojolicious real-time web framework. Every Mojolicious major release has a code name based on a unicode character and this one is lovingly named "Waffle" 🧇!

Every major release I think that there can't be much more to add or change in Mojolicious and then when the next one arrives I reflect on how much has changed since I last thought that. In 9.0 there's far too much to discuss each at length, so I'm going to highlight some of my favorites and then include a distilled list of changes since 8.0.

Perl 5.16 and beyond

Mojolicious now requires Perl version 5.16. This change gives us useful tools to build Mojolicious including the __SUB__ token for building self-referential callbacks without leaking. We were pleased that the community response since the change a few months ago has been ... well crickets. This is great news since we do hope to move to Perl 5.20 in the not so distant future so that we can use Perl's native signatures once that is a sound option for us. We are already encouraging the use of signatures both in the documentation and in our code generation commands.

Asynchronous Functionality

Clearly the highlight of the pre-9.0 development cycle has been the integration of Async/Await. Thanks to Paul Evans (LeoNerd)'s efforts, when you have Future::AsyncAwait installed Mojolicious can use its new keywords async and await to provide the most seamless asynchronous development possible, something that is becoming more and more the standard for asynchronous code in other languages. Writing a non-blocking endpoint is now as simple as

use Mojolicious::Lite -signatures, -async_await;

# Request HTML titles from two sites non-blocking
get '/' => async sub ($c) {
  my $mojo_tx    = await $c->ua->get_p('');
  my $mojo_title = $mojo_tx->result->dom->at('title')->text;
  my $cpan_tx    = await $c->ua->get_p('');
  my $cpan_title = $cpan_tx->result->dom->at('title')->text;

  $c->render(json => {mojo => $mojo_title, cpan => $cpan_title});


When I teach non-blocking to people I can now tell them to follow a trivial recipe for most non-blocking tasks. Simply await any function that returns a promise and mark any function that uses await with the async keyword. Note also that all async functions return promises so await any calls to them. There are some optimizations you can make at times and top-level await (if you aren't in a Mojolicious webapp) can be a little strange but to a first approximation that's all you need to write a non-blocking webapp today!

Speaking of promises (which are at the heart of Async/Await), Mojo::Promise has grown a lot this cycle too, adding all_settled, any, map, timer, and timeout class methods, changing its constructor to be more like the one in JavaScript and to warn when an unhandled rejected promise is destroyed. The latter can help in development where such cases used to hide errors, now, much like the browser, you can at least find where things are going wrong even if you didn't wire all your promises to a catch callback like you should.

Focus on Containerization and Cloud Deployments

The Mojolicious Core Team is aware that more and more deployments are in containers and cloud services these days and we've been thinking of ways to make that easier. With 9.0 we've added two new features specifically designed to help with these types of deployments.

Most existing applications assume a standard deployment by an unprivileged user behind a reverse proxy, but with new deployment strategies the number of configurations an application might expect are too numerous to expect each application to support. To address this we've added support for deployment specific plugins that help you add deployment-level functionality to existing applications. Now the deployer is in control of these features, rather than needing to ask the application author to add support.

We've also extended the existing reverse proxy support to allow for multiple trusted proxies, specifically when you know what the expected IP address or CIDR network you're expecting proxy requests from. This can be useful both when deploying from say Kubernetes where it is likely you'll have several layers of local proxies, or when you're behind a third-party proxy like CloudFlare, who publish a list of their proxy networks. This is an important safety feature in these situations, blindly trusting forwarding headers can lead to big problems.

Also, we've noticed that many cloud deployment tools are based on YAML, so we've added a YAML configuration file loader to our existing Perl and JSON configuration loaders. Because YAML is a tricky target and because ours is technically not-quite-YAML, we've called ours cheekily NotYAMLConfig. In the unlikely event that that distinction should matter for your needs, you can load a specific YAML module from CPAN.

But it isn't just functionality, we've added a Dockerfile generation (myapp generate dockerfile) and added a Container Cookbook recipe to the documentation. We've also added a Cookbook recipe for the popular cloud-native proxy Envoy. If you have other common container or cloud recipes that you think should be added, please let us know!

Documentation Improvements

Speaking of documentation, you might have noticed that both our documentation site and built-in templates for 404 and 500 pages have gotten a face lift.

They're even prettier and easier to read than before, they're responsive, and they have a newer code library (highlight.js) to better render our code. Further the documentation site is now redeployed immediately on any push to master of the website or the main Mojolicious projects, mojolicious, mojo-pg, and minion, so that you always have the most up-to-date docs at your fingertips.

Along with the container and cloud-native changes I've already highlighted, we have updated our documentation to use of upcoming Perl native signatures throughout and paired it with updating our app and lite-app generators to generate applications with signatures too.

Routing (and Rendering)

The routing engine is an often overlooked part of a web framework but it really is at the heart of everything that it does. It can also be a source of confusion or frustration, especially when it doesn't do what you expect. For 9.0 we've made the router both safer and more closely aligned to user expectations.

The biggest change is that Mojolicious now prevents applications from using reserved placeholders in routes. This was originally considered a "feature" because it allowed very terse route definitions for quick apps, but in practice most people that understood how that worked just used the long form while unsuspecting newcomers would trip on it from time to time. We've also made several cases throw exceptions when routing would fail that used to fail silently or unexpectedly. When a route points to a missing controller, a namespace without a controller or a controller without an action, you now get an exception rather than a 404, which would often confuse people. Additionally, you get an exception when when auto-rendering fails or a call to render cannot render a response, which would appear to hang (while it actually was trying to wait for a delayed response that was never going to happen).

We've even simplified some of the routing method names themselves. We've removed the lesser-used detour and route methods in favor of the more generic any method. We've renamed method that designates which HTTP methods to respond to (usually when picking several but not all of them, e.g. GET and POST but not any) from via to methods. We've also renamed the method that designates conditional routing from over to requires. I'm especially excited about this one since over implied a contrast with the commonly used under method, where no such contrast is meant to be implied; they do very different things.


The big one for me is that Mojolicious now is capable of sanely using a contextual logger! Mojolicious applications have a log attribute that holds a Mojo::Log instance, however that logger is global to the application and any context you'd want to add is only for a given request/response cycle. Unlike blocking frameworks, a Mojolicious app might switch between several requests before any are rendered, meaning manually keeping context attached to that global logger was nearly impossible. With 9.0 we now have the ability to use a per-controller "child" logger which descends from the global logger and still logs via it, but can keep its own context information. It feeds that information to its parent when you log from it.

By default, these child loggers now have a random request id attached to it, however you can put whatever context you'd like to add there. You can also pass these loggers (themselves an instance of Mojo::Log) to models or other code that might want to log with that context attached.

We've also improved logging performance in cases where generating the log message is expensive and useless if the log level means that the log message is not emitted. If you pass a closure (code reference) to the logger's debug, info, warn, error, and fatal methods and the logger will only invoke that closure if the log level check is met. In fact this improved Mojolicious' own performance metrics by 10%!

$c->log->debug(sub{ 'Thought you might want to know: ' . generate_expensive_log_output() });

Please note that we have changed the log format slightly so be aware if you care about such things.

And to simplify things, we've removed the confusing behavior of logging to a file if a log directory exists. While it sometimes did what people wanted, it more often than not confused newcomers who couldn't find their logs. The behavior now is to always log to STDERR but you can easily change it to point the file of your choice while your application is starting up.

Proxy Helpers

How often have you written an application which takes a request from a client, makes a request of another server, then responds back to the client with the result? I've done that quite often. While it may seem trivial to do, there can be some subtle problems like what do you do if the read and write speeds mismatch badly enough? Never thought about that? Well you don't have to anymore.

Mojolicious now comes with proxy helpers, the generic proxy->start_p and the shortcuts proxy->get_p and proxy->post_p for common GET and POST requests. Proxying a GET request in an action can now be as simple as

$c->proxy->get_p('')->catch(sub ($err) {
  $c->log->debug("Proxy error: $err");
  $c->render(text => 'Something went wrong!', status => 400);

while using start_p you can customize the transactions to your exact needs by subscribing to available events. See the linked documentation for an example.


CSS Selectors are amazing, but one major failing is that you can only extract the last element in your selector, not something in the middle. Or to put it another way, they lack is the ability to conditionally match an element based on a property of its children. Well CSS4 proposes something that lets you assert just such a condition, the :has pseudo-class.

Without it, if you wanted to find all the links that have images as their content, you'd have to match a > img and traverse back up to the parent (which gets even harder if it isn't a direct child, e.g. a img). However now you can simply do something like

$ mojo get 'a:has(> img)' attr href

for only direct descendants or

$ mojo get 'a:has(img)' attr href

to see all such links. In its syntax, the inner selector is a "relative selector" where the a element in question, called the :scope, is implicitly a "virtual root".

$ mojo get 'a:has(:scope > img)' attr href

would be the same query.

Another neat feature we've imported from CSS4 is the :is pseudo-class. In CSS3 you can "or" together several queries with a comma, but only for each query as a whole. If you wanted to search for h1 elements inside of other tags, you'd have to do for example section h1, article h1, aside h1, nav h1. CSS4 gives you the :is pseudo-class that lets you group logical portions of a selector together so you can make a more concise and comprehensible query. The previous example can be rewritten as :is(section, article, aside, nav) h1.

Now you've seen that I've mentioned that these are CSS4 features but CSS4 is not yet had a stable release, it is still being changed and updated. Mojolicious supports these selectors (and a few others from CSS4) but they will continue to be marked as experimental until the spec is released officially. Until then they might be changed or removed as the CSS4 spec changes.

Team and Process

During the 9.0 development cycle the team grew! We've welcomed CandyAngel and Christopher Rasch-Olsen Raa (mishanti1) and welcomed back Dan Book (Grinnz). We're very happy to have them!

We've also added some automation. In addition to the documentation site that I've already mentioned, and existing test runners, we've also added a perltidy check so that all PRs conform to our rules about it. We also simplified our PR process so that if a PR gets two approvals from the team it is automatically merged. This is a wonderful process boost since it encourages PRs to be reviewed and ensures that PRs with interest can see action quickly.

Finally, We're always looking for more help and to encourage people to do so, we've decided to open the process somewhat. We are encouraging people to review PRs even if you don't have review rights. This will do two things for us, first it will give us more opinions both in regards to community interest and from people who might have technical knowledge of the topic. Secondly, we've started tracking who is reviewing PRs and if you catch our notice, you might be on track to join more officially.

If this sounds interesting, at any level, we hope to see you on our pulls page. While you are there, you might see that we've enabled github discussions as an alternative to (and possible eventual replacement for) our mailing list.

Thanks for reading this! We hope you'll love Mojolicious 9.0!

As promised here is a distillation of all the non-trivial Changes since 8.0.

  • Increased Perl version requirement to 5.16.0. This is just a first step however, at some point in the not so distant future we will increase the Perl version requirement to 5.20.0 for full subroutine signatures support
  • Async/Await
    • Requires Future::AsyncAwait 0.36
    • enable with -async_await
  • Mojo::Promise enhancements
    • Added all_settled, any, map, timer, timeout
    • constructor arguments more like Javascript
    • warn when unhandled rejected promise is destroyed
    • Added MOJO_PROMISE_DEBUG environment variable
    • Improved wait method in Mojo::Promise not to be affected by manually stopped event loops
    • Improved eval command with support for promises
  • Docker
    • generate dockerfile command
    • Container cookbook recipe
  • Envoy deployment recipe
  • Trusted proxy support
  • Added support for deployment specific plugins
  • Disallowed the use of reserved stash values, such as /:action, in route patterns
  • Make routing safer and clearer
    • Throw exceptions for missing controllers
    • Disallowed namespace without controller for routing
    • Throw exceptions for routes with controllers but without action
    • Die if auto rendering failed or call to $c->render cannot render a response
  • Cleaned up lesser-used router methods
    • Removed detour, route (use any)
    • Renamed over to requires, via to methods
  • Contextual logger
    • Added log helper to which builds a child logger with context (request id)
    • Added context method to Mojo::Log
  • Slightly changed log format (datetime format, process id, lines joined with spaces)
  • Removed automatically logging to log/$mode.log if log directory exists, default is always STDERR now
  • Improved log messages generated by Mojolicious to include request ids when possible
  • Improved debug, error, fatal, info and warn methods in Mojo::Log to accept closures to generate log messages on demand, so expensive code for debugging can be deactivated easily in production
    • Improved Mojolicious performance by up to 10% with more efficient logging
  • Added support for YAML config files:
    • Added module Mojolicious::Plugin::NotYAMLConfig
    • Improved app generator command to use a YAML config file
  • Proxying with backpressure monitoring
    • Added proxy->get_p, proxy->post_p and proxy->start_p helpers
    • Added high_water_mark attribute in Mojo::IOLoop::Stream
    • Added bytes_waiting and can_write methods in Mojo::IOLoop::Stream
  • Added EXPERIMENTAL support for SameSite cookies to better protect Mojolicious applications from CSRF attacks
    • Added EXPERIMENTAL samesite attributes to Mojo::Cookie::Response and Mojolicious::Cookies
  • Improved helper performance (Mojo::DynamicMethods)
  • Added before_command hook (expimental)
  • Mojo::DOM
    • all_text method now excludes <script> and <style> from text extraction in HTML documents
  • Mojo::DOM::CSS
    • Added :scope, :has (experimental)
    • Added :is (experimental)
    • Added :any-link (experimental)
    • Case-sensitive attribute selectors like [foo="bar" s]
  • Mojo::IOLoop::Subprocess
    • Added support for progress updates (progress method and event)
    • Added run_p method
    • Added exit_code method
    • Added cleanup event
  • Improved Mojolicious::Commands to treat commands like mojo generate lite_app as mojo generate lite-app
  • Test::Mojo
    • Improved extenability and testability of Test::Mojo with test method and handler attribute
    • Added attr_is, attr_isnt, attr_like and attr_unlike methods
    • Added header_exists and header_exists_not methods
  • Mojolicious::Validator
    • Added not_empty filter
    • Simplify size check
    • Fixed validator to also validate empty string values instead of ignoring them. This behaviour had caused a lot of confusion in the past
  • Added compression (gzip) functionality to Mojolicious::Renderer for dynamic content
  • Improved is_fresh method in Mojolicious::Static with support for weak etags
  • Mojo::Exception
    • Added support for MOJO_EXCEPTION_VERBOSE environment variable
    • raise, check functions
    • include a stack trace in verbose output
  • Mojo::Base
    • Improved Mojo::Base flags not to require a certain order
    • Added support for weak reference accessors to Mojo::Base
    • Improved Mojo::Base to enable the Perl 5.16 feature bundle with unicode_strings, unicode_eval, evalbytes, current_sub and fc
  • Mojo::Server::Daemon (and thus the other servers) now differentiate between inactivity_timeout and keep_alive_timeout
  • Mojo::Reactor::again can now change the timeout for a retry
  • New conveniences
    • Added Mojo::Util::network_contains
    • Added Mojo::Util::scope_guard
    • Added content_type and file_types to Mojolicious::Types
    • Added extname, curfile, stat, lstat, remove, and touch to Mojo::File
    • humanize_bytes in Mojo::Util and Mojo::ByteStream
    • Added head and tail to Mojo::Collection
    • Added l function to ojo
    • Added save_to method to Mojo::Message
  • Website improvements
    • Style
    • Responsiveness
    • Replaced prettify.js with highlight.js
    • auto-deployed on push to master
  • Built-in templates updated too
  • Encouraged signatures
    • Improved app and lite_app generators to use templates with subroutine signatures
    • Updated all documentation to use subroutine signatures
  • Removed module Mojo::IOLoop::Delay
    • Spun out to CPAN for long term compat (but not continued development)
  • Removed deprecated success method from Mojo::Transaction (use !$tx->error or $tx->result)
  • Cleaned up lesser used connection properties in Mojo::UserAgent and Mojo::IOLoop::Client
  • Removed config stash value
  • Removed Mojo::Collection::slice

Tagged in : async/await, non-blocking, promises, documentation, deployment, routing

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Joel Berger

Joel has Ph.D. in Physics from the University of Illinois at Chicago. He an avid Perl user and author and is a member of the Mojolicious Core Team.