PAGI and Async/Await in Mojolicious.

[ajmetz]

What does Mojolicious think about PAGI?

I am completely new to async / await stuff in Perl, and wondering if this new tech is something I can use with Mojolicious?

I see you did stuff based on https://metacpan.org/pod/Future::AsyncAwait
via the blog https://mojolicious.io/ - with posts in both 2021 and 2019 mentioning it.

So will it be possible to write a non-blocking webapp in Mojolicious, and use it with something like PAGI, in the same way I do with PSGI today for a blocking webapp?

Also - I have begun using Mojo::SQLite for a lot of small project webapps, and wonder if a non-blocking approach to databases is possible with it, or only with Mojo::Pg ...?
There's this strange line in the Mojo::SQLite::Database documentation that keeps cropping up: "For API compatibility with Mojo::Pg; the query is still executed in a blocking manner." This doesn't appear in Mojo::Pg::Database documentation.

PAGI is still in beta, and welcoming feedback. Here are a collection of links:

---

John Napiorkowski's blogging:

Asking for feedback here...
https://dev.to/jjn1056/pagi-asgi-for-perl-or-the-spiritual-successor-to-plack-3kb9

Getting philosophical over how some of it was vibe coded here:
https://dev.to/jjn1056/the-joy-of-code-in-the-age-of-vibe-engineering-49j2

And performance testing a hello world server here...
https://dev.to/jjn1056/pagiserver-performance-and-hardening-l8d

John aims to have a beta of PAGI on CPAN soon:
https://metacpan.org/pod/PAGI
...which will not be production ready,
and is more for testing out, and getting feedback on.

It is on Github to play about with:
https://github.com/jjn1056/pagi

There was also a Perl Advent Calendar entry on the subject of PAGI:
https://perladvent.org/2025/2025-12-10.html

He is currently welcoming feedback on his first attempt at a tutorial too:
https://github.com/jjn1056/pagi/blob/main/lib/PAGI/Tutorial.pod

[Grinnz]

Also - I have begun using Mojo::SQLite for a lot of small project webapps, and wonder if a non-blocking approach to databases is possible with it, or only with Mojo::Pg ...? There's this strange line in the Mojo::SQLite::Database documentation that keeps cropping up: "For API compatibility with Mojo::Pg; the query is still executed in a blocking manner." This doesn't appear in Mojo::Pg::Database documentation.

The SQLite API is synchronous, because it's just opening a file. It does not have the ability to execute a non-blocking query. However, you can use the normal mechanisms of https://metacpan.org/pod/Mojo::IOLoop#subprocess or https://metacpan.org/pod/IO::Async::Function to avoid blocking your main loop with excessively slow queries. Just keep in mind this is less useful with SQLite as it is still just a file and cannot handle simultaneous queries as well as Postgres, especially without WAL mode.

[ajmetz]

Makes sense! I'll do any async experimentation then, with Mojo::Pg and PostgreSQL - although I will likely begin with copying some of the simpler examples in the cookbook that don't involve databases in the first instance.

[Grinnz]

What does Mojolicious think about PAGI?

I am completely new to async / await stuff in Perl, and wondering if this new tech is something I can use with Mojolicious?

I see you did stuff based on https://metacpan.org/pod/Future::AsyncAwait via the blog https://mojolicious.io/ - with posts in both 2021 and 2019 mentioning it.

So will it be possible to write a non-blocking webapp in Mojolicious, and use it with something like PAGI, in the same way I do with PSGI today for a blocking webapp?

It should be possible via Future::AsyncAwait, depending on whether it is designed to allow usage of the Mojolicious loop. I haven't investigated it yet.

[kraih]

For async/await stuff you don't need to search the web for blog posts, we have examples right in the documentation: https://docs.mojolicious.org/Mojolicious/Guides/Cookbook#async-await

[ajmetz]

Yes, the cookbook was linked to in the blog posts! =D.

[kraih]

What does Mojolicious think about PAGI?

All the things PAGI promises we've already been doing in Mojolicious for many years, so there isn't really any advantage for us. To be honest, until this discussion thread i didn't know the spec existed in the first place. Nobody told me.

[kraih]

I was also told PAGI is pretty IO::Async based currently, and he hopes it could be loop agnostic in time.

And this is my main issue with the PAGI spec so far. It only solves the simple problems, like how to do middleware, which isn't all that important. But it doesn't touch on how to combine different event loops, which tends to be the main blocker (pun intended) in async Perl applications.

[ajmetz]

Is there a use case for combining different event loops? Or were you getting at the aspiration to eventually be loop agnostic, - presumably accommodating different event loops - such as Mojo::IOLoop? All this stuff is new to me, so am filling in the gaps in my knowledge as I chat to you. A quick google for "combine event loops", brought up this Python and asyncio discussion thread: https://stackoverflow.com/questions/31623194/asyncio-two-loops-for-different-i-o-tasks

[kraih]

It's a very real requirement, and one we have workarounds for already. Combining modules designed for different event loops is very common in async web applications, given the very diverse module landscape in the Perl world. In this area Perl is not comparable to Python, since event loop APIs are not part of core Perl.

[ajmetz]

Happy New Year, =).

Am interested in learning more. Where should I begin for further reading on combining different event loops, and example scenarios where this may be required, and what workarounds were available to navigate the challenge?

Or are you speaking generally, that due to the lack of such an API in core, and the variety of choice on cpan, that it can be common to end up in a situation where two approaches butt up against one another and you need to create a custom workaround for whatever unique combination is at play?

Before posting this I had a quick skim read of some AnyEvent documentation.
I have only read a few paragraphs of a tutorial intro, and so far it appears to simply add a choice of event loops, rather than necessarily combine them. I of course realise I've only read a few paragraphs of it so far, so perhaps something else is revealed further into the docs.

I've seen there's a book on Mojolicious - and because it appears to be about web clients and not necessarily async, I've done a search of youtube, and am about to hit play on Joel Burger's 2020 "Perl Conference In the Cloud" talk after finishing this comment on "Awaiting Mojolicious".

If there's anything else you recommend I look at, I'd be all ears.

Of course, there's loads in the cookbook for me to take in too on getting to grips with the real time web and Futures aspects of Mojolicious.
https://metacpan.org/pod/Mojo::IOLoop#DESCRIPTION
and
https://metacpan.org/pod/distribution/Mojolicious/lib/Mojolicious/Guides/Cookbook.pod#REAL-TIME-WEB

[jberger]

Just quickly reviewed the particular parts of that talk and it does seem pretty relevant still. Hope it helps.