PSGI: What is it and what's the fuss about?

Question

I have been trying to decide if my web project is a candidate for implementation using PSGI, but I don't really see what good it would do for my application at this stage.

I don't really understand all the fuss. To me PSGI seems like a framework that provides a common interface between different Apache modules which lets you move your application between them. e.g Easily move your application from running on mod_perl to fastcgi, and provide the application support for running on both options.

Is that right, or have I missed something?

As I and the team I am a part of not only develop the application, but also pretty much do maintenance and setup of servers I don't see the value for us of being able to run on fastcgi, cgi, and mod_perl, we do just fine with just mod_perl.

Have I misunderstood the PSGI functionality, or is it just not suitable for my project?

Answer 1

Forget the Apache bit. It's a way of writing your application so that the choice of webserver becomes less relevant. At $work we switched to Plack/PSGI after finding our app running with very high CPU load after upgrading to Apache2 - benchmarking various Apache configs and NYTProf'ing were unable to determine the reason, and using PSGI and the Starman webserver worked out much better for us.

Now everything is handled in one place by our PSGI app (URL re-writes, static content, expiry headers, etc) rather than Apache configuration, so it's a) Perl, and b) easily tested via our standard /t/ scripts. Also our tests are now testing exactly what a user sees, rather than just the basic app itself.

It may well not be relevant to you if you're happy with Apache and mod_perl, and I'm sure others will be able to give much better answers, but for us not having to deal with anything Apache-related again is such a relief in itself. The ease of testing, and the ability to just stick in a Data::Dumper and see what's going on rather than wrestling with ModRewrite and friends, is a great boon.

Answer 2

Borrowing from a recent blog post by chromatic, Why PSGI/Plack Matters (Testing), here's what it is:

It's a good idea borrowed from Python's WSGI and Ruby's Rack but made Perlish; it's a simple formalizing of a pattern of web application development, where the entry point into the application is a function reference and the exit point is a tuple of header information and a response body.

That's it. That's as simple as it can be, and that simplicity deceives a lot of people who want to learn it.

An important benefit is, ibid.,

Given a Plack application, you don't have to deploy to a web server—even locally—to test your application as if it were deployed … Plack and TWMP (and Plack::Test) use the well-defined Plack pattern to make something which was previously difficult into something amazingly easy. They're not the first and they won't be the last, but they do demonstrate the value of Plack.