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Web::Machine::Manual(3pm) User Contributed Perl Documentation Web::Machine::Manual(3pm)


Web::Machine::Manual - Learn how to use Web::Machine


version 0.17

Web::Machine IN A NUTSHELL

The basic idea behind "Web::Machine" is that the handling of a web request is implemented as a state machine. If you're not familiar with state machines, think of a flowchart. We look at the request and the resource we provide and ask questions about them. Is our service available? Is this a GET, POST, PUT, etc.? Does the request ask for a content type our resource provides?

The result of each question leads us to the next state (or flowchart box). Eventually we reach a point where we have a response for the client. Since this is all built on top of Plack and PSGI <>, the response consists of a status code, some headers, and an optional body.

The best way to understand the full request/response cycle is to look at the original Erlang webmachine state diagram <>. Each diamond in that diagram corresponds to a method that your Web::Machine::Resource subclass can implement. The return value from your method determines what method to call next.

However, unlike on that diagram, we often support return values beyond simple true/false values for methods. The Web::Machine::Resource documentation describes what each method can return.

Web::Machine and Plack

"Web::Machine" is built on top of Plack and follows the PSGI <> spec. You can mix "Web::Machine" applications with other Plack applications using standard Plack tools like Plack::Builder.

Web::Machine and Plack Middleware

Since "Web::Machine" implements the complete request and response cycle, some Plack middleware is not really needed with "Web::Machine". For example, it wouldn't make sense to use something like "Plack::Middleware::XSLT" with "Web::Machine". "Web::Machine" implements the full content negotiation process, so if you want to handle requests for "text/html" it probably makes more sense to do this in your resources. The benefit of doing so is that with "Web::Machine" you can easily ensure that you return a proper "406 Not Acceptable" status for content types you can't handle.

There are still many pieces of Plack middleware that are useful with "Web::Machine", such as logging middleware, debugging/linting middleware, etc.

That all said, "Web::Machine" won't break if you use an inappropriate middleware; you'll just lose some of the benefits you get from implementing things the "Web::Machine" way.

Bodies Must be Bytes

The PSGI spec requires that the body you return contain bytes, not Perl characters. In other words, strings you return must be passed through "Encode::encode" so that Perl interprets their contents as bytes.

If your data is not binary or ASCII, your resource should make sure to provide "charset_provided()" and "default_charset()" methods. This will make sure that "Web::Machine" knows how to turn your response bodies into bytes.

CAVEAT: Note that currently "Web::Machine" does not provide full charset or encoding support when the body is returned as a CODE ref. This is a bug to be remedied in the future, but currently you are responsible for making sure this code ref returns bytes.


bugs may be submitted through <>.


  • Stevan Little <>
  • Dave Rolsky <>


This software is copyright (c) 2016 by Infinity Interactive, Inc.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

2021-01-08 perl v5.32.0