The middleware stash


The stash is now managed via Catalyst::Middleware::Stash. Why we did this and what it enables is the subject of this article.


In the last year we've started to move code out of Catalyst and into middleware. There's a number of reasons for this. First of all it allows us to share (and to take advantage of) code across all PSGI development platforms. I think its a cleaner interface that is easier for new programmers to understand. It also enables us to aim to build Catalyst in a more moduler way (for example you could someday build a lightweight Catalyst with no stash and without some other features in order to strip it down for speed; this approach works well for other projects like Apache).

By moving the stash to middleware this mean that the stash is now available to applications you mount under your Catalyst controllers (even other Catalyst applications.)

The Catalyst stash middleware allows an application to 'inherit' a stash from a wrapping application. This wrapped application can then add to and change the stash but when the response 'bubbles up' to the wrapping application the stash reverts; its localized properly.


Lets say you have a <Catalyst> application that defines a controller like this

    package MyAppChild::Controller::User;

    use base 'Catalyst::Controller';

    sub stash :Local {
      my ($self, $c) = @_;
      $c->stash->{inner} = "inner";
      $c->res->body( "inner: ${\$c->stash->{inner}}, outer: ${\$c->stash->{outer}}");

And you have another Catalyst application that has a controller which 'mounts' this first application:

    package MyAppParent::Controller::User;

    use base 'Catalyst::Controller';
    use MyAppChild;

    sub stash :Local {
      my ($self, $c) = @_;
      $c->stash->{outer} = "outer";
      $c->res->from_psgi_response( MyAppChild->to_app->($c->req->env) );

If you run the application and issue "GET /user/stash" the result is:

    inner: inner, outer: outer

Since your response is coming from MyAppChild and that application is sharing the stash from its 'wrapping' application MyAppParent.


This technique allows you to break up a complex Catalyst application into several smaller applications and still share a stash. It also allows you to share a stash into mounted applications (such as if you mount a Web::Machine application under Catalyst) or if you are undertaking a major refactor and have two applications side by side.

More Information

You should review Plack::Middleware:: for more details.


John Napiorkowski