Controllers

Controllers are the Erlang modules which decide what happens when a browser requests a page. Zotonic looks at the dispatch rules that match the requested URL, and if a dispatch rule matches, the controller that is named in the dispatch rule is used to handle the request.

Anatomy of a controller

Say we have the following dispatch rule, handling http://localhost/example:

{example_url, ["example"], controller_example, []},

When hitting /example, the controller_example controller will be initialized and various callback functions on the controller will be called, according to the HTTP protocol flow.

Controllers are pretty self-documenting, thanks to the names of the cowmachine callback functions. For instance, when you define a function resource_exists/1, it will be called to decide whether or not the page should return a 404 page.

The simplest controller uses Zotonic’s controller_html_helper.hrl include to serve HTML:

-module(controller_example).

-include_lib("controller_html_helper.hrl").

html(Context) ->
    {<<"<h1>Hello</h1>">>, Context}.

Rendering templates

To return the rendered output of a template file in the module’s priv/templates directory, use z_template:render/3:

-module(controller_example).

-include_lib("controller_html_helper.hrl").

html(Context) ->
    % foo and bam will be available as template variables in mytemplate.tpl.
    Html = z_template:render("mytemplate.tpl", [{foo, 'bar'}, {bam, 1234}], Context),
    z_context:output(Html, Context).

If you need more examples, mod_base contains many controllers, implementing basic HTTP interaction but also redirects, websockets, et cetera. The Controllers page lists all available controllers in the Zotonic core.

The Webmachine documentation itself is also very helpful for understanding controllers.

Differences between Cowmachine and Basho’s Webmachine

Zotonic’s fork has been named cowmachine and lives in its separate repository at https://github.com/zotonic/cowmachine).

The main differences with Basho’s Webmachine are:

  • Used Cowboy instead of MochiWeb
  • All callbacks have a single handler
  • Pluggable dispatch handler
  • Support for the HTTP Upgrade: header
  • Optional caching of controller callbacks results
  • Dispatch handler can redirect requests
  • Use of process dictionary has been removed
  • webmachine_request is now a normal (not parametrized) module
  • Extra logging
  • ping and init callbacks are removed

Together, this gave a significant simplification and speed boost to Cowmachine.