Nitrogen Handlers

Handler Overview

Core Nitrogen behavior has been broken out into well-defined, pluggable behavior modules called /handlers/. Handlers allow you to easily substitute your own logic for things like session, security, routing, and others. Simply create a module that implements one of the existing behaviors, and register it to call nitrogen:handler/2 between the nitrogen:init_request/2 and nitrogen:run/0 calls found inside the nitrogen_xxx (where xxx is the webserver used, for example 'mochiweb', 'yaws', 'cowboy', etc).

For example, let's add custom session and config handlers to our mochiweb install. To do so, let's add our handlers to nitrogen_mochiweb:loop/1:

  loop(Req) ->
    {ok, DocRoot} = application:get_env(mochiweb, document_root),
    RequestBridge = simple_bridge:make_request(mochiweb_request_bridge, Req),
    ResponseBridge = simple_bridge:make_response(mochiweb_response_bridge, {Req, DocRoot}),
    nitrogen:init_request(RequestBridge, ResponseBridge),
    nitrogen:handler(my_config_handler,[]),   %% Add custom config handler
    nitrogen:handler(my_session_handler,[]),  %% Add custom session handler

It may seem odd to you that we're not specifying which handler we're actually loading, but that's because the nitrogen:handler/2 function determines, based on the behavior() defined within the handler module, which handler it is we're actually loading.

nitrogen:handler(HandlerName, Config)

nitrogen:handler/2 is the way to function to connect your custom handler into Nitrogen's handler system. This function takes two arguments:

  • HandlerName - The name of the module.

  • Config - Any configuration settings for this handler. This variable could be just about anything, a proplist containing API keys or IP addresses of related servers (say for a session handler connected to an external memcache or something.

Common Handler Behavior Arguments

In all of the handler behavior functions are found two particular variables:

  • Config - When a handler is initialized using nitrogen:handler/2, the second argument is Config, which is then passed into each handler function.

  • State - The current state of the handler. Each handler maintains its own state.

The Handlers

Note: The init/2 function is called on each handler in a specific order as defined in wf_context:init_context/2.

In the order they are loaded, here are all the handlers, and their documentation:

Core handlers:

  • Config Handler - Controls how and from where the configuration settings are loaded.
  • Log Handler - Controls the logging system for your Nitrogen app.
  • Process Registry Handler - Controls how processes are tracked. The default backend for the process registry is nprocreg.
  • Crash Handler - Controls how to handle page crashes, including logging and how to present the crash to the user.
  • Query Handler - Controls the how wf:q (and its siblings, wf:qs, wf:mq, etc), functions retrieve their values from the POST, GET, or other methods.

Stateful Handlers

  • Session Handler - Controls how session information and variables are set, stored, and/or distributed.
  • State Handler - Controls how the wf:state functions store and retrieve their values. The default uses a simple proplist.
  • Identity Handler - Controls how the identity of the client is stored and retrieved. This is related to the wf:user functions.
  • Role Handler - Controls how the roles of the client are stored and retrieved. This is related to the wf:role functions.

Handlers that possibly redirect

  • Route Handler - Controls how Nitrogen routes requests to modules or static files.
  • Security Handler - Controls whether or not a user has access to a resource, and if not, should determine what to do with the request.

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