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OpenInteract2::Manual::Architecture - Overview of the OpenInteract2 Architecture
This part of the OpenInteract2 manual describes the major pieces of the system and traces a users's request from catching the URL to returning the content.
In case you've been hiding under a rock, MVC stands for Model-View-Controller. It's a method of separating the different concerns of your application for flexibility and other reasons. While too broad to be a pattern it's still extremely useful to keep in mind as you're developing.
MVC was developed in Smalltalk and targeted at desktop applications. It's meant to decouple the various graphical widgets from the actions resulting from them. While some (see link Andy Wardley's discussion in SEE ALSO) have argued that MVC isn't appropriate for web applications, it's still useful to categorize how the different pieces of the application are separated.
This is the smarts of your application and, unless you're a graphic designing, probably the area on which you'll spend the most time. The model consists of your application's core objects and processes. The objects are normally persistent. So the model for a shopping cart application may consist of the products in the cart (persistent), the cart itself (semi-persistent), the customer (persistent), different payment types (persistent) and the order process.
Generally we use relational databases for persistence, but this also encompasses other storage technologies (LDAP, GDBM, CSV, etc.).
In OpenInteract you most often use SPOPS to represent your application's state as persistable objects. There are many hooks for initializing SPOPS classes and adding useful behaviors to the objects. But OI doesn't require that you use SPOPS for everything -- you can easilyuse another CPAN module (like Class::DBI) for your model.
The processes are normally represented as Action objects, although they may be generic enough to be a normal perl object that's instantiated by an Action. To use the hackneyed shopping cart example: the order process would probably be a separate object (e.g., 'OpenInteract2::OrderProcess') so you can use it from a web action and in response to emails or some other input
The view is what the user sees of your application. The view and model communicate in a fairly limited but flexible fashion. In the GUI world a typical MVC example would be a spreadsheet: one view is of tabular data, another view is a pie chart, another view is a line chart. The model stays the same, the view changes.
A typical web application view is a template with some sort of processing engine behind it. The template takes some sort of data structure and extracts the necessary information to display it to the user.
OI is fairly flexible about views. The model (Action) always returns content but can decide to return the content itself. (This is unusual but has its place.) Generally the Action will collect data, do some sort of validation on it and then pass it on to the Content Generator along with the name of a view.
The view name corresponds to a template file somewhere in a
package. So the view 'mypkg::foo' would refer to a file with a name
like $WEBSITE_DIR/pkg/mypkg-x.xx/template/foo. And the Content
Generator is reponsible for taking the data from the model and passing
it to the view so it can be processed. The model doesn't care how the
data are used and the view doesn't care from where the data came.
Also known as a dispatcher, this is what decides which model (Action) is called and how to represent the data from the outside world. These are represented by the Controller object which dispatches the user's request, the Request object which takes the user's inputs (however they arrive) and translates them to a standard format, and the Response object which translates the work done during the request cycle into something the user can understand.
For a web application:
The Request will store the URL requested, pull out useful information from the headers and network information, and parse the GET/POST request into parameters and file uploads.
The Controller instantiates the Action matching the URL as reported by the Request, populates the Action with any necessary information and executes the Action, capturing its output. It doesn't care what the Action does or how the Action does it. The Controller will also place this Action's output in a larger context (e.g., as part of a larger web page) as necessary.
The Response returns the content along with any necessary headers to the user.
The Context (abbrev: CTX) glues the system together and is therefore used everywhere. It holds all application configuration information and provides a central lookup mechanism for actions, content generators, controllers and SPOPS object classes.
It is a singleton (there's only one in the system at any time) and can be imported from the OpenInteract2::Context class since it's used fairly often.
Creating the context is one of the first actions you take when starting an OI2 server. While it can be created without referencing a website it's not much use. (You should only need do this when bootstrapping a new website into existence, and this is already done for you in OpenInteract2::Manage::Website::Create.
So normally it looks something like this:
my $ctx = OpenInteract2::Context->create(
{ website_dir => $website_dir });
Once it's created the CTX symbol can be imported and used anywhere.
The job of the adapter is to translate the world's information to something understandable by OpenInteract and then translate what OpenInteract generates into information for the outside world. So it sits between your interface (e.g., Apache/mod_perl, CGI, etc.) and the OpenInteract server. The information it translates from the outside world includes parameters from the user, user authentication, information about the request (hostname, URL, referer, cookies, etc.) and other data. It places these data into the relevant OpenInteract2::Request subclass.
Once the OpenInteract cycle is complete the adapter translates OpenInteract data (content, headers, etc.) into a response to send back to the user via the relevant OpenInteract2::Response subclass. For an example see Apache::OpenInteract2.
Creating an adapter is not difficult. Adapter classes tend to be fairly short as most of the work is done in in the OpenInteract2::Request/OpenInteract2::Response subclasses. For instance, here's the full adapter for Apache/mod_perl 1.x:
1: package Apache::OpenInteract2;
2:
3: use strict;
4: use Log::Log4perl qw( get_logger );
5: use OpenInteract2::Auth;
6: use OpenInteract2::Constants qw( :log );
7: use OpenInteract2::Context qw( CTX );
8: use OpenInteract2::Request;
9: use OpenInteract2::Response;
10:
11: sub handler($$) {
12: my ( $class, $r ) = @_;
13: my $log = get_logger( LOG_OI );
14:
15: $log->is_info &&
16: $log->info( scalar( localtime ), ": request from ",
17: "[", $r->connection->remote_ip, "] for URL ",
18: "[", $r->uri, '?', scalar( $r->args ), "]" );
19:
20: my $response = OpenInteract2::Response->new({ apache => $r });
21: my $request = OpenInteract2::Request->new({ apache => $r });
22:
23: OpenInteract2::Auth->login( $r->pnotes( 'login_user' ) );
24:
25: my $controller = eval {
26: OpenInteract2::Controller->new( $request, $response )
27: };
28: if ( $@ ) {
29: $response->content( $@ );
30: }
31: else {
32: $controller->execute;
33: }
34: $response->send;
35: return $response->status;
36: }
37:
38: 1;
Very easy -- it's only about 15 lines if you remove the logging! This
even has a little twist by passing in the 'login_user' key from the
Apache pnotes (line 23), which is a hook to the
Apache::OpenInteract2::HttpAuth
class to allow HTTP (rather than cookie) authentication.
Some gotchas to note:
Create response before request - It may seem backwards but you need to create the response object before the request object. (This is due to a dependency.)
Additionally if your adapter is more of a standalone service (like the
oi2_daemon) that spawns off children/threads for requests, you need
to also be aware of the following:
Initialize logging - You'll need to initialize log4perl. This is
normally as simple as passing a parameter to the create method of
OpenInteract2::Context, but you can also use
one of the methods in OpenInteract2::Log.
Close all database connections - Before spawning off
children/threads from the parent you MUST shutdown all database
connections. They won't survive the fork/thread and you'll get very
strange errors. Do this with the shutdown method in
OpenInteract2::DatasourceManager.
Once the adapter has created the request and response it hands off the processing to the OpenInteract2::Controller object. Now we're entirely inside the OI2 server environment. This reads the action and task from the request and creates the relevant action object that will generate the content. It knows which action object to create through a URL-to-action mapping created at Context startup. The most-used controller (OpenInteract2::Controller::MainTemplate) places the generated content in a larger scope so you can control common graphical elements (sidebars, menus, etc.) from once place. Another controller (OpenInteract2::Controller::Raw) returns the content as-is.
Actions are the core of OpenInteract2. Each action provides a discrete set of functionality. What "discrete set" means is up to the developer, but typically this is a set of SCRUD (Search - CReate - Update - Delete) operations on a class of objects.
Each action is represented by zero or more URLs, and each operation is specified by a task referenced in that URL. So if I created a 'news' action my URLs might look like:
Every task returns some sort of content, generally by passing data to a Content Generator which marries it with a template. See OpenInteract2::Action for much more information.
As mentioned above tasks in an Action return content. They normally generate that content by assembling a set of data and passing that data off to a content generator. A content generator is a wrapper around some sort of templating system, such as the Template Toolkit|Template, HTML::Template or Text::Template or even your own homegrown system. (Admit it, you've written your own.)
Each action is associated with a content generator. And you can even associate an action with multiple content generators so you can settle a bet as to which templating system is easiest to use.
Now we'll trace a request throughout OpenInteract.
The adapter or another process (e.g., 'startup.pl' in a mod_perl setup) will run a number of tasks at server startup. This includes:
Initialize logging
Create context with website directory
Assign the proper request/response types to request
If threaded/forking: disconnect all database handles.
This step is a little fuzzy by necessity: we purposefully don't know in what form the request is coming in or how the adapter handles it.
The adapter creates the OpenInteract2::Response and OpenInteract2::Request objects, in that order. Each one has necessary initialization steps. In particular the request object will read the necessary headers, parameters, uploaded files, cookies and create the session from the cookie.
It also finds the 'relevant' part of the URL and determines the action
and task from it. The 'relevant' part is what's leftover after the
URL-space (aka, deployment context, set in the
context_info.deployed_under server configuration key) is lopped off.
It can optionally handle extra authentication as this point such as HTTP auth or some other capability. Generally this will consist of retrieving a user object created from some other part of the system or creating a user object based on trusted information (like a user ID) from another area.
If available this user object is passed to the login method of the
OpenInteract2::Auth class so it has a head
start.
Adapter creates the OpenInteract2::Controller object with the request and response objects created earlier.
The controller asks the request for the action name and asks the
context for an action object of that name. (It's always a subclass of
OpenInteract2::Action.) If the action name
exists but is not found we use the action named in the
action_info.not_found server configuration key. If the action name
doesn't exist we use the action from action_info.none.
Once the action is found we assign it the task (if available) as reported by the request.
If created properly the adapter calls execute() on the
controller. This starts the content generation process running.
The controller will call execute() on the action which returns the
content for it.
The action needs to find which task to execute. Normally this is as
simple as getting the value of the task property. But the
individual action can override this, or if no task was specified we
use the value of task_default.
Find out if the task is invalid. A valid task:
Does not start with a '_'
Is not listed in task_invalid
Is listed among the tasks in in task_valid if that property is
defined.
If the task is valid we also ensure that this user has the proper security level to execute it.
First, we check the cache to see if content exists and if it does, we return it without going any further.
Next we execute the method specified by what we've determined to be the task. (This is almost certainly the method with the same name as the task.)
An action can generate content by itself but most times it just gathers the necessary data and passes it, along with a template specification, to a content generator which returns the content for the action.
If the cache is activated for this method we'll cache the content. In any case we return the content, finishing the flow for the action and moving back up to the controller.
The main action is done and has returned its content to the controller. One controller (OpenInteract2::Controller::Raw) will just return this content and call it a day.
Most times you'll want to take that content and put it into another template. The controller just instantiates a new content generator and goes through the same process as the action, passing it a set of data (of which the generated action content is part) and a template specification (normally from the 'main_template' theme property).
Oftentimes the main template will hold multiple discrete actions of its own. For example, the default main template shipped with OI has an action to generate the list of boxes that appears on the right-hand side. You could trigger an action to get the latest weather conditions, webcam photo, news headlines, whatever you wish.
Each of these actions is just like any other and goes through the same process listed above.
Whether it's the action content or the scoped content (for lack of a better name), we set the content in the response object, which hasn't done much until now except hold the occasional outgoing cookie.
The controller's job is done and flow now returns back up a level to the adapter.
The only job left of the adapter is to ask the response to send the content.
The adapter can do any necessary cleanup.
Andy Wardley's email about MVC and web applications:
http://lists.ourshack.com/pipermail/templates/2002-November/003974.html
Copyright (c) 2002-2003 Chris Winters. All rights reserved.
Chris Winters <chris@cwinters.com>
Generated from the OpenInteract 1.99_03 source.