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Stateful Web Apps in Perl
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README Build README from Continuity.pm

README

NAME
    Continuity - Abstract away statelessness of HTTP, for stateful Web
    applications

SYNOPSIS
      #!/usr/bin/perl

      use strict;
      use Continuity;

      my $server = new Continuity;
      $server->loop;

      sub main {
        my $request = shift;
        $request->print("Your name: <form><input type=text name=name></form>");
        $request->next; # this waits for the form to be submitted!
        my $name = $request->param('name');
        $request->print("Hello $name!");
      }

DESCRIPTION
    Continuity is a library to simplify web applications. Each session is
    written and runs as a persistant application, and is able to request
    additional input at any time without exiting. This is significantly
    different from the traditional CGI model of web applications in which a
    program is restarted for each new request.

    The program is passed a $request variable which holds the request
    (including any form data) sent from the browser. In concept, this is a
    lot like a $cgi object from CGI.pm with one very very significant
    difference. At any point in the code you can call $request->next. Your
    program will then suspend, waiting for the next request in the session.
    Since the program doesn't actually halt, all state is preserved,
    including lexicals -- getting input from the browser is then similar to
    doing "$line = <>" in a command-line application.

GETTING STARTED
    The first thing to make a note of is that your application is a
    continuously running program, basically a self contained webserver. This
    is quite unlike a CGI.pm based application, which is re-started for each
    new request from a client browser. Once you step away from your CGI.pm
    experience this is actually more natural (IMO), more like writing an
    interactive desktop or command-line program.

    Here's a simple example:

      #!/usr/bin/perl

      use strict;
      use Continuity;

      my $server = new Continuity;
      $server->loop;

      sub main {
        my $request = shift;
        while(1) {
          $request->print("Hello, world!");
          $request->next;
          $request->print("Hello again!");
        }
      }

    First, check out the small demo applications in the eg/ directory of the
    distribution. Sample code there ranges from simple counters to more
    complex multi-user ajax applications. All of the basic uses and some of
    the advanced uses of Continuity are covered there.

    Here is an brief explanation of what you will find in a typical
    application.

    Declare all your globals, then declare and create your server.
    Parameters to the server will determine how sessions are tracked, what
    ports it listens on, what will be served as static content, and things
    of that nature. You are literally initializing a web server that will
    serve your application to client browsers. Then call the "loop" method
    of the server, which will get the server listening for incoming requests
    and starting new sessions (this never exits).

      use Continuity;
      my $server = Continuity->new( port => 8080 );
      $server->loop;

    Continuity must have a starting point when starting new sessions for
    your application. The default is "\&::main" (a sub named "main" in the
    default global scope), which is passed the $request handle. See the
    Continuity::Request documentation for details on the methods available
    from the $request object beyond this introduction.

      sub main {
        my $request = shift;
        # ...
      }

    Outputting to the client (that is, sending text to the browser) is done
    by calling the "$request->print(...)" method, rather than the plain
    "print" used in CGI.pm applications.

      $request->print("Hello, guvne'<br>");
      $request->print("'ow ya been?");

    HTTP query parameters (both GET and POST) are also gotten through the
    $request handle, by calling "$p = $request->param('x')", just like in
    CGI.pm.

      # If they go to http://webapp/?x=7
      my $input = $request->param('x');
      # now $input is 7

    Once you have output your HTML, call "$request->next" to wait for the
    next response from the client browser. While waiting other sessions will
    handle other requests, allowing the single process to handle many
    simultaneous sessions.

      $request->print("Name: <form><input type=text name=n></form>");
      $request->next;                   # <-- this is where we suspend execution
      my $name = $request->param('n');  # <-- start here once they submit

    Anything declared lexically (using my) inside of "main" is private to
    the session, and anything you make global is available to all sessions.
    When "main" returns the session is terminated, so that another request
    from the same client will get a new session. Only one continuation is
    ever executing at a given time, so there is no immediate need to worry
    about locking shared global variables when modifying them.

ADVANCED USAGE
    Merely using the above code can completely change the way you think
    about web application infrastructure. But why stop there? Here are a few
    more things to ponder.

  Coro::Event
    Since Continuity is based on Coro, we also get to use Coro::Event. This
    means that you can set timers to wake a continuation up after a while,
    or you can have inner-continuation signaling by watch-events on shared
    variables.

  Multiple sessions per-user
    For AJAX applications, we've found it handy to give each user multiple
    sessions. In the chat-ajax-push demo each user gets a session for
    sending messages, and a session for receiving them. The receiving
    session uses a long-running request (aka COMET) and watches the globally
    shared chat message log. When a new message is put into the log, it
    pushes to all of the ajax listeners.

  Lexical storage and callback links
    Don't forget about those pretty little lexicals you have at your
    disposal. Taking a hint from the Seaside folks, instead of regular links
    you could have callbacks that trigger a anonymous subs. Your code could
    look like:

      use Continuity;
      use strict;
      my @callbacks;
      my $callback_count;
      Continuity->new->loop;
      sub gen_link {
        my ($text, $code) = @_;
        $callbacks[$callback_count++] = $code;
        return qq{<a href="?cb=$callback_count">$text</a>};
      }
      sub process_links {
        my $request = shift;
        my $cb = $request->param('cb');
        if(exists $callbacks[$cb]) {
          $callbacks[$cb]->($request);
          delete $callbacks[$cb];
        }
      }
      sub main {
        my $request = shift;
        my $x;
        my $link1 = gen_link('This is a link to stuff' => sub { $x = 7  });
        my $link2 = gen_link('This is another link'    => sub { $x = 42 });
        $request->print($link1, $link2);
        $request->next;
        process_links($request);
        $request->print("\$x is now: $x");
      }

  Scaling
    To scale a Continuity-based application beyond a single process you need
    to investigate the keywords "session affinity". The Seaside folks have a
    few articles on various experiments they've done for scaling, see the
    wiki for links and ideas. Note, however, that premature optimization is
    evil. We shouldn't even be talking about this.

EXTENDING AND CUSTOMIZING
    This library is designed to be extensible but have good defaults. There
    are two important components which you can extend or replace.

    The Adapter, such as the default Continuity::Adapt::HttpDaemon, actually
    makes the HTTP connections with the client web broswer. If you want to
    use FastCGI or even a non-HTTP protocol, then you will use or create an
    Adapter.

    The Mapper, such as the default Continuity::Mapper, identifies incoming
    requests from The Adapter and maps them to instances of your program. In
    other words, Mappers keep track of sessions, figuring out which requests
    belong to which session. The default mapper can identify sessions based
    on any combination of cookie, ip address, and URL path. Override The
    Mapper to create alternative session identification and management.

METHODS
    The main instance of a continuity server really only has two methods,
    "new" and "loop". These are used at the top of your program to do setup
    and start the server. Please look at Continuity::Request for
    documentation on the $request object that is passed to each session in
    your application.

  $server = Continuity->new(...)
    The "Continuity" object wires together an Adapter and a mapper. Creating
    the "Continuity" object gives you the defaults wired together, or if
    user-supplied instances are provided, it wires those together.

    Arguments:

    *   "callback" -- coderef of the main application to run persistantly
        for each unique visitor -- defaults to "\&::main"

    *   "adapter" -- defaults to an instance of
        "Continuity::Adapt::HttpDaemon"

    *   "mapper" -- defaults to an instance of "Continuity::Mapper"

    *   "docroot" -- defaults to "."

    *   "staticp" -- defaults to "sub { $_[0]->url =~
        m/\.(jpg|jpeg|gif|png|css|ico|js)$/ }", used to indicate whether any
        request is for static content

    *   "debug_level" -- Set level of debugging. 0 for nothing, 1 for
        warnings and system messages, 2 for request status info. Default is
        1

    *   "debug_callback" -- Callback for debug messages. Default is print.

    Arguments passed to the default adapter:

    *   "port" -- the port on which to listen

    *   "no_content_type" -- defaults to 0, set to 1 to disable the
        "Content-Type: text/html" header and similar headers

    Arguments passed to the default mapper:

    *   "cookie_session" -- set to name of cookie or undef for no cookies
        (defaults to 'cid')

    *   "query_session" -- set to the name of a query variable for session
        tracking (defaults to undef)

    *   "assign_session_id" -- coderef of routine to custom generate session
        id numbers (defaults to a simple random string generator)

    *   "cookie_life" -- lifespan of the cookie, as in CGI::set_cookie
        (defaults to "+2d")

    *   "ip_session" -- set to true to enable ip-addresses for session
        tracking (defaults to false)

    *   "path_session" -- set to true to use URL path for session tracking
        (defaults to false)

    *   "implicit_first_next" -- set to false to get an empty first request
        to the main callback (defaults to true)

  $server->loop()
    Calls Coro::Event::loop and sets up session reaping. This never returns!

SEE ALSO
    See the Wiki for development information, more waxing philosophic, and
    links to similar technologies such as <http://seaside.st/>.

    Website/Wiki: <http://continuity.tlt42.org/>

    Continuity::Request, Continuity::RequestCallbacks, Continuity::Mapper,
    Continuity::Adapt::HttpDaemon, Coro

AUTHOR
      Brock Wilcox <awwaiid@thelackthereof.org> - http://thelackthereof.org/
      Scott Walters <scott@slowass.net> - http://slowass.net/
      Special thanks to Marc Lehmann for creating (and maintaining) Coro

COPYRIGHT
      Copyright (c) 2004-2008 Brock Wilcox <awwaiid@thelackthereof.org>. All
      rights reserved.  This program is free software; you can redistribute it
      and/or modify it under the same terms as Perl itself.

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