Brock Wilcox > Continuity-1.6 > Continuity

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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 persistent 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 browser. 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:

Arguments passed to the default adapter:

Arguments passed to the default mapper:

$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

AnyEvent::DBI and Coro::Mysql for concurrent database access.

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) 2004-2014 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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