NAME
    App::MFILE::WWW - Generic web front-end with demo app

VERSION
    Version 0.097

LICENSE
    This software is distributed under the "BSD 3-Clause" license, the text
    of which can be found in the file named `COPYING' in the top-level
    distro directory. The license text is also reprodued at the top of each
    source file.

DESCRIPTION
    This distro contains a generic framework for developing web front-ends
    to REST resources. The framework consists of a web server based on Plack
    and Web::Machine, CSS and HTML for the "app frame" (on-screen area where
    the application's "screens" are displayed), and "widgets" for defining
    the application's login dialog, menus, forms, and actions.

    For illustration, the distro contains a demo app that authenticates
    against App::Dochazka::REST and contains a single menu, a simple form,
    and a sample action.

STACK
    The full stack of which App::MFILE::WWW is a part consists of the
    following components:

    * Database engine
        For storing application data.

    * Perl DBI
        For interfacing between the Perl code and the database engine.

    * REST server
        A REST server, such as App::Dochazka::REST, implements a data model
        and provides a HTTP interface to that model.

    * optional CLI client/frontend
        An optional Command Line Interface (frontend) can provide a command
        line interface to the REST server.

    * WWW client/frontend
        The WWW frontend, built based on this distro, is a web server that
        serves HTML, CSS, and JavaScript code to users to provide them with
        a menu-driven "browser experience" of the application.

    Conceptually, the clients (frontends) act as proxies between the user
    and the REST server. Taking this one step further, the REST server
    itself is a proxy between the client and the database engine.

    From a technical perspective, the strict separation between the REST
    server and its clients makes the application as a whole more robust.

DERIVED WWW CLIENTS
    The philosophy behind the stack design described above is that you, the
    user, have the freedom and the flexibility to write your own client, on
    any platform, in any language -- however you see fit. In other words,
    you are not forced to use any particular client. Conceivably, you can
    even communicate with the REST server without any client at all.

    However, writing a client is time- and labor-intensive. Although
    App::MFILE::WWW is capable of standalone operation, it is designed as a
    "foundation" upon which derived clients can be written.

  Standalone operation
    App::MFILE::WWW can be run as a standalone HTTP server. Authentication
    is disabled by default, so no REST server is needed in this scenario.

    Before a derived client can be written, the developer must first
    understand how App::MFILE::WWW is structured. This is easily understood
    by examining how it works in standalone mode.

    Assuming App::MFILE::WWW has been installed properly, it can be started
    in standalone mode by running `mfile-www', as a normal user, with no
    arguments or options. Here is a basic description of what happens in
    this scenario -- refer to the script source code in `bin/mfile-www' for
    better understanding:

    * by default (in the absence of the `--ddist' option), `$ddist' is set
    to the empty string
    * `$ddist_dir' is not set
    * the script calls the `App::MFILE::WWW::init' routine, which loads the
    configuration parameters stored in `config/WWW_Config.pm' of the core
    distro (App::MFILE::WWW) sharedir
    * since no `sitedir' option was specified on the command line, no other
    configuration files are loaded
    * the configuration parameters and their core default values can be seen
    in `config/WWW_Config.pm' under the core distro (App::MFILE::WWW)
    sharedir
    * a very important configuration parameter is MFILE_WWW_LOG_FILE, which
    is the full path to the log file where the Perl side of App::MFILE::WWW
    will write its log messages -- by default, this is set to
    '.mfile-www.log' in the user's home directory, and the current setting
    is always reported on-screen by the startup script so the user knows
    where to look if something goes wrong
    * the HTTP server is started by calling Plack::Runner, and the script
    reports to the user the port number at which it is listening (5001 by
    default)
    * the HTTP server always interprets URL paths it receives relative to
    its "root" (called `HTTP_ROOT' for the purposes of this document), which
    is set to the core distro (App::MFILE::WWW) sharedir in this case
    * JS and CSS files are considered "static content" and will be served
    from `HTTP_ROOT/js' and `HTTP_ROOT/css', respectively
    * when an HTTP 'GET' request comes in on the port where the HTTP server
    is listening, and it is not requesting static content, the request is
    passed on to the Web::Machine application (effectively,
    App::MFILE::WWW::Resource) for processing
    * POST requests are assumed to be AJAX calls and are handled by the
    `process_post' routine of App::MFILE::WWW::Resource
    * GET requests are assumed to have originated from a web browser running
    on a user's computer -- to handle these, the `main_html' routine of
    `Resource.pm' generates HTML code which is sent back in the HTTP
    response
    * the HTML so generated contains embedded JavaScript code to start up
    RequireJS with the required configuration and pass control over to "the
    JavaScript side" of App::MFILE::WWW

    The embedded JavaScript code does the following:

    * sets the `baseURL' to `$site-'MFILE_WWW_REQUIREJS_BASEURL>, which is
    set to `/js' -- in absolute terms, this means `HTTP_ROOT/js'
    * sets the "`app'" path config to `$site-'MFILE_APPNAME> -- for example,
    if `$site-'MFILE_APPNAME> is set to 'foobar', the path config for `app'
    will be set to `foobar' and a RequireJS dependency `app/bazblat' on the
    JavaScript side will translate to `HTTP_ROOT/js/foobar/bazblat.js'
    * in this particular case, of course, `MFILE_APPNAME' is set to
    `mfile-www'
    * persuades RequireJS via magic incantations to "play nice" together
    with jQuery and QUnit
    * by calling `requirejs.config', brings in site configuration parameters
    needed on the JavaScript side so they can be accessed via the `cf' JS
    module
    * passes control to the `app/main' JS module

    What happens on the JavaScript side is described in a different section
    of this documentation.

  Derived client operation
    In a derived-client scenario, App::MFILE::WWW is basically used as a
    library, or framework, upon which the "real" application is built.

    The derived-client handling is triggered by providing the `--ddist'
    command-line option, i.e.

        $ mfile-www --ddist=App-Dochazka-WWW

    Where 'App-Dochazka-WWW' refers to the Perl module App::Dochazka::WWW,
    which is assumed to contain the derived client source code.

    So, in the first place it is necessary to create such a Perl module. The
    App::MFILE::WWW module can be used as a template. It should have a
    sharedir configured and present.

    Here is a "play-by-play" description of what happens in this scenario
    when the startup script is run. Again, refer to the script source code
    in `bin/mfile-www' for better understanding:

    * `$ddist' is set to the string given in the `--ddist' option, e.g.
    `App-Dochazka-WWW' (or 'App::Dochazka::WWW' in which case it will be
    converted to the correct, hyphen-separated format)
    * `$ddist_dir' is set to `File::ShareDir::dist_dir( $ddist )', i.e. the
    derived distro sharedir (extending the above example, the distro
    sharedir of App::Dochazka::WWW)
    * the presence of the `--ddist' option triggers a special routine whose
    purpose is to ensure that the derived distro exists and that its
    sharedir is properly set up to work with App::MFILE::WWW:
            =over

            =item * error exit if the distro referred to by the C<--ddist> option
            doesn't exist 

            =item * error exit if the distro lacks a sharedir

            =item * C<css> and C<js/core> need to exist and be symlinks to the same
            directories in the L<App::MFILE::WWW> sharedir. If this is not the
            case, the script displays a message asking the user to re-run the
            script as root

            =item * if already running as root, the symlinks are created and the
            script displays a message asking to be re-run as a normal user

            =item * once the symlinks are in place, the script runs some sanity
            checks (mainly verifying the existence of certain files in their
            expected places)

            =back

    * the script calls the `App::MFILE::WWW::init' routine, which loads the
    configuration parameters stored in the following places:
            =over

            =item * the L<App::MFILE::WWW> distro sharedir (under C<config/WWW_Config.pm>)

            =item * the derived distro sharedir (also under C<config/WWW_Config.pm>)

            =item * finally and optionally, if a sitedir was specified on the
            command line -- for example C<--sitedir=/etc/dochazka-www> --,
            configuration parameters are loaded from a file C<WWW_SiteConfig.pm> in
            that directory, overriding the defaults

            =back

    * the derived distro's configuration should override the MFILE_APPNAME
    parameter -- in our example, it could be set to 'dochazka-www'
    * also refer to the previous section to review the explanation of the
    MFILE_WWW_LOG_FILE parameter
    * the HTTP server is started by calling Plack::Runner, and the script
    reports to the user at what port number it is listening (5001 by
    default)
    * the HTTP server always interprets URL paths it receives relative to
    its "root" (called `HTTP_ROOT' for the purposes of this document), which
    is set to the *derived distro's sharedir*
    * the rest of the description is the same as for Standalone operation

REQUEST-RESPONSE CYCLE
    The HTTP request-response cycle is implemented as follows:

    * nginx listens for incoming connections on port 80/443 of the server
    * When a connection comes in, nginx decrypts it and forwards it to a
    high-numbered port where a PSGI-compatible HTTP server (such as Starman)
    is listening
    * The HTTP server takes the connection and passes it to the Plack
    middleware. The key middleware component is Plack::Middleware::Session,
    which assigns an ID to the session, stores whatever data the server-side
    code needs to associate with the session, links the session to the
    user's browser via a cookie, and provides the application a hook (in the
    Plack environment stored in the HTTP request) to access the session data
    * if the connection is asking for static content (defined as anything in
    `images/', `css/', or `js/'), that content is served immediately and the
    request doesn't even make it into our Perl code
    * any other path is considered dynamic content and is passed to
    Web::Machine for processing -- Web::Machine implements the HTTP standard
    as a state machine
    * the Web::Machine state machine takes the incoming request and runs it
    through several functions that are overlayed in
    App::MFILE::WWW::Resource - an appropriate HTTP error code is returned
    if the request doesn't make it through the state machine. Along the way,
    log messages are written to the log.
    * as part of the state machine, all incoming requests are subject to
    "authorization" (in the HTTP sense, which actually means
    authentication). First, the session data is examined to determine if the
    request belongs to an existing authorized session. If it doesn't, the
    request is treated as a login/logout attempt -- the session is cleared
    and control passes to the JavaScript side, which, lacking a currentUser
    object, displays the login dialog.
    * once an authorized session is established, there are two types of
    requests: GET and POST
    * incoming GET requests happen whenever the page is reloaded - in an
    authorized session, this causes the main menu to be displayed, but all
    static content (CSS and JavaScript modules) are reloaded for a "clean
    slate", as if the user had just logged in.
    * Note that App::MFILE::WWW pays no attention to the URI - if the user
    enters a path (e.g. http://mfile.site/some/bogus/path), this will be
    treated like any other page (re)load and the path is simply ignored.
    * if the session is expired or invalid, any incoming GET request will
    cause the login dialog to be displayed.
    * well-formed POST requests are assumed to be AJAX calls and are
    directed to the `process_post' routine, which first examines the request
    body, which must adhere to a simple structure:
            { method: "GET", path: "employee/current", body: { ... } }

        where 'method' is any HTTP method accepted by the REST server,
        'path' is a valid path to a REST server resource, and 'body' is the
        content body to be sent in the HTTP request to the REST server.
        Provided the request is properly authorized and the body is
        well-formed, the request is forwarded to the REST server via the
        App::MFILE package's `rest_req' routine and the REST server's
        response is sent back to the user's browser, where it is processed
        by the JavaScript code.

    * under ordinary operation, the user will spend 99% of her time
    interacting with the JavaScript code running in her browser, which will
    communicate asynchronously as needed with the REST server via AJAX
    calls.

DEVELOPMENT NOTES
    The App::MFILE::WWW codebase has two parts, or "sides": the "Perl side"
    and the "JavaScript side". Control passes from the Perl side to the
    JavaScript side

    * synchronously whenever the user (re)loads the page
    * asynchronously whenever the user triggers an AJAX call

    JavaScript side
    Modular (RequireJS)
    The JavaScript code is modular. Each code module has its own file and
    modules are loaded asynchronously by RequireJS.

    Unit testing (QUnit)
    The JavaScript code included in this package is set up for unit testing
    using the QUnit http://qunitjs.com/ library.

    UTF-8
    In conformance with the JSON standard, all data passing to and from the
    server are assumed to be encoded in UTF-8. Users who need to use
    non-ASCII characters should check their browser's settings.

  Deployment
    To minimize latency, App::MFILE::WWW can be deployed on the same server
    as the back-end (e.g. App::Dochazka::REST), but this is not required.

PACKAGE VARIABLES
    For convenience, the following variables are declared with package
    scope:

FUNCTIONS
  init
    Initialization routine - run from `bin/mfile-www', the server startup
    script. This routine loads configuration parameters from files in the
    distro and site configuration directories, and sets up logging.

