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      user clicks submit (2)
            |
            v
      browser sends a POST request (3)
            |
            v
      server processes the form and replies with HTTP code 200 (4)

    Apache2::POST200 intercepts the server reply, stores the response in a
    database and sends a temporary redirect to the browser. It also
    intercepts the following request from the browser and sends the stored
    reply.

  How it works
    This module inserts an request output filter that looks for replies for
    POST requests with a HTTP code of 200. If it finds one it saves the
    reply in a database and replaces the complete output with a temporary
    redirect (HTTP code 302) to the same URL but with a special marked query
    string appended.

    When the browser follows the redirect the module recognizes the query
    string and routes the request to its own response handler. The handler
    then reads the saved page from the database and sends it to the browser.

    Well, the request routing is actually done by a tricky translation
    handler such as mod_rewrite or Apache2::Translation.

    Note: the redirect must go to the same URL because some WEB application
    forget the "action" attribute in their "<form>" definitions.

  Configuration
    The module itself is loaded from the Apache configuration file via a
    "PerlLoadModule" directive. It then provides a few configuration
    directives of its own. All directives are allowed in server config,
    virtual host and directory contexts.

    Post200Storage dsn user password
        "Post200Storage" describes the database to be used. All 3 parameter
        are passed to the DBI::connect method, see DBI. User and password
        can be omitted if the database supports it.

        "Post200Storage None" disables the output filter. That means replies
        with a HTTP code 200 to a POST request are delivered as is.

    Post200Table table key-column data-column
        "Post200Table" describes the table to be used. The "key" column must
        be able to hold a 41-byte string of printable ascii characters. The
        key length may be extented in future versions of this module but a
        key will always consist of printable characters.

        For best performance create an index on the "key" column.

        The "data" column must be able to hold a variable size data block.
        The maximum size can be limited using "Post200DataBlockSize". If
        "Post200DataBlockSize" is not used the size completely depends on
        your response handlers. If possible use a BLOB type as "data"
        column.

        Although not used by the module it makes sense to add a 3rd column
        to the table. It should be a timestamp column with the default
        attribute set to "now()". Without it it's difficult to decide which
        records can be deleted.

        With a MySQL database a suitable table is created by:

         create table p200 (
           session varchar(50) primary key unique not null,
           data blob,
           tm timestamp not null default 'now'
         );
         create index p200_tm_idx on p200(tm);

        Deletion of expired pages is best done by a simple cron job, e.g.

         45 * * * * echo 'delete from p200 where now()-tm>3600' | mysql post200

    Post200Label marker
        By means of this marker the response handler recognizes a redirected
        request that it is responsible for. When the output filter generates
        a query string it starts with the "marker" as prefix.

        If omitted "-redirect-" is used.

        If the module is used on a forward proxy to repair external WEB
        applications choose a string here that is very likely to be used
        only by your proxy.

    Post200Secret secret initvector
        To make sure the key provided by the browser via the query string
        was generated by the filter it is encrypted. "secret" and
        "initvector" are arbitrary strings, see Crypt::Blowfish.

        If omitted 2 strings are used that once came out of /dev/random on
        my box.

    Post200IpCheck On|Off
        With this directive set the response handler sends a page only to
        the same client where the redirect was sent to. This prevents that
        redirected URLs are mailed around as long as the 2 clients are not
        connected through the same proxy.

        Default is On.

    Post200DataBlockSize Bytes
        This directive defines the maximum size of a data item written to
        the database.

        If omitted the blocksize depends on the response handler.

Some Considerations
  Simple sessions
    One way to look at this module is that it provides some simple session.
    Often a WEB application is simply a collection of forms gathering some
    information. And only after the last form is filled out all of it is to
    be written to a data store.

    With "Apache2::POST200" you can save the information gathered so far in
    hidden fields rather than saving them in a session structure at server
    side.

    Well, the database that this module uses is such a session structure at