Andreas J. König > Apache-HeavyCGI > Apache::HeavyCGI

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NAME ^

Apache::HeavyCGI - Framework to run complex CGI tasks on an Apache server

SYNOPSIS ^

 use Apache::HeavyCGI;

WARNING UNSUPPORTED ALPHA CODE RELEASED FOR DEMO ONLY ^

The release of this software was only for evaluation purposes to people who are actively writing code that deals with Web Application Frameworks. This package is probably just another Web Application Framework and may be worth using or may not be worth using. As of this writing (July 1999) it is by no means clear if this software will be developed further in the future. The author has written it over many years and is deploying it in several places. Update 2006-02-03: Development stalled since 2001 and now discontinued.

There is no official support for this software. If you find it useful or even if you find it useless, please mail the author directly.

But please make sure you remember: THE RELEASE IS FOR DEMONSTRATION PURPOSES ONLY.

DESCRIPTION ^

The Apache::HeavyCGI framework is intended to provide a couple of simple tricks that make it easier to write complex CGI solutions. It has been developed on a site that runs all requests through a single mod_perl handler that in turn uses CGI.pm or Apache::Request as the query interface. So Apache::HeavyCGI is -- as the name implies -- not merely for multi-page CGI scripts (for which there are other solutions), but it is for the integration of many different pages into a single solution. The many different pages can then conveniently share common tasks.

The approach taken by Apache::HeavyCGI is a components-driven one with all components being pure perl. So if you're not looking for yet another embedded perl solution, and aren't intimidated by perl, please read on.

Stacked handlers suck

If you have had a look at stacked handlers, you might have noticed that the model for stacking handlers often is too primitive. The model supposes that the final form of a document can be found by running several passes over a single entity, each pass refining the entity, manipulating some headers, maybe even passing some notes to the next handler, and in the most advanced form passing pnotes between handlers. A lot of Web pages may fit into that model, even complex ones, but it doesn't scale well for pages that result out of a structure that's more complicated than adjacent items. The more complexity you add to a page, the more overhead is generated by the model, because for every handler you push onto the stack, the whole document has to be parsed and recomposed again and headers have to be re-examined and possibly changed.

Why not subclass Apache

Inheritance provokes namespace conflicts. Besides this, I see little reason why one should favor inheritance over a using relationship. The current implementation of Apache::HeavyCGI is very closely coupled with the Apache class anyway, so we could do inheritance too. No big deal I suppose. The downside of the current way of doing it is that we have to write

    my $r = $obj->{R};

very often, but that's about it. The upside is, that we know which manpage to read for the different methods provided by $obj-{R}>, $obj-{CGI}>, and $obj itself.

Composing applications

Apache::HeavyCGI takes an approach that is more ambitious for handling complex tasks. The underlying model for the production of a document is that of a puzzle. An HTML (or XML or SGML or whatever) page is regarded as a sequence of static and dynamic parts, each of which has some influence on the final output. Typically, in today's Webpages, the dynamic parts are filled into table cells, i.e. contents between some <TD></TD> tokens. But this is not necessarily so. The static parts in between typically are some HTML markup, but this also isn't forced by the model. The model simply expects a sequence of static and dynamic parts. Static and dynamic parts can appear in random order. In the extreme case of a picture you would only have one part, either static or dynamic. HeavyCGI could handle this, but I don't see a particular advantage of HeavyCGI over a simple single handler.

In addition to the task of generating the contents of the page, there is the other task of producing correct headers. Header composition is an often neglected task in the CGI world. Because pages are generated dynamically, people believe that pages without a Last-Modified header are fine, and that an If-Modified-Since header in the browser's request can go by unnoticed. This laissez-faire principle gets in the way when you try to establish a server that is entirely driven by dynamic components and the number of hits is significant.

Header Composition, Parameter Processing, and Content Creation

The three big tasks a CGI script has to master are Headers, Parameters and the Content. In general one can say, content creation SHOULD not start before all parameters are processed. In complex scenarios you MUST expect that the whole layout may depend on one parameter. Additionally we can say that some header related data SHOULD be processed very early because they might result in a shortcut that saves us a lot of processing.

Consequently, Apache::HeavyCGI divides the tasks to be done for a request into four phases and distributes the four phases among an arbitrary number of modules. Which modules are participating in the creation of a page is the design decision of the programmer.

The perl model that maps (at least IMHO) ideally to this task description is an object oriented approach that identifies a couple of phases by method names and a couple of components by class names. To create an application with Apache::HeavyCGI, the programmer specifies the names of all classes that are involved. All classes are singleton classes, i.e. they have no identity of their own but can be used to do something useful by working on an object that is passed to them. Singletons have an @ISA relation to Class::Singleton which can be found on CPAN. As such, the classes can only have a single instance which can be found by calling the CLASS->instance method. We'll call these objects after the mod_perl convention handlers.

Every request maps to exactly one Apache::HeavyCGI object. The programmer uses the methods of this object by subclassing. The HeavyCGI constructor creates objects of the AVHV type (pseudo-hashes).

*** Note: after 0.0133 this was changed to an ordinary hash. ***

If the inheriting class needs its own constructor, this needs to be an AVHV compatible constructor. A description of AVHV can be found in fields.

*** Note: after 0.0133 this was changed to be an ordinary hash. ***

An Apache::HeavyCGI object usually is constructed with the new method and after that the programmer calls the dispatch method on this object. HeavyCGI will then perform various initializations and then ask all nominated handlers in turn to perform the header method and in a second round to perform the parameter method. In most cases it will be the case that the availability of a method can be determined at compile time of the handler. If this is true, it is possible to create an execution plan at compile time that determines the sequence of calls such that no runtime is lost to check method availability. Such an execution plan can be created with the Apache::HeavyCGI::ExePlan module. All of the called methods will get the HeavyCGI request object passed as the second parameter.

There are no fixed rules as to what has to happen within the header and parameter method. As a rule of thumb it is recommended to determine and set the object attributes LAST_MODIFIED and EXPIRES (see below) within the header() method. It is also recommended to inject the Apache::HeavyCGI::IfModified module as the last header handler, so that the application can abort early with an Not Modified header. I would recommend that in the header phase you do as little as possible parameter processing except for those parameters that are related to the last modification date of the generated page.

Terminating the handler calls or triggering errors.

Sometimes you want to stop calling the handlers, because you think that processing the request is already done. In that case you can do a

 die Apache::HeavyCGI::Exception->new(HTTP_STATUS => status);

at any point within prepare() and the specified status will be returned to the Apache handler. This is useful for example for the Apache::HeavyCGI::IfModified module which sends the response headers and then dies with HTTP_STATUS set to Apache::Constants::DONE. Redirectors presumably would set up their headers and set it to Apache::Constants::HTTP_MOVED_TEMPORARILY.

Another task for Perl exceptions are errors: In case of an error within the prepare loop, all you need to do is

 die Apache::HeavyCGI::Exception->new(ERROR=>[array_of_error_messages]);

The error is caught at the end of the prepare loop and the anonymous array that is being passed to $@ will then be appended to @{$self->{ERROR}}. You should check for $self->{ERROR} within your layout method to return an appropriate response to the client.

Layout and Text Composition

After the header and the parameter phase, the application should have set up the object that is able to characterize the complete application and its status. No changes to the object should happen from now on.

In the next phase Apache::HeavyCGI will ask this object to perform the layout method that has the duty to generate an Apache::HeavyCGI::Layout (or compatible) object. Please read more about this object in Apache::HeavyCGI::Layout. For our HeavyCGI object it is only relevant that this Layout object can compose itself as a string in the as_string() method. As a layout object can be composed as an abstraction of a layout and independent of request-specific contents, it is recommended to cache the most important layouts. This is part of the reponsibility of the programmer.

In the next step HeavyCGI stores a string representation of current request by calling the as_string() method on the layout object and passing itself to it as the first argument. By passing itself to the Layout object all the request-specific data get married to the layout-specific data and we reach the stage where stacked handlers usually start, we get at a composed content that is ready for shipping.

The last phase deals with setting up the yet unfinished headers, eventually compressing, recoding and measuring the content, and delivering the request to the browser. The two methods finish() and deliver() are responsible for that phase. The default deliver() method is pretty generic, it calls finish(), then sends the headers, and sends the content only if the request method wasn't a HEAD. It then returns Apache's constant DONE to the caller, so that Apache won't do anything except logging on this request. The method finish is more apt to being overridden. The default finish() method sets the content type to text/html, compresses the content if the browser understands compressed data and Compress::Zlib is available, it also sets the headers Vary, Expires, Last-Modified, and Content-Length. You most probably will want to override the finish method.

head2 Summing up +-------------------+ | sub handler {...} | +--------------------+ | (sub init {...}) | |Your::Class |---defines------>| | |ISA Apache::HeavyCGI| | sub layout {...} | +--------------------+ | sub finish {...} | +-------------------+

                                        +-------------------+
                                        | sub new {...}     |
 +--------------------+                 | sub dispatch {...}|
 |Apache::HeavyCGI    |---defines------>| sub prepare {...} |
 +--------------------+                 | sub deliver {...} |
                                        +-------------------+

 +----------------------+               +--------------------+
 |Handler_1 .. Handler_N|               | sub header {...}   |
 |ISA Class::Singleton  |---define----->| sub parameter {...}|
 +----------------------+               +--------------------+

                                                                       +----+
                                                                       |Your|
                                                                       |Duty|
 +----------------------------+----------------------------------------+----+
 |Apache                      | calls Your::Class::handler()           |    |
 +----------------------------+----------------------------------------+----+
 |                            | nominates the handlers,                |    |
 |Your::Class::handler()      | constructs $self,                      | ** |
 |                            | and calls $self->dispatch              |    |
 +----------------------------+----------------------------------------+----+
 |                            |        $self->init     (does nothing)  | ?? |
 |                            |        $self->prepare  (see below)     |    |
 |Apache::HeavyCGI::dispatch()| calls  $self->layout   (sets up layout)| ** |
 |                            |        $self->finish   (headers and    | ** |
 |                            |                         gross content) |    |
 |                            |        $self->deliver  (delivers)      | ?? |
 +----------------------------+----------------------------------------+----+
 |Apache::HeavyCGI::prepare() | calls HANDLER->instance->header($self) | ** |
 |                            | and HANDLER->instance->parameter($self)| ** |
 |                            | on all of your nominated handlers      |    |
 +----------------------------+----------------------------------------+----+

Object Attributes ^

As already mentioned, the HeavyCGI object is a pseudo-hash, i.e. can be treated like a HASH, but all attributes that are being used must be predeclared at compile time with a use fields clause.

The convention regarding attributes is as simple as it can be: uppercase attributes are reserved for the Apache::HeavyCGI class, all other attribute names are at your disposition if you write a subclass.

The following attributes are currently defined. The module author's production environment has a couple of attributes more that seem to work well but most probably need more thought to be implemented in a generic way.

CAN_GZIP

Set by the can_gzip method. True if client is able to handle gzipped data.

CAN_PNG

Set by the can_png method. True if client is able to handle PNG.

CAN_UTF8

Set by the can_utf8 method. True if client is able to handle UTF8 endoded data.

CGI

An object that handles GET and POST parameters and offers the method param() and upload() in a manner compatible with Apache::Request. Needs to be constructed and set by the user typically in the contructor.

CHARSET

Optional attribute to denote the charset in which the outgoing data are being encoded. Only used within the finish method. If it is set, the finish() method will set the content type to text/html with this charset.

CONTENT

Scalar that contains the content that should be sent to the user uncompressed. During te finish() method the content may become compressed.

DOCUMENT_ROOT

Unused.

ERROR

Anonymous array that accumulates error messages. HeavyCGI doesn't handle the error though. It is left to the user to set up a proper response to the user.

EXECUTION_PLAN

Object of type Apache::HeavyCGI::ExePlan. It is recommended to compute the object at startup time and always pass the same execution plan into the constructor.

EXPIRES

Optional Attribute set by the expires() method. If set, HeavyCGI will send an Expires header. The EXPIRES attribute needs to contain an Apache::HeavyCGI::Date object.

HANDLER

If there is an EXECUTION_PLAN, this attribute is ignored. Without an EXECUTION_PLAN, it must be an array of package names. HeavyCGI treats the packages as Class::Singleton classes. During the prepare() method HeavyCGI calls HANDLER->instance->header($self) and HANDLER->instance->parameter($self) on all of your nominated handlers.

LAST_MODIFIED

Optional Attribute set by the last_modified() method. If set, HeavyCGI will send a Last-Modified header of the specified time, otherwise it sends a Last-Modified header of the current time. The attribute needs to contain an Apache::HeavyCGI::Date object.

MYURL

The URL of the running request set by the myurl() method. Contains an URI::URL object.

R

The Apache Request object for the running request. Needs to be set up in the constructor by the user.

REFERER

Unused.

SERVERROOT_URL

The URL of the running request's server-root set by the serverroot_url() method. Contains an URI::URL object.

SERVER_ADMIN

Unused.

TIME

The time when this request started set by the time() method. Please note, that the time() system call is considerable faster than the method call to Apache::HeavyCGI::time. The advantage of calling using the TIME attribute is that it is self-consistent (remains the same during a request).

TODAY

Today's date in the format 9999-99-99 set by the today() method, based on the time() method.

Performance

Don't expect Apache::HeavyCGI to serve 10 million page impressions a day. The server I have developed it for is a double processor machine with 233 MHz, and each request is handled by about 30 different handlers: a few trigonometric, database, formatting, and recoding routines. With this overhead each request takes about a tenth of a second which in many environments will be regarded as slow. On the other hand, the server is well respected for its excellent response times. YMMV.

BUGS ^

The fields pragma doesn't mix very well with Apache::StatINC. When working with HeavyCGI you have to restart your server quite often when you change your main class. I believe, this could be fixed in fields.pm, but I haven't tried. A workaround is to avoid changing the main class, e.g. by delegating the layout() method to a different class.

*** Note: this has no meaning anymore after 0.0133 ***

AUTHOR ^

Andreas Koenig <andreas.koenig@anima.de>. Thanks to Jochen Wiedmann for heavy debates about the code and crucial performance enhancement suggestions. The development of this code was sponsered by www.speed-link.de.

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