The XML Hype
Everybody talks about XML. XML here, XML there. All application servers
support XML, everybody wants to do B2B using XML, web services using
XML, even databases using XML.
Should you care about it? Given the amount of hype, you can't afford to
go around ignoring the argument, would be like ignoring the world wide
web 10 years ago: a clear mistake. But why is this so for XML? What is
this "magic" that XML seems to have to solve my problems? Isn't this
another hype to change once again the IT infrastructure that you spent
so much time implementing and fixing in the last few years? Isn't
another way to spill money out of your pockets?
If you ever asked yourself one of the above questions, this paper is for
you. You won't find singing-and-dancing marketing crap, you won't find
boring and useless feature lists, you won't find the usual acronym
bombing or those good looking vaporware schemas that connect your
databases to your coffee machines via CORBA or stuff like that.
This document will explain you what the Cocoon project is about and what we are
doing to solve the problems that we encountered in our web engineering
experiences, but from an executive perspective, yes, because we all had
the problems of managing a web site, dealing with our colleagues, rushing
to the graphical guru to have the little GIF with the new title, or
calling the web administrator at night because the database is returning
errors without reasons.
It was frustrating to see the best and most clever information
technology ever invented (the web) ruined by the lack of engineering
practices, tortured by those "let's-reinvent-the-wheel-once-again"
craftmen that were great at doing their jobs as individuals but that
couldn't scale and imposed a growth saturation to the whole project.
There had to be a better way of doing things.
Personal Experiences
In 1998, Stefano Mazzocchi volunteered to create the documentation infrastructure for
the java.apache.org project, which is composed by a bunch of different
codebases, maintained by a bunch of different people, with different
skills, different geographical locations and different degree of will
and time to dedicate to the documentation effort.
But pretty soon he realized that no matter how great and well designed the
system was, HTML was a problem: it was *not* designed for those kind of
things. Looking at the main page (http://java.apache.org/) from the
browser and you could clearly identify the areas of the screen: sidebar,
topbar, news, status. But if you opened the HTML, boom: a nightmare or
table tags and nesting and small little tricks to make the HTML appear
the same on every browser.
So he looked around for alternative technologies, but *all* of them were
trying to add more complexity at the GUI level (Microsoft Frontpage,
Macromedia Dreamweaver, Adobe GoLive, etc...) hoping to "hide" the
design problems of HTML under a thick layer of WYSIWYG looks.
What you see is what you get.
But what you see is all you've got.
How can you tell your web server to "extract" the information from the
sitebar? How can you have the news feeds out of a complex HTML page?
Damn, it's easy for a human reader: just look at the page and it's very
easy to distinguish between a sidebar, a banner, a news and a stock
quote. Why is it so hard for a machine?
The HTML Model
HTML is a language that tells your browser how to "draw" things on its
window. An image here, a letter there, a color down here. Nothing more.
The browser doesn't have the "higher level" notion of "sidebar": it
lacks the ability to perform "semantic analysis" on the HTML content.
Semantic analysis? Yeah, it's the kind of thing the human brain is
simply great at doing, while computer programs simply suck big time.
So, with HTML, we went a step up and created a highly visual and
appealing web of HTML content, but we went two steps back by removing
all the higher level semantic information from the content itself.
Ok, let's make an example... most of you have seen an HTML
page... if not, here is an example:
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<html>
<body>
<p>Hi, I'm an HTML page</p>
<p align="center">Written by Stefano</p>
</body>
</html>
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which says to the browser:
-
I'm a HTML page
-
I have a body
-
I have a paragraph
-
I contain the sentence "Hi, I'm an HTML page."
-
I contain the sentence "Written by Stefano"
Suppose you are a chinese guy that doesn't understand our alphabet, try
to answer the following question:
who wrote the page?
You can't perform semantic analysis, you are as blind as a web browser.
The only thing you can do is draw it on the screen since this is what
you were programmed to do. In other words, your semantic capacity is
fixed to the drawing capabilities and a few other things (like linking),
thus limited.
Semantic Markup
Suppose you receive this page:
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<page>
<author>sflkjoiuer</author>
<content>
<para>sofikdjflksj</para>
</content>
</page>
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can you tell me who wrote the page? easy, you say, "sflkjoiuer" did. Good, but later
you receive:
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<dlkj>
<ruijfl>sofikdjflksj</ruijfl>
<wijlkjf>
<oamkfkj>sflkjoiuer</oamkfkj>
</wijlkjf>
</dlkj>
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now, who wrote the page? You could guess by comparing the structure,
but how do you know the two structures reflect the same semantic
information?
The above two pages are both XML documents.
Are they going to help you? Are they doing to simplify your work? Are
they going to simplify your problems?
At this point, clearly not so, rather the opposite.
So, you could be wondering, why did we spend so much effort to
write an XML publishing framework? This document was written exactly
to clear your doubts on this, so let's keep going.
The XML Language
XML is most of the times referred to as the "eXtensible Markup Language"
specification. A fairly small yet complex specification that indicates
how to write languages. It's a syntax. To tell you the truth, nothing fancy at all. So
is correct, while
is not, but
is correct. That's more than this, but I'll skip the technical details here.
XML is the ASCII for the new millenium, it's a step forward from ASCII
or UNICODE (the international extension to ASCII that includes all
characters from all modern languages). It defines a "lingua franca" for
textual languages.
Ok, great, so now instead of having one uniform language with visual
semantics (HTML) we have a babel of languages each with its own
semantics. How this can possibly help you?
XML Transformations
This was the point where Stefano was more or less two years ago for
java.apache.org: I could use XML and define my own semantics with
<sidebar>, <news>, <status>
and all that and I'm sure people would have
found those XML documents much easier to write (since the XML syntax is
very similar to the HTML one and very user friendly)... but I would have
moved from "all browsers" to "no browser".
And having a documentation that nobody can browse is totally useless.
The turning point was the creation of the XSL specification which
included a way to "transform" an XML page into something else. (it's
more complex than this, but, again, I'll skip the technical details).
So now you have:
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XML page ---(transformation)--> HTML page
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transformation rules
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that allows you to write your pages in XML, create your "graphics" as
transformation rules and generate HTML pages on the fly directly from your
web server.
Apache Cocoon 1.0 did exactly this.
The Model Evolves
If XML is a lingua franca, it means that XML software can work on almost
anything without caring about what it is. So, if a cell phone requests
the page, Cocoon just has to change transformation rules and send the
WAP page to the phone. Or, if you want a nice PDF to printout your
monthly report, you change the transformation rules and Cocoon creates
the PDF for you, or the VRML, or the VoiceML, or your own proprietary
B2B markup.
Anything without changing the basic architecture that is simply based on
the simple "angle bracket" XML syntax.
Separation of Concerns (SoC)
Cocoon was not the first product to perform server side XML
transformations, nor will be the last one (in a few years, these
solutions will be the rule rather than the exception). So, what is the
"plus" that the Cocoon project adds?
We believe the single most important Cocoon innovation is SoC-based design.
SoC is something that you've always been aware of: not everybody is
equal, not everybody performs the same job with the same ability.
It can be observed that separating people with common skills in
different working groups increases productivity and reduces management
costs, but only if the groups do not overlap and have clear "contracts"
that define their operability and their concerns.
For a web publishing system, the Cocoon project uses what we call the
pyramid of contacts which outlines four major concern areas and five
contracts between them. Here is the picture:
Cocoon is engineered to provide you a way to isolate these four
concern areas using just those 5 contracts, removing the contract
between style and logic that has been bugging web site development since
the beginning of the web.
Why? because programmers and graphic people have very different skills
and work habits... so, instead of creating GUIs to hide the things that
can be harmful (like graphic to programmers or logic to designers),
Cocoon allows you to separate the things into different files, allowing
you to "seal" your working groups into separate virtual rooms connected
with the other rooms only by those "pipes" (the contracts), that you
give them from the management area.
Let's have an example:
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<page>
<content>
<para>Today is <dynamic:today/></para>
</content>
</page>
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is written by the content writers and you give them the
"contract" that states that the tag
<dynamic:today/> prints out the time of the day
when included in the page. Content writers don't care (nor
should) about what language has been used for that, nor they
can mess up with the programming logic that generates the
content since it's stored in another part of the system they
don't have access to.
So <dynamic:today/> is the "logic - content" contract.
At the same time, the structure of the page is given as a contract to
the graphic designers who have to come up with the transformation rules
that transform this structure in a language that the browser can
understand (HTML, for example).
So, the page structure is the "content - style" contract.
As long as these contract don't change, the three areas can work in a
completely parallel way without saturating the human resources used to
manage them: costs decrease because time to market is reduced and
maintenance costs is decreased because errors do not propagate out of
the concern areas.
For example, you can tell your designers to come up with a "Xmas look"
for your web site, without even telling the other people: just switch
the XMas transformation rules at XMas morning and you're done.... just
imagine how painful it would be to do this on your web site today.
With the Cocoon architecture all this is a couple of line changes away.
Here we go
If you reached this far by reading all sections, you should have grasped the
value of the Cocoon Project and distinguish most of the marketing
hype that surrounds XML and friends.
Just like you shouldn't care if somebody offers you a software that is
"ASCII compliant" or "ASCII based", you shouldn't care about "XML
compliant" or "XML based": it doesn't mean anything.
Cocoon uses XML as a core piece of its framework, but improves the model
to give you the tools you need and is designed to be flexible enough to
follow your needs as well as paradigm shifts that will happen in the
future.
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