DTD vs. XML Schema

AShocka writes "The W3C XML Schema Working Group has released the first public Working Draft of Requirements for XML Schema 1.1. Schemas are technology for specifying and constraining the structure of XML documents. The draft adds functionality and clarifies the XML Schema Recommendation Part 1 and Part 2. The XML Schema Valid FAQ highlights development issues and resources using XML Schema. This article at webmasterbase.com addresses the XML DTDs Vs XML Schema issue. Also see the W3C Conversion Tool from DTD to XML Schema and other XML Schema/DTD Editors."

Comments

Power

[sethadam1]

There's no "vs."

XML Schema are much more flexible and powerful.

There're also about 100 times more difficult and confusing.

Re:Power

[goatasaur]

"There're also about 100 times more difficult and confusing."

The phrase "difficult and confusing" goes hand-in-hand with any flexible or powerful computer utilities.

Full utilization of XML (and myriad programming languages) takes time.

They call them "languages" for a reason. You can't write a sonnet in French if you have only studied it for a year; and you can't write a full-featured browser suite if you started coding a month ago. :)

All this hype about XML

[MimsyBoro]

I am a programmer for a commercial company (yes I like to make money, and I program on WinTel). I year ago we had the XML craze we converted all our internal protocols to XML. I discovered that XML was just a lot of hype about nothing. There is nothing self-describing about it. Or maybe there is, just like the section names in an INI file describe the keys in them...
On the other hand the one thing that I did find XML useful for is easy parsing. If you use XML to develop a lower level protocol you end up with bloated 10k messages. But for high-level protocols or for configuration files it's great for only one reason: There are lots of ready-made tools. If you want to parse XML in Windows just load the IXMLDocument interface and it works at lightening speed. If you want to parse the messages in a web-browser through together a quick DOM parser or even use the build in DOM one! If you want to parse XML in PERL or C/C++ there are great libs. The only reason XML is good is because all the hype got people developing very neat tools. In one of my latest projects that needs to pass information between two programs written in different languages a used a Home-Made SOAP and designed a base class the persists using XML. I developed it in both langauges in under an hour!
So although it wastes bandwidth and there really isn't anything neat about it, it is comfortable I'll give it that.

Re:All this hype about XML

[sporty]

I year ago we had the XML craze we converted all our internal protocols to XML. I discovered that XML was just a lot of hype about nothing. There is nothing self-describing about it. Or maybe there is, just like the section names in an INI file describe the keys in them...

Great thing about XML, is if you need to convert your communications, you can write XSLT against it to convert it while you convert your XML source.. easily. For instance, one vendor I worked with decided that the old protocol didn't work well anymore, and a ne one would be better. Forget the reasons for the change, good or bad.

I plopped an XSLT processor in front of it. Took minutes to implement. In the mean time, I was able to properly rewrite the XML producing code. So I had some flexibility in terms of patching the protocol quickly, while taking the weeks I needed to fix things right.

As for self describing, what is more self describing than HTML? You see a bold and italics tag around an element, you can easily figure out what style the text would be in. Yes, I know about CSS, but the point is, XML IS descriptive, so long as you use good names. Naming elements a, b and c is just developer fault.

If you use XML to develop a lower level protocol you end up with bloated 10k messages.

If in today's age of gigabit ethernet and cheap parts, you really really need to squeeze that extra bit through, compress the line. Seriously. Simplest case, is using ssh. Hell, it auth's AND encrypts. If you are worried about anonymous access, there are other tools.

Re:All this hype about XML

[plierhead]

I discovered that XML was just a lot of hype about nothing.

Amen to that. Sad to say, but certain parts of the IT industry (and in particular, anything to with one computer (or piece of code) magically talking to another one owned by a different organization) are constantly buying into the bogus claims of snakeoil salesman with silver bullet technologies. XML is merely the latest in a long line.

The only new things about XML, IMHO, are that is has spawned more sub-specifications than any previous pretender to the crown.

Anyone remember CORBA ? Or any of the other zillions of RPC-type mechanisms that people have jumped on the bandwagon of ?

I'm not blaiming the people who push these agendas. I too would love to spend my weekends sunning on the beaches of exotic European tourist destinations and chugging beers on my expense account. The price of sitting through a ferw stiflingly boring and pointless standards meetings seems a small price to pay. All large IT companies employ 2 or 3 people whose job it is to front up to these meetings. Typically these people are articulate and highly versed ex-programmers but architecurally challenged and with little understanding of the real nature of building complex IT systems.

Ultimately, these RPC mechanisms all end up as nothing - or rather, as only perhaps 1% of the eventual solution.

All that XML is, is an easy-to-parse, text based data transfer mechanism. And as the parent posting says, there are some nice tools around for it. Big deal. Probably you'd be silly to use anything else if designing a data transfer. But is it ever going to change the world ? Or even rock it a little ? No.

Re:All this hype about XML

[ttfkam]

Yup, I remember CORBA. It's still used. In fact, it can be used with XML. CORBA provides the interfaces for programmatic logic. There is nothing really required to know about how the ORBs communicate with one another. If an ORB decides that its transport mechanism is XML, so be it.

As for SOAP and XML-RPC, what's so hard about compressing it before sending the message? The whole point about XML is that you don't need to write a new parser. You don't need to write a new broadcaster. Your project is about getting a task done, not micromanaging implementation details.

If (and only if) your higher level API/transport is insufficient for the task do you roll your sleeves up and dig in. Do you write everything in assembly? Why not? It would be faster than whatever language you are using now. The reason you don't is that you have better things to do with your time. The goal is important, not the tool. Everyone has standardized on and is optimizing this one particular tool and it works well. So many people have done work so that you don't have to.

Will it change the world? Of course not. It's just a markup language. Will any other computing tool change the world? Of course not. The end users have never cared how you got to the solution. They cared only if you got to the solution faster than the other guy.

Schemas are often a bad idea

[mesocyclone]

XML is a very powerful tool.

On very important use is in creating interfaces between heterogeneous systems. Areadable character set and meaningful tags is very handy for developers. The hierarchical structure is extremely powerful. And, of course, the fact that it is a standard with common tools is invaluable.

However, one useful principle of such interfaces is "if you don't understand it, ignore it." In other words, when you get a message, look for what you want in it and use it. Ignore anything that isn't what you want. XML is ideally suited for this approach - especially if you use path based access rather than DOM tree traversal.

This approach to interfaces allows systems to interchange messages without exact version consistency, and without requiring a tight congruence of the applications. It allows a system to "tell what it knows" and another system to "read what it needs" without further ado.

Unfortunately, the use of schemas goes against this idea. It is IMHO a more old fashioned approach of rigidly constraining the messages to an exact specification. This can make interfaces far less robust and flexible, and increase the amount of work.

Schema processing may also be promoted to "verify" message integrity before processing. However, it only does so in the most primitive ways. Real world messages, especially in the business world, tend to have integrity rules that go far beyond what can be expressed in anything short of a complex computer program or equivalent declarations.

I am sure there are plenty of places where schemas make sense, but in the areas of commercial message interchange, they take a powerful and flexible construct and hobble it.

Not really

[Codex The Sloth]

This approach to interfaces allows systems to interchange messages without exact version consistency, and without requiring a tight congruence of the applications. It allows a system to "tell what it knows" and another system to "read what it needs" without further ado.

Unfortunately, the use of schemas goes against this idea. It is IMHO a more old fashioned approach of rigidly constraining the messages to an exact specification. This can make interfaces far less robust and flexible, and increase the amount of work.


If your talking about using XML for data messaging not using schemas is just lazy. XML Schema allows optional elements and attributes and/or default values. So if it isn't required, then just make it optional. If you want multiversion interfaces, you have a different XMLSchema for each version. Then each side knows explicitly what the messaging protocol is.

While it's probably true that things mostly kinda work if the versions don't match, you shouldn't be relying on this. There's lots of software out there that does this but that doesn't mean it's the ideal.

If your using XML for markup of documents, schemas are somewhat less useful since the underlying semantics of the tags is usually more important.

The hell I won't

[ttfkam]

Trimming bloat like namespaces and comments? Are you nuts?

How do you embed MathML in another document (like XHTML)? Currently it's with namespaces. How do you propose to do that without namespaces? Just the prefixes? What happens when two different markups use the same prefix? Wups! You're screwed!

No comments? This is supposed to make a better alternative to XML? It won't help readability, and it certainly isn't a major bottleneck during parsing.

Don't want the "bloat" of namespaces and comments? Wait for it... Wait for it... Don't use namespaces and comments in your documents! Wow! What a concept!

Maybe no Unicode in PXML hunh? So much for interoperability for any kind of data. You don't ever want your pet project used in East Asia (or Russia or Greece or most other places in the world) do you? Unicode too bloated? Why not just use ISO-8859-15 (basically ASCII w/ a Euro character -- which incidentally a Euro character isn't available in ASCII)? Oh wait! That's right. You don't want to allow processing instructions, which in XML tell you what encoding is used.

What happens if you want to change some of the basic syntax of PXML? Because you've nuked processing instructions, you can't specify a markup version like you can in XML.

Yes, yes. We've all seen your little pet project. I hope it was just a class assignment.

Use both!

[Capsaicin]

This is true, but DTD's are more human readable IMHO.

Absolutely. All the possible attributes, and kids of any element are there in one (OK, two) place(s) and you can garner the information about any element in a matter of seconds. With XML Schema you have to keep track of the levels of nesting and rifle through a series of name/value pairs to get the same information. It is in its greater expressiveness that the advantage of XSD is seen to lie. And there might be applications where this expressiveness necessitates the use of XSD.

However, XML Schema, has besides this expressivenss, one other great advantage. It is XML. As such it can be processed with the same XML tools one uses elsewhere with an XML application.

As an example, in one application, I take a DTD, translate it into XSD, and then run an XSL stylesheet over the XSD file to generate some base code used in my application. In this way I can ensure that my code will automatically be changed to reflect any minor changes made to my Schema.

So while I continue to write DTDs, I look on XML Schema as a way to translate, and bring my DTD into the XML universe, with all its attendant advantages.