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What is a XML schema in plain simple terms? What will be the problem if an XML doesn't have a schema defined? An example can be very helpful.

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You might try to process arbitrary XML. What will your app do if I send <xml><sillyTag>some random text</sillyTag></xml>? – Dan Pichelman Apr 18 '13 at 19:05
@Dan: most programs out them will exit with an error. I don't see a big issue here. – Codism Apr 18 '13 at 21:50
up vote 5 down vote accepted

The main problem I see is that it cannot be validated either against a DTD, or against an XML Schema. As such, you don't have any guarantee that what you received is what you expected.

For example, if you are sending this XML file to someone:

<x id="2">

How can he be certain that:

  1. is "id" always a number ?
  2. is "id" mandatory or optional ?
  3. is "id" the only attribute of the "x" element ?
  4. how many "yyy" elements can be contained in a "x" element ?
  5. is "yyy" the only element that can be inside "x" ?
  6. how many "zzz" elements in a "yyy" element ?
  7. etc.

There is an unlimited list of questions that person can ask to you. The goal of an XML Schema is to hopefully answer all these questions and also to provide a way to validate a document before or during the parsing phase.

Not fond of w3schools, but here is a list of short and interesting reads on XML Schema:

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It's the same question as using a static or dynamically-typed language. Schema is a declarative check on the correctness of your data. If you have a field that has to be an integer, you can mark as such in your schema and then invalid data won't get past the validator. Without a schema, you would have to check in your code which is more error-prone. Sometimes, you might have a preference for flexibility in which case a schema is a hindrance. Notably there is no standard schema language for JSON, yet it is growing in popularity.

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