Recursion in an XML schema?

Question

I need to create an XML schema that validates a tree structure of an XML document. I don\'t know exactly the occurrences or depth level of the tree.

XML example:

Answer 1

The other solutions work great for making root elements recursive. However, in order to make a non-root element recursive without turning it into a valid root element in the process, a slightly different approach is needed.

Let's say you want to define an XML message format for exchanging structured data between nodes in a distributed application. It contains the following elements:

• - the root element;
• - the message's origin;
• - the message's destination;
• - the data structure type encoded in the message;
• - the data contained in the message.

In order to support complex data types, is a recursive element. This makes possible to write messages as below, for sending e.g. a geometry_msgs/TwistStamped message to a flying drone specifying its linear and angular (i.e. rotating) speeds:

  controller:8080
  drone:8080
  geometry_msgs/TwistStamped
  
    0
    
      1
      0
    
    base_link
  
  
    
      1.0
      0
      1.0
    
    
      0.3
      0
      0
    
  

We can easily write an XML schema to validate this format:

The problem with the schema above is that it makes a root element, which means it also validates the document below:

  
    0
    
      1
      0
    
    base_link
  
  
    
      1.0
      0
      1.0
    
    
      0.3
      0
      0
    
  

In order to avoid this side-effect, instead of defining the element directly at the global level, we first define a data type, then define a data element of that type inside message:

Notice that we end up having to define the element twice — once inside the data type, and again inside — but apart a little work duplication this is of no consequence.