Best Practices Guide for Documenting XML Schemas

This guide provides a set of best practices that will help you with the task of documenting an XML schema.  These best practices were established by the developers at Adivo who have extensive experience in technical documentation, XML data modeling and development.

Start With a Good XML Data Model

A good XML data model can make documenting your XML schema a much easier task.   Although it might not always be an option, if you have any influence over the data model, here are some key areas that you should look out for:

  • Naming conventions - The lack of naming conventions can result in very inconsistent and cryptic names for the elements, attributes and types defined by your data model making it more difficult to document them properly.  Applying good naming conventions to the data model will promote better readability of the documentation.
  • Global components - Some XML schema grammars allow schema components to be defined either locally within the parent component or globally so that the component can be reused. By using global components in your data model, you will maximize reuse and reduce the number of components that you need to document.

And even if you do not have any control over the data model, a quick review will help you better estimate how much time it will take and set expectations for the completion of the documentation.

Embed Comments inside Your XML Schema

Most XML schema grammars provide a mechanism for adding comments to elements, types and other schema components.  These comments are included as part of the definition of the schema component, making them easy to maintain as the XML schema changes over time.

By embedding comments inside your XML schema, you create a "self-documenting" data model that will be easier for XML data modelers and XML programmers to understand.

Comment What You Can and Annotate Everything Else

Depending on the commenting capabilities of your XML schema grammar, you should add comments to the following XML schema components:

  • Elements - Elements are typically used to model real world objects.  The comments for these types of elements should describe what that real world object is.  Your data model may also include elements, sometimes referred to as "association elements", that are used to create many-to-many relationships between objects.  For these types of elements, your comment should describe the purpose of the relationship.
  • Attributes - As elements typically represent real world objects, attributes are typically used to represent properties of those objects.  Attribute comments should describe what those properties are and how they are represented.
  • Types - Most types are used to create specific elements or attributes. In these cases the types should be commented the same way as elements and attributes.
  • Enumerations - For elements, attributes and types that have a list of enumerated values defined, you should comment each of these values.

The descriptions that you add to these XML schema components should be concise and consist of one or two sentences.  If there is additional information that is important to document, you should consider structuring your comments into two parts: a summary description followed by a remarks section.  The remarks section can be used to include the additional information.

Not all XML schema grammars will have commenting capabilities robust enough to handle documenting all of the XML schema components mentioned here.  To get around these limitations, you can use a tool like TechWriter to annotate the XML schema with external comments that can be merged together with the embedded comments to produce the complete documentation.

Use a Documentation Generator

Having a self-documented XML schema is helpful for data model design and maintenance, but there are many instances where you are going to need actual data model documentation. Here are a few examples:

  • Data Model Documentation - If you are creating an XML schema for a client, data model documentation is typically a required deliverable.
  • End-User Documentation - While your XML data modeler may have the XML schema grammar knowledge and expertise needed to read and understand the data model, many users of the XML may not.  These users may be consumers or producers of the XML data that is defined by the XML schema. You will need to provide them with some form of documentation that will help them understand the data model. 
  • Data Model Reviews - Not all participants in a data model review are going to have the same level of XML schema knowledge and expertise.  Your subject matter experts may not be technical at all.  To facilitate such a data model review, you will need to provide documentation that anybody can read and understand.

Fortunately there are tools like TechWriter that can be used to generate the XML schema documentation automatically, leveraging the XML schema definitions and the comments you've embedded.

Make Documentation Part of Your Development Process

If your project is using automated build tools like Ant, NAnt and MSBuild, you can make generating the documentation part of the automated build process for the system.   By having the documentation as up-to-date as the system itself, your project will benefit in the following areas:

  • Knowledge Capture - While some development methodologies such as Agile ascribe to a "document late" philosophy, projects can suffer greatly when they lose a team member.  The knowledge of a system literally walks out the door.  Documenting as you go minimizes the impact of losing a team member and makes it easier for new team members to ramp up.
  • Communication - Documentation can help communication among project team members.  The project manager can better assess the status of the project.  Subject matter experts can verify that developers are on the right track.  And mistakes can be identified earlier in the project life-cycle.
  • Documentation Quality - Scrambling at the end of the project to throw the documentation together for the final deliverable can result in the data model not being documented properly, if documented at all. 

By making documentation part of your development process you will ensure that the documentation is always in sync with the data model, even in extreme programming environments where the XML schema is constantly changing. 

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