Configuration Service Validation Design
The major form of configuration in the Carbon system is XML-based.
This provides the ability to use various forms of XML validation
technology to help developers create properly defined configuration
documents without the need to start the Carbon system to and
allow each Component to validate its own configuration upon
There are three major forms of validation that can be performed
against XML documents. Each level of the validation is layered
upon the success of the previous layer.
Structural validation checks for the well-formedness of an
XML document. This means that the document is properly
layered with each element opening and closing itself in
a proper hierarchy.
Additional structural validation can occur with the
presence of a validation document such as a DTD or
XML Schema. These documents can define the list of
valid XML elements within the document, the order
of the elements and what is allowed to be sub-elements
of each. They also define the attributes that can
be applied to the various elements.
Data validation is the validation of the information
appearing within elements and attributes. Data
validation may restrict the content of elements or
attributes to be numbers, match regular expressions
or various other content based restrictions.
Data validation requires the use of validation document
which supports complex type definitions. DTDs do not
have this capability, and thus the use of XML Schema
is needed to support automatic data validation.
Programmatic validation, often called business validation,
is the most complex level. There is no formal method
of externalizing the rules of programmatic validation
into a validation document. This type of validation
is performed by a business application that reads in
the XML file and performs programmatic checks against
"XML 1.0 included a set of tools for defining XML document
structures, called Document Type Definitions (DTDs). DTDs
provide a set of tools for defining which element and
attribute structures are permitted in a document, as well
as mechanisms for providing default values for attributes,
defining reusable constants (entities), and some kinds of
metadata information (notations). While DTDs are widely
supported and used, many XML developers quickly outgrew
the capabilities DTDs provide."
- XML Schema (Eric van der Vlist).
A complete description of DTDs can be found in the
Extensible Markup Language (XML) 1.0 (Second Edition)
technical documentation at the W3C.
Carbon supports the use of DTDs as validation documents
inside of Configuration files, but does not recommend
them. DTDs can only provide structural validation of an
XML document and therefore do not add a significant level of
value to the average developer of the configuration files.
XML Schema was created by the W3C as a method of adding
data validation information to documents and providing
an XML-based extensible way of defining the structure
and content of the document.
A complete description of XML Schema can be found in the
technical documentation at the W3C.
Attaching an XML Schema to a Carbon configuration document
is a simple process that requires only two additional
attributes to be added to the top of the Configuration
document inside the <Configuration> element.
<?xml version="1.0" encoding="UTF-8"?>
<!-- ... -->
- The first item defines a namespace to the XML
processor stating that the document may include
XML Schema based attributes or elements.
- The second item associates a schema to this document.
The value given is often called a "hint" to the location of
the actual schema. Currently in carbon the value must be
a valid URL (or a classpath location) to locate the schema.
(Note: This is a Carbon specific enhancement
to the resolution of the Schema location. Normal
XML processing programs will not be able to resolve
this URI without the help of XML Catalogs.)
The use of the XML Schema documents with Carbon is most
useful during application development. When using a
XML editing program (like Altova XMLSPY or TIBCO TurboXML)
a developer can instantly know their document is meeting
the structural and data validation requirements of
the configuration. The editor will also provide
useful IDE like features of automatically closing elements,
listing possible sub-elements, etc.
It is also possible to develop the configuration XML
in a standard text editor and use one of the many
XML validation tools available on the web to quickly
check if the document is validating properly.
Run-time validation of the XML document is considered
less useful and is somewhat disabled inside the Carbon
Structural validation of XML documents is required since
it is impossible for a parser to properly read in the
information contained within the document if it is not
Data validation errors will not break the Carbon system.
If an XML document contains a validation document
(DTD or XML Schema) and attempt will be made to validate
the document. If validation fails, an error message
will be logged, but the system will continue to process
the XML document ignoring the validation rules applied.
XML Namespaces provide a method using multiple schemas
to define a single XML document. With Carbon's concept
of nested configurations, it seems like having multiple
namespaces would be a natural extension.
For example, a Cache config document usually includes a
nested DataLoader element. It would appear that this
configuration should actually be validated against
a Cache XML Schema as well a a DataLoader XML Schema
each in its own namespace.
This solution is possible on input, but provides huge
problems when attempting to manipulate the document
through JMX or other management tools. When adding
elements to the document there is no way for Carbon's
configuration service to known what namespace the
element is suppose to occur under. It can make
educated guesses, but may often be incorrect. Therefore
it was decided to only allow XML Schemas that do
not include a namespace.
When including the location of an XML schema in a
configuration document with the
xsi:noNamespaceSchemaLocation you must give
a location that the XML processor can use to find
the XML Schema document. The default entity resolver
used by Xalan (Carbon's default XML processor) is
only able to find locations with a valid URL. It
does not do additional processing to attempt to determine
the location of the document.
Future versions of Carbon may include the Apache
XML Commons Resolver which provides support for
OASIS XML Catalogs that allow a user to map
a given URI for a schema location to a URL
using a configuration file.