XML Web Services Hints and Tips

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XML Web ServicesHints and Tips

• Bob SteemsonProduct ArchitectiSOFT plc

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Introduction

• Why use XML Web Services?
• Some thoughts on XML
• Building Web Service Code
• Summary
• Questions

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Why Use XML Web Services?

• Facilitates loose-coupling of applications
• Platform Independent
• Powerful XML tools available
• Standard Internet technology lowers costs
• Independent of proprietary platforms
• Independent of expensive third party software
• Open-source solutions available

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Some Thoughts on XML

• XML is not about text
• Understand and use namespaces
• Make use of XML Schemas
• Make use of XSLT

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XML is not about text

• Dont build XML Strings by hand
• Use appropriate tools
• For example MXXMLWriter, XmlWriter, JDOM
• Will handle illegal characters and buffer data
• Will help ensure your XML is well formed
• Dont pretty print XML strings unnecessarily
• Most XML will be passed from method to method or
  from machine to machine
• View with tools like XML Spy or Internet Explorer
• If necessary, include an option in test tools
• Dont keep parsing and serialising documents
• Once parsed, keep as a DOM as long as possible

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Understanding XML Namespaces

• Prevent element and attribute names from
  different applications from clashing
• XML Namespaces associate element and attribute
  names with a URI (URI or URN)
• The URI is a way of defining a globally unique
  string
• Two namespace URIs are equal if they are
  identical character-by-character (case sensitive)
• There need not be a corresponding resource
• Namespace prefix is a local synonym for URI
• Avoids problems if URI contains illegal
  characters
• Two names with different prefixes are identical
  if local names and URIs are the same

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Using XML Namespaces

• Always use namespaces for external XML
• Anything that may be seen outside this
  application
• Internal XML such as configurations may have no
  namespace if you are certain it will not escape
• Avoid using the default namespace with no prefix
• Reserve no prefix to mean no namespace
• Can make writing XSLT more difficult
• Beware of domain name changes if using a URL
• Do not hard code the URL
• URL changes require coordination with partners
• Dont forget Schema and XSL documents

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Make use of XML Schemas

• Create and publish a schema for each document
• Validate input XML before processing by default
• Allow validation to be turned off
• Have an option to validate output documents
• Use mainly for testing
• Turn this off by default Buyer beware

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Make use of XSL Transformations

• XSLT is a powerful way of transforming XML
• Much more than a way of turning XML into HTML
• Can change or remove document namespaces
• Restructure elements or convert codes
• Simplify complex external schemas
• Can make serialisation deserialisation simpler

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Building Web Services

• Designing Web Services
• Security requirements
• Think about testing
• Dont leave namespaces at the default
• .NET Web Services Enhancements

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Designing Web Services

• Web Services should be Coarse Grained
• Think of complex transactions that involve
  multiple co-operating objects, rather than object
  methods
• Try to have fewer methods and extend
  functionality with options if necessary
• Try to make stabilise service signatures - WSDL
• Both ESI and the CHI Web Services have a single
  method that includes a service type parameter
• Pass complex data as XML
• Consider passing XML as a DOM Element rather than
  a string

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Web Services Security

• Keep security and other parameters separate
• Both may change but probably not at the same
  time
• If possible, keep security credentials out of the
  SOAP body
• Consider using WS-Security (see below)

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Web Services Testing

• Ensure that services can be tested in-house
• When writing a new service
• Write a simple interactive client for prototyping
  and early development
• Create an automatic test client for long-term use
• When writing a client for an external service
• Provide a simple emulator for in-house testing
• Allows progress while connections are set up
• Allows future debugging in a closed environment
• Update the emulator after external testing

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Web Services Tracing

• Use Web Services Tracing if all else fails
• Can be useful in sorting out namespace issues
• MS SOAP Toolkit (VB6) includes Trace Utility
• Apache SOAP includes TcpTunnelGui class
• .NET Web Services Enhancements classes have built
  in trace facilities

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Use of Namespaces with SOAP

• Tools such as the MS SOAP Toolkit and .NET will
  create WSDL files with default XML namespaces
• Always take the time to override these with
  specific namespaces for your application

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.NET Web Services Enhancements

• Extension to standard .NET Web Services classes
• Download from MSDN (current version 1.0 SP1)
• Supports additional Web Services specifications
  that have been developed by Microsoft, IBM and
  other partners
• WS-Security
• Supports security tokens for authentication
• Supports digital signing for message integrity
• Supports encryption for confidentiality
• WS-Routing
• Route of Web Services requests via intermediaries
• WS-Attachments
• Allows attachments to SOAP messages

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Summary

• XML Web Services provide a simple and reliable
  way to connect applications
• Based on well-established technologies
• Cross-platform but there are still grey areas
• Easier to design and deploy than many
  alternatives such as DCOM or CORBA

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Questions

• Contact Details
• Bob.Steemson_at_isoftplc.com