Document/Literal vs. RPC/Encoded SOAP

1
Document/Literal vs. RPC/Encoded SOAP

• Dan Gunter
• Lawrence Berkeley National Laboratory

2
Background - What is SOAP?

• Simple Object Access Protocol, but..
• Not really Simple
• Doesnt really Access Objects
• Not really a Protocol
• What is it really?
• A simple messaging framework for transferring
  information specified in the form of an XML
  infoset between an initial SOAP sender and an
  ultimate SOAP receiver (SOAP 1.2 Primer)
• SOAP 1.2 will not spell out the acronym.
  (ibid.)

3
Very Brief history of SOAP

• From an article by Don Box 1
• 1998 XML had no type system (no XML Schema),
  SOAP focused on providing one
• Dave Winer invents XML-RPC using original SOAP
  type system
• 1999-2000 SOAP adds support for arrays and
  references XML Schema still in flux
• 2001-? SOAP also adopts XSD, providing two type
  systems
• Present SOAP Encoding is going away..

4
What is WSDL?

• Web Services Description Language
• Describes the message exchanges supported by Web
  Services in an abstract way
• May be bound to different underlying
  transports, e.g. SOAP/HTTP, REST/HTTP, SMTP,
• Most software I know of supports the SOAP/HTTP
  binding

5
What is RPC/Encoded?

• Remote Procedure Call style invocation
• Changes the Fault elements slightly
• (WSDL 1.2) Each element in the input document
  is a parameter
• Top-level structure MUST be a sequence (no
  choice)
• SOAP Encoding
• Serialization of XML adds ID and HREF to allow a
  directed graph (not just a tree)
• Resulting XML document may no longer validate
  against the XML Schema

6
RPC/Encoded WSDL (1.1)

• Input
• ltwsdlmessage name"DistanceInput"gt
• ltwsdlpart name"p1" type"tnsPoint" /gt
• ltwsdlpart name"p2" type"tnsPoint" /gt
• lt/wsdlmessagegt
• Output
• ltwsdlmessage name"DistanceOutput"gt
• ltwsdlpart name"result type"xsdfloat/gt
• lt/wsdlmessagegt

Perhaps XSD ltelementsgt in WSDL 2.0
7
RPC/Encoded SOAP

• Input
• ltsoapEnvelope ..gt
• ltsoapBody soapencodingStyle
• "http//schemas.xmlsoap.org/soap/encoding/
  "gt
• ltnsDistance ..gt
• ltp1 HREF"id1"/gt
• ltp2 HREF"id1"/gt
• lt/nsDistancegt
• ltnsPoint id"id1"gt
• ltxgt10lt/xgt
• ltygt20lt/ygt
• lt/nsPointgt
• lt/soapBodygt
• lt/soapEnvelopegt

Hmm, doesnt look much like a sequence of integer
elements!
8
What is Document/Literal?

• Document-style invocation
• Input is any XML document
• Output, if present, is another one
• Both are described with XML Schema (in theory,
  other schema languages are supported)
• Literal encoding
• The schema can validate the XML in the SOAP
  request

9
Document/Literal WSDL (1.1)
..in theory, non-XSD schema languages can be used

• Input
• ltrncDistancegt
• distance p1 Point , p2 Point
• Point x xsdfloat, y xsdfloat
• lt/rncDistancegt
• ltwsdlmessage name"DistanceInput"gt
• ltwsdlpart namebody" typerncDistance" /gt
• lt/wsdlmessagegt
• Output
• ltwsdlmessage name"DistanceOutput"gt
• ltwsdlpart name"result" type"xsdfloat" /gt
• lt/wsdlmessagegt

10
Document/Literal SOAP

• Input
• ltsoapEnvelopegt
• ltsoapBodygt
• ltnsdistancegt
• ltp1gt
• ltxgt10lt/xgt ltygt20lt/ygt
• lt/p1gt
• ltp2gt
• ltxgt100lt/xgt ltygt200lt/ygt
• lt/p2gt
• lt/nsdistancegt
• lt/soapBodygt
• lt/soapEnvelopegt

Contents of soapBody will validate against the
schema
11
Schemas and Interoperability

• Schema validation provides a starting point
• Platform-independent, widely-implemented
• Note Schema validation is imperfect
• Slow as molasses
• Schemas can be brittle (especially if the schema
  has a lot of datatype information)
• Message validity ! Semantic validity
• Versioning not automatic
• But Better than eyeballing it!

12
What does this mean for NM-WG?

• Schemas have 3 uses
• Contracts Agreeing on structure of data
• Tooling Generate code stubs
• Validation Did I get what I wanted?
• The WG is dealing with 1
• In its full sense, 3 includes constraints not
  expressible with schemas need informal or formal
  specifications for this
• E.g., if user is requesting a UDP measurement,
  ignore TCP parameters

13
Additional thoughts..

• Schemas are good for describing the shape of an
  XML document, but data types may be overkill
• Especially if youre using a language like Perl
  or Python
• Many useful constraints are still hard to
  express
• If packetType is UDP, ignore numStreams
• Note that data-types may inadvertently make life
  harder
• Need something like XSLT to evaluate
  co-occurrence constraints?
• All the tools are, at best, first-generation

14
References

1. http//webservices.xml.com/pub/a/ws/2001/04/04/soa
   p.html