3'1 SOAP Introduction

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3.1 SOAP Introduction
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What is SOAP?

• SOAP is an XML-based protocol for exchanging
  information between computers
• Mainly use for performing remote procedure calls
  transported via HTTP
• Different from CORBA, DCOM or Java RMI, SOAP
  messages are entirely written in XML
• Hence platform and language independent
• E.g. SOAP java client running on Linux can
  connect to a Microsoft SOAP server running on
  Windows XP
• At the same time, a Perl client running on
  Solaris can also connect to the same SOAP server

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What does SOAP Define?

• SOAP envelope specification
• Specify the rules for encapsulating data being
  transferred between computers
• In case of failure, define how to encode error
  messages
• Data encoding rules
• Define how the data are encoded
• E.g. the rule to encode floating point numbers
• Most conventions are based on the W3C XML Schema
• RPC conventions
• Define how a RPC can be proceeded
• E.g. how to specify the procedure name, pass
  parameters and receive response (returned results)

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SOAP vs XML-RPC

• Similarities
• Use XML for messaging
• Messages are usually embedded into HTTP header
• Use request/response mechanism
• Mainly use in remote procedure call
• Platform independent
• Language independent
• Differences
• SOAP messages are more complicated than XML-RPC
• Make use of XML namespaces and XML Schemas
• Hence give a standard way for data encoding and
  RPC
• Thus allow automatic method invocation on the Web

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SOAP in Action
XML
HTTP
SOAP Request
SOAP client
SOAP server
SOAP Response
HTTP
XML
Hence SOAP messages can be delivered via the Web
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SOAP Communication Styles

• SOAP supports two different communication styles
• RPC Style
• Uses the SOAP RPC conventions, hence the
  communication method and procedure are governed
  by the conventions
• Conceptually similar to other RPCs
• The default of early SOAP implementations
• Document Style
• Also known as message-oriented style
• Sending non-encoded XML content
• Require more programming work on server and
  client sides to interpret the messages
• More flexible communication and provides the best
  interoperability

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A Sample Request in RPC Style
lt?xml version1.0 encodingUTF-8?gt ltSOAP-ENVE
nvelope xmlnsSOAP-ENVhttp//schemas.xmlsoap.or
g/soap/envelope/ xmlnsxsihttp//www.w3.org/20
01/XMLSchema-instance xmlnsxsdhttp//www.w3.o
rg/2001/XMLSchemagt ltSOAP-ENVBodygt
ltns1getTemp xmlnsns1urnxmethods-Temperatu
re SOAP-ENVencodingStyle
http//schemas.xmlsoap.org/soap/encoding/gt
ltzipcode xsitypexsdstringgt10016lt/zipcodegt
lt/ns1getTempgt lt/SOAP-ENVBodygt lt/SOAP-ENVEnve
lopegt
Try to call the remote method getTemp with input
parameter, a zipcode 10016
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A Sample Response in RPC Style
lt?xml version1.0 encodingUTF-8?gt ltSOAP-ENVE
nvelope xmlnsSOAP-ENVhttp//schemas.xmlsoap.or
g/soap/envelope/ xmlnsxsihttp//www.w3.org/20
01/XMLSchema-instance xmlnsxsdhttp//www.w3.o
rg/2001/XMLSchemagt ltSOAP-ENVBodygt
ltns1getTempResponse xmlnsns1urnxmethods-T
emperature SOAP-ENVencodingStyle
http//schemas.xmlsoap.org/soap/encoding/gt
ltreturn xsitypexsdfloatgt71.0lt/returngt
lt/ns1getTempResponsegt lt/SOAP-ENVBodygt lt/SOAP-E
NVEnvelopegt
Return a double 71.0 which represents the
temperature
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SOAP Messages

• In most cases, we do not need to directly
  programming in SOAP
• Dozens of SOAP implementations now freely exist
  on the Internet
• Apache SOAP (Axis)
• Microsoft SOAP ToolKit
• Allow generating SOAP messages automatically
  using High Level Language, e.g. Java, C, Perl
• Understanding of SOAP message structure enables
  us to intercept a SOAP transaction and to debug a
  SOAP application

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Message Structure
SOAP Message
Envelope (mandatory)
Header (optional)
Body (mandatory)
Fault (optional)
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lt?xml version1.0 encodingUTF-8?gt ltSOAP-ENVE
nvelope ltSOAP-ENVHeadergt
lt/SOAP-ENVHeadergt ltSOAP-ENVBodygt
ltSOAP-ENVFaultgt
lt/SOAP-ENVFaultgt
lt/SOAP-ENVBodygt lt/SOAP-ENVEnvelopegt
Example
Envelope Mandatory
Body Mandatory
Header Optional
Fault Optional
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SOAP message Envelope

• Every SOAP message has a root Envelope element
• Does not define the SOAP version
• Rather, use namespaces to differentiate versions
• For SOAP 1.2, the namespace URI is
• http//www.w3.org/2001/09/soap-envelope

Namespace for SOAP 1.1
ltSOAP-ENVEnvelope xmlnsSOAP-ENV
http//schemas.xmlsoap.org/soap/envelope/
lt/SOAP-ENVEnvelopegt
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XML Namespaces

• XML document can define its own markup language
  or vocabularies
• XML uses namespace to uniquely identify XML
  vocabularies

Define the namespace of SOAP-ENV is
ltSOAP-ENVEnvelope xmlnsSOAP-ENV
http//schemas.xmlsoap.org/soap/envelope/
lt/SOAP-ENVEnvelopegt
Define also element Envelope is part of namespace
SOAP-ENV
Attribute for defining namespace
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• We do not need to really go to
• http//schemas.xmlsoap.org/soap/envelope/
• to do anything
• Since this URL is unique, the word SOAP-ENV is
  also unique in this document
• If there is another XML document that has the
  same word SOAP-ENV but referring to different
  URL, it means that SOAP-ENV is different from
  this SOAP-ENV
• In contrary, if they refer to the same URL, it
  means that the two SOAP-ENV are referring to the
  same thing

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ltSOAP-ENVEnvelope xmlnsSOAP-ENV
http//schemas.xmlsoap.org/soap/envelope/
xmlnsxsihttp//www.w3.org/2001/XMLSchema-instan
ce xmlnsxsdhttp//www.w3.org/2001/XMLSchemagt
lt/SOAP-ENVEnvelopegt

• Define the namespaces for data encoding
• Need to specify this to server to declare the
  data in the document are encoded based on a
  particular convention
• Again, we do not need to really go to these Web
  sites to do anything. Just to imply uniqueness
  and for verification purpose

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lt?xml version1.0 encodingUTF-8?gt ltSOAP-ENVE
nvelope ltSOAP-ENVHeadergt
lt/SOAP-ENVHeadergt ltSOAP-ENVBodygt
ltSOAP-ENVFaultgt
lt/SOAP-ENVFaultgt
lt/SOAP-ENVBodygt lt/SOAP-ENVEnvelopegt
Header

• Optional. For specifying additional
  application-level requirements
• For example (1) specify a digital signature (2)
  specify an account number

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lt?xml version1.0 encodingUTF-8?gt ltSOAP-ENVE
nvelope ltSOAP-ENVHeadergt
lt/SOAP-ENVHeadergt ltSOAP-ENVBodygt
ltSOAP-ENVFaultgt
lt/SOAP-ENVFaultgt
lt/SOAP-ENVBodygt lt/SOAP-ENVEnvelopegt
Body

• Mandatory to all SOAP messages
• Carry the actual content such as the RPC requests
  or responses

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lt?xml version1.0 encodingUTF-8?gt ltSOAP-ENVE
nvelope ltSOAP-ENVHeadergt
lt/SOAP-ENVHeadergt ltSOAP-ENVBodygt
ltSOAP-ENVFaultgt
lt/SOAP-ENVFaultgt
lt/SOAP-ENVBodygt lt/SOAP-ENVEnvelopegt
Fault

• In case of error, the body element should contain
  a fault sub-element
• Indicate the error code and the possible cause of
  error

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• May contain the following elements

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• For example

ltSOAP-ENVFaultgt ltfaultcode xsitype"xsdstring
"gt SOAP-ENVClientlt/faultcodegt
ltfaultstring xsitype"xsdstring"gt Failed
to locate method (ValidateCreditCard) in
class (examplesCreditCard) at
/usr/local/ActivePerl-5.6/lib/
site_perl/5.6.0/SOAP/Lite.pm line 1555.
lt/faultstringgt lt/SOAP-ENVFaultgt
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• Samples of fault codes

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SOAP Data Encoding

• A data is just a sequence of 1 and 0
• To allow different computers understand the
  meaning of a data (string, integer, float ), a
  standard data type encoding method is required
• The original XML 1.0 specification does not
  include rules for encoding data type
• Later W3C released the XML Schema and provided a
  standard framework for encoding data type
• SOAP specification adopted the XML Schema, with
  exception such as arrays and structs

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Scalar Types

• A scalar type data contains exactly one value
• e.g. string, Boolean, float, double. int, date,
  time, etc
• SOAP adopts all the built-in simple types
  specified by XML Schema
• For details, see
• http//www.w3.org/TR/2000/WD-xmlschema-0-20000407
  /

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Follow the SOAP 1.2 encoding method. For SOAP
1.1, use schemas.xmlsoap.org/soap/encoding/

• For example

ltSOAP-ENVBodygt ltns1getPriceResponse
xmlnsns1"urnexamplespriceservice
SOAP-ENVencodingStyle "http//www.w3.org/200
1/09/soap-encoding"gt ltreturn
xsitype"xsddouble"gt 54.99lt/returngt
lt/ns1getPriceResponsegt lt/SOAP-ENVBodygt
xsitype is set to xsddouble, means a double
number is to be returned
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Compound Types

• Compound type data contains multiple values
• Can be further divided into arrays and structs
• Arrays contain multiple values of the same type
• Some implementations support multidimensional
  array
• Structs contain multiple values, but each element
  is specified by a name

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Arrays

• Need to specify both the element type and array
  size

ltreturn xmlnsns2 http//www.w3.org/2001/09/
soap-encoding xsitypens2Array
ns2arrayTypexsddouble2gt ltitem
xsitype"xsddouble"gt54.99lt/itemgt ltitem
xsitype"xsddouble"gt19.99lt/itemgt lt/returngt
Specify the type is array and there are two
double numbers in the array
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Structs

• Struct contains multiple values, but each element
  is specified with a unique accessor element

Accessor names
ltreturn xmlnsns2"urnexamples"
xsitype"ns2product"gt ltname
xsitype"xsdstring"gt Red Hat Linuxlt/namegt
ltprice xsitype"xsddouble"gt54.99lt/pricegt
ltdescription xsitype"xsdstring"gt Red Hat
Linux Operating System lt/descriptiongt ltSKU
xsitype"xsdstring"gtA358185lt/SKUgt lt/returngt
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Literal Encoding

• It is possible to ignore all the SOAP
  specification and embed a generic XML document
  directly into a SOAP message
• Doing so is referred to as literal encoding
• Different implementation may have different ways
  to encode literal XML document
• For Apache SOAP, need to specify the namespace
• http//xml.apache.org/xml-soap/literalxml

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Specify the encoding style
SOAP-ENVencodingStyle "http//xml.apache.org/xml
-soap/literalxml"gt ltreturngt ltproduct
sku"A358185"gt ltnamegtRed Hat Linuxlt/namegt
ltdescriptiongtRed Hat Linux Operating
Systemlt/descriptiongt
ltpricegt54.99lt/pricegtlt/productgt lt/returngt
Doing the same as the struct example but without
the SOAP rules. Solely in generic XML format
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• The two communication styles (RPC, document) and
  two encodings (encoded, literal) methods can be
  freely intermixed
• RPC / encoded
• Following SOAP RPC convention for messaging and
  also following SOAP data encoding method
• Most straightforward to implement but also most
  restrictive
• Can introduce interoperability problem
• Most popularly used in early version of SOAP
  implementation (WebSphere 4 and 5.0)
• RPC / literal
• Following SOAP RPC convention for messaging but
  sending literal XML data
• Less restrictive

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• Document / literal
• Sending non-encoded XML message with literal XML
  data
• Need more programming work for the server and
  client to interpret the data
• Provide the best interoperability between Java
  and non-Java implementations
• Default for recently implementation of SOAP
  (WebSphere 5.1 and Microsoft Toolkit 3.0)
• Document / encoded
• Not used in practice

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SOAP Interoperability

• Although SOAP was developed to solve the
  interoperability problem when doing RPC, it also
  has interoperability problem
• For example, there is known problem between
  Apache SOAP, SOAPLite for Perl and the
  Microsoft SOAP Toolkit
• Apache SOAP requires all parameters to be typed
  via the xsitype attribute, while others dont
• Microsoft SOAP Toolkit supports multidimensional
  array while others dont
• Much effort has been made, but still some way to
  go to totally solve the problem

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Web Services Interoperability (WS-I)

• WS-I is an organization chartered to promote Web
  service interoperability across platforms, OS and
  programming languages (see http//www.ws-i.org/)
• Establish WS-I Basic Profile which outlines the
  requirements to which WSDL and Web service
  protocol (SOAP/HTTP) traffic must comply
• Provide WS-I validation tools currently support
  WS-I Basic Profile 1.0
• One of the important suggestions made is the
  banning of RPC / encoded style
• However, the use of RPC / encoded style provides
  much convenience in exposing preexisting classes
  and methods
• Much work is being performed to make sure the RPC
  style works between most SOAP implementations