eie424 distributed systems and networking programming –part ii

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Distributed Systems and Networking Programming –Part II3.1 SOAP – Introduction

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 messagesData 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-RPCSimilarities

– 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

SOAPclient

SOAPclient

SOAPserver

SOAPserver

XML

XML

HTTP

HTTPSOAP Response

SOAP Request

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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<?xml version=‘1.0’ encoding=‘UTF-8’?><SOAP-ENV:Envelope xmlns:SOAP-ENV=“http://schemas.xmlsoap.org/soap/envelope/” xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance” xmlns:xsd=“http://www.w3.org/2001/XMLSchema”> <SOAP-ENV:Body> <ns1:getTemp xmlns:ns1=“urn:xmethods-Temperature” SOAP-ENV:encodingStyle= “http://schemas.xmlsoap.org/soap/encoding/”> <zipcode xsi:type=“xsd:string”>10016</zipcode> </ns1:getTemp> </SOAP-ENV:Body></SOAP-ENV:Envelope>

<?xml version=‘1.0’ encoding=‘UTF-8’?><SOAP-ENV:Envelope xmlns:SOAP-ENV=“http://schemas.xmlsoap.org/soap/envelope/” xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance” xmlns:xsd=“http://www.w3.org/2001/XMLSchema”> <SOAP-ENV:Body> <ns1:getTemp xmlns:ns1=“urn:xmethods-Temperature” SOAP-ENV:encodingStyle= “http://schemas.xmlsoap.org/soap/encoding/”> <zipcode xsi:type=“xsd:string”>10016</zipcode> </ns1:getTemp> </SOAP-ENV:Body></SOAP-ENV:Envelope>

A Sample Request in RPC Style

Try to call the remote method getTemp with input parameter, a zipcode: 10016

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A Sample Response in RPC Style

<?xml version=‘1.0’ encoding=‘UTF-8’?><SOAP-ENV:Envelope xmlns:SOAP-ENV=“http://schemas.xmlsoap.org/soap/envelope/” xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance” xmlns:xsd=“http://www.w3.org/2001/XMLSchema”> <SOAP-ENV:Body> <ns1:getTempResponse xmlns:ns1=“urn:xmethods-Temperature” SOAP-ENV:encodingStyle= “http://schemas.xmlsoap.org/soap/encoding/”> <return xsi:type=“xsd:float”>71.0</return> </ns1:getTempResponse> </SOAP-ENV:Body></SOAP-ENV:Envelope>

<?xml version=‘1.0’ encoding=‘UTF-8’?><SOAP-ENV:Envelope xmlns:SOAP-ENV=“http://schemas.xmlsoap.org/soap/envelope/” xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance” xmlns:xsd=“http://www.w3.org/2001/XMLSchema”> <SOAP-ENV:Body> <ns1:getTempResponse xmlns:ns1=“urn:xmethods-Temperature” SOAP-ENV:encodingStyle= “http://schemas.xmlsoap.org/soap/encoding/”> <return xsi:type=“xsd:float”>71.0</return> </ns1:getTempResponse> </SOAP-ENV:Body></SOAP-ENV:Envelope>

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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<?xml version=‘1.0’ encoding=‘UTF-8’?><SOAP-ENV:Envelope

: <SOAP-ENV:Header>

: </SOAP-ENV:Header>

: <SOAP-ENV:Body>

: <SOAP-ENV:Fault>

: : </SOAP-ENV:Fault>

: </SOAP-ENV:Body></SOAP-ENV:Envelope>


: <SOAP-ENV:Header>


: <SOAP-ENV:Body>

: <SOAP-ENV:Fault>



Header: Optional

Envelope: Mandatory

Body: Mandatory

Fault: Optional

Example

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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

<SOAP-ENV:Envelope xmlns:SOAP-ENV= “http://schemas.xmlsoap.org/soap/envelope/” :</SOAP-ENV:Envelope>


Namespace for SOAP 1.1

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XML NamespacesXML document can define its own markup

language or vocabulariesXML uses namespace to uniquely identify XML

vocabularies



Define the namespace of SOAP-ENV is

Define also element Envelope is part of namespace SOAP-ENVAttribute for

defining namespace

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We do not need to really go to http://schemas.xmlsoap.org/soap/envelope/”

to do anythingSince this URL is unique, the word SOAP-ENV is also

unique in this documentIf 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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<SOAP-ENV:Envelope xmlns:SOAP-ENV= “http://schemas.xmlsoap.org/soap/envelope/” xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance” xmlns:xsd=“http://www.w3.org/2001/XMLSchema”> :</SOAP-ENV:Envelope>

<SOAP-ENV:Envelope xmlns:SOAP-ENV= “http://schemas.xmlsoap.org/soap/envelope/” xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance” xmlns:xsd=“http://www.w3.org/2001/XMLSchema”> :</SOAP-ENV:Envelope>

• 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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Header

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: <SOAP-ENV:Header>


: <SOAP-ENV:Body>

: <SOAP-ENV:Fault>




: <SOAP-ENV:Header>


: <SOAP-ENV:Body>

: <SOAP-ENV:Fault>



• Optional. For specifying additional application-level requirements

• For example: (1) specify a digital signature; (2) specify an account number

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Body <?xml version=‘1.0’ encoding=‘UTF-8’?><SOAP-ENV:Envelope

: <SOAP-ENV:Header>


: <SOAP-ENV:Body>

: <SOAP-ENV:Fault>




: <SOAP-ENV:Header>


: <SOAP-ENV:Body>

: <SOAP-ENV:Fault>



• Mandatory to all SOAP messages

• Carry the actual content such as the RPC requests or responses

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Fault <?xml version=‘1.0’ encoding=‘UTF-8’?><SOAP-ENV:Envelope

: <SOAP-ENV:Header>


: <SOAP-ENV:Body>

: <SOAP-ENV:Fault>




: <SOAP-ENV:Header>


: <SOAP-ENV:Body>

: <SOAP-ENV: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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Element name Description

faultCode A text code to indicate the class of error

faultString A human-readable explanation of error

faultActor A text string indicating who caused the fault. Particularly useful when the SOAP message travels a few nodes

detail Carry application-specific error message

May contain the following elements

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<SOAP-ENV:Fault> <faultcode xsi:type="xsd:string"> SOAP-ENV:Client</faultcode> <faultstring xsi:type="xsd:string"> 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. </faultstring></SOAP-ENV:Fault>

<SOAP-ENV:Fault> <faultcode xsi:type="xsd:string"> SOAP-ENV:Client</faultcode> <faultstring xsi:type="xsd:string"> 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. </faultstring></SOAP-ENV:Fault>

For example

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Samples of fault codes

Name Description

SOAP-ENV:VersionMismatch Envelope element includes an invalid namespace

SOAP-ENV:Client Indicate the client request contained an error

SOAP-ENV:Server Indicate the server is unable to process the client request

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SOAP Data Encoding A data is just a sequence of 1 and 0To 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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<SOAP-ENV:Body> <ns1:getPriceResponse xmlns:ns1="urn:examples:priceservice“ SOAP-ENV:encodingStyle= "http://www.w3.org/2001/09/soap-encoding"> <return xsi:type="xsd:double">

54.99</return>

</ns1:getPriceResponse> </SOAP-ENV:Body>

<SOAP-ENV:Body> <ns1:getPriceResponse xmlns:ns1="urn:examples:priceservice“ SOAP-ENV:encodingStyle= "http://www.w3.org/2001/09/soap-encoding"> <return xsi:type="xsd:double">

54.99</return>

</ns1:getPriceResponse> </SOAP-ENV:Body>

For example

xsi:type is set to xsd:double, means a double number is to be returned

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

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Compound Types

Compound type data contains multiple valuesCan be further divided into arrays and structsArrays contain multiple values of the same type

– Some implementations support multidimensional arrayStructs 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

<return xmlns:ns2= “http://www.w3.org/2001/09/soap-encoding” xsi:type=“ns2:Array” ns2:arrayType=“xsd:double[2]”> <item xsi:type="xsd:double">54.99</item> <item xsi:type="xsd:double">19.99</item></return>

<return xmlns:ns2= “http://www.w3.org/2001/09/soap-encoding” xsi:type=“ns2:Array” ns2:arrayType=“xsd:double[2]”> <item xsi:type="xsd:double">54.99</item> <item xsi:type="xsd:double">19.99</item></return>

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

<return xmlns:ns2="urn:examples" xsi:type="ns2:product"> <name xsi:type="xsd:string"> Red Hat Linux</name> <price xsi:type="xsd:double">54.99</price> <description xsi:type="xsd:string"> Red Hat Linux Operating System </description> <SKU xsi:type="xsd:string">A358185</SKU> </return>

<return xmlns:ns2="urn:examples" xsi:type="ns2:product"> <name xsi:type="xsd:string"> Red Hat Linux</name> <price xsi:type="xsd:double">54.99</price> <description xsi:type="xsd:string"> Red Hat Linux Operating System </description> <SKU xsi:type="xsd:string">A358185</SKU> </return>

Accessor names

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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 encodingDifferent implementation may have different ways to

encode literal XML documentFor Apache SOAP, need to specify the namespace

http://xml.apache.org/xml-soap/literalxml

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SOAP-ENV:encodingStyle= "http://xml.apache.org/xml-soap/literalxml"> <return> <product sku="A358185"> <name>Red Hat Linux</name> <description>Red Hat Linux Operating System</description> <price>54.99</price></product></return>

SOAP-ENV:encodingStyle= "http://xml.apache.org/xml-soap/literalxml"> <return> <product sku="A358185"> <name>Red Hat Linux</name> <description>Red Hat Linux Operating System</description> <price>54.99</price></product></return>

Doing the same as the struct example but without the SOAP rules. Solely in generic XML format

Specify the encoding style

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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

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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, SOAP::Lite for Perl and the Microsoft SOAP Toolkit

Apache SOAP requires all parameters to be typed via the xsi:type attribute, while others don’t

Microsoft SOAP Toolkit supports multidimensional array while others don’t

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

eie424 distributed systems and networking programming –part ii

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