Web Service Foundations: WSDL and SOAP

Marlon Pierce

Community Grids Lab, Indiana University

mpierce@cs.indiana.edu

What Are Web Services? Web services framework is an XML-based distributed services

system. SOAP, WSDL, UDDI WS-Interoperability Intended to support machine-to-machine interactions over the

network using messages. Basic ideas is to build a platform and programming language-

independent distributed invocation system out of existing Web standards. Most standards defined by W3C, OASIS (IP considerations) Interoperability really works, as long as you can map XML message

to a programming language type, structure, class, etc. We regularly use Java-C++ and Java-Perl communication

Very loosely defined, when compared to CORBA, etc. Inherit both good and bad of the web

Scalable, simple, distributed But no centralized management, not high performance, client

applications must be tolerant of failures.

Servlets/CGI Compared to Web Services

Browser

WebServer

HTTP GET/POST

DB

JDBC

WebServer

DB

JDBC

Browser

WebServer

SOAP

GUIClient

SOAPWSDL

WSDL

WSD

LWSD

L

Explanation of Previous Slide The diagram on the left represents a standard web

application. Browsers converse with web servers using HTTP

GET/POST methods. Servlets or CGI scripts process the parameters and take

action, like connect to a DB. On the right, we have a Web services system.

Separates visual from non-visual components Interactions may be either through the browser or

through a desktop client (Java Swing, Python, Windows, etc.)

Some Terminology The diagram on the left is called a client/server system. The diagram on the right is called a multi-tiered

architecture. SOAP: Simple Object Access Protocol

No longer an abbreviation in SOAP 1.2 XML Message format between client and service.

WSDL: Web Service Description Language. Describes how the service is to be used Compare (for example) to Java Interface. Guideline for constructing SOAP messages. WSDL is an XML language for writing Application

Programmer Interfaces (APIs).

More Examples of Web Services Geographical Information Systems are perfect candidates for WS

The Open Geospatial Consortium defines several relevant standards Geographic Markup Language (GML) exchanges info. Web Feature Service works with abstract GML feature data. Web Map Service creates maps (images) Lots more at http://www.opengeospatial.org/specs/?page=specs

XMethods Lots and lots of contributed examples, live demos Try them

http://www.xmethods.com/ Lots more for bioinformatics.

Easiest way to find is to download Taverna from SourceForge. Then check out http://communitygrids.blogspot.com for

guidelines. CICC is building many new one for chemical informatics.

RDAHMM: GPS Time Series SegmentationSlide Courtesy of Robert Granat, JPL

Complex data with subtle signals is difficult for humans to analyze, leading to gaps in analysis

HMM segmentation provides an automatic way to focus attention on the most interesting parts of the time

GPS displacement (3D) length two years.

Divided automaticallyby HMM into 7 classes.

Features:• Dip due to aquifer

drainage (days 120-250)

• Hector Mine earthquake (day 626)

• Noisy period at end of time series

Making RDAHMM into a Web Service RDAHMM takes GPS (or

other) time-series data as input, along with various command line parameters.

GPS data comes from GRWS or other services. http://geoapp.ucsd.edu/

scignDataPortal/grwsSummary.jsp

It creates 11 output files. Results are

superimposed on the input time series.

USAGE: GEMCodes/RDAHMM2/bin/rdahmm -data 'input observation sequence file'

[-L 'output model log likelihood file']

[-Q 'output optimal state sequence file']

[-pi 'output model initial state probability file']

[-A 'output model transition probability file']

[-B 'output model output distribution file']

[-minvalfile 'data minimum value file']

[-maxvalfile 'data maximum value file file']

[-rangefile 'data range file']

[-covarsweightsfile 'covariance component weightings file']

[-covgraphfile 'covariance graph connectivity file']

-T 'number of observations'

-D 'dimension of observations'

-N 'number of model states'

-output_type 'type of HMM output distribution {gauss}'

[-init_type 'type of HMM parameter initialization {random}']

.....

This is a portal client to a data mining service that I built. The web service analyzes GPS signal data to look for modes.

Portal courtesy of NASA REASoN project.

GPS data comes from the Scripps GRWS Web Service. Instead of defining a data type for this file, we just pass around URLs. The RDAHMM service receives the URL as input.

The service returns output result files as URLs.

The lesson: don’t go overboard with XML message definitions. You will regret it. Use URLs and keep your SOAP/WSDL simple.

How Do You Design the RDAHMM Service? First, you need an engine to run RDAHMM.

I develop Java services, so I have found Apache Ant very useful for wrapping binary executables, managing command-lines, and interacting with the Unix shell.

You can embed Ant in other Java programs. Second, you need an appropriate Web Service

container for your development environment. I use Apache Axis (examples will use version 1.4).

This runs in Apache Tomcat. .NET, C/C++, Python, Ruby, Perl, etc all have Web

Service containers. Ex: gSOAP for C/C++ from FSU, ZSI for Python

Writing the Service Writing a Web Service is easy

Just write a Java program In our case, the Java program must

Grab GPS data from GRWS service We pass this around using URLs.

Collect command line parameter values as input. Run the code. Send back a response as a Java Bean that

encapsulates URLs. Can either block or not-block, depending on how you want to

execute things.

This is a mixture of REST and XML-RPC styles.

Service Code Example public RDAHMMResultsBean runNonblockingRDAHMM2(String

siteCode,String resource, String contextGroup, String contextId, String minMaxLatLon, String beginDate, String endDate, numModelStates) throws

Exception {

try {

String dataUrl=querySOPACGetURL(siteCode, resource, contextGroup,

contextId, minMaxLatLon, beginDate, endDate);

return createRDAHMMBean( dataUrl,numModelStates);

}

catch (Exception ex) {...}

}

RDAHMMResultBean Codepublic class RDAHMMResultsBean

implements java.io.Serializable {

private java.lang.String AUrl;

private java.lang.String BUrl;

private java.lang.String LUrl;

private java.lang.String QUrl;

private java.lang.String inputUrl;

....

public RDAHMMResultsBean() {

}

//Plus all of the getters and setters

public java.lang.String getInputUrl() { return inputUrl; }

public void setInputUrl(java.lang.String inputUrl) { this.inputUrl = inputUrl; }

......

}

Nothing special about this code.

Note all the returned values are actually URLs.

AUrl, BUrl, LUrl, etc are all URLs to files generated by RDAHMM.

http://crisisgrid.svn.sourceforge.net/viewvc/crisisgrid/QuakeSim2/ExecutionServices/RDAHMMService/src/main/java/

Deploying an Axis 1 Service Now that you have written the code, you

follow these steps to make it into a service. Download Axis and install it into Tomcat.

That is, create a subdirectory of webapps and put all the Axis jars in WEB-INF/lib/.

Create a service descriptor file, service-config.wsdd and put this in WEB-INF/ Axis gives you tools to help.

Compile your code and put it in WEB-INF/classes or WEB-INF/lib (if jarred).

Creating an Axis Client Axis will inspect your newly deployed service and create a

WSDL file out of it. More on this in a minute.

WSDL is an XML API description. It tells clients how to invoke your service. Typically the service is invoked by sending a SOAP message, so

WSDL tells you how to construct SOAP. Clients typically discover and download the WSDL (UDDI,

wget, whatever). Axis has a tool called WSDL2Java that will convert the

WSDL into client stubs. Stubs give you local objects that invoke the remote service.

Clients can be anything JSP pages, Java Portlets, PHP clients, Swing or SWT GUIs, etc.

Some Notes on Axis 2 Axis 2 is a redesign of Axis 1 that has

Greater performance (using StAX XML parsers) Extensiblity to support Web Service Quality of Service

add-ons. Better support for Java-to-XML binding frameworks.

Allows you to send and receive more complicated XML messages. But I think you should avoid this.

See my notes: http://communitygrids.blogspot.com/2007/02/some-

notes-on-axis2-version-11.html

Some Additional Notes Typically, you don’t need to import any Axis

specific packages. Exception: finding and loading a property file.

If you are familiar with JSP, servlets, or similar things, you will notice that you also don’t Need to manage HTTP request, response, and

session variables. This style of programming is similar to the

Inversion of Control Pattern (IOC). Very useful when dealing with Java Beans.

What Have We Gained from This? We have decoupled the RDAHMM client and the

service. Now separated using well-defined interfaces. One service can be used by multiple, independently

developed clients. Services just do one thing really well. Application

“smarts” are in the client. Multiple services can be linked together into

composite applications. Workflow See for example Taverna

Google “Taverna SourceForge” to find it. Others: Kepler, XBaya (from IU)

Some General Advice Keep you services self-contained with simple interfaces.

Core problem in distributed systems is scalability. Services, like mash-ups, are intended to be put to unexpected

uses. Complication is the enemy. Services are NOT Distributed Objects

http://www.allthingsdistributed.com/historical/archives/000343.html Use XML Simple Types and URLs for input and output

rather than attachments. Collect your input/output into Java Beans, C structs, etc,

but don’t go overboard. Interoperability can suffer if your I/O types are too complicated.

Java<-->C, Axis 1<-->Axis2 JavaBeans/POJOs are used frequently in IOC systems like

Spring and Java Server Faces. Db4o is a really nice JavaBean database.

Web Service Extensions Web Services communicate with SOAP, and SOAP is

designed to be extensible. Examples of Extensions

Addressing: describes how SOAP messages can be conveyed across multiple hops.

Security: how to authenticate clients and servers, how to authorize usage, etc.

Reliability/ReliableMessaging: provides guaranteed delivery through acknowledgements

Most of these are defined by specifications published by OASIS. For more discussion, see

http://grids.ucs.indiana.edu/ptliupages/presentations/GGF15WebServices/

For a critique by Shrideep Pallickara, see http://grids.ucs.indiana.edu/ptliupages/presentations/GGF15WebServices/GGF-Slides.ppt

WSDL 1.1 Overview

Marlon Pierce Community Grids LabIndiana Universitympierce@cs.indiana.edu

What Is WSDL?

Web Service Description Language W3C specification See http://www.w3.org/TR/wsdl for the official “note” for

WSDL 1.1. WSDL 1.1 never became a full “recommendation”. WSDL 2.0 working draft just completed it’s public call for

comments. This slide set will review WSDL 1.1, which is still the

“standard”. WSDL 2.0 should replace this soon.

Why Use WSDL? WSDL uses XML to describe interfaces

Programming language independent way to do this. So you can use (for example) C++ programs to remotely invoke Java

programs and vice versa. Consider Web browsers and Web servers:

All web browsers work pretty well with all web sites. You don’t care what kind of web server Amazon.com uses. Amazon doesn’t care if you use IE, Mozilla, Konqueror, Safari, etc. You all speak HTTP.

WSDL (and SOAP) are a generalization of this. Note I will describe WSDL from an Remote Procedure

Call/Remote Method Invocation point of view. But WSDL and SOAP also support more a more message-centric

point of view. C.f. Java Messaging System.

A Very Simple Example: Echopublic class echoService implements echoServiceInterface{

public String echo(String msg) {return msg;

}public static void main(String[] args) {

new echoService().echo(“hello”);}

}

The Echo Interface

/*** All implementers of this interface must* implement the echo() method.*/public interface echoServiceInterface {

public String echo(String toEcho);}

Now Use Echo As A Remote Service We can take the previous

Java program and deploy it in Tomcat as a service.

Clients can then invoke the echo service. WSDL tells them how to

do it. Clients don’t need to

know anything about the service implementation or even language.

WSDL is the latest IDL DCE and CORBA IDL

were two older examples.

C#Client

WSDL

Tomcat+Axis+Echo

WSDL

SOAP(Echo “hello”) “hello”

What Does echoServiceInterface Look Like In WSDL? <?xml version="1.0" encoding="UTF-8" ?>

<wsdl:definitions targetNamespace="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" xmlns="http://schemas.xmlsoap.org/wsdl/" xmlns:apachesoap="http://xml.apache.org/xml-soap" xmlns:impl="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" xmlns:intf="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/" xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/" xmlns:wsdlsoap="http://schemas.xmlsoap.org/wsdl/soap/" xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <wsdl:types /> <wsdl:message name="echoResponse">  <wsdl:part name="echoReturn" type="xsd:string" />   </wsdl:message>

<wsdl:message name="echoRequest">  <wsdl:part name="in0" type="xsd:string" />   </wsdl:message><wsdl:portType name="Echo">

<wsdl:operation name="echo" parameterOrder="in0">  <wsdl:input message="impl:echoRequest" name="echoRequest" />   <wsdl:output message="impl:echoResponse" name="echoResponse" />   </wsdl:operation>  </wsdl:portType> There’s more…

What Does This Look Like In WSDL, Continued?

  <wsdl:binding name="EchoSoapBinding" type="impl:Echo"><wsdlsoap:binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http" /> <wsdl:operation name="echo">

  <wsdlsoap:operation soapAction="" /> <wsdl:input name="echoRequest">

  <wsdlsoap:body encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" namespace="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo"

use="encoded" />   </wsdl:input>

<wsdl:output name="echoResponse">  <wsdlsoap:body encodingStyle="http://schemas.xmlsoap.org/soap/encoding

namespace="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" use="encoded" />

  </wsdl:output>  </wsdl:operation>  </wsdl:binding><wsdl:service name="EchoService">

<wsdl:port binding="impl:EchoSoapBinding" name="Echo">  <wsdlsoap:address location="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" />   </wsdl:port>  </wsdl:service></wsdl:definitions>

Don’t strain your eyes. We will break this down

Writing WSDL I’m sure you are impressed with the previous two

slides. One could write WSDL by hand, but this is not the

usual way. It was automatically generated by Apache Axis. Most

other Web service tools will do the same from your service code.

We will go through the construction, though, for understanding.

You should not think of WSDL (and SOAP) as programming languages. They are just assertions, or descriptions.

WSDL Parts Types

Used to define custom message types Messages

Abstraction of request and response messages that my client and service need to communicate.

PortTypes Contains a set of operations. Operations organize WSDL messages. Operation->method name, portType->java interface

Bindings Binds the portType to a specific protocol (typically SOAP over

http). You can bind one portType to several different protocols by using

more than one port. Services

Gives you one or more URLs for the service. Go here to execute “echo”.

Echo Service WSDL, Section by Section

Namespaces The WSDL document begins with several XML namespace

definitions.

Namespaces allow you to compose a single XML document from

several XML schemas.

Namespaces allow you to identify which schema an XML tag comes

from.

Avoids name conflicts.

See earlier XML lectures

As we will see, the Axis namespace generator went overboard.

Not all of these are used.

Front Matters: Namespace Definitions<?xml version="1.0" encoding="UTF-8" ?>

<wsdl:definitions targetNamespace="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" xmlns="http://schemas.xmlsoap.org/wsdl/" xmlns:apachesoap="http://xml.apache.org/xml-soap" xmlns:impl="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" xmlns:intf="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/" xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/" xmlns:wsdlsoap="http://schemas.xmlsoap.org/wsdl/soap/" xmlns:xsd="http://www.w3.org/2001/XMLSchema">

…</wsdl:definitions>

WSDL Types

Use <types/> to declare local message structures.

What Does echoServiceInterface Look Like In WSDL?<?xml version="1.0" encoding="UTF-8" ?>

<wsdl:definitions …>  <wsdl:types /> <wsdl:message name="echoResponse">  <wsdl:part name="echoReturn" type="xsd:string" />   </wsdl:message>

<wsdl:message name="echoRequest">  <wsdl:part name="in0" type="xsd:string" />  </wsdl:message>…</wsdl:definitions>

It’s empty...

WSDL Types

WSDL messages don’t need to declare types when just sending XML Schema primitive objects.

EchoService just has string messages. So no special types definitions are

needed in our WSDL. Strings are an XML schema built-in type.

Schema Built In Types

When Would I Need A Custom Type? Any time your Web Service needs to send data formatted by anything other than XML Schema built-in types, you must define the type in WSDL.

Example: Arrays are not built-in types! Arrays of strings, ints, etc., must be defined in the WSDL

<type></type> structure. Another example: JavaBeans (or C structs or any

data classes with get/set methods) can be serialized to XML. Pass as messages to the remote endpoint. Support for this in implementations is variable.

AXIS has limited support because they use their own serializers.

Sun has better support but it won’t work with Axis.

How Does WSDL Encode String Arrays? Imagine that my echo service actually echoes

back an array of strings. Arrays are not part of the built-in types, so I

will have to define them myself. Luckily for us, SOAP defines arrays, so we

can import this definition. Next slide shows what this looks like.

String Array Example<wsdl:types>

<schema targetNamespace="http://.../GCWS/services/EchoArray"

xmlns="http://www.w3.org/2001/XMLSchema"> <import namespace="http://schemas.xmlsoap.org/soap/encoding/" /> <complexType name="ArrayOf_xsd_string">

<complexContent> <restriction base="soapenc:Array"> <attribute ref="soapenc:arrayType"

wsdl:arrayType="xsd:string[]" />   </restriction>  </complexContent>  </complexType>  <element name="ArrayOf_xsd_string" nillable="true"

type="impl:ArrayOf_xsd_string" />   </schema>  </wsdl:types>

Create a new data type, “ArrayOf_xsd_string” that is a restricted extension of the general SOAP array class.

WSDL String Array Types WSDL <type/> is nothing more than an extensibility

placeholder in WSDL. Technically, the WSDL schema specifies that

<type> </type> can contain a <sequence> of 0 or more <any> tags. Look at the WSDL schema.

And note that the <any/> tag acts like wildcard. You can insert any sort of xml here. This is a common XML/Web Service trick.

Type allows us to strongly type messages Compare: strong versus weak typing in

programming languages

Inserting a Type Between <type></type>, we insert a <schema>. Since arrays are defined in SOAP encoding rules, I next

import the appropriate schema. I import the definition of the SOAP Array and extend it

to a String array. Typically imports also have “location” attributes

“This namespace is located here for download.” Next, insert our own local definition of a type called

“ArrayOf_xsd_string”. This is a restricted extension of the SOAP Array complex

type. We only allow 1 dimensional string arrays It is also nillable—I am allowed to returna “null” value

for the string.

Handling Other XML Types You can also express other message arguments

as XML. Examples: a purchase order, an SVG description of an

image, a GML description of a map. In practice, these are handled by automatic

Bean serializers/deserializers. Castor is an example: http://www.castor.org/ XMLBeans is another http://xml.apache.org/xmlbeans/

These are tools that make it easy to convert between XML and JavaBeans.

By “JavaBeans” I mean objects that associate simple get/set methods with all data.

Implementation dependent.

WSDL Messages

WSDL Messages The “message” section specifies

communications that will go on between endpoints. Gives each message a name (to be

used later for reference). Specifies the type of message

Can be primitive types, like strings Can be defined types, as we saw

previously.

The echoServiceInterface messages

<?xml version="1.0" encoding="UTF-8" ?> <wsdl:definitions>  <wsdl:types /> <wsdl:message name="echoResponse">  <wsdl:part name="echoReturn" type="xsd:string" /> </wsdl:message><wsdl:message name="echoRequest">  <wsdl:part name="in0" type="xsd:string" /> </wsdl:message><wsdl:portType name="Echo">

<wsdl:operation name="echo" parameterOrder="in0">  <wsdl:input message="impl:echoRequest" name="echoRequest" />   <wsdl:output message="impl:echoResponse"

name="echoResponse" />   </wsdl:operation>  </wsdl:portType>…</wsdl:definitions>

Our Echo Messages<wsdl:message name="echoResponse">

  <wsdl:part name="echoReturn" type="xsd:string" />

</wsdl:message>

<wsdl:message name="echoRequest">

<wsdl:part name="in0" type="xsd:string" />

</wsdl:message>

Echo Service Messages Our echo service takes a string argument

and returns a string answer. In WSDL, I first abstract these as

messages. Echo needs two messages: request and

response Note we have not yet said message is the

request and which is the response. That is the job of the portType

operations, coming up.

Structure of a Message WSDL <message> elements have name attributes

and one or more parts. The message name should be unique for the

document. <operation> elements will refer to messages by

name. I need one <part> for each piece of data I need to

send in that message. Each <part> is given a name and specifies its type.

<part> types can point to <wsdl:type> definitions if necessary.

Our service just needs xsd:strings, so no problem.

PortTypes and Operations

WSDL portTypes WSDL messages are only abstract

messages. We bind them to operations within the portType.

The structure of the portType specifies (still abstractly) how the messages are to be used. Think of operations-->java methods and

portTypes-->java interfaces.

The echoServiceInterface portType<?xml version="1.0" encoding="UTF-8" ?>

<wsdl:definitions>  <wsdl:types /> <wsdl:message name="echoResponse">  <wsdl:part name="echoReturn" type="xsd:string" />   </wsdl:message>

<wsdl:message name="echoRequest">  <wsdl:part name="in0" type="xsd:string" />   </wsdl:message><wsdl:portType name="Echo">

<wsdl:operation name="echo" parameterOrder="in0">  <wsdl:input message="impl:echoRequest"

name="echoRequest" />   <wsdl:output message="impl:echoResponse"

name="echoResponse" />   </wsdl:operation>  </wsdl:portType>…</wsdl:definition>

EchoService portType<wsdl:portType name="Echo">

<wsdl:operation name="echo" parameterOrder="in0">

  <wsdl:input message="impl:echoRequest"

name="echoRequest" />

  <wsdl:output message="impl:echoResponse"

name="echoResponse" />

  </wsdl:operation>

  </wsdl:portType>

portType Message Patterns PortTypes support four types of messaging:

One way: Client send a message to the service and doesn’t want a response. <input> only.

Request-Response: Client sends a message and waits for a response. <input>, then <output>

Solicit-Response: Service sends a message to the client first, then the client responds. <output>, then <input>

Notification: <output> only. These still are abstract. We must implement them using some

message protocol. HTTP units of transmission are request and response, so mapping

Solicit-Response to HTTP will take some work.

portType for EchoService The echo service has one method, echo. It takes one string argument and returns one

string. In WSDL, the portType is “Echo”, the operation

is “echo”. The messages are organized into input and

output. Messages are placed here as appropriate. That is, <input> takes the <echoRequest>

message.

Parameter Order

This attribute of operation is used to specify zero or more space-separated values.

The values give the order that the input messages must be sent.

Echo is a bad example, since it only has one input parameter, named in0.

WSDL Self-Referencing

The WSDL <input> and <output> tags need to point back to the <message> definitions above:

<wsdl:message name="echoResponse">  <wsdl:part name="echoReturn" type="xsd:string" />  </wsdl:message>…<wsdl:portType name="Echo">

<wsdl:operation name="echo" parameterOrder="in0">  …  <wsdl:output message="impl:echoResponse"

name="echoResponse" />   </wsdl:operation>  </wsdl:portType>

The Picture So Far…

Part

Input Message

Part

Input Message

PartPart

Output Message

portType

Operation

Input

Ouput

hasInput

hasOutput

Bindings

WSDL SOAP Bindings

In the previous slide, we specify several things: We will use SOAP/HTTP We will use RPC encoding style

Other choice is literal “document” style. We specify the namespace associated with

the Echo service input and output messages. All of this corresponds to SOAP message parts.

We will expand this in the next lecture.

Binding Section of WSDL<wsdl:definitions>…  <wsdl:binding name="EchoSoapBinding" type="impl:Echo">

<wsdlsoap:binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http" /> <wsdl:operation name="echo">

  <wsdlsoap:operation soapAction="" /> <wsdl:input name="echoRequest">

  <wsdlsoap:body encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"

namespace="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" use="encoded" />

  </wsdl:input><wsdl:output name="echoResponse">

  <wsdlsoap:body encodingStyle="http://schemas.xmlsoap.org/soap/encoding

namespace="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" use="encoded" />

  </wsdl:output>  </wsdl:operation>  </wsdl:binding><wsdl:service name="EchoService">

<wsdl:port binding="impl:EchoSoapBinding" name="Echo">  <wsdlsoap:address location="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" />   </wsdl:port>  </wsdl:service></wsdl:definitions>

Don’t strain your eyes--we will zoom in.

So Far…

We have defined abstract messages, which have XML values. Simple or custom-defined types.

We have grouped messages into operations and operations into portTypes.

We are now ready to bind the portTypes to specific protocols.

The Binding for Echo<wsdl:binding name="EchoSoapBinding" type="impl:Echo">  <wsdlsoap:binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http" /> <wsdl:operation name="echo">  <wsdl:input name="echoRequest">  <wsdlsoap:body

encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" namespace=“[echo service namespace URI]" use="encoded" />

  </wsdl:input> <wsdl:output name="echoResponse">  <wsdlsoap:body encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"

namespace=“[echo service namespace URI]" use="encoded" />

  </wsdl:output> </wsdl:operation></wsdl:binding> The highlighted “wsdlsoap:” tags are

extensions for SOAP message bindingand not part of the WSDL schema.

Binding tags Binding tags are meant to bind the parts of

portTypes to sections of specific protocols. SOAP, HTTP GET/POST, and MIME are provided in the

WSDL specification. Bindings refer back to portTypes by name, just as

operations point to messages. They are mirror images of the portTypes. Each part is extended by schema elements for a particular

binding protocol (i.e. SOAP). In our WSDL bindings, we will have two messages

(input and output). Each corresponds to SOAP body sections, described later. Additionally, we specify that the body should be encoded.

That is, RPC encoded. Alternatively, could also be “literal” (or “document”).

WSDL Internal References

portType

Operation

Input

Ouput

binding

Operation

Input

Output

Structure of the Binding

<binding> tags are really just placeholders. They are meant to be extended at specific places by

wsdl protocol bindings. These protocol binding rules are defined in supplemental

schemas. The following box figure summarizes these things

Green boxes are part of WSDL From the wsdl namespace, that is.

Red boxes are parts of the document from other schemas From wsdlsoap namespace in the echo example.

Binding Structure

binding

Non-wsdl extension

operation

Non-wsdl extension

input output

Non-wsdl extension

Non-wsdl extension

A little more on encoding...

We specify SOAP encoding SOAP is a message format and needs a transport

protocol, so we specify HTTP. Operation styles may be either “RPC” or “Document”.

We use RPC. SOAP Body elements will be used to actually convey

message payloads. RPC requires “encoded” payloads.

Each value (echo strings) is wrapped in an element named after the operation.

Useful RPC processing on the server side. Documents are literal (unencoded)

Use to just send a payload of XML inside SOAP.

Binding Associations to SOAP

Binding SOAP RPC

Operation

WSDL SOAP

SOAP Action

Input

Output

SOAP Body

SOAP Body

Binding Restrictions

Binding elements point by name to portTypes.

WSDL allows more than one binding element to point to the same port type. Why? Because a service may support multiple,

alternative protocol bindings.

What Does It Mean? WSDL is not a programming language. A service that exposes an WSDL interface is just

telling a client what it needs to do to communicate with the service. Send me strings and I will return strings. I expect SOAP messages that include the strings in

the body. I expect this body to be RPC encoded with the

operation name so that I will know which operation the body contents belong to.

I will return SOAP messages that include Strings in the body.

These will also be encoded so that you know what to do with them.

Ports and Services

What Does This Look Like In WSDL?

<wsdl:definitions>…

<wsdl:binding>… 

  </wsdl:binding><wsdl:service name="EchoService">

<wsdl:port binding="impl:EchoSoapBinding" name="Echo">

  <wsdlsoap:address location="http://grids.ucs.indiana.edu:8045/GCWS/services/Echo" />

  </wsdl:port>  </wsdl:service></wsdl:definitions>

Ports and Services

<wsdl:service name="EchoService">

<wsdl:port binding="impl:EchoSoapBinding" name="Echo"> <wsdlsoap:address

location=“http://..../"/>

  </wsdl:port>

</wsdl:service>

Port and Service Tags

The service element is a collection of ports. That’s all it is for.

Ports are intended to point to actual Web service locations The location depends on the binding. For SOAP bindings, this is a URL.

Ports and Services

A service can have more than one port. Two ports can point back to the same binding

element. Ports refer to bindings by name This allows you to provide alternative service

locations. The figure on next slide conceptually depicts

associating two ports to a single binding. The ports differ only in the URLs of their services.

Port Associations to Bindings

Binding Service

OperationPort #1

Input

Output

URL #1

URL #2

Port #2

Summary of WSDL WSDL decouples remote service operations.

Types=custom message definitions. Any data types not in the XML schema.

Message=name the messages that must be exchanged and their data types, possibly defined by <type>.

PortTypes=service interfaces Operations=remote method signatures.

Bindings=mappings of portType operations to real message formats

Ports=locations (URLs) of real services.

SOAP Intro and Message Formats

Marlon PierceCommunity Grids LabIndiana Universitympierce@cs.indiana.edu

SOAP Primary References

SOAP is defined by a number of links http://www.w3.org/TR/soap/

See primarily the “Primer” and “Messaging Framework” links.

The actual SOAP schema is available from http://www.w3.org/2003/05/soap-envelope/ It is pretty small, as these things go.

SOAP and Web Services

Our previous lectures have looked at WSDL Defines the interfaces for

remote services. Provides guidelines for

constructing clients to the service.

Tells the client how to communicate with the service.

The actual communications are encoded with SOAP. Transported by HTTP

Client

Service

WSDL

WSDL

SOAPRequest

SOAPResponse

Beyond Client-Server SOAP assumes messages

have an originator, one or more ultimate receivers, and zero or more intermediaries.

The reason is to support distributed message processing.

Implementing this message routing is out of scope for SOAP. Assume each node is a

Tomcat server or JMS broker.

That is, we can go beyond client-server messaging.

Originator Recipient

Intermediary

Intermediary

Intermediary

SOAP in One Slide SOAP is just a message format.

Must transport with HTTP, TCP, etc. SOAP is independent of but can be connected

to WSDL. SOAP provides rules for processing the

message as it passes through multiple steps. SOAP payloads

SOAP carries arbitrary XML payloads as a body. SOAP headers contain any additional information These are encoded using optional conventions

Defining SOAP Messages

Given what you have learned about WSDL, imagine it is your job to design the message interchange layer. What are the requirements?

Note SOAP actually predates WSDL, so this is in reverse order.

Web Service Messaging Infrastructure Requirements?

Define a message format Define a messaging XML schema Allow the message to contain arbitrary XML from other schemas.

Keep It Simple and Extensible Messages may require advanced features like security, reliability, conversational

state, etc. KISS, so don’t design these but do design a place where this sort of advanced

information can go. Add these capabilities in further specifications: WS-Security, WS-ReliableMessaging, etc.

Tell the message originator is something goes wrong. Define data encodings

That is, you need to tell the message recipient the types of each piece of data. Define some RPC conventions that match WSDL

Your service will need to process the message, so you need to provide some simple conventions for matching the message content to the WSDL service.

Decide how to transport the message. Generalize it, since messages may pass through many entities.

Decide what to do about non-XML payloads (movies, images, arbitrary documents).

SOAP Messaging

SOAP Basics

SOAP is often thought of as a protocol extension for doing Remote Procedure Calls (RPC) over HTTP. This is how it is often used.

This is not accurate: SOAP is an XML message format for exchanging structured, typed data. It may be used for RPC in client-server applications May be used to send XML documents Also suitable for messaging systems (like JMS) that

follow one-to-many (or publish-subscribe) models. SOAP is not a transport protocol. You must attach

your message to a transport mechanism like HTTP.

What Does SOAP Look Like? The next two slides shows examples of SOAP

message from our Echo service. It’s just XML

First slide is an example message that might be sent from a client to the echo service.

Second slide is an example response. I have highlighted the actual message payload.

<?xml version=‘1.0’ ?><soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd=http://www.w3.org/2001/XMLSchema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> <soapenv:Body> <ns1:echo soapenv:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" xmlns:ns1="http://.../axis/services/EchoService">

<in0 xsi:type="xsd:string">Hollow World</in0> </ns1:echo> </soapenv:Body></soapenv:Envelope>

SOAP Request

<?xml version=‘1.0’ ?><soapenv:Envelope xmlns:soapenv=http://schemas.xmlsoap.org/soap/envelope/ xmlns:xsd=http://www.w3.org/2001/XMLSchema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> <soapenv:Body> <ns1:echoResponse soapenv:encodingStyle=http://schemas.xmlsoap.org/soap/encoding/ xmlns:ns1="http://../axis/services/echoService">

<echoReturn xsi:type=“String“>Hollow World

</echoReturn> </ns1:echoResponse> </soapenv:Body></soapenv:Envelope>

SOAP Response

SOAP Structure SOAP structure is very

simple. 0 or 1 header elements 1 body element Envelop that wraps it all.

Body contains XML payload.

Headers are structured the same way. Can contain additional

payloads of “metadata” Security information,

quality of service, etc.

Envelope

Body

MessagePayload

Header

SOAP Schema Notes All of this is expressed formally

in the SOAP schema. Which in turn derives from the

SOAP Infoset XML on the right is taken

directly from the SOAP schema.

This just encodes the previously stated rules.

Also, note that the SOAP envelope can contain other attributes. <anyAttribute> tag is the

wildcard

<xs:complexType name="Envelope">

<xs:sequence>  <xs:element

ref="tns:Header" minOccurs="0" />

  <xs:element ref="tns:Body" minOccurs="1" />

</xs:sequence>  <xs:anyAttribute

namespace="##other" processContents="lax" />

</xs:complexType>

SOAP Envelop The envelop is the root container of the SOAP

message. Things to put in the envelop:

Namespaces you will need. http://schemas.xmlsoap.org/soap/envelope is required, so

that the recipient knows it has gotten a SOAP message. Others as necessary

Encoding rules (optional) Specific rules for deserializing the encoded SOAP data. More later on this.

Header and body elements. Headers are optional, body is mandatory. Headers come first in the message, but we will look at

the body first.

Options on <xsd:any/> The <xsd:any/> element takes the usual optional

maxOccurs, minOccurs attributes. Allows a namespace attribute taking one of the values:

##any (the default), ##other (any namespace except the target

namespace), List of namespace names, optionally including either

##targetNamespace or ##local. Controls what elements the wildcard matches, according to

namespace. It also allows a processContents attribute taking one of the

values strict, skip, lax (default strict), controlling the extent to which the contents of the matched element are validated. SOAP is lax.

Lax

“If the item, or any items among its children if it's an element information item, has a uniquely determined declaration available, it must be ·valid· with respect to that definition.”

That is, ·validate· message payloads when you can, don't worry when you can't.

SOAP Headers

SOAP Body elements contain the primary message contents. Headers are really just extension points where you can

include elements from other namespaces. i.e., headers can contain arbitrary XML.

Headers may be processed independently of the body. Headers may optionally define encodingStyle. Headers may optionally have a “role” attribute Header entries may optionally have a “mustUnderstand”

attribute. mustUnderstand=1 means the message recipient must

process the header element. If mustUnderstand=0 or is missing, the header element is

optional. Headers may also have a “relay” attribute.

Header Definition From SOAP Schema

<xs:element name="Header" type="tns:Header" /> <xs:complexType name="Header">

<xs:annotation> <xs:documentation>Elements replacing the wildcard MUST be

namespace qualified, but can be in the targetNamespace</xs:documentation>

  </xs:annotation><xs:sequence>

<xs:any namespace="##any" processContents="lax" minOccurs="0" maxOccurs="unbounded" />

</xs:sequence>  <xs:anyAttribute namespace="##other" processContents="lax" />   </xs:complexType>

Example Uses of Headers

Security: WS-Security and SAML place additional security information (like digital signatures and public keys) in the header.

Quality of Service: SOAP headers can be used if we want to negotiate particular qualities of service such as reliable message delivery and transactions.

Session State Support: Many services require several steps and so will require maintenance of session state. Equivalent to cookies in HTTP. Put session identifier in the header.

Example Header from SOAP Primer<?xml version='1.0' ?> <env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-

envelope"> <env:Header> <m:reservation xmlns:m=“http://my.example.com/"

env:role="http://www.w3.org/2003/05/soap-envelope/role/next" env:mustUnderstand="true">

<m:reference>uuid:093a2da1-q345-739r-ba5d-pqff98fe8j7d </m:reference>

<m:dateAndTime>2001-11-29T13:20:00.000-05:00 </m:dateAndTime>

</m:reservation> <n:passenger xmlns:n=“…"

env:role="http://www.w3.org/2003/05/soap-envelope/role/next" env:mustUnderstand="true">

<n:name>Åke Jógvan Øyvind</n:name> </n:passenger> </env:Header>

Explanation of Header Example In general, we can import tags into the header from name

spaces outside of soap. <reservation/>, <reference/>, <dataAndTime/>,<passenger/>

SOAP doesn’t need to worry to much about these. It is the node’s job to process these things.

In this particular case, we may imagine an ongoing transaction for making an airline reservation. Involves several steps and messages, so client must remind

the server of this state information when sending a message. The actual header content all comes from other

namespaces. The role and mustUnderstand attributes are from SOAP.

Header Processing

SOAP messages are allowed to pass through many intermediaries before reaching their destination. Intermediary=some unspecified routing application. Imagine SOAP messages being passed through many

distinct nodes. The final destination processes the body of the

message. Headers are allowed to be processed independently of

the body. May be processed by intermediaries.

This allows an intermediary application to determine if it can process the body, provide the required security, session, or reliability requirements, etc.

Roles, Understanding, and Relays

Role?must

Understand

Relay?ForwardHeader

No

Yes ProcessHeader

Yes

No

Yes No RemoveHeader

Header Roles SOAP nodes may be assigned role designations. SOAP headers then specify which role or roles

should process. Standard SOAP roles:

None: SOAP nodes MUST NOT act in this role. Next: Each SOAP intermediary and the ultimate SOAP

receiver MUST act in this role. UltimateReceiver: The ultimate receiver MUST act in

this role. In our example, all nodes must process the

header entries.

SOAP Body Body entries are really just placeholders for XML

from some other namespace. The body contains the XML message that you

are transmitting. It may also define encodingStyle, just as the

envelop. The message format is not specified by SOAP.

The <Body></Body> tag pairs are just a way to notify the recipient that the actual XML message is contained therein.

The recipient decides what to do with the message.

SOAP Body Element Definition<xs:element name="Body" type="tns:Body" /> <xs:complexType name="Body">

<xs:sequence> <xs:any namespace="##any"

processContents="lax" minOccurs="0“ maxOccurs="unbounded" />

  </xs:sequence>  <xs:anyAttribute namespace="##other"

processContents="lax" /> </xs:complexType>

SOAP Body Example

<soapenv:Body> <ns1:echo soapenv:encodingStyle=

"http://schemas.xmlsoap.org/soap/encoding/"

xmlns:ns1="http://.../axis/services/EchoService">

<in0 xsi:type="xsd:string">HollowWorld</in0>

</ns1:echo></soapenv:Body.

Example SOAP Body Details The <Body> tag is extended to include elements

defined in our Echo Service WSDL schema. This particular style is called RPC.

Maps WSDL bindings to SOAP body elements. Guidelines will be given in next lecture.

xsi-type is used to specify that the <in0> element takes a string value. This is data encoding Data encoding rules will also be examined in next lectures.

A Broader View of Web Services

Beyond WSDL, SOAP, and the OASIS Swamp

Representational State Transfer (REST) The term REST was proposed by Roy Fielding

Dissertation is available from http://roy.gbiv.com/pubs/dissertation/top.htm

Fielding is the first author of the HTTP 1.1 Spec, IETF RFC 2616) The primary concept of REST is statelessness (or

idempotence). All invocations of the same method should give identical

responses. Note REST does not have to transfer HTML

One could build other client server applications on top of this. Famous example: WebDAV (actually predates Fielding’s

dissertation). Modern examples: Atom and RSS feeds emit XML data that can

be easily parsed and used in applications. Commonly used in mash-ups. See Yahoo’s Pipes examples. That is, RSS and Atom XML don’t have to be immediately

rendered for presentations.

<?xml version='1.0' encoding='UTF-8'?>'<feed xmlns='http://www.w3.org/2005/Atom' xmlns:openSearch='http://a9.com/-/spec/opensearchrss/1.0/'><id>tag:blogger.com,1999:blog-19457310</id><updated>2007-03-30T14:10:58.184-07:00</updated><title type='text'>Marlon Pierce's Community Grids Lab Blog</title><link rel='alternate' type='text/html' href='http://communitygrids.blogspot.com/index.html'></link><link rel='next' type='application/atom+xml' href='http://communitygrids.blogspot.com/feeds/posts/default?start-index=26&amp;max-results=25'></link><link rel='http://schemas.google.com/g/2005#feed' type='application/atom+xml' href='http://communitygrids.blogspot.com/feeds/posts/default'></link><link rel='self' type='application/atom+xml' href='http://communitygrids.blogspot.com/feeds/posts/default'></link><author><name>Marlon Pierce</name></author><generator version='7.00' uri='http://www2.blogger.com'>Blogger</generator><openSearch:totalResults>76</openSearch:totalResults><openSearch:startIndex>1</openSearch:startIndex><entry><id>tag:blogger.com,1999:blog-19457310.post-4399342799160789059</id><published>2007-03-29T18:24:00.000-07:00</published><updated>2007-03-30T14:10:58.271-07:00</updated><title type='text'>db4o Java Bean Database</title><content type='html'>The db4o &lt;a href="http://www.db4o.com/"&gt;http://www.db4o.com/&lt;/a&gt; project makes a very simple, light-weight object database. It's free, too. Maybe too free, as they use GPL. But see comment below. Obviously this sort of thing is great if you do a lot of POJO development (say, with JSF) and need some persistent storage.&lt;br /&gt;&lt;br /&gt;The downloadable documentation is pretty good, as is this article:

Atom and RSS Services? Sure, why not? You can convey a lot of information

in a news feed. Good for sequential science data

Seismic events Recently docked drug-like molecules in on-going

simulations. Also it is easy to build HTTP GET query interfaces. Useful for conveying metadata about projects

http://www.chembiogrid.org/wsrss/wsdlrss/getFeed lists available Web services, which are frequently updated.

Useful tools: Atomsphere Java libraries SimplePie RSS libraries for PHP

REST Vs. SOAP? Actually, I think it is really REST versus WSDL. REST applies the same API to all applications:

PUT, GET, POST, DELETE These operations are applied to URLs The actual URLs can point to XML files.

XML could be SOAP, ATOM, RSS, ... Clients and services may do additional processing of

the transmitted XML that is not in the API’s scope. WSDL and XML-RPC define custom APIs

specific to the application.

Realistic REST From my experience, REST’s stateless

philosophy is generally a good idea, even if you design and build SOAP+WSDL Web Services.

Some useful principals: Make each service completely self-contained. The

WSDL should define everything you need to invoke the service.

Use URLs as return values. This is especially useful for scientific services that use input and output files. It’s much easier and more maintainable to use URLs to

transfer files that your service needs or generates. Don’t use stateful communication protocols. Park

state information in a URL or in a WS-Context service.

This is a portal client to a data mining service that I built. The web service analyzes GPS signal data to look for modes.

Portal courtesy of NASA REASoN project.

GPS data comes from the Scripps GRWS Web Service. Instead of defining a data type for this file, we just pass around URLs. The RDAHMM service receives the URL as input.

The service returns output result files as URLs.

The lesson: don’t go overboard with XML message definitions. You will regret it. Use URLs and keep your SOAP/WSDL simple.

Apache S3 REST

Amazon’s Simple Storage Service (S3) provides a simple for-fee online storage service.

Files are uploaded and downloaded using HTTP PUT and GET operations.

Shared symmetric session keys are used to generate unique, un-guessable URLs for files that can be reproduced by the client without having to contact the service.

It is all amazingly simple. See docs at

http://docs.amazonwebservices.com/AmazonS3/ See my notes at

http://communitygrids.blogspot.com/2007/02/notes-on-amazon-s3-file-services.html