Chapter 1 - Introduction to Web Services

Chapter 1 - Introduction to Web Services
Objectives
After completing this unit, you will be able to:
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Describe the basic design for Web service interaction.
Explain how Web services can be applied to support various
business models.
Identify the core Web service technology standards.
1.1 A Conceptual Look at Services
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standard interfaces (serial,
monolithic/proprietary design
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parallel,
USB)
expensive
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fixed architecture and capabilities
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competitive pricing
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technology locked
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component-based design
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vendor locked
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adaptable to technology
innovation
vendor-neutral
A Conceptual Look at Services
The evolution that has taken place in the computing industry is analogous to the evolution that service
orientation represents for enterprise systems. It is about transitioning from closed, proprietary solutions
to open, standards-based solutions. This carries with it a host of benefits with respect to flexibility and
cost.
Chapter 1 - Introduction to Web Services
1.2 Defining Services
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The ‘S’ in SOA is embodied as the configurable enterprise assets that
comprise the various layers.
Abstractly, services can be described as follows:
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Performs one or more well-defined functions.
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Is loosely coupled and self contained.
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Is almost always stateless.
Various enterprise elements can function as services:
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CICS/IMS mainframe systems
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Distributed computing components (CORBA, EJB, DCOM, etc.)
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Messaging systems (MSMQ, IBM MQ, etc.)
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Packaged applications (ERP, CRM, etc.)
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Home-grown applications
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Web services (most popular)
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1.3 Service Communication Analogy
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Analogy: Calling an automated system to get flight information or the
status of a bank transaction.
What? Sequence
of
prompts, keystrokes,
and voice responses Inc.
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necessary to achieve the desired goal
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How? Communication mechanism selection (landline, cellular, or VOIP)
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Where? Phone number dialed to initiate call
Parallel: Sending electronic messages to a remote system in order to
achieve a desired goal.
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What? Typically an XML-based interface format.
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How? Typically XML messages sent via HTTP.
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Where? Some network address (IP, Web, etc.).
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1.4 Three Key Service Questions
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What?
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What capabilities does this service provide?
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What requirements does this service have for potential consumers?
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What data, if any, will this service return?
How?
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How should messages be formatted for this service?
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How should messages be submitted to this service (i.e. the transport
protocol)?
Where?
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Where is the service located (IP address or endpoint URL)?
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1.5 Connecting the Dots
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What?
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How?
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Service interface
Service binding
Where?
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Service address
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Interface
Service
Binding
Service Provider
Service Consumer
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1.6 SOA: Runtime Implementation
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Interface
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Describes the generic interface.
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Consist of generic operations and messages that describe a service.
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Contains no information on how to invoke the service.
Binding
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Contains information on how to bind a service to a specific
implementation.
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Binds operations to actual methods.
Address/Endpoint
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Describes binding of communication information for calling a service.
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Binds service name to implementation details.
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SOA: Runtime Implementation
At runtime, only the Binding and Service information is used.
The Interface, Binding and Service definitions exist as Web Services Definition Language (WSDL)
files.
WSDL is discussed in the next few slides.
1.7 Web
What Is aAge
Web Service?
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“Loosely coupled software components that encapsulate discrete
functionality and that are accessible over standard Internet protocols.”
– The Stencil Group
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“Web Services are a new breed of Web application. They are selfcontained, self-describing, modular applications that can be published,
located, and invoked across the Web.”
– IBM’s Tutorial on Web Services
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Chapter 1 - Introduction to Web Services
Any
Anyapplication
applicationthat
thataccepts
acceptsXML-formatted
XML-formattedrequests
requestsfrom
fromother
other
systems
systemsacross
acrossaanetwork
network(Internet
(InternetororIntranet)
Intranet)via
vialightweight,
lightweight,
vendor-neutral
vendor-neutralcommunications
communicationsprotocols.
protocols.
1.8 Enterprise Assets as Services
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Typically, a Web service is used as a mechanism to expose functionality
made available by another application.
As such, a Web service operates as a standards-based façade or wrapper
for accessing non-standardized middleware components.
Web Server
Web
Service
middleware
XML Response
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Listener
XML Request
Enterprise
Asset
Enterprise Assets as Services
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Listener: Intercepts the client request and routes Web service requests to the appropriate service.
Web Service: Unmarshalls the XML sent by the client, parses the parameters and method call, and
invokes the client request via the underlying middleware. Then the Web service retrieves the result
from the middleware layer, encodes it in XML, marshalls the XML document, and sends the response
back to the client.
Middleware: (This layer is optional.) The middleware layer acts as an intermediary between the Web
service and the business service that the client is ultimately interested in accessing.
Enterprise Asset: The resource the client is ultimately trying to access.
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1.9 Typical Development Workflow
1. A service provider will create an interface that captures the data format,
operations, and input/output message structure.
Service
Provider
design
Interface
Interface
2. Next, the business logic for the service is implemented and put into
production.
Service
Provider
develop
Service
Implementation
Production
Environment
deploy
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3. The production service is then registered in a service registry.
Service
Provider
register
Interface
Interface
Service
Registry
Typical Development Workflow
This development workflow represents a top-down approach to service development (WSDL is created
first). A bottom-up approach (existing system is exposed as a service) is also possible.
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Development Workflow
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A client developer will locate the service in the registry, download the
interface file, learn about the service, and then build the client application.
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Client
Developer
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ver
disco
Interface
Interface
Interface
Interface
develop
ins
pe
ct
Service
Registry
Client
Application
At runtime, the client application invokes the service by sending an
appropriately formatted message to the designated endpoint.
Client
Application
XML
S
S
Production
Environment
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Typical Development Workflow
The URL where the service request should be posted may be supplied to the consumer in one of several
ways:
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Hard coded in the client
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Obtained from the registry
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Obtained from within the service interface file
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of Web Services
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Deliver new IT solutions faster and at a lower cost by focusing code
development on core business and using Web services applications for
non-core business programming.
Protect investment in IT legacy systems by using Web services to wrap
legacy software systems for integration with modern IT systems.
Integrate business processes with customers and partners at lower cost.
Web services makes this integration feasible by allowing businesses to
share processes without sharing technology. With lower costs, even small
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Chapter 1 - Introduction to Web Services
business will be able to participate in B2B integration.
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Enter new markets and widen customer base. Web services listed in
registries can be “discovered” and thus are “visible” to the entire Web
community.
1.12 Web Service Business Models
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Web Services make it possible for businesses to externalize automated
business processes.
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This is a step after Web-enabling business processes.
In a more advanced scenario, buyers and sellers can discover each other
and connect dynamically to execute transactions.
The following business models can be easily implemented using Web
services:
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Business Information
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Internal System Integration
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Business Process Externalization
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Business Aggregation
Web Service Business Models
Web enabling business processes allowed customers to directly take part in such processes. For
example, customers could directly manage their orders without having to fax it or place the order on the
phone. The process was still inefficient. The customer may have to place that order first in an internal
system. Once the order is approved, she needs to place that order again in the supplier's Web site.
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If the supplier makes the order taking process available over Web services, the internal application can
directly place the order once the order is approved internally.
1.13 Web Service Business Models
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Following models are presented in a progressive order of complexity. A
business can implement simpler models first before considering a more
complex one.
1. Business Information: Provide read-only information to consumers.
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Chapter 1 - Introduction to Web Services
Example: Corporate news, financial report, weather forecast.
2. Internal System Integration: A business may have automated sales
process, order taking Web site and warehouse management system.
But they may not be integrated. Diverse systems within the same
business can communicate using Web services.
3. Business Process Externalization: Allow automated systems of
partner or customer organizations to communicate with an application.
4. Business Aggregation: Multiple travel agent systems can adhere to
the same Web service specification. This allows an automated system
to look for the cheapest ticket.
1.14 Example: Internal System Integration
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Acme Inc. has these automated systems:
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Sales: Allows sales agents to set prices and discounts for preferred
buyer organizations.
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Order taking Web site: Allows customers to browse through products
and place orders.
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Warehouse: Orders placed in the Web site are sent to the warehouse.
Each system is sophisticated and mature in its own right. Significant
problems remain.
When a new product is entered in the Sales system, who enters it in the
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Web site andof
the warehouse?
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When the price or description of an item is changed, who updates the
Web site?
Solution: Have each system offer a service called "AddProduct". When a
product is added to the Sales system, it can invoke this service in the Web
site and warehouse system. A standardized specification will allow
additional systems to participate in the process. The Sales system does
not need to contain external system specific business logic.
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Similarly, the Web system can offer a service called "UpdateProduct".
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1.15 Example: Business Process Externalization
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Acme Inc. is a printing company. Currently, staff from the client
organizations need to upload documents via Acme's Web site and place a
print order.
See things from a client organization's point of view. They are probably
receiving orders for a book or course material over their own Web site.
Each order requires a staff member to upload documents using Acme's
Web site and place an order.
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The process is time consuming and error prone. Once in a while, a staff
uploads the wrong documents.
Acme can externalize the order taking process using a Web service. The
order-taking application of the client can invoke this service as soon as a
new order is placed. Proper documents are sent over as a part of the
service invocation.
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1.16 Web Services Standards
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We will now take a brief look at some of the basic standards:
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SOAP
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WSDL
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UDDI
WS-I
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1.17 Binding via SOAP
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The message format most often used for the binding portion of a Web
service is SOAP (Simple Object Access Protocol).
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A simple, lightweight XML-based data format for exchanging
information in a distributed environment.
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Messages exchanged in a request and response pattern.
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SOAP is managed by the W3C: http://www.w3.org/TR/soap/.
SOAP can be used with multiple communication protocols:
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HTTP, SMTP, FTP, JMS, etc. (HTTP is most common).
SOAP is language and platform neutral.
XML
Client
Web
Service
Binding via SOAP
SOAP specifies the XML request and response message format only. It does not describe any network
communication protocol. A message constructed in SOAP format can be communicated over any
protocol: HTTP, FTP, SMTP or JMS.
1.18 SOAP in Protocol Stack
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SOAP
HTTP/JMS/SMTP
Data payload format
Client server handshake,
authentication, encryption
TCP/IP
protocol
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SOAP in Protocol Stack
SOAP covers just the request and response data. We will learn shortly how a
SOAP message contains the following key information:
1. The operation that needs to be invoked corresponding to the request.
2. The input or output data for the message.
SOAP also covers more advanced issues such as:
1. Encryption of certain pieces of request or response data. Note: This is different from HTTPS,
which encrypts the entire request/response data.
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2. User identity and credentials (password).
1.19 SOAP Structure
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A SOAP message consists of three parts:
1. An envelope that contains a message and specifies processing
instructions.
2. An optional header section for transmitting auxiliary information such as
client preferences, security, or transaction semantics.
3. A mandatory body section that carries the message payload.
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The SOAP specification also defines the following:
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A set of encoding rules for expressing instances of application-defined
data types.
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A convention for representing remote procedure calls and responses.
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1.20 SOAP Message Architecture
Protocol Headers
The complete SOAP Message
Standard Protocol
(HTTP, SMTP, etc.)
SOAP Envelope
<Envelope> encloses payload
SOAP Message
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<Header>
encloses
headers
SOAP Header
Headers
SOAP Body
Message Name
& Data
Individual headers
<Body> contains SOAP
Message Name & Data
XML Encoded SOAP
Message Name & Data
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SOAP Message Architecture
There are three components to the SOAP spec:
1. The SOAP envelope.
2. A set of encoding rules.
3. A means of interaction between request and response.
Think about a SOAP message as an actual letter:
1. The document is enclosed in an envelope.
2. The envelope is transmitted via mail system (the transport).
3. The HTTP header data acts as the address which provides routing information so that the
message is delivered to the appropriate server resource.
4. The “last mile service” is provided by the XML encoding.
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1.21 Applying SOAP
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SOAP is an XML-based document that contains a message payload to be
exchanged between parties.
Although there are various alternatives (REST, XML-RPC, JSON, etc.),
SOAP has emerged as the de facto standard for Web service exchanges.
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More ‘enterprise grade’ extensions and capabilities (security,
transactions, reliable delivery, etc.).
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Strong vendor tool support.
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Extensive literature
on the subject,
including
best practice guidelines.
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1.22 Interface via WSDL
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The message format most often used for the binding portion of a Web
service is WSDL (Web Services Description Language)
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An XML-based language the defines a service interface.
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Describes how to locate and invoke a service.
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Managed by the W3C: http://www.w3.org/TR/wsdl.
The WSDL document specifies the location of the service and the
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operations (or methods) the service exposes.
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For every operation, the input/output message format is also specified.
WSDL is essentially the IDL of Web services.
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Makes a neutral declaration of one or more services.
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Describes how a client can interact with those services.
Interface via WSDL
Here, “location” means information about how a consumer can invoke the service. For example, for an
implementation of a service that can be invoked by posting a SOAP message, the consumer needs to
know the URL.
1.23 WSDL Structure
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A WSDL file has two major sections:
1. An abstract description of operations and their input/output data.
2. A concrete definition of how to bind to the service on a given port.
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An abstract description contains programming language and protocolneutral description of operation names and message formats.
Information about a concrete implementation includes enough detail for a
consumer to actually invoke the service.
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WSDL Structure
A WSDL file can contain a purely abstract service description. A specific service implementation can
provide its own WSDL that imports the abstract WSDL and supplies concrete details.
1.24 Applying WSDL
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WSDL is an XML-based document that describes the service being
provided.
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The provider records this document with the registry.
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It provides information on what the service is about, where it resides,
and how it can be invoked.
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It is extensible, allowing for services other than SOAP to be declared.
In a Web services exchange, the WSDL serves as a contract between
client and service.
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Changes made to the service implementation do not have to impact the
client and vice-versa.
Applying WSDL
In the WSDL document, the following information is defined:
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What are services?
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What are the ports (network endpoints/address) in each service?
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What is the protocol binding for each port?
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What is the portType (operation/method) in each binding?
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What is the operation type (one way, request-response, solicit-response, or notification)?
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What are the messages (parameters) for the operation?
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What is the data type of the parameter?
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1.25 Locating a Service
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How do you currently find businesses and/or services on the Web?
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Type in the address (URL) directly.
Use a search engine.
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Web Services work the same way.
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Type in the address (URL) directly.
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Query a registry (a search engine for services).
1.26 UDDI Overview
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UDDI (Universal Discovery Description Integration) is a specification for
publishing and discovering service information.
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It represents a search engine for online businesses and services that
are registered with the registry.
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The goal of the UDDI project is to accelerate the interoperability and
adoption of Web Services through standards-based specifications for
service description and discovery.
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Service providers publish the service description and the Service
Requestors search for the required services and obtain binding
information to connect to the actual service.
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UDDI is based on XML and SOAP.
For the most part, public registries have died in favor of private registries
within intranets and B2B extranets.
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Xmethods.com is one of the few remaining UDDI registries.
1.27 UDDI Terminology
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UDDI terminology involves two main terms:
1. Business
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General term referring to an organization or operating concern.
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Defined in the UDDI registry.
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The business information is primarily used to answer “who, what,
where, and how”.
2. Service
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Service refer to Web services.
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Businesses act as a container for services.
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A business is first registered in the registry and the various services are
then listed under that particular business.
UDDI Terminology
“Who” and “what” refer to general information about the products and services.
“Where” involves registering information about the URL or email address (or other address) through
which each type of service is accessed.
“How” refers to registering references to information about interfaces and other properties of a given
service.
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1.28 UDDI Structure
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UDDI is often compared to a phone book that stores various classifications
of information.
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White Pages
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Yellow Pages
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Allows searches for businesses by name or some other unique
identifier such as a DUNS, Thomas Register, or other ID.
UDDI also supports the categorization of businesses by industry,
product/service offering, and location.
Green Pages
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Supports the registration and searching of businesses by the service
types and capabilities they provide.
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1.29 Applying UDDI
SOAP
processor
SOAP request
Client
application
SOAP response
Web
server
UDDI registry
service
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UDDI
database
UDDI registry node
Applying UDDI
The service invocation process is simple:
1. The client queries a UDDI registry node.
2. The server returns a list of records matching request.
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3. The client identifies a specific service of interest.
4. The server provides the client with binding information so the client can invoke service
methods.
1.30 WS-I Overview
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Early work with Web services ran into significant problems with service
interoperability.
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Differences in SOAP messaging formats.
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Differences in data type encoding within SOAP.
The Web Services Interoperability (WS-I) organization was created in
2002 by a vendor consortium to address this problem.
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Its goal is to promote Web service interoperability across platforms,
operating systems, and programming languages.
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It delivers practical guidance, best practices, and resources for
developing interoperable services solutions in the form of ‘profiles’.
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With so many variables in a service-oriented exchange, risk mitigation
through the adoption of WS-I basic profiles becomes essential.
WS-I Overview
The WS-I Web site can be accessed at http://www.ws-i.org/
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WS-I Deliverables
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WS-I’s deliverables provide resources for Web services developers to
create interoperable Web services and verify that their results are
compliant with WS-I guidelines. They include:
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Profiles
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Provide implementation guidelines for how related Web services
specifications should be used together for best interoperability.
Sample Applications
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Demonstrate Web services applications that are compliant with WS-I
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Chapter 1 - Introduction to Web Services
guidelines.
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Testing Tools
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Test whether the messages exchanged with a Web service conform
to WS-I guidelines.
WS-I Deliverables
Profiles: To date, WS-I has finalized the Basic Profile, Attachments Profile, and Simple SOAP Binding
Profile. More information can be found at http://www.ws-i.org/deliverables/.
The WS-I Basic Profile is an outline of requirements to which WSDL and Web service protocol
(SOAP/HTTP) traffic must comply in order to claim WS-I conformance.
1.32 Summary
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The service aspect of SOA comes in many different forms:
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Packaged applications (COTS)
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Home-grown solutions
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Mainframe systems
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Web services
Web services represent the most popular approach to service enabling an
enterprise.
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Web services facilitate various business models:
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Business Information
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Internal system integration
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Business Process Externalization
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Business Aggregation
Technology standards play a big part in Web services, especially SOAP,
WSDL, and UDDI.
WS-I is a vendor-neutral organization dedicated to promoting service
interoperability.
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