Marco Piccioni Whats a web service? A software system designed to - - PowerPoint PPT Presentation

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Marco Piccioni Whats a web service? A software system designed to - - PowerPoint PPT Presentation

Feb 07, 2023 278 likes •597 views

Chair of Software Engineering Languages in Depth Series: Java Programming Prof. Dr. Bertrand Meyer Web Services Marco Piccioni Whats a web service? A software system designed to support interoperable machine to machine interaction over

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Languages in Depth Series: Java Programming

  • Prof. Dr. Bertrand Meyer

Chair of Software Engineering

Web Services Marco Piccioni

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What’s a web service?

  • “A software system designed to support interoperable

machine to machine interaction over a network” (W3C)

  • A self-contained, self-describing client-server system that

enforces communication via XML messages

  • A web API accessible over a network, and executed on a

remote system hosting the requested service

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The Web services protocol stack

A collection of protocols that are used to define, locate, implement, and make web services interact with each other

  • Description protocol layer: WSDL
  • Discovery protocol layer: UDDI
  • XML messaging protocol layer: SOAP, …
  • Transport protocol layer: HTTP(S), SMTP, FTP, RSS,

XMPP

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A high- level view

Picture from http://en.wikipedia.org/wiki/Web_services

Endpoint Endpoint 1 3 2

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The WSDL protocol

Web Services Description Language

  • XML-based, platform independent
  • 2.0 is endorsed by W3C

Describes

  • A web service public interface
  • Protocol bindings details
  • Configuration

Used

  • For generating client and server code
  • By Microsoft for .NET
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The WSDL protocol (2)

Web services are described as collections of network endpoints, or ports A port type is defined:

  • As an abstract collection of supported operations
  • By associating a network address with a reusable binding
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The WSDL protocol (3)

  • A reusable binding is a concrete protocol and data format

specification for a specific port type

  • Through the binding, operations and messages are bound

to a concrete network protocol and message format

  • A message is an abstract description of the exchanged

data

  • Ports and messages abstract definition is separated from

their implementation, allowing reuse

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The UDDI protocol

Universal Description, Discovery and Integration

  • XML-based, platform independent

A registry for businesses to list themselves on the www

  • Each business publishes a list of services

Each UDDI business registration consists of:

  • White pages: address, contact and known identifiers
  • Yellow pages: industrial categorizations
  • Green pages: technical info about exposed services
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The UDDI protocol (2)

  • Written in Y2K, enforces a vision in which consumers link

to providers through a public brokerage system

  • By the end of 2005, 70% of Fortune 500 companies were

planning to use it

  • In January 2006, IBM, Microsoft and SAP announced they

were closing their public UDDI nodes

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The UDDI protocol (3)

“The idea was that companies could publish how they wanted to interact, and other companies could find that information and use it to establish a relationship. Needless to say, this isn't how companies do business. There's always a human element to establishing a relationship.” Jason Bloomberg, senior analyst at ZapThink

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The UDDI protocol (3)

Anyway, according to Microsoft and IBM

  • The interoperability and robustness of the UDDI was

proven What now?

  • Mostly used from within-the firewall (intranet)
  • Used as metadata management standard for SOA (more
  • n this later)
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The SOAP protocol

Service Oriented Architecture Protocol (former Simple Object Access Protocol)

  • Simple and extensible XML-based communication

protocol

  • Platform and language independent
  • Basic message-wrapping framework
  • Has bindings to underlying protocols like HTTP, HTTPS,

SMTP, FTP, RSS, XMPP

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Remote Procedure Call

  • The most used SOAP messaging pattern
  • Enables remote calls to be treated as local calls from the

programmer

  • In object-oriented terms: Remote Method Invocation
  • Microsoft DCOM, .NET remoting, OMG‘s CORBA and

Java RMI API all use RPC

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Remote Procedure Call (2)

  • Implements the classical client-server model of distributed

computing

  • An RPC is initiated by a client (network node) sending a

request to a server (another network node)

  • The server immediately responds, while the client is

blocked

  • Remote calls can fail due to unpredictable network issues
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Java RPC-style implementation

  • JAX-RPC (Java API for XML-based Remote Procedure

Calls) provides a standard way for accessing and exposing RPC-style SOAP web services

  • JAX-RPC 2.0 has been renamed to JAX-WS 2.0 (Java

API for XML Web Services)

  • Apache Axis, fully JAX-RPC compliant, is the most

popular JAX-RPC flavor

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High-level view of Java JAX

  • A Java program invokes a method on a stub (local object

representing the remote service)

  • The stub invokes routines in the JAX-WS Runtime
  • JAX-WS converts the RMI into a SOAP message
  • JAX-WS transmits the message as an HTTP request
  • Therefore the web service can be implemented on the

server side as a servlet or EJB container

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A general framework: SOA

Service Oriented Architecture

  • A message is the basic unit of communication
  • Focus is on WSDL contract, specifying
  • Header (name, version, owner, responsible, type…)
  • Functional (functionality accomplished, service operations,

invocation details)

  • Non-functional (security constraints, allowed failure rate,

transactional, service level agreement, semantics)

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One more style of use: RESTful

REpresentational State Transfer is a collection of network architecture principles

  • Application state and functionality are divided into

resources

  • Every resource is uniquely addressable using a universal

syntax for use in hypermedia links

  • All resources share a uniform interface for the transfer of

state between client and resource

  • A constrained set of well-defined operations
  • A constrained set of content types
  • A client-server, stateless, cacheable and layered protocol
  • Does non use SOAP
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Quiz What could be an example of RESTful design?

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Quiz solution What could be an example of RESTful design?

Yes, the World Wide Web, using URIs, MIME types, HTTP, HTML, DNS, …

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RESTful vs. RPC

  • RESTful resources
  • Commands are defined in simple terms of resources to be

retrieved, stored, get, set

  • Difficult to do many joins
  • RPC commands
  • Commands are defined in methods with varying complexity

depending on “standard”

  • RESTful nouns exchange resources and concepts
  • RPC verbs exchange methods
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Spring Web Services support

Spring Web Services is a separate Spring subproject

  • 1.5.1 released on 04/05/2008
  • Document-driven, contract-first development
  • Loose coupling between contract and implementation
  • Standard JAXP API support (DOM, SAX,…)
  • Mappings from incoming XML requests to any object
  • Flexible XML marshalling (using JAXB, Castor, …)
  • Support for security (sign, encrypt, decrypt and

authenticate against SOAP messages)

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Contract-first and contract-last WS

  • Contract-First approach
  • Write contract (WSDL) first, and use the language (Java)

to implement that contract

  • Contract-Last approach
  • Write (Java) code first (interfaces), and let the Web

Service contract (WSDL) be generated from that

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Issues with contract-last development

  • Object/XML impedance mismatch
  • Fragility
  • Performance
  • Reusability
  • Versioning
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Object-XML impedance mismatch

XML (and XSD) are hierarchical, OO languages are modeled by graphs XSD extensions are not like inheritance

  • A XSD restriction that uses a regular expression is lost

Unportable types

  • e.g. java.util.TreeMap, time and dates

Cyclic graphs

  • A refers to B that refers to A
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Fragility and performance

  • Not all SOAP stacks generate the same web service

contract from a Java contract

  • By automatically transforming Java into XML, there is no

guarantee on what is sent over the wire (dependencies can be very deep and unwanted)

  • Using contract-first you can choose what information has

to be sent

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Reusability and versioning

  • Defining your schema in a separate file you can reuse it

in different scenarios

  • Starting from Java, when a contract evolves you have no

choice but to assign a different name to your interface. The old contract must be kept around for clients that need the older version

  • Starting from contracts, you can implement the old and

the new version of the contract in one class using XSLT for conversions

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Web Services assessment

  • Provide interoperability between different web

applications running on different platforms

  • Represent one of many possible solutions for remoting
  • Adopt a specific tradeoff, namely interoperability and

extensibility over ease-of-use and performance

  • RMI/EJB or Hessian/Burlap are better for plain Java to

Java communication

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References

  • http://en.wikipedia.org
  • http://www.springframework.org
  • http://static.springframework.org/spring-ws/site/

To write a contract-first web service see tutorial at:

http://static.springframework.org/spring-ws/site/ reference/html/tutorial.html

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More references

  • http://www.w3.org/TR/ws-arch
  • http://ws.apache.org/axis
  • http://www.w3schools.com/xml/default.asp
  • http://www.w3schools.com/schema/default.asp
  • http://www.w3schools.com/soap/default.asp
  • http://www.w3schools.com/wsdl/default.asp
  • http://www.soapui.org/