Distributed Systems 2006

1
Distributed Systems 2006

• Web Services
• (With material from Møller Schwartzbach, 2003)

2
Plan

• Basics
• SOAP
• WSDL
• UDDI
• Related specifications

3
A Web Service

• A software system designed to support
  interoperable machine-to-machine interaction over
  a network
• It has an interface described in a
  machine-processable format (specifically WSDL)
• Other systems interact with the Web service in a
  manner prescribed by its description using SOAP
  messages
• These are typically conveyed using HTTP with an
  XML serialization in conjunction with other
  Web-related standards
• W3C

4
Motivation

• Heterogeneous, distributed systems
• Scalability
• Interoperability
• Modifiability
• The main motivation is interoperability
• The ability for separately developed applications
  to work in together

5
SOAP, SOAP, UDDI

• SOAP
• A framework for exchanging XML-based information
  in a network
• SOAP used to be an acronym Simple Object Access
  Protocol
• "This is no longer the case." (it is neither
  simple nor has anything to do with objects)
• The currently most hyped XML/Web service
  technology
• WSDL (Web Service Description Language)
• An XML-based language for describing network
  services
• WSDL descriptions of capabilities and locations
  of services
• Like an Interface Description Language for Web
  services
• Often communication using SOAP HTTP
• UDDI (Universal Description, Discovery, and
  Integration)
• Provides a registry mechanism for clients and
  servers to find each other
• Uses SOAP for communication

6
(web services)
web services
(web services)
web services
7
An Example
8
A Result
9
SOAP Request
Message transport
Message intent
Message contents
Application data
10
SOAP Response
11
org.apache.axis.utils.tcpmon

• Part of Apache Axis toolset

12
Anatomy of SOAP

• Lightweight, stateless protocol framework for
  exchanging XML messages
• Applications can create complex interaction
  patterns
• Request/response
• Request/multiple responses
• ...
• by combining one-way exchanges with features
  provided by an underlying protocol and/or
  application-specific information
• Silent on the semantics of any application-specifi
  c data it conveys
• Message contents
• xmlnssoapenv"http//schemas.xmlsoap.org/soap/env
  elope/
• soapenvEnvelope
• soapenvHeader
• Application-specific control information
• May be processed by intermediaries
• soapenvBody
• Information exchanged between sender and receiver
• Typical protocols
• HTTP

13

• (Really, the Google search API is a bad example
  of a web service design
• E.g., http//www.prescod.net/rest/googleapi/ ?
• But has its merits, e.g., in interaction with
  other web services)

14
WSDL

• WSDL is an XML-based language that allows formal
  descriptions of the interfaces of Web services
• which interactions does the service provide?
• which arguments and results are involved in the
  interactions?
• which network addresses are used to locate the
  service?
• which communication protocol should be used?
• which data formats are the messages represented
  in?
• So what are the benefits?
• an interface description is a contract between
  the server developers and the client developers
• having formal descriptions allows tool support,
  e.g. code template generators

15
A WSDL Example (1)
16
A WSDL Example (2)
Data types defined using XML Schema
Message types definedfrom data types
Binding to SOAP RPCover HTTP
Port where the runningservice may be accessed
Port type defining operations
17
WSDL Layers

• The language can be described as having two
  layers
• the service definition layer describes abstract
  properties
• data types
• message types
• operations
• services
• The binding layer describes concrete properties
• Protocols
• Data formats
• (using SOAP, HTTP, MIME)

18
WSDL Elements

• An actual WSDL document consists of a set of
  definitions of the following kinds
• types - containing XML Schema element and type
  definitions
• message - consisting of either
• a number of named parts typed by XML Schema
  elements, or
• a single part typed by a XML Schema type
• portType - describing a set of operations, each
  being either
• one-way receiving an input message,
• request-response receiving an input message and
  then responding with an output message (like
  Remote Procedure Calls),
• solicit-response sending an output message and
  then receiving an input message, or
• notification sending an output message
• binding - selects communication protocol and data
  formats for each operation and message
• service - describes a collection of named ports,
  each associated with a binding and a network
  address
• An import mechanism allows modularization of
  definitions.

19
SOAP in WSDL

• The SOAP binding in WSDL
• selects document or rpc style (rpc wraps message
  parts)
• selects HTTP/SMTP/... protocol
• selects encoding type
• Encoded specifies data formats
• Literal
• places message parts in header or body parts of
  the envelope

20
Examples of style versus encoding

• public void myMethod(int x, float y)
• RPC/Encoded
• ltsoapenvelopegt ltsoapbodygt ltmyMethodgt ltx
  xsitype"xsdint"gt5lt/xgt lty xsitype"xsdfloat"gt5
  .0lt/ygt lt/myMethodgt lt/soapbodygt lt/soapenvelopegt
• RPC/Literal
• ltsoapenvelopegt ltsoapbodygt ltmyMethodgt ltxgt5lt/xgt
  ltygt5.0lt/ygt lt/myMethodgt lt/soapbodygt
  lt/soapenvelopegt
• (Document/Encoded)
• Document/Literal
• ltsoapenvelopegt ltsoapbodygt ltxElementgt5lt/xElementgt
  ltyElementgt5.0lt/yElementgt lt/soapbodygt
  lt/soapenvelopegt
• WS-I Compliance
• Tries to clean up to web service mess so that web
  services may actually interoperate
• Recommends RPC/Literal or Document/Literal
• (Why not use Document/Literal all the time?)

21
Problems with WSDL

• The HTTP and MIME bindings are too loosely
  specified (so not all WSDL documents "make
  sense")
• WSDL is closely tied with SOAP
• SOAP is big and complicated without offering very
  much
• Complex interaction patterns cannot be described
  (e.g. transactions)

22
Using Web Services with Java

• JAX-RPC (JSR-101)
• Java API for XML-Based RPC
• Including WSDL lt-gt Java mapping
• Apache AXIS (Apache Extensible Interaction
  System)
• http//ws.apache.org/axis/
• a Java-based implementation of SOAP WSDL
• largely allows the programmer to forget these
  technologies
• WSDL lt-gt Java
• Used together with a servlet container such as
  Tomcat or Jetty
• Java Web Services Developer Pack
• JAX-WS 2.0 (JSR-224)
• Java API for XML Web Services was JAX-RPC 2.0
• Heavy use of Java 5.0 annotations to customize
  mapping from WSDL and at runtime
• _at_WebService_at_BindingType(value"http//www.w3.org/
  2003/05/soap/bindings/HTTP/")public class
  AddNumbersImpl
• Similar to .NET, will (probably) be built into
  Java 6.0

23
UDDI - Universal Description, Discovery, and
Integration

• A Web service registry mechanism
• "a meta service for locating Web services by
  enabling robust queries against rich metadata"
• UDDI business registration XML files used to
  describe business entities and their Web services
• white pages - business address, contact info,
  etc.
• yellow pages - industrial categorizations based
  on standard taxonomies (allows search within
  particular industry, product category,
  geographical region, ...)
• green pages - more technical information, for
  instance WSDL descriptions
• UDDI also provides a SOAPWSDL-based registry API
  for registering ("publish") and discovering
  ("inquire") Web services
• A UDDI browser http//www.soapclient.com/uddisear
  ch.html

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An Example

• ltbusinessEntity businessKey"ba744ed0-3aaf-11d5-80
  dc-002035229c64"
• operator"www.ibm.com/services/udd
  i"
• authorizedName"0100001QS1"gt
• ltdiscoveryURLsgt
• ltdiscoveryURL useType"businessEntity"gthttp//
  www.ibm.com/services/uddi/uddiget?businessKeyBA74
  4ED0-3AAF-11D5-80DC- 002035229C64lt/discove
  ryURLgt
• lt/discoveryURLsgt
• ltnamegtXMethodslt/namegt
• ltdescription xmllang"en"gtWeb services
  resource sitelt/descriptiongt
• ltcontactsgt
• ltcontact useType"Founder"gt
• ltpersonNamegtTony Honglt/personNamegt
• ltphone useType"Founder" /gt
• ltemail useType"Founder"gtthong_at_xmethods.netlt
  /emailgt
• lt/contactgt
• lt/contactsgt
• ...

25
An Example

• ltbusinessServicesgt
• ltbusinessService serviceKey"d5921160-3e16-11d
  5-98bf-002035229c64"
• businessKey"ba744ed0-3aaf-11
  d5-80dc-002035229c64"gt
• ltnamegtXMethods Delayed Stock Quoteslt/namegt
• ltdescription xmllang"en"gt20-minute
  delayed stock quoteslt/descriptiongt
• ltbindingTemplatesgt
• ltbindingTemplate bindingKey"d594a970-3e16
  -11d5-98bf-002035229c64"
• serviceKey"d5921160-3e16
  -11d5-98bf-002035229c64"gt
• ltdescription xmllang"en"gtSOAP binding
  for delayed stock quotes servicelt/descriptiongt
• ltaccessPoint URLType"http"gthttp//servi
  ces.xmethods.net80/soaplt/accessPointgt
• lttModelInstanceDetailsgt
• lttModelInstanceInfo
  tModelKey"uuid0e727db0-3e14-11d5-98bf-002035229c
  64" /gt
• lt/tModelInstanceDetailsgt
• lt/bindingTemplategt
• lt/bindingTemplatesgt
• lt/businessServicegt
• lt/businessServicesgt
• lt/businessEntitygt

26
UDDI Elements

• A UDDI businessEntity describes a business and
  its services
• Key attributes locate various data structures
• a businessService describes a particular service
  (or family of services)
• a bindingTemplate describes where and how a
  service is accessed
• a tModel describes compliance with a
  specification (e.g. a WSDL description)
• This tModel for the single service described
  above refers to bindings in a WSDL description
• lttModel tModelKey"uuid0e727db0-3e14-11d5-98bf-00
  2035229c64"
• operator"www.ibm.com/services/uddi"
• authorizedName"0100001QS1"gt
• ltnamegtXMethods Simple Stock Quotelt/namegt
• ltdescription xmllang"en"gtSimple stock quote
  interfacelt/descriptiongt
• ltoverviewDocgt
• ltdescription xmllang"en"gtwsdl
  linklt/descriptiongt
• ltoverviewURLgthttp//www.xmethods.net/tmodels/S
  impleStockQuote.wsdllt/overviewURLgt
• lt/overviewDocgt
• ltcategoryBaggt ltkeyedReference
  tModelKey"uuidc1acf26d-9672-4404-9d70- 39b756e
  62ab4 ... lt/categoryBaggt
• lt/tModelgt

27
SOAP to UDDI

• The following SOAP message could be sent to a
  UDDI registry to inquire about services named
  "delayed stock quotes"
• lt?xml version"1.0" encoding"UTF-8"?gt
• ltEnvelope xmlns"http//schemas.xmlsoap.org/s
  oap/envelope/"gt
• ltBodygt
• ltfind_service businessKey"" generic"1.0"
  xmlns"urnuddi-orgapi"gt
• ltnamegtdelayed stock quoteslt/namegt
• lt/find_servicegt
• lt/Bodygt
• lt/Envelopegt
• Alternatively, we could search by category codes
  of various kinds
• The find_service meta-service operation is
  specified in the WSDL for the UDDI API
• Of course, the UDDI registries are also
  registered services -)

28
Sidetrack SOA

• Service-Oriented Architecture
• Practices and technology that enable application
  functionality to be provided and consumed as
  services
• Services can be invoked, published and
  discovered, and are abstracted away from the
  implementation using a single, standards-based
  form of interface.
• Resources
• http//www.cbdiforum.com
• http//www.google.com/search?qoasissoa

29
SOA and Web Services

• SOA is not equivalent to web services
• Other possible underlying technologies
  J2EE/CORBA, .NET,
• Concepts map well to web services

30
SOA Concepts

• Service
• Contractually defined behavior
• Implemented and provided by a component, used by
  another component
• Web services
• Maps to a web service
• Service descriptions
• Each service should include a service description
  in standardized format
• Technical parameters, constraints, and policies
  that define the terms to invoke the service
• Web services
• Maps to a WSDL description of a web service
• Advertising and discovery
• Services should be able to be discovered
• Services need to be advertised to be discovered
• Web services
• May be provided by a UDDI repository
• Data model
• Invoker and invokee need to have a common
  conception of data
• Web services
• XML Schemas may define data as used by services

31
Internal Service Layer

• Defines an applications boundary with a layer of
  services that establishes a set of available
  operations and coordinates the applications
  response in each situation

32
Internal Services

• Service guidelines
• Self-contained coarse-grained functionality
• Stateless should not retain client state
  between invocations
• Context-independence should not depend on other
  services

33
External Architecture

• Service-Oriented Architecture does not define a
  software architecture per se
• the structure or structures of a system,
  which comprise software elements, the externally
  visible properties of those elements, and the
  relationships among them Bass et al., 2003
• Interaction among services still need to be
  designed at an architectural level

34
External Deutschs Eight Fallacies of
Distributed Computing

• 1. The network is reliable
• 2. Latency is zero
• 3. Bandwidth is infinite
• 4. The network is secure
• 5. Topology doesn't change
• 6. There is one administrator
• 7. Transport cost is zero
• 8. The network is homogeneous

35
Seven Fallacies of SOA

• Fallacy 1
• There's Nothing New Under the Sun, and SOA Is No
  Exception
• Fallacy 2
• SOA is a Revolutionary Paradigm Shift
• Fallacy 3
• SOAs are All Hype, No Substance
• Fallacy 4
• SOA is a Panacea
• Fallacy 5
• The Overhead from SOA Leads to Unacceptably Poor
  Performance
• Fallacy 6
• A Bottom-Up Approach to SOA is Good Enough
• Fallacy 7
• SOA is Optional
• http//www.zapthink.com/report.html?idZAPFLASH-08
  052004

36
FastWS

• Application are unaware of which protocol they
  use
• Implementations may serve XML and Fast
  concurrently
• http//java.sun.com/developer/technicalArticles/We
  bServices/fastWS/

37
The Web Service Stack
38
Status of the Stack
39
Summary

• Web services
• Machine-to-machine interaction over a network
• Typically WSDL SOAP HTTP
• A large number of supporting specifications

40
References

• Møller Schwartzbach, 2003
• Møller, A., and Schwartzbach, M. (2003).
  Interactive Web Services with Java.
  http//www.brics.dk/amoeller/WWW, accessed
  2006-02-14