1. Inventory of Distributed Computing Concepts and Web services
Chapter 3

2. Motivation Earlier attempts to replace existing systems failed;
Distributed computing environment (DCE): emphasis -- distribution
Common object request broker architecture (CORBA): emphasis -- interoperability
Uniqueness of SOA is that it leverages existing middleware and distributed computing concepts
RPC, distributed objects, message-oriented middleware (MOM), application coupling, etc.
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3. Technical Layers Participant A Participant B Participant C XML ORB
Core
Assets
Participant A
Participant B
Participant C
Business
Logic
Technology
Independent
Interface
Description
Technology
Independent
Interface
Description
Technology
Independent
Interface
Description
Middleware
Mapping
XML
ORB
Technology
Adapters
Middleware
Buses
Communication
facilities
CORBA
XML Web services
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4. Technical Layer 2 : Chapter 3
Core
Assets
Participant A
Participant B
Participant C
Business
Logic
Technology
Independent
Interface
Description
Technology
Independent
Interface
Description
Technology
Independent
Interface
Description
Middleware
Mapping
XML
ORB
Technology
Adapters
Middleware
Buses
Communication
facilities
CORBA
XML Web services
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5. Technical Layer 1: Ch. 4 -7 Participant A Participant B Participant C
Core
Assets
Participant A
Participant B
Participant C
Business
Logic
Technology
Independent
Interface
Description
Technology
Independent
Interface
Description
Technology
Independent
Interface
Description
Middleware
Mapping
XML
ORB
Technology
Adapters
Middleware
Buses
Communication
facilities
CORBA
XML Web services
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6. Communication Network
Application
Application
A communication middleware framework isolates the application developers from the details of the network protocol.
Network Protocol Stack
Network Protocol Stack
A communication middleware framework isolates the application developers from the details of the network protocol.
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7. Communication Middleware
Application
Application
Middleware
Middleware
Network Protocol Stack
Network Protocol Stack
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8. Remote procedure call (RPC)
Client
Application
Server
Application
Procedure
call
Execute
call
RPC Stub code
RPC stub code
RPC library/runtime
RPC library/runtime
RPC stubs and runtime enable location transparency, encapsulate RPC comm infrastructure and provide a proceudre call interface.
Network Protocol Stack
Network Protocol Stack
RPC stubs and runtime enable location transparency, encapsulate RPC communication
infrastructure and provide a procedure call interface.
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9. Distributed Objects Client Application Server Application
invoke
method
Execute
method
Client proxies
Server skeletons
ORB
ORB
RPC stubs and runtime enable location transparency, encapsulate RPC comm infrastructure and provide a proceudre call interface.
Network Protocol Stack
Network Protocol Stack
ORBs enable client applications to remotely instantiate, locate, invoke methods and
Delete server objects; Java RMI, Microsoft’s DCOM; CORBA is meant to be platform independent.
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10. Separation of Interface and Implementation
An important contribution of CORBA is IDL or Interface Definition Language.
Interface and implementation can be in two different languages.
Interface abstracts and protects details (trade secrets) from client
Interface offers a means of expressing design without worrying about implementation.
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11. J2EE Application Programming Model for Web-based applications
Web Service
Business Logic
Web
Application
Database
Server
Enterprise
Java Beans
EJB container
Web Container
Web
client
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12. J2EE Application Programming Model for Three-tier Applications
Presentation
Components
Database
Server
Enterprise
Java Beans
EJB container
Application
Container
Business Logic
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13. Interface vs Payload Semantics
Typically interaction between a client and a server results in the execution of an activity (ot transaction)
Request needs to be specified by the request.
Interface semantics: Requested activity can be encoded in the operation signature in the server’s “interface” or
Payload semantics: It can be embedded in the message itself
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14. Interface Semantics
Process1
Process2
getCustomer()
retrieveCustomerData()
returnResult()
Semantics of the activity is explicitly stated in the message/method call
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15. Payload Semantics Process 1 Process 2
Envelop
With message
Process 1
Process 2
Requested transaction/activity is embedded in the message
Details of the activity not explicit; the semantics are embedded in the message
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16. Payload Semantics
onMessage()
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17. Payload semantics is generic
String transferMoney (amt: decimal, accTo: String)
{ …}
String executeService (message: String)
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18. Document-centric Messages
With emergence of self-descriptive data structures such as XML, document-centric has become popular
Semantically rich messages where operation name, its parameters, return type are self descriptive.
SOAP (Simple Object Access Protocol) over XML is an example (look at the example in p.45 of your text)
Lets look at XML, SOAP, WS evolution. WSSOA
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19. Tight vs. Loose Coupling
An important characteristics of an SOA that is a loosely coupled system.
On the technology front this is driven by dynamic discovery and binding enabled by Universal Description, Discovery and Integration (UDDI)
On the business front loose coupling addresses the growing need for companies to be flexible and agile with respect changes in their own processes and those of their partners (read p.46)
How does loose coupling help in improving agility, flexibility and performance?
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20. Tight vs. Loose coupling
Level
Tight coupling
Loose coupling
Physical coupling
Direct physical link required
Physical intermediary
Communication style
synchronous
asynchronous
Type system
Strongly typed (interface semantics)
Weak type system (payload semantics)
Interaction pattern
OO-style navigation of complex object trees
Data-centric, self-contained messages
Control of process logic
Central control of process logic
Distributed logic components
Service discovery and binding
Statically bound services
Dynamically bound services
Platform dependencies
Strong OS and programming language dependencies
OS- and programming language dependent
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