1. Inventory of Distributed Computing Concepts
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. Issues in distributed systems
Heterogeneity in various aspects of a distributed systems
Communication modes
Synchronous: RPC, ORB
Asynchronous: P2P, Publish and subscribe
Variations in products
Many vendors: IBM, IONA, TIBCO, Apache, Adobe
Additional runtime features:
fault tolerance, load balancing, transaction handling, usage metering, auditing, ..
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4. 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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5. 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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6. 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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7. Technical Layer 3: Chapter 9
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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8. 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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9. Communication Middleware
Application
Application
Middleware
Middleware
Network Protocol Stack
Network Protocol Stack
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10. 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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11. 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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12. What is CORBA?
Common Object Request Broker Architecture (CORBA) specification (/standard) defines a framework for object-oriented distributed applications.
It is defined by a consortium of vendors under the direction of Object Management Group (OMG).
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13. What is CORBA? (contd.)
Allows distributed programs in different languages and different platforms to interact as though they were in a single programming language on one computer.
Brings advantages of OO to distributed systems.
Allows you design a distributed application as a set of cooperating objects and to reuse existing objects.
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14. Object Request Broker (ORB)
A software component that mediates transfer of messages from a program to an object located on a remote host.
Hides underlying network communications from a programmer.
ORB allows you to create software objects whose member functions can be invoked by client programs located anywhere.
A server program contains instances of CORBA objects.
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15. Clients, severs and objects
Server0
Client
CORBA Object
invocation
Server1
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16. CORBA Objects and IDL
These are standard software objects implemented in any supported language including Java, C++ and Smalltalk.
Each CORBA object has a clearly defined interface specified in CORBA interface definition language (IDL).
The interface definition specifies the member functions available to the client without any assumption about the implementation of the object.
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17. Client and IDL
To call a member function on a CORBA object the client needs only the object’s IDL.
Client need not know the object’s implementation, location or operating system on which the object runs.
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18. IDL facilitates internetworking
Specify in IDL
Use IDL-language compiler
Client sees only IDL interface
C++
Java
target object
smalltalk
OLE
(VB, PB,Delphi)
Ada, Cobol etc.
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19. Separation of Interface and Implementation
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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20. ORB : Conceptual View
When a client invokes a member function on a CORBA object, the ORB intercepts the function call.
ORB directs the function call across the network to the target object.
The ORB then collects the results from the function call returns these to the function call.
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21. Implementation Details
Client
ORB
Object
Stub
Access to the services provided by an Object
ORB : (Object-oriented middleware) Object Request Broker
ORB mediates transfer between client program and server object.
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22. Message-oriented Middleware (MOM)
Made famous by IBM’s MQseries and TIBCO’s Rendezvous products.
Based on messages and queues.
A message contains a header and a payload.
A queue can store and distribute messages.
Publish/subscribe model of communication:
A topic offers another model of communication between subscribers and publishers.
MOM allows for loose coupling between message consumers and message producers enabling dynamic, reliable, flexible, high-performance systems to be built.
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23. Components and Application Servers
An application server mediates between a web server and backend systems.
Request from a web client is passed onto an application server by the web server.
Programmer productivity, cost-effective deployment, rapid time to market, seamless integration, application portability, scalability, security are some of the challenges that component technology tries to address head on.
Enterprise Java Beans is Sun’s server component model that provides portability across application servers, and supports complex systems features such as transactions, security, etc. on behalf of the application components.
EJB is a specification provided by Sun and many third party vendors have products compliant with this specification: BEA systems, IONA, IBM, Oracle, Sybase, Gemstone.
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24. 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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25. J2EE Application Programming Model for Three-tier Applications
Presentation
Components
Database
Server
Enterprise
Java Beans
EJB container
Application
Container
Business Logic
Study Fig. 3-8 of your text for J2EE technology suite.
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26. On to more fundamental concepts: Synchrony
Synchronous and asynchronous communications
Synchronous:
immediate response of communicating partners
Server process/thread blocks until response is completed
Follows request/response pattern
Used when servers are available all the time
Typically communicating partners are tightly coupled
Examples:
request from web client to a web browser for “search” or for “information”
CORBA procedure invocation
Java RMI (remote method invocation)
Traditional remote procedure call (RPC)
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27. Asynchronous communication
Communicating partners are decoupled
Message driven:
sender creates a message and delivers it to a mediator who then sends it to “a” recipient
Server need not be available all the time
Sender and receiver loosely coupled
Can facilitate high-performance message-based system
Example:
Any event-driven system
Any messaging system (instant messenger)
Publish-subscribe mode communications
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28. 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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29. Interface Semantics
Process1
Process2
getCustomer()
retrieveCustomerData()
returnResult()
Semantics of the activity is explicitly stated in the message/method call
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30. 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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31. Payload Semantics
onMessage()
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32. Payload semantics is generic
String transferMoney (amt: decimal, accTo: String)
{ …}
String executeService (message: String)
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33. Tool to explore Rational rose demo model Windows rosecppdemo.exe
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34. 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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35. 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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36. 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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37. Summary
We discussed the fundamental choices available to a designer in assembling a distributed system
A designer must choose appropriate communication infrastructure, synchrony, call semantics, use of intermediary, object-oriented versus data-centric interfaces.
All these factors impact the coupling of the distributed systems.
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