IntroJan14.ppt

B I N A R A M A M U R T H Y
4/15/2023
Copyright 2010 B. Ramamurthy
1
Distributed Systems

Introduction
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 A distributed system is a network of
autonomous computers cooperating to
accomplish a task.
 Hardware and software components of a
distributed system coordinate their activity by
sharing resources such as data, computation,
compute cycles, bandwidth and storage.
 Examples: Internet, intranet, grid and mobile
computing systems.

Topics
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 Some fundamental terms
 Advances in
 Client-side technologies
 Communication network
 Middleware concept
 Server-side technology
 Application models (web, web2.0, web3.0)

Copyright 2010 B. Ramamurthy 4/15/2023
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Terminologies

Protocol
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 Protocol is a set of rules that end points in a
telecommunication system use when
exchanging information.
 IP: Internet protocol defines an unreliable packet
transfer protocol.
 TCP: Transmission Control Protocol builds on IP to
define a reliable data delivery protocol.
 LDAP: Lightweight Directory Access Protocol builds
on TCP to define a query-response protocol for
querying the state of a remote database.
 HTTP: Hyper Text Transfer Protocol builds on TCP
to facilitate hyper-text document exchange.

Service
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 Service is a network-enabled entity that provides a
specific capability.
 Service = Protocol + Behavior
 A service definition permits many
implementations.
 Examples: ability to move files, create processes,
verify access rights
 An FTP server speaks File Transfer Protocol and
supports remote read and write access to a
collection of files.

API
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 Application Program Interface (API) defines a
standard interface for invoking a specified set of
functionality.
 Examples: The Generic Security Service (GSS) API
defines standard functions for verifying identity of
communicating parties, encrypting messages and so
forth.

SDK
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 Software Development Kit (SDK) denotes a
set of code designed to be linked with, and invoked
from within, an application program to provide
specified functionality.
 An SDK typically implements an API.
 Example: Different SDKs implement GSS-API
using the Kerberos or PKI protocols, respectively.

Internet
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 Internet is a very large distributed system.
 Interconnection of a collection of heterogeneous
networks of computers.
 Protocols: IP, TCP, HTTP
 Services: world wide web (www), file transfers (ftp),
email, etc.

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Advances in Client-side
programming

Client programming
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 Simple programs written as a single module
 Single entry point typically in a “main” function
 Procedural, functional and object-oriented
 Applications, applets (web-based)
 More recently the focus is on rich content: Rich
Internet Application (RIA)
 Adobe Flash, Adobe Flex, Microsoft Silverlight, Ajax
 Model-View-Controller (MVC) model for design and
deployment flexibility. Ex: java swing, struts..

Client/Server
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 Server: refers to a process on a networked computer that
accepts requests from other (local or remote) processes to
perform a service and responds appropriately.
 Client: requesting process in the above is referred to as the
client.
 Request and response are in the form of messages.
 Request/response model
 Client is said to invoke an operation on the server.
 Many distributed systems today are constructed out of
interacting clients/servers.
 Issues: connectivity (speed and accessibility), addressing,
naming

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Advances in Networking

Internetworking
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 Internet stack
 Standardization
 IPV4, IPV6: Internet protocol version 4, 6
 Tremendous increase in network bandwidth:
measured in bits per second
 From few kilobits per second (56kb/s dial up lines to
1.5Mb/s T1 to 100Gb/s ethernet)
 What can you do with such fast delivery speed?
Connect them up.. Networked application models

Communication Network
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A communication middleware framework isolates the application
developers from the details of the network protocol.
Application Application
Network Protocol Stack Network Protocol Stack

Client/server Issues
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 Basic object-technology could not fulfill the promises
such as reusability and interoperability fully in the
context internet and enterprise level applications.
Deployment was still a major problem and as a result
portability and mobility were impaired.

Issues in networked systems
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 Heterogeneity in various aspects of a distributed
systems
 Communication modes different
 Synchronous: RPC
 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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Advances in Middleware

Communication Middleware
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Application Application
Middleware Middleware

Remote procedure call (RPC)
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Client
Application
Server
Application
RPC Stub code RPC stub code
RPC library/runtime
RPC library/runtime
Procedure
call
Execute
call
RPC stubs and runtime enable location transparency, encapsulate RPC communication
infrastructure and provide a procedure call interface.

Distributed Objects
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Client
Application
Server
Application
Client proxies Server skeletons
ORB
ORB
invoke
method
Execute
method
ORB (Object Request broker) enables 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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Middleware (Eg. CORBA, Grid)
“network”
client
server
middleware
middleware
client
server
“desktop”
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Technical Layers
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Participant A Participant C
Participant B
Technology
Independent
Interface
Description
Technology
Independent
Interface
Description
Technology
Independent
Interface
Description
Core
Assets
XML ORB Technology
Adapters
Middleware
Mapping
CORBA
XML Web services
Middleware
Buses
Communication
facilities
Business
Logic
GRID

What is CORBA?
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 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).

What is CORBA? (contd.)
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 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.

Object Request Broker (ORB)
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 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.

Clients, severs and objects
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Client
Server0
Server1
CORBA Object
invocation

CORBA Objects and IDL
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 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.

Client and IDL
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 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.

IDL facilitates internetworking
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C++
Java
smalltalk
OLE
(VB, PB,Delphi)
Ada, Cobol etc.
target object
Specify in IDL
Use IDL-language compiler
Client sees only IDL interface

Separation of Interface and Implementation
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 Interface and implementation can be in two different
languages.
 Interface abstracts and protects details (trade
secrets) from client
 Design by contract
 Interface offers a means of expressing design without
worrying about implementation.

ORB : Conceptual View
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 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.

Implementation Details
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Access to the services provided by an Object
ORB : (Object-oriented middleware) Object Request Broker
ORB mediates transfer between client program and server object.
Client
ORB
Object
Object
Stub
Client
Stub

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What is a grid?
 Grid is a sophisticated framework that enables sharing
of a variety of resources among distributed applications.
 Open standard
 Large scale operations
 Automatic
 Intelligent
 Spontaneous
 Interoperable
 Service-oriented

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What is a grid? (A formal definition)
 Grid specifies a standard architecture, infrastructure,
protocols and application program interface (API) for
building an open enterprise system.
 It can provide
 Middleware supporting network of systems to facilitate
sharing, standardization and openness.
 Infrastructure and application model dealing with sharing
of compute cycles, data, storage and other resources.
 A framework for high reliability, availability and security.
 Interoperation of batch-oriented and service-based
architectures.
 Standard service level feature definitions and higher level
concepts for inter and intra-business collaboration.

4/15/2023 Copyright 2010 B. Ramamurthy
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Beginnings of The Grid
 Beginnings of the grid in Search for Extra Terrestrial
Intelligence (seti@home project)
 http://planetary.org/html/UPDATES/seti/index.html
 The Wow signal
http://planetary.org/html/UPDATES/seti/SETI@h
ome/wowsignal.html

Message-oriented Middleware (MOM)
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 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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Server-side Advances

Server-side advances: Two-tier applications
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Presentation
Logic
Business
Logic Database
Server

Server-side: Three-tier Applications
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Presentation
Logic
Business
Logic Database
Server
Tremendous advances in DBMS: relational, query languages,
Object-relational…ACID property transactional systems

Programming Model for Web-based
applications
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Web
client
Web
Application
Database
Server
Enterprise
components
Logic container
Web Container
Business Logic
Web Service

Components and Application Servers
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 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 (bought out by oracle), IONA,
IBM, Oracle, Sybase (bought out by IBM).

Application Programming Model for Three-
tier Applications
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Presentation
Components Database
Server
Enterprise
Components
Container
Application
Container
Business Logic

Expectations of a Distributed System
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•Access transparency: enables local and remote resources to be accessed using
identical operations.
•Location transparency: enables resources to be accessed without knowledge of
their location.
•Concurrency transparency: enables several processes to operate concurrently
using shared resources without interference between them.
•Replication transparency: enables multiple instances of resources to be used to
increase reliability and performance without knowledge of the replicas by users or
application programmers.
•Failure transparency: enables the concealment of faults, allowing users and
application programs to complete their tasks despite the failure of hardware or
software components.
•Mobility transparency: allows the movement of resources and clients within a
system without affecting the operation of users or programs.
•Performance transparency: allows the system to be reconfigured to improve
performance as loads vary. “Scalability”
•Expansion transparency: allows the system and applications to expand in scale
without change to the system structure or the application algorithms.

Issues contd.
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 Heterogeneity of components
 Scalability : ability to perform well under
increased loads and data sizes
 Failure handling
 Concurrency
 Transparency
 Reliability
 Interoperability
 Performance
 Openness
 Security and protection

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Emerging Application Models

Large scale systems
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 Most emerging distributed applications are very large
demanding large amounts of storage and data resources
 E-commerce systems and online businesses
 Applications connecting communities: from search to social
networking
 See world’s 10 largest data bases
 Amount of data collected by various sources from terrorism
monitoring to environmental monitoring
 Data deluge: most of the data is write once read many
(WORM)  Analytics resulting in data-intensive computing
models and big-data computing (CSE487 material)
 We will look just one success story of distributed system:
amazon.com

Amazon.com
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 Werner Vogels’ talk “Order in the Chaos: Building the Amazon.com
Platform."
 1995: Started out with a single web service on a single server. Today
amazon has about 150 web services on its homepage alone.
 1 million merchant partners; 60 million customers
 One server of customers and inventory grew into two servers; more
database servers were added as the business expanded
 1999: A mistep during this exponential growth period was moving to
mainframe from distributed server. Failed to meet scalability, reliability
and performance; it was scratched in 2000.

Amazon (contd.)
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 Robustness: Shopping cart is tested for 20000 items by a single customer,
for example!
 Amazon’s secret sauce is “operating reliably at scale”.
 After “the denial of service” debacle in 1999, they decided to use Web
services to insulate the databases from being overwhelmed by direct
interaction with online applications.
 Each web service is the responsibility of a team of developers:
 “And they are not just responsible for writing the service and then tossing
it over the wall for testing and eventual entry into production where some
poor maintenance geek has to look after it.
 The Amazon CTO tells his Web services team members: "You build it.
You own it."
 That means the team is responsible for its Web service's on-going
operation. If a Web service stops working in the middle of the night, team
members are called to fix it.”
 Web services are kept simple: complexity is the notorious enemy of
reliability
 No attachment to one technology or standard: what ever customer
wants, give it.

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On to more fundamental
concepts

Synchrony
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 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)

Asynchronous communication
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 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

Interface vs Payload Semantics
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 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

Interface Semantics
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Process1 Process2
getCustomer()
retrieveCustomerData()
returnResult()
Semantics of the activity is explicitly stated in the
message/method call

Payload Semantics
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Process 1 Process 2
Envelop
With
message
Requested transaction/activity is embedded in the message
Details of the activity not explicit; the semantics are
embedded in the message

Payload Semantics
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Queue
Client Server
createMessage()
sendMessage()
receiveMessage()
receiveMessage()
onMessage()

Payload semantics is generic
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String transferMoney (amt: decimal, accTo: String)
{ …}
String executeService (message: String)
{ …}

Document-centric Messages
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 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
 Lets look at XML, SOAP, WS evolution. WSSOA

Tight vs. Loose Coupling
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 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
 How does loose coupling help in improving agility,
flexibility and performance?

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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Summary
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 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.
 In this course, we will study different types of
distributed systems and learn to design, develop
and implement distributed systems.

Tools to explore
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 Design tool: Rational rose demo model
 Windows
http://www.cse.buffalo.edu/bina/rosecppdemo.exe
 Language for your distributed system development:
Java
 IDE for development needs of your projects:
Netbeans? Eclipse?

IntroJan14.ppt

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