Client/Server Distributed Systems

240-322 Cli/Serv.: Models/1 1

Client/Server Distributed SystemsClient/Server Distributed Systems

ObjectivesObjectives– introduce the client/server modelintroduce the client/server model– give an overview of DCE and Java RMIgive an overview of DCE and Java RMI

240-322, Semester 1, 2005-2006

1. The Client Server Model(Chapters 1 and 2, Berson)


OverviewOverview

1.1. Client/Server BasicsClient/Server Basics

2.2. Client/Server in More DetailClient/Server in More Detail

3.3. Client/Server SummaryClient/Server Summary

4.4. Standards OrganisationsStandards Organisations

5.5. DCEDCE


1. Client/Server Basics1. Client/Server Basics

A first examination of client/server A first examination of client/server functionality.functionality.

A brief definition:A brief definition:– A A serverserver is a program (or collection of is a program (or collection of

cooperating programs) that provides services cooperating programs) that provides services and/or manages resources on the behalf of other and/or manages resources on the behalf of other programs (its programs (its clientsclients).).


1.1. Client/Server Environment1.1. Client/Server Environment

LAN or WAN

Server DataBerson,Fig 1.4, p.8

clients

network


1.2. Example1.2. Example

The ATM network:The ATM network:– the clients are the ATM machines the clients are the ATM machines

user interfaces;user interfaces;some simple application processingsome simple application processing

– the server is at the bank the server is at the bank most application processing;most application processing;

very large database of customer accountsvery large database of customer accounts


1.3. Architectural Requirements1.3. Architectural Requirements

Reliable, robust communication between the clients Reliable, robust communication between the clients and server.and server.

Client/server cooperationClient/server cooperation– started by the clientstarted by the client

Application processing is usually distributed Application processing is usually distributed between a client and the server.between a client and the server.

Server Server controlscontrols services/data that the client accesses. services/data that the client accesses. Server handles conflicting requests.Server handles conflicting requests.


1.4. Recent Client/Servers Architectures1.4. Recent Client/Servers Architectures

More complex networkingMore complex networking– LAN, WAN LAN, WAN Web, InternetWeb, Internet– client mobilityclient mobility

More complex data structuresMore complex data structures– relational relational multimedia, OO, deductivemultimedia, OO, deductive– sizesize– distributed databasesdistributed databases– parallelismparallelism

continued


Separation of ‘business logic’ (i.e. program Separation of ‘business logic’ (i.e. program code for manipulating data) from the datacode for manipulating data) from the data– 3-tier (multi-tier) architectures3-tier (multi-tier) architectures– distributed business logicdistributed business logic


2. Client/Server in More Detail2. Client/Server in More Detail

2.1. Converting a Database App.2.1. Converting a Database App.

2.2. Component Placement?2.2. Component Placement?

2.3. The 2-tier Model2.3. The 2-tier Model

2.4. The 3-tier Model2.4. The 3-tier Model

2.5. Locating the Business Logic2.5. Locating the Business Logic

2.6. Locating the Data2.6. Locating the Data

2.7. Multi-tier Model2.7. Multi-tier Model


2.1. Converting a Database App.2.1. Converting a Database App.

Presentation Logic

Business Logic

Database Logic

DBMSDatabase

Stand-aloneApplication


Different client/server models are obtained by Different client/server models are obtained by locating different components and combinations locating different components and combinations of the application on the client and server(s).of the application on the client and server(s).

In general:In general:– presentation logic stays on the clientpresentation logic stays on the client– DBMS and database move to the serverDBMS and database move to the server

– parts of the business and database logic that can be used by parts of the business and database logic that can be used by several clients are placed on the serverseveral clients are placed on the server


2.2. Component Placement?2.2. Component Placement?

How much data is required by the local How much data is required by the local application?application?

How many application users require the same How many application users require the same data?data?

How many interactions occur between the How many interactions occur between the application parts?application parts?

Technical issuesTechnical issues– platforms, networking platforms, networking


2.3. The 2-tier Model2.3. The 2-tier Model Fig. 2.3, p.41 Fig. 2.3, p.41

Presentation Logic

Business Logic

Database LogicDBMS

Database

Database Logic

ClientServer


PointsPoints

The database is on the serverThe database is on the server– could some of it be moved to the client?could some of it be moved to the client?

Distributed database logicDistributed database logic– most of it is on the clientmost of it is on the client

The client does the presentation.The client does the presentation. ‘‘Fat’ versus ‘thin’ clients.Fat’ versus ‘thin’ clients. Much simpler if all the database servers are Much simpler if all the database servers are

the same (the same (homogenoushomogenous).).


DrawbacksDrawbacks

It is difficult to build It is difficult to build heterogeneousheterogeneous database database environments.environments.

Transaction processing is limited by the DBMS.Transaction processing is limited by the DBMS.

Asynchronous processing is difficultAsynchronous processing is difficult– i.e. the client doesn’t wait for the server’s answeri.e. the client doesn’t wait for the server’s answer

Scaleability?Scaleability?


2.4. The 3-tier Model2.4. The 3-tier Model Fig. 1.6, p.12 Fig. 1.6, p.12

ServerData

Application servers Data Servers

ServerData

UNIX

Win/NTClients


The 3-tier Model AgainThe 3-tier Model AgainFig.2.6, p.48Fig.2.6, p.48

Server

Mainframehost(s)

Tier 1

Tier 2

Tier 3

LAN

Price/PerformanceFunctionalityLocal Autonomy

Greater IntegritySecurityCentral Control


PointsPoints

The “Mainframe host” is usually a very The “Mainframe host” is usually a very large database (or databases)large database (or databases)– sometimes called a sometimes called a back-end serverback-end server

The “server” usually holds shared The “server” usually holds shared applications (application/business logic)applications (application/business logic)– sometimes called the sometimes called the middle-tier servermiddle-tier server


Benefits of 3-tier over 2-tierBenefits of 3-tier over 2-tier

The application logic in the middle-tier is more The application logic in the middle-tier is more independent of the client and the back-end serverindependent of the client and the back-end server– it should be more robustit should be more robust

The application logic in the middle-tier can work The application logic in the middle-tier can work more easily with data from multiple sources.more easily with data from multiple sources.

Encourages multiple back-end serversEncourages multiple back-end servers– encourages data distributionencourages data distribution


ProblemsProblems

Much more complex:Much more complex:– network management, data integrity, network management, data integrity,

maintenance, developmentmaintenance, development

Still (partially) dependent on platformsStill (partially) dependent on platforms– e.g. the client may still be restricted to a certain e.g. the client may still be restricted to a certain

application server, but not (maybe) to any data application server, but not (maybe) to any data serverserver


ExamplesExamples A ‘real’ ATM networkA ‘real’ ATM network

– the ATM machines are the clients (as before)the ATM machines are the clients (as before)

– the middle-tier servers provide certain processingthe middle-tier servers provide certain processing checking balances, money transfer requestschecking balances, money transfer requests directing queries to the relevant back-end serverdirecting queries to the relevant back-end server

– back-end server(s)back-end server(s) specialized by account typespecialized by account type very robust concurrency control, transaction processingvery robust concurrency control, transaction processing

continued


Many Web applications are 3-tier:Many Web applications are 3-tier:– the Web browser is the client softwarethe Web browser is the client software

– the embedded components in Web pages (e.g. the embedded components in Web pages (e.g. Java applets) come from the middle-tierJava applets) come from the middle-tier

– the back-end server contains the the back-end server contains the database/groupwaredatabase/groupware


2.5. Locating the Business Logic2.5. Locating the Business Logic Fig.2.12, p.60 Fig.2.12, p.60

Presentation Logic

Business LogicDBMS

Database

Database Logic

ClientServer

Business Logic


Three ways of distributing the ‘business Three ways of distributing the ‘business logic’ (i.e. the program code):logic’ (i.e. the program code):– locate it entirely on the client (‘fat’ client)locate it entirely on the client (‘fat’ client)– locate it entirely on the server (‘fat’ server)locate it entirely on the server (‘fat’ server)– split it between the client and serversplit it between the client and server


Fat Server AdvantagesFat Server Advantages

Easier to update the application logic since Easier to update the application logic since clients not involved.clients not involved.

Data is better hidden from clients.Data is better hidden from clients.

Easier to manage and debug since data and Easier to manage and debug since data and code is centrally located.code is centrally located.

Reduces bandwidth problems since data Reduces bandwidth problems since data processing stays on the server.processing stays on the server.

continued


Better for mission-critical applications Better for mission-critical applications when when fault-tolerance and stability are important.fault-tolerance and stability are important.

Encourages client simplicity and Encourages client simplicity and compatibility since the server must be able compatibility since the server must be able to work with many types of client.to work with many types of client.– e.g. serve Web pages e.g. serve Web pages without ActiveX ActiveX


Fat Client AdvantagesFat Client Advantages

The server is unaffected when updates The server is unaffected when updates are done to the client’s application logicare done to the client’s application logic– the server will be more stablethe server will be more stable

Easier to programEasier to program– less networkingless networking– more direct access to client platform more direct access to client platform

features, such as GUIfeatures, such as GUI


Database

2.6. Locating the Data 2.6. Locating the Data Fig.2.14,p.69Fig.2.14,p.69

Presentation Logic

Business LogicDBMS

Database

Database LogicClient

Multiple Servers

DBMS

Database Logic

Database


IssuesIssues

Dividing up the data.Dividing up the data.

Transparency of the distribution.Transparency of the distribution.

Data integrity / synchronisation / Data integrity / synchronisation / consistency.consistency.

Data administration / management.Data administration / management.


Transaction ProcessingTransaction Processing

A transaction is a sequence of actions which A transaction is a sequence of actions which takes a system (usually a database) from takes a system (usually a database) from one consistent state to another.one consistent state to another.– e.g. change a customer’s recorde.g. change a customer’s record

A transaction should possess the “A transaction should possess the “ACIDACID” ” properties:properties:– AAtomicity, tomicity, CConsistency, onsistency, IIsolation, solation, DDurabilityurability

continued


Recovery and concurrency mechanisms are Recovery and concurrency mechanisms are necessary, typically implemented in a necessary, typically implemented in a Transaction Processing Management (TPM) Transaction Processing Management (TPM) system.system.

TPMs become very complex when data is TPMs become very complex when data is distributed.distributed.– ACID must be distributed as wellACID must be distributed as well


2.7. Multi-tier Model 2.7. Multi-tier Model Fig.2.9,p.53Fig.2.9,p.53

Middleware

Physical Network


Common New FeaturesCommon New Features

Asynchronous connectivityAsynchronous connectivity– e.g. asynchronous RPCse.g. asynchronous RPCs

Data distribution using replicationData distribution using replication

Name/directory services for resource location Name/directory services for resource location independenceindependence

More complex data typesMore complex data typescontinued


More complex analysisMore complex analysis– e.g. data mining, network characteristicse.g. data mining, network characteristics

Authentication servicesAuthentication services– you must 'prove' who you are to the systemyou must 'prove' who you are to the system

Distributed file system(s)Distributed file system(s)

Time servicesTime services


ExamplesExamples

Domain-specific:Domain-specific:– ODBC, SQL, Oracle GlueODBC, SQL, Oracle Glue

Groupware middleware:Groupware middleware:– Microsoft Exchange, Lotus NotesMicrosoft Exchange, Lotus Notes

Object middleware:Object middleware:– CORBA, DCOM (more on these in part 2)CORBA, DCOM (more on these in part 2)


Mult-tier Web ApplicationsMult-tier Web Applications

The Web browser is the client software on the The Web browser is the client software on the first tier.first tier.

Web page components come from the second Web page components come from the second tier.tier.

The third tier is a database front-end for a series The third tier is a database front-end for a series of fourth tier heterogeneous databasesof fourth tier heterogeneous databases– the third tier database may have been constructed the third tier database may have been constructed

with data mining techniqueswith data mining techniques


3. Client/Server Summary3. Client/Server Summary

3.1. Recurring Issues3.1. Recurring Issues

3.2. Advantages of Client/Server3.2. Advantages of Client/Server

3.3. Disadvantages of Client/Server3.3. Disadvantages of Client/Server


3.1. Recurring Issues3.1. Recurring Issues

LAN, WAN, Internet scalingLAN, WAN, Internet scaling Data distribution/replicationData distribution/replication Distributed processingDistributed processing System management/maintenanceSystem management/maintenance Choice of middlewareChoice of middleware Standards / open systemsStandards / open systems


What’s an Open System?What’s an Open System?

An open system:An open system:– complies with industry standards for complies with industry standards for

programming, communication, networking, programming, communication, networking, presentation, etc.presentation, etc.

– is designed with portability/interoperability in is designed with portability/interoperability in mindmind

– is scaleableis scaleable


3.2. Advantages of Client/Server3.2. Advantages of Client/Server

Mainframe functionality can be made widely Mainframe functionality can be made widely availableavailable– cost benefitscost benefits

Processing and data are localised on the serverProcessing and data are localised on the server– reduces network traffic, response time,reduces network traffic, response time,

bandwidth requirementsbandwidth requirements

Business logic can be distributed (in 3-tier model)Business logic can be distributed (in 3-tier model)– reuse, portabilityreuse, portability

continued


Encourages open systemsEncourages open systems

Present-day systems are too large and Present-day systems are too large and involve too many users to be located on one involve too many users to be located on one machine.machine.


3.3. Disadvantages of Client/Server3.3. Disadvantages of Client/Server

The server becomes a bottleneckThe server becomes a bottleneck

Distributed applications are much more complex Distributed applications are much more complex than non-distributed onesthan non-distributed ones– i.e. in development, run time, maintenance, upgradesi.e. in development, run time, maintenance, upgrades

Requires a shift in business practisesRequires a shift in business practises– local, simple data local, simple data distributed, open, complex datadistributed, open, complex data


4. Standards Organizations4. Standards Organizations

POSIXPOSIX: Portable Operating Systems Interface: Portable Operating Systems Interface– family of standards developed by IEEEfamily of standards developed by IEEE

– POSIX working groups have standardised C, UNIX POSIX working groups have standardised C, UNIX shell, networking API for sockets, real-time, threads, shell, networking API for sockets, real-time, threads, etc.etc.

continued


The The Open GroupOpen Group– consolidation of X/Open company and Open consolidation of X/Open company and Open

Software Foundation (OSF)Software Foundation (OSF)

– consortium of vendors and consortium of vendors and industrial/government usersindustrial/government users

– developed API for UNIX, including XTI developed API for UNIX, including XTI (X/Open Transport Interface) network protocol(X/Open Transport Interface) network protocol

continued


Internet Engineering Task Force (Internet Engineering Task Force (IETFIETF))– community of network designers, operators, community of network designers, operators,

vendors, and researchersvendors, and researchers– concerned with the evolution of the Internet concerned with the evolution of the Internet

architecturearchitecture– e.g. sockets API for IP version 6 (IPv6)e.g. sockets API for IP version 6 (IPv6)

128-bit addresses, simpler header, multicasting, 128-bit addresses, simpler header, multicasting, authentication and securityauthentication and security

continued


ISOISO (International Organization for Standards) (International Organization for Standards)– main standards organizationmain standards organization– e.g. communications protocol: the Open Systems e.g. communications protocol: the Open Systems

Interconnection (OSI) reference modelInterconnection (OSI) reference model

OMGOMG (Object Management Group) (Object Management Group)– object-oriented standardsobject-oriented standards– e.g. CORBAe.g. CORBA


5. DCE5. DCE

The Distributed Computing EnvironmentThe Distributed Computing Environment– developed by the OSF (late 1980’s)developed by the OSF (late 1980’s)– openopen– very extensive features (and complex)very extensive features (and complex)

Basic programming feature is the Remote Basic programming feature is the Remote Procedure Call (Procedure Call (RPCRPC))– other features: name/directory services, other features: name/directory services,

authentication, security, data pipesauthentication, security, data pipes


DCE RPCDCE RPC

ClientApplicationcode

RPC

Stub Code

RPC runtime library

Server

Network

Input Output

Applicationcode

RemoteProcedure

Stub Code

RPC runtime library


Using RPCUsing RPC

The data structures to be passed as arguments of The data structures to be passed as arguments of the RPC call are used by a compiler to generate the RPC call are used by a compiler to generate the client and server stub code.the client and server stub code.

The data structures are written in a special high-The data structures are written in a special high-level language (simplified C types), which are level language (simplified C types), which are easily converted to network packets.easily converted to network packets.


DCE Client/Server Model (v.2)DCE Client/Server Model (v.2)

DistributedDCE Application

Executive

OS and NetworkInterface

DistributedDCE Application

Services Data Sharing

OS and NetworkInterface

Client Server

Network

Fig 1.10, p.27


DCE ArchitectureDCE ArchitectureApplications

PC Integration Dist. Services

MgmtSecurity DFS: Dist. File Services

Naming Services

Time Services

Future CoreServices

RPCs

Presentation Services

Threads Services

OS

Network Services

Fig. 1.9, p.23


5.1. Distributed File System (DFS)5.1. Distributed File System (DFS)

Problem: how to allow a user on one system to Problem: how to allow a user on one system to modify data stored on a file in another system?modify data stored on a file in another system?– User = clientUser = client– Distant data is managed by a file serverDistant data is managed by a file server


IssuesIssues

Access security and protectionAccess security and protection– user authentication (Kerberos) + user controluser authentication (Kerberos) + user control

Data reliabilityData reliability Data availabilityData availability PerformancePerformance ManagementManagement


5.3. More Details5.3. More Details

There are several books from O’Reilly on different There are several books from O’Reilly on different aspects of DCE:aspects of DCE:– "Understanding DCE""Understanding DCE"

– "Guide to Writing DCE Applications""Guide to Writing DCE Applications"

– "Distributed Applications Across DCE "Distributed Applications Across DCE and Windows NT"and Windows NT"

– "Introduction to OSF DCE""Introduction to OSF DCE" (in PSU library)(in PSU library)

We will look at simple RPC on UNIX later in the We will look at simple RPC on UNIX later in the course.course.

Documents

Client/Server Distributed Systems