ClientServer Software Engineering

1
Client-Server Software Engineering

• architectural styles
• specialized tools

2
History

• Batch - cards and printers
• On-line
• character oriented video display
• dumb ASCII terminal / green screen
• PCs as terminals - need to be integrated
• PC is client / other computer is server
• two-tier
• three-tier

3
Client/server usually means business

• Business systems
• Complex and arbitrary business rules
• Persistent data (RDBMS)
• Client/server
• GUI
• Distributed systems

4
Two-tiered client-server
Clients - implement GUI and maybe business rules
SQL
Server - stores data and maybe business rules
5
Three-tiered Client-Server
Clients - implement GUI
middleware
Middle tier - business object server
SQL
Database server - stores data
6
Variations on Client-Server

• Fat client
• Fat server
• Thin client
• Two-tiered client/server
• Three-tiered client/server

7
Two or three tiers?

• Advantages of three tiered
• smaller client programs
• can change business rules without changing client
  programs
• can change database without changing client
  programs

8
Two or three tiers?

• Advantages of two tiered
• More traditional design
• Less object design
• More database design
• Let DBMS handle concurrency
• Less tools required
• Connection to RDBMS

9
Middleware

• Software that lets clients and servers
  communicate
• Lives on both clients and servers. It is in the
  middle.
• CORBA, DCOM, JavaBeans

10
The Big Lie

• Object-oriented programming is well suited to
  building client-server systems because sending a
  message to an object is just like sending a
  message to a client.
• Take good object design, partition objects
  between clients and servers, and iterate until it
  is fast enough.

11
The Cruel Facts

• Client/server communication is much slower than
  object communication in an OO language
• Server must deal with concurrency
• Clients must deal with failure of server
• Good OO architecture is usually not a good
  client/server architecture

12
Concurrency

• Airline reservation system
• get list of available seats
• choose one
• reserve it
• Maybe someone took it while you were waiting
• Reserve it can fail.

13
Concurrency in DBMS

• Database transactions ensure atomicity
• Begin transaction
• search database
• change database
• End transaction
• If transaction fails, start again

14
Concurrency in object server

• Only handle one request at a time
• little concurrency
• requests on different data should be OK
• Store all shared data in database
• retry transaction until it works
• Implement your own transaction system

15
Architecture for 3-tier

• Servers are big objects.
• Not created nor destroyed
• Lots of operations
• Interface grows, operations dont go away
• Clients use value objects to communicate with
  servers.
• Immutable
• strings, numbers, SS, location, ...

16
COM

• A program accesses a COM object using one or more
  interfaces
• Interface is a set of functions.
• First function is always QueryInterface(i)
• returns interface i if the object supports it,
  nil if it does not.
• program does not know implementation of COM
  object, just its interfaces

17
DCOM

• objects on another machine (remote objects) have
  special implementations of interfaces
• interface function becomes a remote procedure
  call
• remote objects are called just like local objects

18
MTS

• COM objects can be stored on an MTS server
• Implements transactions
• Transactions are initialized and committed
• When a transaction is committed,
• all objects are notified
• all objects are deleted from memory
• interfaces changed to return errors

19
MTS
MTS
20
MTS

• High volume of transactions
• Handles concurrency
• Distribution, language independent
• Can be accessed by VB clients, web server, other
  COM programs
• Not pure OO, difficult to use

21
Database

• Usually a RDBMS
• industry standard SQL
• many vendors, skilled developers, experience
• widely used in most companies
• must map to object system
• Sometimes an OODBMS
• less standards
• less is known about them
• eliminates mapping problem

22
Database

• Database is central to most client/server systems
• Data design is the heart of the system design
• Design the data early

23
Process

• Standard roles
• business analyst
• database analyst/administrator
• GUI developer
• programmer
• architect
• tester

24
Summary

• Client-server has
• several architectural styles
• specialized tools
• specialized techniques
• changes/restrictions on development process

25
Next time

• Read http//jerry.cs.uiuc.edu/plop/plop97/
  Proceedings/renzel.pdf