1. Distributed system (Lecture 02)
Dr.S.Sridhar, Director, RVCT, RVCE, Bangalore

2. Dr.S.Sridhar, Director, RVCT, RVCE, Bangalore-560059
A distributed system B A C D
Dr.S.Sridhar, Director, RVCT, RVCE, Bangalore

3. Dr.S.Sridhar, Director, RVCT, RVCE, Bangalore-560059
client-server system
Client
Client
Client
Server
Client
Dr.S.Sridhar, Director, RVCT, RVCE, Bangalore

4. Dr.S.Sridhar, Director, RVCT, RVCE, Bangalore-560059
multiple servers
Server
Server
Server
Examples: amazon, google, airline reservations, finance (e-transactions, e-banking, stock exchange, military)
Server
Server
Dr.S.Sridhar, Director, RVCT, RVCE, Bangalore

5. Why distributed systems?
What are the advantages?
distributed vs centralized?
multi-server vs client-server?

6. Why distributed systems?
What are the advantages?
distributed vs centralized?
multi-server vs client-server?
Geography
Concurrency => Speed
High-availability (if failures occur).

7. Why not distributed systems?
What are the disadvantages?
distributed vs centralized?
multi-server vs client-server?

8. Why not distributed systems?
What are the disadvantages?
distributed vs centralized?
multi-server vs client-server?
Expensive (to have redundancy)
Concurrency => Interleaving => Bugs
Failures lead to incorrectness.

9. What’s a Distributed System?
Centralized:
everything in one place
stand-alone PC or Mainframe
Distributed:
some parts remote
distributed users
distributed execution
distributed data

10. Why Distribute? No best organization
Companies constantly swing between
Centralized: focus, control, economy
Decentralized: adaptive, responsive, competitive
Why distribute?
reflect organization or application structure
empower users / producers
improve service (response / availability)
distributed load
use PC technology (economics)

11. What Should Be Distributed?
Users and User Interface
Thin client
Processing
Trim client
Data
Fat client
Will discuss tradeoffs later
Presentation
workflow
Business Objects
Database

12. Transparency in Distributed Systems
Make distributed system as easy to use and manage as a centralized system
Give a Single-System Image
Location transparency:
hide fact that object is remote
hide fact that object has moved
hide fact that object is partitioned or replicated
Name doesn’t change if object is replicated, partitioned or moved.

13. Work Distribution Spectrum
Fat
Thin
Presentation and plug-ins
Workflow manages session & invokes objects
Business objects
Database
Presentation
workflow
Business Objects
Database

14. PC Evolution to Three Tier Intelligence migrated to server
Stand-alone PC (centralized)
PC + File & print server message per I/O
PC + Database server message per SQL statement
PC + App server message per transaction
ActiveX Client, ORB ActiveX server, Xscript
IO request
reply
disk I/O
SQL
Statement
Transaction

15. The Pattern: Three Tier Computing
Presentation
Clients do presentation, gather input
Clients do some workflow (Xscript)
Clients send high-level requests to ORB (Object Request Broker)
ORB dispatches workflows and business objects -- proxies for client, orchestrate flows & queues
Server-side workflow scripts call on distributed business objects to execute task
workflow
Business
Objects
Database

16. Dr.S.Sridhar, Director, RVCT, RVCE, Bangalore-560059
The Three Tiers
Web Client
HTML
VB or Java
Script Engine
Virt Machine
VBscritpt
JavaScrpt
VB Java
plug-ins
Internet
ORB
HTTP+
DCOM
Object
server
Pool
Middleware
TP Monitor
Web Server...
DCOM (oleDB, ODBC,...)
Object & Data
server.
LU6.2
IBM
Legacy
Gateways
Dr.S.Sridhar, Director, RVCT, RVCE, Bangalore

17. Why Did Everyone Go To Three-Tier?
Manageability
Business rules must be with data
Middleware operations tools
Performance (scalability)
Server resources are precious
ORB dispatches requests to server pools
Technology & Physics
Put UI processing near user
Put shared data processing near shared data
Presentation
workflow
Business
Objects
Database