I hear and I forget,

I see and I remember,

I do and I understand.

-Chinese Proverb

Tip

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Distributed Systems

Fall Semester

Raihan Ur Rasool

Session 01: Characteristics of Distributed

Systems

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Information

• Office #: +51-9280439 (129)

• Email: dr.raihan@niit.edu.pk

• Email: vitalfact @ {gmail, hotmail}.com• Group:

http://groups.yahoo.com/group/bit_8

• Group Email: bit_8@yahoogroups.com

• TA: Shahid Mehmood  – shahid.mahmood@niit.edu.pk

Overview• This course focuses on the issues fundamental to the

understanding of distributed systems. • Text and reference books

– Distributed Systems - Concept and Design, G. Coulouris, J, Dollimore, and T. Kindberg, Addison-Wesley, 4th Edition, 2005.

– Distributed Systems – Principles and Paradigms, A. Tanenbaum and M. van Steen, Prentice Hall, 2002.

• Course Webpage: http://www.niit.edu.pk/~dr.raihan/BIT8_DC.html• Check Regularly

What we’ll learn

• Characteristics of Distributed Systems• Distributed System Models• Interprocess Communication• Grid Computing• Distributed Objects and Remote Invocation• Peer to Peer Systems• Web Services• Service Oriented Architecture• Socket Programming, MPI, RMI, MPICHG2

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Tentative Grading

• Midterm : 25 %

• Quizzes : 5%

• Assignment & Lab : 10 %

• Concept report/ Project : 10 %

• Final Exam : 45 %

• Open : 5%

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Class Information

• Class groups: 3 students/ group• For CR: Send me the list by 28th Nov [5:00 PM]

• ** Five minute rule **

• Two 50 mins Lectures (Tue)• Two Hrs Lab (Thur)• Paper Discussion Session: – One 20 mins paper-discussion sessions (Thur)

Academic Integrity

• Permitted Collaboration: – encouraged and allowed at all times– Examples– Discussion of material covered during lecture / handouts– Discussion of the requirements of an assignment– Discussion of the use of tools or development environments– Discussion of general approaches to solving problems– Discussion of general techniques of coding or debugging– Discussion between a student & a TA or instructor

Academic Integrity• Collaboration Requiring Citation:

– must be able to explain the solution – properly credit contribution, just like citing a reference in a

paper

• Examples– Discussing the “key” to a problem – Discussing the design of a programming project– Assistance in debugging code – Sharing advice for testing – Research from alternative sources

Academic Integrity

• Non-permitted Collaboration:– submissions must represent original, independent work

• Examples– Copying solutions from others– Using work from past quarters – Studying another student's solution – Debugging code for someone else – Cut & paste of solution/code from Internet

Overview • Introduction and History• Examples of distributed systems

– The internet– Interanets– Moble and ubiquitous computing

• Resource sharing and the web– The world wide web

• HTML

• URL

• HTTP

• Dynamic Pages

• Web Services

• Discussion of the web

• Challenges– Heterogeneity – Openness– Security– Scalability– Failure Handling– Concurrency– Transparency

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Our Aim• explain the characteristics of networked computer

Lets start our journey !

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History

• Uni-processory monolithic computing– Tightly coupled computers

• RFC 1– ARPANET (four node network) –1972

• Increase in the network bandwidth in 1980

• Distributed Systems– Loosely coupled computers– Network service (FTP, email) , Client Server– Network Application

• Resource Sharing

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Definition of Distributed System

• A distributed system is:– Components located at networked

computers communicate and coordinate their actions by passing messages

• A distributed system is:– Collection of independent computers that

appear to the users of the system as a single computer [Tanenbaum]

Forms of computing

• Monolithic Computing– Single-user monolithic computing (word, excel)– Multiuser monolithic computing

• Distributed Computing– WWW, IE and web server interaction

• Parallel Computing– CPUs can execute deferent portions without

interfering

• Cooperative Computing– Sharing surplus CPU cycles –SETI@Home

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Definition of Distributed System

• "a collection of autonomous computers linked by a computer network with distributed system software"

• "a collection of processors interconnected by a communication network in which each processor has its own local memory and other peripherals and the communication between any two processors of the system takes place by message passing over the communication network"

• "one in which hardware or software components located at networked computers communicate and coordinate their actions only by passing messages"

Definition(s) of a Distributed System• A distributed system is:

• one in which hardware or software components at networked computers communicate and coordinate their actions only by passing messages [CDK]

• collection of independent computers that appears to its users as a single coherent system. [DS]

• several computers doing something together; multiple components, interconnections and shared state [Schroeder]

• fundamental properties are fault tolerance and parallelism [Mullender]

• one that stops you from getting any work done when a machine you’ve never heard of crashes [Lamport]

• A set of communicating autonomous entities functioning together to achieve a common goal [Cheriton]

• Collection of computer nodes connected by a network running software that makes it function as one system.

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Definition of Distributed System

• In other words, to the user, a DS looks like a single system – a virtual uniprocessor

• The user doesn't know (or care) – - where (on what machine) files are located – - where a job is executed

• A Distributed System is made of several computers which – have no shared memory – have no shared clock – communicate with each other via messages – have their own operating systems

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Advantages

• Why distributed systems– Resource sharing– Communication– Inherent distribution– Speed

• 10000 CPU running at speed of 50 MIPS • 500,000 MIPS

– Economy– Incremental growth– Reliability

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Characteristics of DS

• Concurrency– Distributed systems are concurrent systems

• Every software or hardware component is autonomous– Process

• Concurrent tasks

– Synchronization and coordination by message passing

• Lack of global clock– Coordination and time

• Independent failure– Failures of individual processes may remain undetected

– Each component can individually fail, leaving the others still running

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Examples of Distributed Systems1. The internet

– Heterogeneous network of computers and applications

– Implemented through Internet protocol

intranet

ISP

desktop computer:

backbone

satellite link

%

server:

network link:

%

%

%

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Examples of Distributed Systems 2. Distributed multimedia

•Often use the Internet infrastructure

•Characteristics

•Heterogeneous data sources that need to be synchronized in real time

•Video, Audio, Text

•Often: distribution services

•Examples

•Tele teaching, Video conferencing

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Examples of Distributed Systems3. Intranet

•Locally administered network

•Usually proprietary

•Interfaces with the Internet via router

•Firewall

the rest of

email server

Web server

Desktopcomputers

File server

router/firewall

print and other servers

other servers

print

Local areanetwork

email server

the Internet

Issues- File services need to share data-Firewall -Legitimate access

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Examples of Distributed Systems (1) 4. Mobile and Ubiquitous computing

•Resource Sharing while on the move –Nomadic computing

•Access home intranet

•Primary concerns:

•resource discovery

•efficient use of limited bandwidth

•security (privacy)

Laptop

Mobile

PrinterCamera

Internet

Host intranet Home intranetWAP

Wireless LAN

phone

gateway

Host site

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Resource sharing and the Web Resource sharing pattern

•Sharing of the higher level resources

•Shared database or set of WebPages

•Search engine (Google search() ) Service

•Software that manages a collection of resources and presents them to users and applications via a well-defined interface•E-payment service•File service (methods: read, write, delete)

•Server•Running program (a process that provide one or more services)

•Remote invocation•Client’s request to server.

Overview • Introduction and History• Examples of distributed systems

– The internet– Intranets– Mobile and ubiquitous computing

• Resource sharing and the web

– The world wide web• HTML• URL• HTTP• Dynamic Pages• Web Services• Discussion of the web

• Challenges– Heterogeneity – Openness– Security– Scalability– Failure Handling– Concurrency– Transparency

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Challenges

• A number of challenges arises:

• Heterogeneity

• Openness

• Security

• Scalability

• Failure handling

• Concurrency of components

• Transparency

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Challenges• Heterogeneity

– networks– computer hardware– Operating systems– Programming languages

• Data types representation/ byte order• Middleware

– To overcome differences in network, OS and languages

• RMI, CORBA

• Heterogeneity and mobile code• Applets

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Definition of Distributed System

A distributed system organized as middleware.Note that the middleware layer extends over multiple machines.

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Challenges

• Openness– Ensures extensibility and maintainability of the

system• Standard interfaces & their publication• Addition of new resources

– RFC specification for internet protocols. – key interfaces are published

• Benefits of ODS– uniform communication mechanisms and access

to shared resources– Construction

• Heterogeneous components

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Challenges

• Security– Confidentiality – integrity– availability– Example

• A doctor might request access to hospital patient data

• In electronic commerce and banking, users send their credit card numbers across the internet

• Current security challenges– Denial of service attack– Security of mobile code (Email attachment)

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Challenges

• Scalability– Controlling the cost of the physical resources

• Does the system remain effective given the expected growth

– Controlling the performance loss• www.amzon.com is more than one computer

– Preventing the software resources running out• IP addressees :32 bits to 128• Avoiding performance bottleneck

– Decentralization of data/information

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Challenges

• Failure handling– Detecting failure

• Checksum can be used to detect corrupted data• System crash

– Masking• Retransmission• Redundant storage

– Tolerating failure• Exception handling (timeout when waiting for

web source)

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Challenges• Failure handling

– Recovery from failure• Roll back

– Redundancy (services can be made tolerate failures)

• Redundant routes in network• Replication of DNS tables in multiple domain name

servers• Database replication

– Availability • Measure of the proportion of the time a server is available• User may move to another computer if the one they were

using fails

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Challenges

• Concurrency• Data structure for bids is accessed very frequently

near the deadline– One client request a time –Throughput ?– Resources can be encapsulated in an object and

that invocations are executed in concurrent threads– Avoidance of deadlock

• Transparency– Concealing the heterogeneous and distributed

nature of the system so that it appears to the user like one system

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Challenges :Transparency• 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.

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Challenges :Transparency

• 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.

• Scaling transparency: allows the system and applications to expand in scale without change to the system structure or the application algorithms.

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Reading Assignment1

Interview of Fred B. Schneider on “Distributed Computing”

'It's all distributed computing to me... http://www.computerworld.com/hardwaretopics/hardware/gridcomputing/story/0,10801,104937,00.html

2 From the text book answer

1.7

1.8

1.9

More yet to Come !

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Useful Research Papers

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Papers

• Eric A. Brewer. Lessons from Giant-Scale Services. IEEE Internet Computing, 5(4): pp.46-55

• A. D. Birrell and B. J. Nelson. Implementing remote procedure calls. ACM Transactions on Computer Systems 2(1):39-59, February 1984

• P.V. Mockapetris and K.J. Dunlap. Development of the Domain Name System. Proceedings of ACM SIGCOMM 1988

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Papers

• Leslie Lamport. Time, Clocks, and the Ordering of Events in a Distributed System. Communications of the ACM, 21(7):558-565, July 1978

• Yasushi Saito, Brian Bershad, and Henry Levy. Manageability, Availability and Performance in Porcupine: A Highly Scalable Cluster-Based Mail Service. Proc. of the 17th ACM Symp. on Operating Systems Principles, December 1999

• D. B. Terry, et al. Managing Update Conflicts in Bayou, a Weakly Connected Replicated Storage System. Proceedings of the Fifteenth ACM Symposium on Operating Systems Principles, 1995

• John Ousterhout. The Role of Distributed State. CMU Computer Science: A 25th Anniversary Commemorative, ACM Press Anthology Series, R. Rashid (Ed.), July 1991

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Papers

• George Candea et al. Microreboot -- A Technique for Cheap Recovery. Proc. 6th Symposium on Operating Systems Design and Implementation (OSDI), San Francisco, CA, December 2004

• Stuart Staniford, Vern Paxson, and Nicholas Weaver. How to Own the Internet in Your Spare Time. Proceedings of the 2002 USENIX Security Symposium, San Francisco, CA, August 2002.

• Butler W. Lampson. Hints for computer system design.  Proceedings of the Ninth ACM Symposium on Operating Systems Principles (SOSP).