Lecture 1-1 2002, M. T. Harandi, J. Hou, and I. Gupta (modified Y. Hu) ECE 428 Distributed Systems Yih-Chun Hu August 25, 2005

2002, M. T. Harandi, J. Hou, and I. Gupta (modified Y. Hu) Lecture 1-1Lecture 1-1

ECE 428Distributed

Systems

ECE 428Distributed

Systems

Yih-Chun Hu

August 25, 2005

2002, M. T. Harandi, J. Hou, and I. Gupta (modified Y. Hu)

Lecture 1-2Lecture 1-2

The “Constitution” of Distributed Systems

(and the First Amendment)





Our First Aim TodayOur First Aim Today

To Define the Term Distributed System



Can you name some examples of Operating Systems?Can you name some examples of Operating Systems?



Can you name some examples of Operating Systems?Can you name some examples of Operating Systems?

…

Linux WinXP Unix FreeBSD Mac

2K Aegis Scout Hydra Mach SPIN

OS/2 Express Flux Hope Spring

AntaresOS EOS LOS SQOS LittleOS TINOS

PalmOS WinCE TinyOS

…



What is an Operating System?What is an Operating System?



What is an Operating System?What is an Operating System?

• User interface to hardware (device driver)

• Provides abstractions (processes, file system)

• Resource manager (scheduler)

• Means of communication (networking)

• …



FOLDOC definitionFOLDOC definition

• The low-level software which handles the interface to peripheral hardware, schedules tasks, allocates storage, and presents a default interface to the user when no application program is running.

• The OS may be split into a kernel which is always present and various system programs which use facilities provided by the kernel to perform higher-level house-keeping tasks, often acting as servers in a client-server relationship.

• Some would include a graphical user interface and window system as part of the OS, others would not. The operating system loader, BIOS, or other firmware required at boot time or when installing the operating system would generally not be considered part of the operating system, though this distinction is unclear in the case of a roamable operating system such as RISC OS.

• The facilities an operating system provides and its general design philosophy exert an extremely strong influence on programming style and on the technical cultures that grow up around the machines on which it runs.



Can you name some examples of Distributed Systems?






• Client-server (e.g., NFS)• The Internet• The Web• An ad-hoc network• A sensor network• DNS• Kazaa (peer to peer overlays)• Titan Lander-Orbiter-Earth Station• (Society?)• (Food Chain?)



What is a Distributed System?What is a Distributed System?



FOLDOC definitionFOLDOC definition

A collection of (probably heterogeneous) automata whose distribution is transparent to the user so that the system appears as one local machine. This is in contrast to a network, where the user is aware that there are several machines, and their location, storage replication, load balancing and functionality is not transparent. Distributed systems usually use some kind of client-server organization.



Textbook definitionsTextbook definitions

• A distributed system is a collection of independent computers that appear to the users of the system as a single computer

[Andrew Tanenbaum]

• A distributed system is several computers doing something together. Thus, a distributed system has three primary characteristics: multiple computers, interconnections, and shared state

[Michael Schroeder]



UnsatisfactoryUnsatisfactory

• Why are these definitions short?

• Why do these definitions look inadequate to us?

• Because we are interested in the insides of a distributed system– design and implementation

– maintenance

– study

– algorithmics



“I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.”

[Potter Stewart, Associate Justice, US Supreme Court (talking about his interpretation of a technical term laid down in the law, case Jacobellis versus Ohio 1964) ]



(A) Plants and Animals interacting in the Food Chain

Which is a Distributed System – (A) or (B)?Which is a Distributed System – (A) or (B)?

(A)



(B) The Internet (Internet Mapping Project, color coded by ISPs)

(B)



A working definition for usA working definition for us

A distributed system is a collection of entities, each of which is autonomous, programmable, asynchronous and failure-prone, and communicating through an unreliable communication medium.

• Our interest in distributed systems involves – design and implementation, maintenance, study, algorithmics

• Entity=a process on a device (PC, PDA)

• Communication Medium=Wired or wireless network



A range of interesting problems for Distributed System designersA range of interesting problems for Distributed System designers

• • • Basic Concepts [Asynchrony,Consensus,…]• Routing [IP,BGP]• Large-scale Systems [The Grid,Gnutella,Kazaa]• Distributed File Systems [NFS,AFS]• Protocols, e.g., multicast [IP multicast, SRM, RMTP]• Coordination[SETI@Home,Multiplayer online games]• Storage and Databases• Security• •



Distributed Systems Design GoalsDistributed Systems Design Goals• Common Goals:

– Heterogeneity – can the system handle different types of PCs and devices?

– Robustness – is the system resilient to crashes and failures?

– Availability – are data, services always there?

– Transparency – can the system hide its internal workings from the users?

– Concurrency – can the server handle multiple clients simultaneously?

– Efficiency – is it fast enough?

– Scalability – can it handle 100 million nodes?

– Security – can the system withstand hacker attacks?

– Openness – is the system extensible?



intranet

ISP

desktop:

backbone

satellite link

server:

network link:

Distributed System Example -- the Internet Distributed System Example -- the Internet



The InternetThe Internet

• A vast interconnected collection of computer networks of many types.

• Intranets – subnetworks operated by companies and organizations.

• ISPs – companies that provide modem links and other types of connections to users.

• Intranets are linked by backbones – network links of large bandwidth, such as satellite connections, fiber optic cables, and other high-bandwidth circuits.



A Typical Intranet A Typical Intranet

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



IntranetsIntranets

• Composed of several local area networks (LANs) linked by backbone connections.

• Connected to the Internet via a router/multiple routers.

• A firewall is used to protect an intranet by preventing unauthorized message leaving/entering, and is implemented by filtering incoming and outgoing messages.



Internet Apps: Their Protocols and Transport ProtocolsInternet Apps: Their Protocols and Transport Protocols

Application

e-mailremote terminal access

Web file transfer

streaming multimedia

remote file serverInternet telephony

Applicationlayer protocol

smtp [RFC 821]telnet [RFC 854]http [RFC 2068]ftp [RFC 959]rtsp [RFC 2326],proprietaryNFSH.323, SIP [RFC 2543], …

Underlyingtransport protocol

TCPTCPTCPTCPTCP or UDP

TCP or UDPtypically UDP



WWW: the HTTP ProtocolWWW: the HTTP Protocol

HTTP: hypertext transfer protocol

• WWW’s application layer protocol

• client/server model– client: browser that requests,

receives, and “displays” WWW objects

– server: WWW server sends objects in response to requests

• http1.0: RFC 1945

• http1.1: RFC 2068

PC runningExplorer

Server Runningcnn.com

Webserver

Mac runningNavigator

http request

http re

quest

http response

http re

sponse



The HTTP Protocol: MoreThe HTTP Protocol: More

http: TCP transport service:

• client initiates a TCP connection (creates socket) to server, port 80

• server accepts the TCP connection from client

• http messages (application-layer protocol messages) exchanged between browser (http client) and WWW server (http server)

• TCP connection closed

http is “stateless”• server maintains no

information about past client requests

Protocols that maintain “state” are complex!

• past history (state) must be maintained

• if server/client crashes, their views of “state” may be inconsistent, and hence must be reconciled.

aside



HTTP ExampleHTTP ExampleSuppose user enters URL www.cs.uiuc.edu/

1a. http client initiates a TCP connection to http server (process) at www.cs.uiuc.edu. Port 80 is default for http server.

2. http client sends a http request message (containing URL) into TCP connection socket

1b. http server at host www.cs.uiuc.edu waiting for a TCP connection at port 80. “accepts” connection, notifying client

3. http server receives request messages, forms a response message containing requested object (index.html), sends message into socket

time

(contains text, references to 10

jpeg images)



HTTP Example (cont.)HTTP Example (cont.)

• non-persistent connection: one object in each TCP connection– some browsers create multiple TCP connections

simultaneously - one per object

• persistent connection: multiple objects transferred within one TCP connection

5. http client receives a response message containing html file, displays html, Parses html file, finds 10 referenced jpeg objects

6. Steps 1-5 repeated for each of 10 jpeg objects

4. http server closes the TCP connection.

time



Trying Out HTTP (Client Side) for YourselfTrying Out HTTP (Client Side) for Yourself1. Telnet to your favorite WWW server:

Opens TCP connection to port 80(default http server port) at www.eurecom.fr.Anything typed in sent to port 80 at www.eurecom.fr

telnet www.eurecom.fr 80

2. Type in a GET http request:

GET /~ross/index.html HTTP/1.0 By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to http server

3. Look at response message sent by http server!



Does our Working Definition work for the http Web?Does our Working Definition work for the http Web?

A distributed system is a collection of entities, each of which is autonomous, programmable, asynchronous and failure-prone, and communicating through an unreliable communication medium.

• Our interest in distributed systems involves – design and implementation, maintenance, study, algorithmics

• Entity=a process on a device (PC, PDA)

• Communication Medium=Wired or wireless network



“Important” Distributed Systems Issues“Important” Distributed Systems Issues

• No global clock; no single global notion of the correct time (asynchrony)

• Unpredictable failures of components: lack of response may be due to either failure of a network component, network path or a computer crash (failure-prone, unreliable)

• Highly variable bandwidth

• Possibly large and variable latency

• Large numbers of hosts



“Important” Issues“Important” Issues

• If you’re already complaining that the list of topics we’ve discussed so far has been perplexing…

– You’re right!

– It was meant to be (perplexing)

• The Goal for the Rest of the Course: see enough examples and learn enough concepts so these topics and issues will make sense



“Concepts”?“Concepts”?

• Which of the following inventions do you think is the most important?1. The PDA2. The PC3. The transistor

• Which of the following inventions do you think is the most important?1. Email2. The Web3. TCP/IP

“What lies beneath?” Concepts!



How will you Learn?How will you Learn?

Take a look at handout “Course Information and Schedule”

• Text: Coulouris, Dollimore, and Kindberg

• Lectures

• Homeworks– Approx. one every two weeks

– Need to be typed, figures can be hand-drawn

• Programming assignments – Incremental, in 5-6 stages

– We will probably build a peer to peer system!

• Exams/quizzes– Midterm, and final



What assistance is available to you?What assistance is available to you?

• Lectures – lecture slides will be placed online at course website

» “Tentative” version before lecture

» “Final” version after lecture

• Homeworks – office hours to help you (without giving you the solution)

• Programming Assignments – program templates will be given to you. C (or C++) programming.

– The last assignment will be open-ended, and can turn into a research project.



If you’re still thinking, “Everything you’ve said so far is so ho-hum...”…

If you’re still thinking, “Everything you’ve said so far is so ho-hum...”…

CS425 is about enjoying distributed systems, learning a few new things, and designing some cool new systems that you can boast about to your friends (and job interviewers).

We’re here to help you achieve all these things.



For Next ClassFor Next Class

• Read sections 10.1-10.4

• Fill out and return Student InfoSheet









Additional SlidesAdditional Slides



Design Challenges: HeterogeneityDesign Challenges: Heterogeneity

• Variety and differences among:

– Networks

– Hardware and Operating Systems

– Programming Languages

– Implementations

• Middleware – a software layer that provides a programming abstraction as well as

– (i) masking the heterogeneity of the underlying networks, hardware, operating systems and programming languages.

– (ii) providing uniform computational models, e.g., remote object invocation, remote event notification.

• Virtual machine – The compiler for a particular language generates code for a virtual

machine, instead of a particular hardware code to assist execution of mobile code.



Design Challenges: OpennessDesign Challenges: Openness

• Degree to which new resource-sharing services can be added & used.

• Requires publication of interfaces for access to shared resources

• Requires uniform communication mechanism

• Conformance of each component to the published standard must be tested and verified



Design Challenges: SecurityDesign Challenges: Security

• Security has three components:

– Confidentiality (protection against disclosure to unauthorized individuals).

– Integrity (protection against alternation or corruption).

– Availability (protection against interference with the means to access the resources).

• Two security challenges:– Denial of service attacks: bombarding the service with a

large number of pointless requests.

– Mobile code security: mobile codes may be accessing local resources.



Design Challenges: ScalabilityDesign Challenges: Scalability

• A system is said to be scalable if it will remain effective when there is a significant increase in the number of resources and users:

– Controlling the cost of resources

– Controlling the performance loss

– Preventing software resources running out (e.g., IP addresses)

– Avoiding performance bottlenecks



Design Challenges: Failure HandlingDesign Challenges: Failure Handling

• Failure in DS is partial – some component fails while the rest is functional;

– Detecting failures (remote site crash or delay in message transmission?)

– Masking failures (message retransmission, file replication)

– Tolerance for failure (clients give up after a pre-determined number of attempts and take other actions)

– Failure recovery (checkpoint and rollback recovery)

– Redundancy (multipath routing, replicated database, replicated DNS)



Design Challenges: ConcurrencyDesign Challenges: Concurrency

• Is a problem when two or more users access to the same resource by two at the same time

– Each resource is encapsulated as an object and invocations are executed in concurrent threads

– Concurrency can be maintained by use of semaphores and other mutual exclusion mechanisms.



Design Challenges: TransparencyDesign Challenges: Transparency

• Concealment of the separation of components from users:

– Access transparency: local and remote resources can be accessed using identical operations.

– Location transparency: resources can be accessed without knowing their whereabouts.

– Concurrency transparency: processes can operate concurrently using shared resources without interferences.

– Failure transparency: faults can be concealed from users/applications.

– Mobility transparency: resources/users can move within a system without affecting their operations.

– Performance transparency: system can be reconfigured to improve the performance.

– Scaling transparency: system can be expanded in scale without change to the applications.



Design Challenges: TransparencyDesign Challenges: Transparency

• Distributed file system allows access transparency and location transparency.

• URLs are location transparent, but are not mobility-transparent (someone’s personal web page cannot move to a new place and still be accessed using the same URL).

• Message retransmission governed by TCP is a mechanism for providing failure transparency.

• Mobile phone is an example of mobility transparency.

Documents

Lecture 1-1 2002, M. T. Harandi, J. Hou, and I. Gupta (modified Y. Hu) ECE 428 Distributed Systems Yih-Chun Hu August 25, 2005