DistributedSoftware Engineering

Lecture 1Introduction

Sam MalekSWE 622, Fall 2012

George Mason University

outline

course mechanics

what is a distributed system?

challenges & advantages of

distribution

the rest of the course

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what is a distributed system?[Tanenbaum] distributed system:

A collection of independent computers that appears to its users as a single coherent system

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Three key characteristics:Multiple machines are autonomousSoftware lets users see a single systemSystem easy to expand without user noticing

what is a distributed system?[Webopedia] distributed computing:

A type of computing in which different components and objects comprising an application can be located on different computers connected to a network.

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Key requirement:set of standards that specify how objects communicate with one another(e.g. CORBA and DCOM).

what is a distributed system?[Wikipedia] distributed computing:

decentralized and parallel computing, using two or more computers communicating over a network to accomplish a common objective or task.

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Note:The types of hardware, programming languages, operating systems and other resources may vary drastically. It is similar to computer clustering with the main difference being a wide geographic dispersion of the resources.

what is a distributed system?

[you] distributed (software) system: ?

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other properties?scalability?transparency?human interaction?

assembly of (software) components that serve a

purpose

scope of distribution?

example: eCommerce

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LAN

DB client

DB server

internet

is this a DS? how is it a DS?distributed components?kinds of interaction?

app server

app client (UI)

http

example: clusters

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LAN

app server

app client (UI)

gateway

high-speed network

homogeneous machines

is this a DS? how is it a DS?distributed components?kinds of interaction?

example: timesharing

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dumb terminals

mainframe

keystrokes over wire

is this a DS? how is it a DS?distributed components?kinds of interaction?

example: parallel computing

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dumb terminals

mainframe

multiprocessor

keystrokes over wireapplications:

aerodynamics simulationsweather forecasting…number crunching

is this a DS? how is it a DS?distributed components?kinds of interaction?

example: local networks

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LAN

application

ApplicationDistributed Application

access resources, such as files,

across the network

is this a DS? how is it a DS?distributed components?kinds of interaction?

hard to develop distributed apps!

factor common interaction capabilitiesinto middleware

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LAN

raise level of abstraction(easier-to-use primitives)

example: middleware

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…

is middleware a DS? how is it a DS?distributed components?kinds of interaction?

theme of this course:

styles of interactionoffered by middleware

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…data (sharing)-orientedservice-orientedprocess oriented

…and

example: ubiquitous computing

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distributed applications?

internetLAN

wireless

radiotelephony

more powerful middleware!

outline

course mechanics

what is a distributed system?

challenges & advantages of distribution

replication

hiding, aka “transparency”

scalability

the rest of the course

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challenges of DS

latency of communicationcoordination

shared resources and mutual exclusion ordering, deadlock and live-locktiming

adaptation to changefailures, soft faults, and optimizationservice discovery and configurationheterogeneity and third-party softwarescalability and evolution

security and privacytrust on machines, software, communications & other users

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advantages of DS

latency of communication processing capacitycoordination

shared resources and mutual exclusion ordering, deadlock and live-locktiming

adaptation to change fault tolerant, evolving, scalablefailures, soft faults, and optimizationservice discovery and configurationheterogeneity and third-party softwarescalability and evolution

security and privacy explicit control, preferencestrust on machines, software, communications & other users

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example: replication helps with failures

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advantages of DS

server appfailure rate: F

F = probability server fails

what is the probability that all servers fail?how many replicas needed to assure failure rate less than goal G?

challenges of DSexample: replication has downsides

buy more hardwareadministration costssoftware upgradesload balancing

performance overheadmore complex software

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challenges of DSexample: hiding is a technique to reduce complexity

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[Tanenbaum] “transparency” goals:

challenges of DSexample: hiding is a technique to reduce complexity

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hiding consists of:

(1) push handling

complexity to a lower level

Application

Middleware

Network OS

OS Kernel

Network

(2) implementing a one-size-fits-all solution

challenges of DSexample: hiding has severe limitations

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conversion of complex formats, e.g. medialatency vs. fidelity of access, e.g. DBs, Web…

trusted hosts (security and privacy)different performancedifferent capabilitiesdifferent network access (bandwidth & latency)

select server based on QoS (e.g., mirrors)

cannot hide sharing of resources:resources are consumed, data is modified by othersunexplained behavior

someone needs to decide whether an object is persistent, and someone needs to commit it to disk

new techniques to address DS complexityawareness and adaptation

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key idea:separate decisions from (controllable) mechanisms

lower layer

upper layer

’80s: spaghettilower layer

upper layer

’90s: hidinglower layer

upper layer

’00s: adaptation

narrow accessfull exposurehidden

mechanismsmechanisms

info control

Challenges of DSexample: Scalability Problems

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Examples of scalability limitations.

Scalability Technique (1)

Leverage decentralized algorithms:No machine has complete information about the system state.Machines make decisions based only on local information.Failure of one machine does not ruin the algorithm.There is no implicit assumption that a global clock exists.

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Scaling Techniques (2)

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The difference between letting (a) a server or (b) a client check forms as they are being filled.

Scaling Techniques (3)

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An example of dividing the DNS name space into zones.

outline

course mechanics

what is a distributed system?

challenges & advantages of

distribution

the rest of the course

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