Chap13

7/18/2019 Chap13

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Distributed Computing• Spectrum of possibilities

– Communications Architecture

• Set of independent machines talking over a

network

• Example: C!"#! $email% &!% etc'(

– )etwork *perating S+stem

• Set of independent networked machines

with network,wide services $file and

printer sharing% etc'(

• Examples: )ovell )etware

– Distributed *perating S+stem

• )etworked computers that appear as a

single s+stem to users

• Client"Server Computing – &igure -.'-% page /01

– 2ser,friendl+ clients $usuall+ !Cs(

– *ne or more servers

– )etwork to connect machines

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Client"Server Characteristics

• Common application – 2sers

accessing a shared database• Differences from distributed

processing:

– 2ser,friendl+ applications running on

each client

• 2sers have control over usage

– Databases and network management

often centrali3ed• 4elps maintain interoperabilit+

• #ndividual departments don5t have to

worr+ about sophisticated servers

– *pen and modular products• &lexibilit+ in mixing e6uipment from

various vendors

– )etworking is fundamental

• 7anagement and securit+ is vital

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Client"Server Example• Database Application

– &igure -.'.% page /01

• Server holds database

– 4andles searching and storing data

• Client sends re6uest to server% server

replies with information

• #deall+ each re6uest should generate a

relativel+ small response

– 8eturning most or all of a database re6uires a

lot of network traffic

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Application Classes• &igure -.'/% page /00

• 4ost,9ased

– Client servers onl+ as a dumb displa+

• Server,9ased

– Client handles 2# user interface – Server does all processing

– ;hin Client<

• Client,9ased

– Client does processing

– Server onl+ supplies data

– 8e6uires more network capabilit+

– ;&at Client<• Cooperative:

– 9oth handle processing

– 2se strengths of both server and client – 7ore complex% but more efficient

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Client"Server • hree,ier Architecture

– &igure -.'=% page /0>

– 2ser Client• enerall+ thin client

– 7iddle,tier server • atewa+ between thin client and backend

database servers• Also converts protocols and ma+ merge

data from different servers

– 9ack,end server

• &ile cache and consistenc+ – Caching helps network performance

– 7ultiple clients should have the sameview of the file $cache consistenc+(

– !roblems if one or more modif+ it• #f one client using file% no problem – client

can cache file% send data to server later

• Can lock portions of the file

• Can tell other readers not to cache the file%writer sends all updates to server

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7iddleware• Customers tend to focus in what

applications the+ want% not servers

• 4elps provide a standard interface to

multivendor configurations

– Client can decide on a middleware

package% see who supports it

• &igure -.'>% page />-

• 4elps make portable applications – Also ma+ combine server information

• Adds another la+er to the s+stem

• Example: S?@ for database use

– 7an+ vendors support S?@ 6ueries

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Distributed 7essage !assing• 2sed to implement client"server

functions

• Send$( – Send message – ive contents and destination

– 2se network protocol $such as C!"#!(to send it to the target s+stem

• 8eceive$( – et a message – ive buffer and $optional( sender

• 8eliable"2nreliable

– nown failures reported to sender – #f deliver+ is not guaranteed%

application handles acknowledgements

• 9locking")onblocking – )onblocking – Call copies message%

then returns $before message delivered(• 7ore flexible% programs harder to debug

– 9locking – Baits for deliver+

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8emote !rocedure Calls• Allow processes to interact using standard

procedure call"return

• 8easons: – !rocedure call widel+ understood

– 8emote interfaces can be represented as

named operations with designated t+pes –

compiler can t+pe,check – Standardi3ed interface is more portable

• Sample S+ntax:

– !rocname$SendArgs%8ecvArgs(

• Arguments usuall+ passed b+ value

• &igure -.'-.% page />0

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8emote !rocedure Calls• !arameter 8epresentation

– )umber"String formats ma+ differ

– Send values across network in a

standard form

– Each node converts to"from local form

• Client"Server 9inding – )onpersistent – Create new connection

for each call

– !ersistent – eep the connection open

• @ess overhead for multiple connections

• S+nchronous"As+nchronous

– S+nchronous – Bait for response

– As+nchronous – Caller is not blocked• Allows greater parallelism

• 7ust s+nchroni3e at some point

• *bect,*rientated Design

– Send messages between obects

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Clusters• roup of interconnected computers

working together a one resource

– Alternative to S7! – Each node is a complete computer

• 9enefits

– Scales to large s+stems easil+

– Can scale in small increments – 4igh availabilit+

• *ne failure doesn5t halt s+stem

– !rice"!erformance

• Configuration $&igure -.'-1% able -.'( – !assive Standb+

• *ne s+stem does work% second one waits to take

over if necessar+ $not considered a cluster(

– Active Standb+ – All s+stems do work • Separate servers – )o disks shared

– 7ust cop+ data using network

• Shared nothing – Disks shared% but partitions

$volumes( not shared

• Shared disk – Folumes shared – 8e6uires distributed locking

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Cluster *'S' #ssues• &ailure 7anagement

– Bhat happens to current operations if a

failure occursG – &ailover – Switch to a new node

– &ailback – 8epeat when node comes up

• @oad 9alancing – r+ to keep all nodes bus+

• !aralleli3ing Computation – Done b+ compiler

– Done b+ programmer – !arametric computing – 2sed when

running one application man+ timeswith different parameters

• Clusters vs' S7! – S7! closer to uniprocessor model

– S7! re6uires less space and power

– Cluster easier to scale

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Cluster #mage• Single entr+ $login( point

• Single file hierarch+

• Single control point $control s+stem(

• Single virtual network

– An+ node can access an+ other node

• Single memor+ space

– Distributed shared memor+

• Single ob,management s+stem

• Single user interface

• Single #"* space

– Access #"* devices on an+ node

• Single process space

– S+stemwide,uni6ue process #Ds

• Checkpointing

• !rocess 7igration $load balancing(

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BinHHH Cluster Server • &ormerl+ called Bolfpack

• Shared,nothing cluster • Concepts

– Cluster Service – Software that

manages cluster,specific activit+

– 8esource – #tem to be managed

• !h+sical: Disk drives% )etwork cards

• @ogical: Folumes% C!"#! addresses%

Applications% Databases

– *nline – A resource is online at a node

when it is providing service on that

node

– roup – A collection of resourcesmanaged as a unit

• 4elps manage resources% load balancing

• 2suall+ a set of resources needed for a

specific application

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BinHHH Cluster Server • Components

– )ode 7anager – 7aintain membership

• 2ses heartbeat messages to track s+stem

– Database 7anager – 7aintain cluster

configuration database $resources% groups%

ownership(

• 2ses fault,tolerant transaction software

– 8esource 7anager – 7ake decisions

concerning resources% startup% failover

– Event !rocessor – Connect components

• &ig -.'-=% pg' /I0

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Sun Cluster • Extension to Solaris $&ig -.'-0(

• *bect and communication support – *bect orientation using 8!Cs

• !rocess management – !rocess location is transparent to the

user

• )etworking – #ncoming packets sent to correct node

– *utgoing packets routed to node withexternal connection

– Database tracks traffic to each node

• lobal distributed file s+stem

– Firtual node $vnode( – eneric fileaccess

– Firtual file s+stem – Deals with generalfiles+stem calls

– All nodes see same files+stem

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9eowulf • &rom )ASA 4igh !erformance

Computing and Communications !roect

• 9ased on commodit+ hardware• #mplemented as an add,on to @inux

• Each node runs a cop+ of @inux% with

kernel extensions for global namespaces

• Examples of 9eowulf s+stem software:

– 9eowulf Distributed !rocess Space –

Allow process #D space to span

machines% provide single s+stemimage% start processes on other nodes

– 9eowulf Ethernet Channel 9onding –

Combine multiple low,cost links into a

single logical network – !vms+nc – !provide s+nchroni3ation

and shared data obects in a cluster

– En&u3ion – Set of tools for parametric

computing

Chap13

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