Multiprossesors SystemsMultiprossesors Systems

..

What are Distributed Databases ?What are Distributed Databases ?

“ “ A Logically interrelated collection of shared data ( and a A Logically interrelated collection of shared data ( and a description of this data) physically distributed over a description of this data) physically distributed over a computer network”computer network”

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

Characteristics of Distributed DBMSCharacteristics of Distributed DBMS

A collection of Logically related shared dataA collection of Logically related shared data Data is split into a no. of fragmentsData is split into a no. of fragments Fragments may be replicatedFragments may be replicated Fragments are allocated to sitesFragments are allocated to sites Sites are linked by comm. NetworkSites are linked by comm. Network Data at each site is under control of DBMSData at each site is under control of DBMS

Characteristics of Distributed DatabasesCharacteristics of Distributed Databases

Users access DDBMS via applications :Users access DDBMS via applications :

Local AppsLocal Apps : That do not require data : That do not require data from other sitesfrom other sites

Global AppsGlobal Apps : That do require data : That do require data from other sitesfrom other sites

Distributed DatabasesDistributed Databases

Computer Network

Site 1

Site 3

Site 2Site 4

DB

DBDB

FRAGMENTATIONFRAGMENTATION

FragmentationFragmentation - A relation may be divided - A relation may be divided into no. of subrelations called fragments, into no. of subrelations called fragments, which are distributedwhich are distributed

AllocationAllocation – Each Fragment is stored at the – Each Fragment is stored at the site with ‘optimal’ distributionsite with ‘optimal’ distribution

Replication Replication – DDBMS may maintain a copy – DDBMS may maintain a copy of a fragment at several different sitesof a fragment at several different sites

DATA REPLICATIONDATA REPLICATION

Process of storing data in more than one Process of storing data in more than one site or nodesite or node

Improves the availability of dataImproves the availability of data

Full (Complete) ReplicationFull (Complete) Replication – Copy of – Copy of whole database is stored at every sitewhole database is stored at every site

Partial ReplicationPartial Replication – Some Fragments of – Some Fragments of the database are replicatedthe database are replicated

A Distributed SystemA Distributed System

FRAGMENTATIONFRAGMENTATION

Data can be stored in different computers Data can be stored in different computers by fragmenting the whole database into by fragmenting the whole database into several pieces called fragmentsseveral pieces called fragments

Fragments are logical data units stored at Fragments are logical data units stored at various sites in a distributed database various sites in a distributed database systemsystem

FRAGMENTATION - AdvantagesFRAGMENTATION - Advantages

UsageUsage -- Appropriate to work with subsets -- Appropriate to work with subsets of relation as the unit of distribution.of relation as the unit of distribution.

EfficiencyEfficiency -- Data is stored close to -- Data is stored close to where it is to be frequently usedwhere it is to be frequently used

ParallelismParallelism – A Transaction can be – A Transaction can be divided into several subqueries that divided into several subqueries that operate on fragmentsoperate on fragments

Security Security – Data not required by local – Data not required by local applications is not stored, so not available applications is not stored, so not available to unauthorized usersto unauthorized users

Network TopologyNetwork Topology

Local-Area Network (LAN) – designed to cover small geographical area.

Multiaccess bus, ring, or star network.Speed 10 megabits/second, or higher.Broadcast is fast and cheap.

Network TypesNetwork Types

Depiction of typical LAN:

Hardware ConceptsHardware Concepts

1.6

Different basic organizations and memories in distributed computer

systems

MultiprocessorsMultiprocessors

A bus-based multiprocessorA bus-based multiprocessor

1.7

MultiprocessorsMultiprocessors

a)a) A crossbar switchA crossbar switchb)b) An omega switching networkAn omega switching network

1.8

Homogeneous Multicomputer SystemsHomogeneous Multicomputer Systems

a)a) GridGridb)b) HypercubeHypercube

1-9

Distributed-Operating Systems (Cont.)Distributed-Operating Systems (Cont.)

Process Migration – execute an entire process, or parts of it, at Process Migration – execute an entire process, or parts of it, at different sites.different sites.• Load balancing – distribute processes across network to even Load balancing – distribute processes across network to even

the workload.the workload.• Computation speedup – subprocesses can run concurrently on Computation speedup – subprocesses can run concurrently on

different sites.different sites.• Hardware preference – process execution may require Hardware preference – process execution may require

specialized processor.specialized processor.• Software preference – required software may be available at Software preference – required software may be available at

only a particular site.only a particular site.• Data access – run process remotely, rather than transfer all Data access – run process remotely, rather than transfer all

data locally.data locally.

TopologyTopology

Sites in the system can be physically connected in a variety of Sites in the system can be physically connected in a variety of ways; they are compared with respect to the following criteria:ways; they are compared with respect to the following criteria:

• Basic costBasic cost. How expensive is it to link the various sites in the . How expensive is it to link the various sites in the system?system?

• Communication costCommunication cost. How long does it take to send a . How long does it take to send a message from site message from site AA to site to site BB??

• ReliabilityReliability. If a link or a site in the system fails, can the . If a link or a site in the system fails, can the remaining sites still communicate with each other?remaining sites still communicate with each other?

The various topologies are depicted as graphs whose nodes The various topologies are depicted as graphs whose nodes correspond to sites. The following six items depict various network correspond to sites. The following six items depict various network topologies. topologies.

CommunicationCommunication

Naming and name resolutionNaming and name resolution: How do two : How do two processes locate each other to communicate?processes locate each other to communicate?

Routing strategiesRouting strategies. How are messages sent through . How are messages sent through the network?the network?

Connection strategiesConnection strategies. How do two processes send a . How do two processes send a sequence of messages?sequence of messages?

ContentionContention. The network is a shared resource, so how . The network is a shared resource, so how do we resolve conflicting demands for its use?do we resolve conflicting demands for its use?

The design of a communication network must address four basic issues:

Design IssuesDesign Issues

TransparencyTransparency – the distributed system should appear as a – the distributed system should appear as a conventional, centralized system to the user.conventional, centralized system to the user.

Fault toleranceFault tolerance – the distributed system should continue to – the distributed system should continue to function in the face of failure.function in the face of failure.

ScalabilityScalability – as demands increase, the system should easily – as demands increase, the system should easily accept the addition of new resources to accommodate the accept the addition of new resources to accommodate the increased demand.increased demand.

ClustersClusters – a collection of semi-autonomous machines that acts as – a collection of semi-autonomous machines that acts as a single system.a single system.

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