Multiprossesors Systems.

What are Distributed Databases ? “ A Logically interrelated collection of shared data ( and a description of this data) physically distributed over a computer network”  A collection of independent computers that appears to its users as a single coherent system

Characteristics of Distributed DBMS  A collection of Logically related shared data  Data is split into a no. of fragments  Fragments may be replicated  Fragments are allocated to sites  Sites are linked by comm. Network  Data at each site is under control of DBMS

Characteristics of Distributed Databases Users access DDBMS via applications :  Local Apps : That do not require data from other sites  Global Apps : That do require data from other sites

Distributed Databases Computer Network Site 1 Site 3 Site 2 Site 4 DB

FRAGMENTATION  Fragmentation - A relation may be divided into no. of subrelations called fragments, which are distributed  Allocation – Each Fragment is stored at the site with ‘optimal’ distribution  Replication – DDBMS may maintain a copy of a fragment at several different sites

DATA REPLICATION  Process of storing data in more than one site or node  Improves the availability of data  Full (Complete) Replication – Copy of whole database is stored at every site  Partial Replication – Some Fragments of the database are replicated

A Distributed System

FRAGMENTATION Data can be stored in different computers by fragmenting the whole database into several pieces called fragments Data can be stored in different computers by fragmenting the whole database into several pieces called fragments Fragments are logical data units stored at various sites in a distributed database system Fragments are logical data units stored at various sites in a distributed database system

FRAGMENTATION - Advantages Usage -- Appropriate to work with subsets of relation as the unit of distribution. Usage -- Appropriate to work with subsets of relation as the unit of distribution. Efficiency -- Data is stored close to where it is to be frequently used Efficiency -- Data is stored close to where it is to be frequently used Parallelism – A Transaction can be divided into several subqueries that operate on fragments Parallelism – A Transaction can be divided into several subqueries that operate on fragments Security – Data not required by local applications is not stored, so not available to unauthorized users Security – Data not required by local applications is not stored, so not available to unauthorized users

Network 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 Types

n Depiction of typical LAN:

Hardware Concepts 1.6 Different basic organizations and memories in distributed computer systems

Multiprocessors  A bus-based multiprocessor 1.7

Multiprocessors a) A crossbar switch b) An omega switching network 1.8

Homogeneous Multicomputer Systems a) Grid b) Hypercube 1-9

Distributed-Operating Systems (Cont.)  Process Migration – execute an entire process, or parts of it, at different sites. Load balancing – distribute processes across network to even the workload.Load balancing – distribute processes across network to even the workload. Computation speedup – subprocesses can run concurrently on different sites.Computation speedup – subprocesses can run concurrently on different sites. Hardware preference – process execution may require specialized processor.Hardware preference – process execution may require specialized processor. Software preference – required software may be available at only a particular site.Software preference – required software may be available at only a particular site. Data access – run process remotely, rather than transfer all data locally.Data access – run process remotely, rather than transfer all data locally.

Topology  Sites in the system can be physically connected in a variety of ways; they are compared with respect to the following criteria: Basic cost. How expensive is it to link the various sites in the system?Basic cost. How expensive is it to link the various sites in the system? Communication cost. How long does it take to send a message from site A to site B?Communication cost. How long does it take to send a message from site A to site B? Reliability. If a link or a site in the system fails, can the remaining sites still communicate with each other?Reliability. If a link or a site in the system fails, can the remaining sites still communicate with each other?  The various topologies are depicted as graphs whose nodes correspond to sites. The following six items depict various network topologies.

Communication  Naming and name resolution: How do two processes locate each other to communicate?  Routing strategies. How are messages sent through the network?  Connection strategies. How do two processes send a sequence of messages?  Contention. The network is a shared resource, so how do we resolve conflicting demands for its use? The design of a communication network must address four basic issues:

Design Issues  Transparency – the distributed system should appear as a conventional, centralized system to the user.  Fault tolerance – the distributed system should continue to function in the face of failure.  Scalability – as demands increase, the system should easily accept the addition of new resources to accommodate the increased demand.  Clusters – a collection of semi-autonomous machines that acts as a single system.

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