databases: concurrency - ucl · databases: concurrency p.a.rounce room 6.18 [email protected]...
TRANSCRIPT
1
1P.Rounce Introduction
Databases: Concurrency
P.A.RounceRoom 6.18
2P.Rounce Introduction
Concurrency in Databases: Distr ibuted Processing and Distr ibuted Database Systems
� Concepts.� Advantages and disadvantages of distr ibuted databases.� Functions and architecture for a DDBMS.� Aspects of Distr ibuted database design.
© Pearson Education Limited 1995, 2005
– A DBMS running across multiple processors and disks designed to execute operations in parallel, whenever possible, to improve per formance: based on premise that single processor systems can no longer meet requirements for cost-effective scalability, reliability, and per formance.
2
3P.Rounce Introduction
Concepts
Distr ibuted ProcessingCentralised database with distr ibuted processing
Distr ibuted DatabaseA logically inter related collection of shared data (and a descr iption of this data), physically distr ibuted over a computer network.
Distr ibuted DBMSSoftware system that permits the management of the distr ibuted database and makes the distr ibution transparent to users.
© Pearson Education Limited 1995, 2005
4P.Rounce Introduction
Distr ibuted Database
� Collection of logically-related shared data.� Data split into fragments.� Fragments may be replicated.� Fragments/replicas allocated to sites.� Sites linked by a communications network.� Data at each site is under control of a DBMS.� DBMSs handle local applications autonomously.
© Pearson Education Limited 1995, 2005
3
5P.Rounce Introduction
Distr ibuted ProcessingA centralized database that can be accessed over a computer network: multiple processing units, centralised data.
© Pearson Education Limited 1995, 2005
Single data store
6P.Rounce Introduction
Distr ibuted DBMS
© Pearson Education Limited 1995, 2005
Multiple processing units
Multiple storage units
– Parallel DBMSs link multiple, smaller machines to achieve same throughput as single, larger machine, with greater scalability and reliability.
4
7P.Rounce Introduction
Parallel DBMS
� Main architectures for parallel DBMSs are:
– Shared memory,
– Shared disk,
– Shared nothing.
© Pearson Education Limited 1995, 2005
8P.Rounce Introduction
Parallel DBMS
© Pearson Education Limited 1995, 2005© Pearson Education Limited 1995, 2005
shared memoryshared disk
shared nothing
Distributed processing
5
9P.Rounce Introduction
Advantages of DDBMSs
� Reflects organizational structure� Improved shareability and local autonomy� Improved availability� Improved reliability� Improved per formance� Economics� Modular growth
© Pearson Education Limited 1995, 2005
10P.Rounce Introduction
Disadvantages of DDBMSs
� Complexity� Cost� Secur ity� Integr ity control more difficult� Lack of standards� Lack of exper ience� Database design more complex
© Pearson Education Limited 1995, 2005
6
11P.Rounce Introduction
Functions of a DDBMS
� Expect DDBMS to have at least the functionality of a DBMS.
� Also to have following functionality:– Extended communication services.
– Extended Data Dictionary.
– Distr ibuted query processing.
– Extended concurrency control.
– Extended recovery services.
© Pearson Education Limited 1995, 2005
Look at these
12P.Rounce Introduction
Reference Architecture for DDBMS
© Pearson Education Limited 1995, 2005
7
13P.Rounce Introduction
Components of a DDBMS
© Pearson Education Limited 1995, 2005
DDBMS-Distributed DatabaseManagemenrtSystemLDBMS-Local Database Management SystemGSC – Global System CatalogDC – Data Communications
14P.Rounce Introduction
Distr ibuted Database Design
� Three key issues:
FragmentationRelation may be divided into a number of sub-
relations, which are then distr ibuted.
AllocationEach fragment is stored at site with “ optimal”
distr ibution.
ReplicationCopy of fragment may be maintained at several sites.
© Pearson Education Limited 1995, 2005
8
15P.Rounce Introduction
Fragmentation
� Definition and allocation of fragments car r ied out strategically to achieve:– Locality of Reference.– Improved Reliability and Availability.– Improved Per formance.– Balanced Storage Capacities and Costs.– Minimal Communication Costs.
� Involves analyzing most impor tant applications, based on quantitative/qualitative information.
© Pearson Education Limited 1995, 2005
16P.Rounce Introduction
Why Fragment?
� Advantages� Parallelism
– With fragments as unit of distr ibution, transaction can be divided into several sub-quer ies that operate on fragments.
� Secur ity
– Data not required by local applications is not stored and so notavailable to unauthor ized users.
� Disadvantages
– complexity
– Integr ity.
© Pearson Education Limited 1995, 2005