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Distributed DBMSs-Concept and Design

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Title: Distributed DBMSs-Concept and Design


1
Distributed DBMSs-Concept and Design
  • Jing Luo
  • CS 157B
  • Dr. Lee
  • Fall, 2003

2
DBMSs
  • Distributed DBMS
  • It allows users to access not only the data at
    their own site but also data stored at remote
    sites.
  • Centralized DBMS
  • It allows users to access only a single logical
    database located at one site under its control.

3
Definitions
  • Distributed database A logically interrelated
    collection of shared data (and a description of
    this data) physically distributed over a computer
    network.
  • Distributed DBMS The software system that
    permits the management of the distributed
    database and makes the distribution transparent
    to users.

4
Users access the distributed database via
applications
  • Local applications
  • Applications are those do not require data from
    other sites.
  • Global applications
  • Applications are those do require data from
    other sites.

5
Characteristics of DDBMS
  • A collection of logically related shared data
  • The data is split into a number of fragments
  • Fragments may be replicated
  • Fragments/replicas are allocated to sites
  • The sites are linked by a communications network
  • The data at each site is under the control of a
    DBMS
  • The DBMS at each site can handle local
    applications, autonomously
  • Each DBMS participates in at lease one global
    application.

6
A DDBMS is required to have at least one global
application. It is not necessary for every site
in the system to have its own local database.
DB
Site 1
  • DDBMS

Site 2
Computer network
Site 4
DB
DB
Site 3
7
Distributed processing A centralized database
that can be accessed over a computer network.
8
Distributed Processing (contd)
  • Distributed Processing

Site 1
Site 2
Computer network
Site 4
DB
Site 3
9
Distributed DBMS vs. Distributed Processing
  • Distributed DBMS
  • System consists of data that is physically
    distributed across a number of sites in the
    network.
  • Distributed processing
  • Data is centralized, even though other users may
    be accessing the data over the network.

10
Parallel DBMSs A DBMS running across
multiple processors and disks that is designed to
execute operations in parallel, whenever
possible, in order to improve performance
11
Three Main Architectures for Parallel DBMSs
  • To provide multiple processors with
  • common access to a single database, a
  • parallel DBMS must provide for shared
  • resource management.
  • Shared memory
  • Shared disk
  • Shared nothing

12
Shared memory is a tightly coupled architecture
in which multiple processors within a single
system share system memory.
  • Symmetric multiprocessing (SMP)
  • This approach has become popular on platforms
    ranging from personal workstations that support a
    few microprocessors in parallel, to RISC (Reduced
    Instruction Set Computer) based machines, all the
    way up to the largest mainframes.
  • The architecture provides high-speed data access
    for a limited number of processors, but it is not
    scalable beyond about 64 processors when the
    interconnection network becomes a bottleneck.

13
Shared Memory (contd)
  • Shared Memory

CPU
CPU
CPU
CPU
Interconnection network
DB
DB
DB
Memory
14
Shared disk is a loosely-coupled architecture
optimized for applications that are inherently
centralized and require high availability and
performance.
  • Each processor can access all disks directly, but
    each has its own private memory.
  • Shared disk architecture eliminates the shared
    memory performance bottleneck without introducing
    the overhead associated with physically
    partitioned data.

15
Shared Disk (contd)
  • Shared Disk

Memory
Memory
Memory
Memory
CPU
CPU
CPU
CPU
Interconnection network
DB
DB
DB
16
Shared nothing known as massively parallel
processing, is a multiple processor architecture
in which each processor is part of a complete
system, with its own memory and disk storage.
  • The database is partitioned among all the disks
    on each system associated with the database, and
    data is transparently available to users on all
    system.
  • This architecture can easily support a large
    number of processors.

17
Shared nothing (contd)
  • SN

Memory
Memory
DB
DB
CPU
CPU
Interconnection network
DB
DB
CPU
CPU
Memory
Memory
18
Homogeneous Heterogeneous DDBMSs
  • Homogeneous system
  • All sites use the same DBMS product.
  • Heterogeneous system
  • Sites may run different DBMS products, which need
    not be based on the same underlying data model,
    and so the system may be composed of relational,
    network, hierarchical, and object-oriented DBMSs.

19
Heterogeneous system problems
  • In a heterogeneous system, translations are
    required to
  • allow communication between different DBMSs.
  • The system has the task of locating the data and
  • performing any necessary translation.
  • Data required from another site may have
  • Different hardware
  • Different DBMS products
  • Different hardware and different DBMS products
  • If the hardware is different but the DBMS
    products are the same,
  • involving the change of codes and word length.
  • If the DBMS products are different, involving the
    mapping of data
  • structures in one data model to the equivalent
    data structures in
  • another data model.

20
Heterogeneous system problems (contd)
  • An additional complexity is the provision of a
    common
  • Conceptual schemas. The integration of data
    models can be very difficult owing to the
    semantic heterogeneity.
  • For example, attributes with the same name in two
  • Schemas may represent different things. Equally
    well,
  • Attributes with different names may model the
    same thing.

21
Solution Gateways, which convert the language
and model of each different DBMS into the
language and model of the relational system.
  • Limitation
  • It may not support transaction management. The
    gateway between two systems may be only a query
    translator. For example, a system may not
    coordinate concurrency control and recovery of
    transactions that involve updates to the pair of
    databases.
  • The gateway approach is concerned only with the
    problem of translating a query expressed in one
    language into an equivalent expression in another
    language. As such, generally it does not address
    the issues of homogenizing the structural and
    representational differences between different
    schemas.

22
A multidatabase system (MDBS) is a distributed
DBMS in which each site maintains complete
autonomy. An MDBS resides transparently on top
of existing database and file systems, and
presents a single database to its users. It
maintains a global schema against which users
issue queries and updates an MDBS maintains only
the global schema and the local DBMSs themselves
maintain all user data.
23
Concepts of Networking
  • Network
  • An interconnected collection of autonomous
  • computers that are capable of exchanging
  • information.
  • For our purposes, the DDBMS is built on top
  • of a network in such of a way that the
  • Network is hidden from the user.

24
Classification of network LAN a local area
network is intended for connecting computers at
the same site. WAN a wide area network is
used when computers or LANs need to be connected
over long distances. A special case of the WAN
is a metropolitan area network (MAN), which
generally covers a city or suburb.
25
Summary of WAN and LAN characteristics
  • WAN
  • Distances up to thousands of kilometers link
    autonomous computers
  • Network managed by independent organization
    (using telephone or satellite links)
  • Data rate up to 33.6 kbits/(dial-up via modem),
    45 Mbit/s (T3 circuit)
  • Complex protocol
  • Use point-to-point routing
  • Use irregular topology
  • Error rate about 1105
  • LAN
  • Distances up to a few kilometers
  • Link computers that cooperate in distributed
    applications
  • Network managed by users (using privately owned
    cables)
  • Data rate up to 2500 Mbit/s (ATM)
  • Simpler protocol
  • Use broadcast routing
  • Use bus or ring topology
  • Error rate about 1109

26
Network protocols a set of rules that determines
how messages between computers are sent,
interpreted, and processed.
  • TCP/IP (Transmission Control Protocol/Internet
    Protocol)
  • SPX/IPX (Sequenced Packet Exchange/Internetwork
    Package Exchange)
  • NetBIOS (Network Basic Input/Output System)
  • APPC (Advanced Program-to-Program Communications)

27
Network protocol (contd)
  • DECnet
  • AppleTalk
  • WAP (Wireless Application Protocol)
  • SPX/IPX (Sequenced Packet Exchange/Internetwork
    Package Exchange)
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