Distributed Database System

Chapter 10

Distributed databases

Concepts
Distributed Database.
A logically interrelated collection of shared data
(and a description of this data), physically
distributed over a computer network.

Distributed DBMS.
Software system that permits the management of
the distributed database and makes the
distribution transparent to users.

Concepts
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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.
Each DBMS participates in at least one global
application.

Component Architecture for a DDBMS
site 1

GDD

DDBMS
DC

LDBMS
GDD

Computer Network
DDBMS
DC
site 2

LDBMS : Local DBMS component
DC
: Data communication component
GDD
: Global Data Dictionary

DB

The Ideal Situation


A single application should be able to operate
transparently on data that is:
spread across a variety of different DBMS's
running on a variety of different machines
supported by a variety of different operating
systems
connected together by a variety of different
communication networks



The distribution can be geographical or local

Workable definition
A distributed database system consists of a collection of
sites connected together via some kind of
communications network, in which :
each site is a database system site in its own right;
the sites agree to work together, so that a user at any
site can access data anywhere in the network exactly
as if the data were all stored at the user's own site
It is a logical union of real databases
 It can be seen as a kind of partnership among individual
local DBMS's
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Difference with remote access or distributed processing
systems

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Temporary assumption: strict homogeneity

Distributed Processing
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A centralized database that can be accessed
over a computer network.

6

Parallel DBMS
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A DBMS running across multiple processors
and disks designed to execute operations in
parallel, whenever possible, to improve
performance.
Based on premise that single processor
systems can no longer meet requirements for
cost-effective scalability, reliability, and
performance.
Parallel DBMSs link multiple, smaller machines
to achieve same throughput as single, larger
machine, with greater scalability and reliability.

Parallel DBMS
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Main architectures for parallel DBMSs are:
a:
b:
c:

Shared memory.
Shared disk.
Shared nothing.

Advantages of DDBMSs
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Organizational Structure
Shareability and Local Autonomy
Improved Availability
Improved Reliability
Improved Performance
Economics
Modular Growth

Disadvantages of DDBMSs
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Complexity
Cost
Security
Integrity Control More Difficult
Lack of Standards
Lack of Experience
Database Design More Complex

Types of DDBMS
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Homogeneous DDBMS
Heterogeneous DDBMS

Homogeneous DDBMS
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All sites use same DBMS product.
Much easier to design and manage.
Approach provides incremental growth and
allows increased performance.

Heterogeneous DDBMS
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Sites may run different DBMS products, with
possibly different underlying data models.
Occurs when sites have implemented their own
databases and integration is considered later.
Translations required to allow for:
Different hardware.
Different DBMS products.
Different hardware and different DBMS products.

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Typical solution is to use gateways.

Open Database Access and Interoperability
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Open Group has formed a Working Group to provide
specifications that will create database infrastructure
environment where there is:
Common SQL API that allows client applications to be
written that do not need to know vendor of DBMS they
are accessing.
 Common database protocol that enables DBMS from one
vendor to communicate directly with DBMS from another
vendor without the need for a gateway.
 A common network protocol that allows communications
between different DBMSs.

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Most ambitious goal is to find a way to enable
transaction to span DBMSs from different vendors
without use of a gateway.

Multidatabase System (MDBS)
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DDBMS in which each site maintains complete
autonomy.

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DBMS that resides transparently on top of
existing database and file systems and
presents a single database to its users.
Allows users to access and share data without
requiring physical database integration.
Non-federated MDBS (no local users) and
federated MDBS (FMDBS).

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Functions of a DDBMS
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Expect DDBMS to have at least the
functionality of a DBMS.
Also to have following functionality:
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Extended communication services.
Extended Data Dictionary.
Distributed query processing.
Extended concurrency control.
Extended recovery services.

Reference Architecture for DDBMS
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Due to diversity, no universally accepted
architecture such as the ANSI/SPARC 3-level
architecture.
A reference architecture consists of:
Set of global external schemas.
Global conceptual schema (GCS).
Fragmentation schema and allocation schema.
Set of schemas for each local DBMS conforming to
3-level ANSI/SPARC .

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Some levels may be missing, depending on
levels of transparency supported.

Reference Architecture for DDBMS

Reference Architecture for MDBS
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In DDBMS, GCS is union of all local conceptual
schemas.
In FMDBS, GCS is subset of local conceptual
schemas (LCS), consisting of data that each
local system agrees to share.
GCS of tightly coupled system involves
integration of either parts of LCSs or local
external schemas.
FMDBS with no GCS is called loosely coupled.

Reference Architecture for TightlyCoupled Federated MDBS

Distributed Database Design
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Three key issues:
Fragmentation.
Allocation
Replication

Distributed Database Design
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Fragmentation
Relation may be divided into a number of subrelations, which are then distributed.

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Allocation
Each fragment is stored at site with "optimal"
distribution.

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Replication
Copy of fragment may be maintained at several sites.

Fragmentation
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Definition and allocation of fragments carried
out strategically to achieve:
Locality of Reference
Improved Reliability and Availability
Improved Performance
Balanced Storage Capacities and Costs
Minimal Communication Costs.

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Involves analyzing most important
applications, based on quantitative/qualitative
information.

Fragmentation
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Quantitative information may include:
frequency with which an application is run;
site from which an application is run;
performance criteria for transactions and
applications.

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Qualitative information may include
transactions that are executed by application,
type of access (read or write), and predicates
of read operations.

Data Allocation
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Four alternative strategies regarding
placement of data:
Centralized
Partitioned (or Fragmented)
Complete Replication
Selective Replication

Data Allocation
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Centralized
Consists of single database and DBMS stored at
one site with users distributed across the network.

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Partitioned
Database partitioned into disjoint fragments, each
fragment assigned to one site.

Data Allocation
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Complete Replication
Consists of maintaining complete copy of database
at each site.

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Selective Replication
Combination of partitioning, replication, and
centralization.

Comparison of Strategies for Data
Distribution

33

Why Fragment?
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Usage
Applications work with views rather than entire
relations.

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Efficiency
Data is stored close to where it is most frequently
used.
Data that is not needed by local applications is not
stored.

Why Fragment?
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Parallelism
With fragments as unit of distribution, transaction
can be divided into several subqueries that operate
on fragments.

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Security
Data not required by local applications is not stored
and so not available to unauthorized users.

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Disadvantages
Performance
Integrity.

Correctness of Fragmentation
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Three correctness rules:
Completeness
Reconstruction
Disjointness.

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Completeness
 If relation R is decomposed into fragments R1,
R2, ... Rn, each data item that can be found in R
must appear in at least one fragment.

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Reconstruction
Must be possible to define a relational
operation that will reconstruct R from the
fragments.
Reconstruction for horizontal fragmentation is
Union operation and Join for vertical .

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Disjointness
If data item di appears in fragment Ri, then it
should not appear in any other fragment.
Exception: vertical fragmentation, where
primary key attributes must be repeated to
allow reconstruction.
For horizontal fragmentation, data item is a
tuple
For vertical fragmentation, data item is an
attribute.

Types of Fragmentation
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Four types of fragmentation:
Horizontal
Vertical
Mixed
Derived.

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Other possibility is no fragmentation:
If relation is small and not updated frequently, may
be better not to fragment relation.

Horizontal and Vertical Fragmentation

41

Horizontal Fragmentation
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This strategy is determined by looking at
predicates used by transactions.
Involves finding set of minimal (complete and
relevant) predicates.
Set of predicates is complete, if and only if, any
two tuples in same fragment are referenced
with same probability by any application.
Predicate is relevant if there is at least one
application that accesses fragments
differently.

Transparencies in a DDBMS
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Distribution Transparency
Fragmentation Transparency
Location Transparency
Replication Transparency
Local Mapping Transparency
Naming Transparency

Transparencies in a DDBMS
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Transaction Transparency
Concurrency Transparency
Failure Transparency

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Performance Transparency

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DBMS Transparency

Distribution Transparency
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Distribution transparency allows user to
perceive database as single, logical entity.
If DDBMS exhibits distribution transparency,
user does not need to know:
data is fragmented (fragmentation transparency),
location of data items (location transparency),
otherwise call this local mapping transparency.

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With replication transparency, user is unaware
of replication of fragments .

Naming Transparency
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Each item in a DDB must have a unique name.
DDBMS must ensure that no two sites create a
database object with same name.
One solution is to create central name server.
However, this results in:
loss of some local autonomy;
central site may become a bottleneck;
low availability; if the central site fails, remaining
sites cannot create any new objects.

Transaction Transparency
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Ensures that all distributed transactions
maintain distributed database’s integrity and
consistency.
Distributed transaction accesses data stored at
more than one location.
Each transaction is divided into number of subtransactions, one for each site that has to be
accessed.
DDBMS must ensure the indivisibility of both
the global transaction and each
subtransactions.

Concurrency Transparency
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All transactions must execute independently
and be logically consistent with results
obtained if transactions executed one at a time,
in some arbitrary serial order.
Same fundamental principles as for centralized
DBMS.
DDBMS must ensure both global and local
transactions do not interfere with each other.
Similarly, DDBMS must ensure consistency of
all sub-transactions of global transaction.

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Replication makes concurrency more complex.
If a copy of a replicated data item is updated,
update must be propagated to all copies.
Could propagate changes as part of original
transaction, making it an atomic operation.
However, if one site holding copy is not
reachable, then transaction is delayed until site
is reachable.

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Could limit update propagation to only those
sites currently available. Remaining sites
updated when they become available again.

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Could allow updates to copies to happen
asynchronously, sometime after the original
update. Delay in regaining consistency may
range from a few seconds to several hours.

Failure Transparency
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DDBMS must ensure atomicity and durability of
global transaction.
Means ensuring that sub-transactions of global
transaction either all commit or all abort.
Thus, DDBMS must synchronize global
transaction to ensure that all sub-transactions
have completed successfully before recording
a final COMMIT for global transaction.
Must do this in presence of site and network
failures.

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DDBMS must perform as if it were a centralized
DBMS.
DDBMS should not suffer any performance
degradation due to distributed architecture.
DDBMS should determine most cost-effective
strategy to execute a request.

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Distributed Query Processor (DQP) maps data
request into ordered sequence of operations
on local databases.
Must consider fragmentation, replication, and
allocation schemas.
DQP has to decide:
which fragment to access;
which copy of a fragment to use;
which location to use.

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DQP produces execution strategy optimized
with respect to some cost function.
Typically, costs associated with a distributed
request include:
I/O cost;
CPU cost;
communication cost.

Date’s 12 Rules for a DDBMS
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0.

Fundamental Principle
 To the user, a distributed system should look
exactly like a non-distributed system.

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1.
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4.
5.
6.

Local Autonomy
No Reliance on a Central Site
Continuous Operation
Location Independence
Fragmentation Independence
Replication Independence

Date’s 12 Rules for a DDBMS
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7.
8.
9.
10.
11.
12.

Distributed Query Processing
Distributed Transaction Processing
Hardware Independence
Operating System Independence
Network Independence
Database Independence

Last four rules are ideals.

Distributed Database System

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