Outline
n Introduction & architectural issues
l What is a distributed DBMS
l Problems
l Current state-of-affairs
q Data distribution
q Distributed query processing
q Distributed query optimization
q Distributed transactions & concurrency control
q Distributed reliability
q Database replication
q Parallel database systems
q Database integration & querying
q Advanced topics
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.1
File Systems
program 1
File 1
data description 1
program 2
data description 2 File 2
program 3
data description 3 File 3
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.2
Page 1
Database Management
Application
program 1
(with data
semantics)
DBMS
description
Application
program 2 manipulation
(with data database
semantics) control
Application
program 3
(with data
semantics)
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.3
Motivation
Database Computer
Technology Networks
integration distribution
Distributed
Database
Systems
integration
integration ≠ centralization
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.4
Page 2
Distributed Computing
n A number of autonomous processing elements
(not necessarily homogeneous) that are
interconnected by a computer network and
that cooperate in performing their assigned
tasks.
n What is being distributed?
l Processing logic
l Function
l Data
l Control
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.5
What is a Distributed Database
System?
A distributed database (DDB) is a collection of multiple,
logically interrelated databases distributed over a
computer network.
A distributed database management system (D–DBMS)
is the software that manages the DDB and provides an
access mechanism that makes this distribution
transparent to the users.
Distributed database system (DDBS) = DDB + D–DBMS
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.6
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What is not a DDBS?
n A timesharing computer system
n A loosely or tightly coupled multiprocessor
system
n A database system which resides at one of the
nodes of a network of computers - this is a
centralized database on a network node
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.7
Centralized DBMS on a
Network
Site 1
Site 2
Site 5
Communication
Network
Site 4 Site 3
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.8
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Distributed DBMS
Environment
Site 1
Site 2
Site 5
Communication
Network
Site 4 Site 3
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.9
Implicit Assumptions
n Data stored at a number of sites à each site
logically consists of a single processor.
n Processors at different sites are interconnected
by a computer network à not a multiprocessor
system
l Parallel database systems
n Distributed database is a database, not a
collection of files à data logically related as
exhibited in the users’ access patterns
l Relational data model
n D-DBMS is a full-fledged DBMS
l Not remote file system, not a TP system
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.10
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Data Delivery Alternatives
n Delivery modes
l Pull-only
l Push-only
l Hybrid
n Frequency
l Periodic
l Conditional
l Ad-hoc or irregular
n Communication Methods
l Unicast
l One-to-many
n Note: not all combinations make sense
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.11
Distributed DBMS Promises
Transparent management of distributed,
fragmented, and replicated data
Improved reliability/availability through
distributed transactions
Improved performance
Easier and more economical system expansion
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.12
Ch.x/
12
Page 6
Transparency
n Transparency is the separation of the higher
level semantics of a system from the lower
level implementation issues.
n Fundamental issue is to provide
data independence
in the distributed environment
l Network (distribution) transparency
l Replication transparency
l Fragmentation transparency
u horizontal fragmentation: selection
u vertical fragmentation: projection
u hybrid
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.13
Ch.x/
13
Example
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.14
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Transparent Access
SELECT ENAME,SAL
Tokyo
FROM EMP,ASG,PAY
WHERE DUR > 12
Boston Paris
AND EMP.ENO = ASG.ENO
Paris projects
AND PAY.TITLE = EMP.TITLE Paris employees
Communication Paris assignments
Network Boston employees
Boston projects
Boston employees
Boston assignments
Montreal
New
Montreal projects
York Paris projects
Boston projects New York projects
New York employees with budget > 200000
New York projects Montreal employees
New York assignments Montreal assignments
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.15
Distributed Database - User
View
Distributed Database
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Distributed DBMS - Reality
User
Query
DBMS User
Application
Software
DBMS
Software
DBMS Communication
Software Subsystem
User
DBMS User Application
Software Query
DBMS
Software
User
Query
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.17
Types of Transparency
n Data independence
n Network transparency (or distribution
transparency)
l Location transparency
l Fragmentation transparency
n Replication transparency
n Fragmentation transparency
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Reliability Through
Transactions
n Replicated components and data should make distributed
DBMS more reliable.
n Distributed transactions provide
l Concurrency transparency
l Failure atomicity
• Distributed transaction support requires implementation of
l Distributed concurrency control protocols
l Commit protocols
n Data replication
l Great for read-intensive workloads, problematic for updates
l Replication protocols
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.19
Potentially Improved
Performance
n Proximity of data to its points of use
l Requires some support for fragmentation and replication
n Parallelism in execution
l Inter-query parallelism
l Intra-query parallelism
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Parallelism Requirements
n Have as much of the data required by each
application at the site where the application
executes
l Full replication
n How about updates?
l Mutual consistency
l Freshness of copies
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.21
System Expansion
n Issue is database scaling
n Emergence of microprocessor and workstation
technologies
l Demise of Grosh's law
l Client-server model of computing
n Data communication cost vs
telecommunication cost
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Distributed DBMS Issues
n Distributed Database Design
l How to distribute the database
l Replicated & non-replicated database distribution
l A related problem in directory management
n Query Processing
l Convert user transactions to data manipulation
instructions
l Optimization problem
u min{cost = data transmission + local processing}
l General formulation is NP-hard
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.23
Distributed DBMS Issues
n Concurrency Control
l Synchronization of concurrent accesses
l Consistency and isolation of transactions' effects
l Deadlock management
n Reliability
l How to make the system resilient to failures
l Atomicity and durability
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Relationship Between Issues
Directory
Management
Query Distribution
Reliability
Processing Design
Concurrency
Control
Deadlock
Management
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.25
Related Issues
n Operating System Support
l Operating system with proper support for database
operations
l Dichotomy between general purpose processing
requirements and database processing requirements
n Open Systems and Interoperability
l Distributed Multidatabase Systems
l More probable scenario
l Parallel issues
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Architecture
n Defines the structure of the system
l components identified
l functions of each component defined
l interrelationships and interactions between components
defined
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ANSI/SPARC Architecture
Users
External External External External
Schema view view view
Conceptual Conceptual
view
Schema
Internal Internal view
Schema
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Generic DBMS Architecture
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.29
DBMS Implementation
Alternatives
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Dimensions of the Problem
n Distribution
l Whether the components of the system are located on the same
machine or not
n Heterogeneity
l Various levels (hardware, communications, operating system)
l DBMS important one
u data model, query language,transaction management algorithms
n Autonomy
l Not well understood and most troublesome
l Various versions
u Design autonomy: Ability of a component DBMS to decide on
issues related to its own design.
u Communication autonomy: Ability of a component DBMS to
decide whether and how to communicate with other DBMSs.
u Execution autonomy: Ability of a component DBMS to execute
local operations in any manner it wants to.
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.31
Client/Server Architecture
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Advantages of Client-Server
Architectures
n More efficient division of labor
n Horizontal and vertical scaling of resources
n Better price/performance on client machines
n Ability to use familiar tools on client machines
n Client access to remote data (via standards)
n Full DBMS functionality provided to client
workstations
n Overall better system price/performance
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.33
Database Server
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Distributed Database
Servers
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.35
Datalogical Distributed
DBMS Architecture
ES1 ES2 ... ESn
GCS
LCS1 LCS2 ... LCSn
LIS1 LIS2 ... LISn
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Peer-to-Peer Component
Architecture
USER PROCESSOR DATA PROCESSOR
Global Local System Local
External
Conceptual Conceptual Log Internal
Schema Schema GD/D Schema Schema
User
requests Database
Semantic Data
User Interface
Local Recovery
Global Query
Local Query
Controller
Optimizer
Execution
Handler
Processor
Processor
Manager
Runtime
Monitor
Support
Global
USER
System
responses
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.37
Datalogical Multi-DBMS
Architecture
GES1 GES2 ... GESn
LES11 … LES1n GCS LESn1 … LESnm
LCS1 LCS2 … LCSn
LIS1 LIS2 … LISn
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MDBS Components & Execution
Global
User
Request
Local Local
User Multi-DBMS User
Request Layer Request
Global Global Global
Subrequest Subrequest Subrequest
DBMS1 DBMS2 DBMS3
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.39
Mediator/Wrapper Architecture
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.40
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