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CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.1
Outline Introduction & architectural issues
What is a distributed DBMSProblemsCurrent state-of-affairs
Data distribution Distributed query processing Distributed query optimization Distributed transactions & concurrency control Distributed reliability Database replication Parallel database systems Database integration & querying Advanced topics
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.2
File Systems
program 1
data description 1
program 2
data description 2
program 3
data description 3
File 1
File 2
File 3
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.3
Database Management
database
DBMS
Applicationprogram 1(with datasemantics)
Applicationprogram 2(with datasemantics)
Applicationprogram 3(with datasemantics)
descriptionmanipulation
control
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.4
Motivation
DatabaseTechnology
ComputerNetworks
integration distribution
integration
integration ≠ centralization
DistributedDatabaseSystems
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.5
Distributed Computing
A number of autonomous processing elements (not necessarily homogeneous) that are interconnected by a computer network and that cooperate in performing their assigned tasks.
What is being distributed?Processing logicFunctionDataControl
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.6
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 1.7
What is not a DDBS?
A timesharing computer system
A loosely or tightly coupled multiprocessor system
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 1.8
Centralized DBMS on a Network
Site 5
Site 1
Site 2
Site 3Site 4
CommunicationNetwork
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.9
Distributed DBMS Environment
Site 5
Site 1
Site 2
Site 3Site 4
CommunicationNetwork
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.10
Implicit Assumptions
Data stored at a number of sites each site logically consists of a single processor.
Processors at different sites are interconnected by a computer network not a multiprocessor system
Parallel database systems
Distributed database is a database, not a collection of files data logically related as exhibited in the users’ access patterns
Relational data model
D-DBMS is a full-fledged DBMSNot remote file system, not a TP system
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.11
Data Delivery Alternatives
Delivery modesPull-onlyPush-onlyHybrid
FrequencyPeriodicConditionalAd-hoc or irregular
Communication MethodsUnicastOne-to-many
Note: not all combinations make sense
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.12
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
Ch.x/12
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.13
Transparency
Transparency is the separation of the higher level semantics of a system from the lower level implementation issues.
Fundamental issue is to providedata independence
in the distributed environment Network (distribution) transparency
Replication transparency
Fragmentation transparency horizontal fragmentation: selection vertical fragmentation: projection hybrid
Ch.x/13
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.14
Example
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.15
Transparent Access
SELECT ENAME,SALFROM EMP,ASG,PAYWHERE DUR > 12AND EMP.ENO = ASG.ENOAND PAY.TITLE =
EMP.TITLEParis projectsParis employeesParis assignmentsBoston employees
Montreal projectsParis projectsNew York projects with budget > 200000Montreal employeesMontreal assignments
Boston
CommunicationNetwork
Montreal
Paris
NewYork
Boston projectsBoston employeesBoston assignments
Boston projectsNew York employeesNew York projectsNew York assignments
Tokyo
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.16
Distributed Database - User View
Distributed Database
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.17
Distributed DBMS - Reality
CommunicationSubsystem
DBMSSoftware
UserApplicationUser
Query
DBMSSoftware
DBMSSoftware
DBMSSoftware
UserQuery
DBMSSoftware
UserQuery
UserApplication
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.18
Types of Transparency
Data independence Network transparency (or distribution
transparency)Location transparencyFragmentation transparency
Replication transparency Fragmentation transparency
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.19
Reliability Through Transactions
Replicated components and data should make distributed DBMS more reliable.
Distributed transactions provideConcurrency transparencyFailure atomicity
•Distributed transaction support requires implementation of Distributed concurrency control protocolsCommit protocols
Data replicationGreat for read-intensive workloads, problematic for updatesReplication protocols
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.20
Potentially Improved Performance
Proximity of data to its points of use
Requires some support for fragmentation and replication
Parallelism in execution
Inter-query parallelism
Intra-query parallelism
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.21
Parallelism Requirements
Have as much of the data required by each application at the site where the application executes
Full replication
How about updates?
Mutual consistency
Freshness of copies
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.22
System Expansion
Issue is database scaling
Emergence of microprocessor and workstation technologies
Demise of Grosh's law
Client-server model of computing
Data communication cost vs telecommunication cost
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.23
Distributed DBMS Issues
Distributed Database DesignHow to distribute the database
Replicated & non-replicated database distribution
A related problem in directory management
Query ProcessingConvert user transactions to data manipulation
instructions
Optimization problem min{cost = data transmission + local processing}
General formulation is NP-hard
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.24
Distributed DBMS Issues
Concurrency ControlSynchronization of concurrent accesses
Consistency and isolation of transactions' effects
Deadlock management
ReliabilityHow to make the system resilient to failures
Atomicity and durability
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.25
DirectoryManagement
Relationship Between Issues
Reliability
DeadlockManagement
QueryProcessing
ConcurrencyControl
DistributionDesign
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.26
Related Issues
Operating System SupportOperating system with proper support for database
operationsDichotomy between general purpose processing
requirements and database processing requirements
Open Systems and InteroperabilityDistributed Multidatabase SystemsMore probable scenarioParallel issues
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.27
Architecture
Defines the structure of the systemcomponents identified
functions of each component defined
interrelationships and interactions between components defined
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.28
ANSI/SPARC Architecture
ExternalSchema
ConceptualSchema
InternalSchema
Internal view
Users
External view
Conceptual view
External view
External view
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.29
Generic DBMS Architecture
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.30
DBMS Implementation Alternatives
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.31
Dimensions of the Problem
Distribution Whether the components of the system are located on the same
machine or not Heterogeneity
Various levels (hardware, communications, operating system) DBMS important one
data model, query language,transaction management algorithms
Autonomy Not well understood and most troublesome Various versions
Design autonomy: Ability of a component DBMS to decide on issues related to its own design.
Communication autonomy: Ability of a component DBMS to decide whether and how to communicate with other DBMSs.
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 1.32
Client/Server Architecture
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.33
Advantages of Client-Server Architectures
More efficient division of labor Horizontal and vertical scaling of resources Better price/performance on client
machines Ability to use familiar tools on client
machines Client access to remote data (via
standards) Full DBMS functionality provided to client
workstations Overall better system price/performance
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.34
Database Server
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.35
Distributed Database Servers
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.36
Datalogical Distributed DBMS Architecture
...
...
...
ES1 ES2 ESn
GCS
LCS1 LCS2 LCSn
LIS1 LIS2 LISn
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.37
Peer-to-Peer Component Architecture
Database
DATA PROCESSORUSER PROCESSOR
USER
Userrequests
Systemresponses
ExternalSchema
User
Inte
rface
Han
dle
r
GlobalConceptual
Schema
Sem
an
tic D
ata
Con
troller
Glo
bal
Execu
tion
Mon
itor
SystemLog
Local R
ecovery
Man
ag
er
LocalInternalSchema
Ru
nti
me
Su
pp
ort
Pro
cessor
Local Q
uery
Pro
cessor
LocalConceptual
Schema
Glo
bal Q
uery
Op
tim
izer
GD/D
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.38
Datalogical Multi-DBMS Architecture
...
GCS… …
GES1
LCS2 LCSn…
…LIS2 LISn
LES11 LES1n LESn1 LESnm
GES2 GESn
LIS1
LCS1
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.39
MDBS Components & Execution
Multi-DBMSLayer
DBMS1 DBMS3DBMS2
GlobalUser
Request
LocalUser
Request
GlobalSubrequest
GlobalSubrequest
GlobalSubrequest
LocalUser
Request
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.40
Mediator/Wrapper Architecture
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