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CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
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.
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.
Database Management
database
DBMS
Application
program 1
(with data
semantics)
Application
program 2
(with data
semantics)
Application
program 3
(with data
semantics)
description
manipulation
control
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
Motivation
Database
Technology
Computer
Networks
integration distribution
integration
integration ≠ centralization
Distributed
Database
Systems
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
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.
Centralized DBMS on a
Network
Site 5
Site 1
Site 2
Site 4^ Site 3
Communication
Network
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
Distributed DBMS
Environment
Site 5
Site 1
Site 2
Site 4 Site 3
Communication
Network
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
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.
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 Ch.x/ 13 CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
Example
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
Transparent Access
SELECT ENAME,SAL
FROM EMP,ASG,PAY
WHERE DUR > 12
AND EMP.ENO = ASG.ENO
AND PAY.TITLE = EMP.TITLE Paris projects Paris employees Paris assignments Boston employees Montreal projects Paris projects New York projects with budget > 200000 Montreal employees Montreal assignments Boston Communication Network Montreal Paris New York Boston projects Boston employees Boston assignments Boston projects New York employees New York projects New York assignments Tokyo CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
Distributed Database - User
View
Distributed Database
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
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.
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
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
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.
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
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1. Directory Management
Relationship Between Issues
Reliability Deadlock Management Query Processing Concurrency Control Distribution Design CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
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
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
Architecture
n Defines the structure of the system
l components identified l functions of each component defined l interrelationships and interactions between components defined CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
ANSI/SPARC Architecture
External Schema Conceptual Schema Internal Schema Internal view Users External view Conceptual view External view External view
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
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.
Client/Server Architecture
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
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.
Database Server
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
Peer-to-Peer Component
Architecture
Database
USER PROCESSOR DATA PROCESSOR
USER User requests System responses External Schema User Interface Handler Global Conceptual Schema Semantic Data Controller^ Global Execution^ Monitor System Log Local Recovery Manager Local Internal Schema Local Query^ Processor^ Runtime^ Support^ Processor Local Conceptual Schema Global Query Optimizer GD/D CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
Datalogical Multi-DBMS
Architecture
…^ GCS …
GES 1
LCS 2 … LCS n
LIS 2^ … LIS n
LES 11 LES 1 n LES n 1 LES nm GES 2 GES n LIS 1
LCS 1
CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
MDBS Components & Execution
Multi-DBMS Layer DBMS 1 DBMS 2 DBMS 3 Global User Request Local User Request Global Subrequest Global Subrequest Global Subrequest Local User Request CS742 – Distributed & Parallel DBMS M. Tamer Özsu Page 1.
Mediator/Wrapper Architecture
