Basis for Distributed Database Technology

1. Basis for Distributed Database Technology • Database System Technology (DST) • controlled access to structured data • aims towards centralized (single site) computing • Computer Networking Technology (CNT) • facilitates distributed computing • goes against centralized computing • Distributed Database Technology = DST + CNT • aims to achieve integration without centralization

2. What is distributed? • Processing Logic • Function • Data • Control All the above modes of distribution are necessary and important for distributed database technology

3. Distributed database system A distributed database is a collection of multiple, logically interrelated databases distributed over a computer network. A distributed database management system (DDBMS) is a software system that permits the management of the distributed databases and makes the distribution transparent to the users.

4. What is not a DDBMS? A DDBMS is not a “collection of files” that can be stored at each node of a computer network. A multiprocessor system based DBMS (parallel database system) is not a DDBMS. A DDBMS is not a system wherein data resides only at one node.

5. Aims of Distributed DBMS - Transparent Management of Distributed & Replicated Data Transparency refers to separation of the higher-level semantics of a system from lower-level implementation details. From data independence in centralized DBMS to fragmentation transparency in DDBMS. Who should provide transparency? - DDBMS!

6. Aims of Distributed DBMS - Reliability through Distributed Transactions Distributed DBMS can use replicated components to eliminate single point failure. The users can still access part of the distributed database with “proper care” even though some of the data is unreachable. Distributed transactions facilitate maintenance of consistent database state even when failures occur.

7. Aims of Distributed DBMS - Improved Performance Since each site handles only a portion of a database, the contention for CPU and I/O resources is not that severe. Data localization reduces communication overheads. Inherent parallelism of distributed systems may be exploited for inter-query and intra-query parallelism. Performance models are not sufficiently developed.

8. Aims of Distributed DBMS - Easier System Expansion Ability to add new sites, data, and users over time without major restructuring. Huge centralized database systems (mainframes) are history (almost!). PC revolution (Compaq buying Digital, 1998) will make natural distributed processing environments. New applications (such as, supply chain) are naturally distributed - centralized systems will just not work.

9. Complicating Factors Data may be replicated in a distributed environment. Therefore, DDBMS is responsible for (i) choosing one of the stored copies of the requested data, and (ii) making sure that the effect of an update is reflected on each and every copy of that data item. Maintaining consistency of distributed/replicated data. Since each site cannot have instantaneous information on the actions currently carried out in other sites, the synchronization of transactions at multiple sites is harder than centralized system. and Complexity, Cost, Distribution of control, Security,...

10. Problem Areas Distributed Database Design Distributed Query Processing Distributed Directory Management Distributed Concurrency Control Distributed Deadlock Management Reliability of Distributed Databases Operating Systems Support Heterogeneous Databases

11. Relationship among Problems Directory Management Query Processing Distributed DB Design Reliability Concurrency Control Deadlock Management

12. Transparency and Architecture issues in DDBMSs

13. Global Schema Fragmentation Schema Allocation Schema Local Mapping Schema I DBMS I Local Mapping Schema I DBMS I Local Database I Top-Down DDBMS Architecture - Classical Site Independent Schemas Other sites Local Database 2 Site 1 Site 2

14. Top-Down DDBMS Architecture - Classical Global Schema: a set of global relations as if database were not distributed at all Fragmentation Schema: global relation is split into “non-overlapping” (logical) fragments. 1:n mapping from relation R to fragments Ri. Allocation Schema: 1:1 or 1:n (redundant) mapping from fragments to sites. All fragments corresponding to the same relation R at a site j constitute the physical image Rj. A copy of a fragment is denoted by Rji. Local Mapping Schema: a mapping from physical images to physical objects, which are manipulated by local DBMSs.

15. R R1 R11 R1 (Site 1) R12 R2 R21 R2 (Site2) R3 R22 R32 R3 (Site3) Global Relation Fragments R33 Physical Images Global Relations, Fragments and Physical Images • Separating concepts of fragmentation and allocation • Explicit control of redundancy • Independence from local databases • Allows for: • Fragmentation Transparency • Location Transparency • Local Mapping Transparency

16. Rules for Data Fragmentation Completeness: All the data of the global relation must be mapped into fragments. Reconstruction: It must always be possible to reconstruct each global relation from its fragments. Disjointedness: It is convenient if the fragments are disjoint so that the replication of data can be controlled explicitly.

17. Types of Data Fragmentation • Vertical Fragmentation • Projection on relation (subset of attributes) • Reconstruction by join • Updates require no tuple migration • Horizontal Fragmentation • Selection on relation (subset of tuples) • Reconstruction by union • Updates may requires tuple migration • Mixed Fragmentation • A fragment is a Select-Project query on relation. Vertical Fragmentation Horizontal Fragmentation

18. Levels of Distribution Transparency Fragmentation Transparency: Just like using global relations. Location Transparency: Need to know fragmentation schema; but need not know where fragments are located. Applications access fragments (no need to specify sites where fragments are located). Local Mapping Transparency: Need to know both fragmentation and allocation schema; no need to know what the underlying local DBMSs are. Applications access fragments explicitly specifying where the fragments are located. No Transparency: Need to know local DBMS query languages, and write applications using functionality provided by the Local DBMS

19. Why is support for transparency difficult? There are tough problems in query optimization and transaction management that need to be tackled (in terms of system support and implementation) before fragmentation transparency can be supported. Less distribution transparency the more the end-application developer needs to know about fragmentation and allocation schemes, and how to maintain database consistency. Higher levels of distribution transparency require appropriate DDBMS support, but makes end-application developers work easy.

20. Some Aspects of top-down architecture Distributed database technology is an “add-on” technology, most users already have populated centralized DBMSs. Whereas top down design assumes implementation of new DDBMS from scratch. In case of OODBMs, top-down architecture makes sense because most OODBMs are going to be built from scratch. In many application environments, such as semi-structured databases, continuous multimedia data, the notion of fragment is difficult to define. Current relational DBMS products provide for some form of location transparency (such as, by using nicknames).

21. Bottom up Architecture - Present & Future Possible ways in which multiple databases may be put together for sharing by multiple DBMSs. The DBMSs are characterized according to • Autonomy - degree to which individual DBMSs can operate independently. Tightly coupled - integrated (A0), Semiautonomous -federated (A1), Total Isolation - multidatabase systems(A2) • Distribution - no distribution - single site (D0), client-server - distribution of DBMS functionality (D1), full distribution - peer to peer distributed architecture(D2) • Heterogeneity - homogeneous (H0) or heterogeneous (H1)

22. Distributed DBMS Implementation Alternatives Distribution (A0,D2,H0) (A2,D2,H1) Autonomy Heterogeneity

23. Architectural Alternatives (A0,D0,H0): multiple DBMSs that are logically integrated at single site - composite systems. (A0,D0,H1): multiple database managers that are heterogeneous but provide integrated view to the user. (A0,D1,H0): client-server based DBMS. (A0,D2,H0): Classical distributed database system architecture. (A1,D0,H0): Single site, homogeneous, federated database systems - not realistic. (A1,D0,H1): heterogeneous federated DBMS, having common interface over disparate cooperating specialized database systems.

24. Architectural Alternatives (A1,D1,H1): heterogeneous federated database systems with components of the systems placed at different sites. (A2,D0,H0): homogeneous multidatabase systems at a single site. (A2,D0,H1): heterogeneous multidatabase systems at a single site. (A2,D1,H1) & (A2,D2,H1): distributed heterogeneous multidatabase systems. In case of client-server environments it creates a three layer architecture. Interoperability is the major issue. Autonomy, distribution, heterogeneity are orthogonal issues.

25. Client/Server Database Systems Distinguish and divide the functionality to be provided into two classes: server functions and client functions. That is, two level architecture. Made popular by relational DBMS implementations. DBMS client: user interface, application, consistency checking of queries, and caching and managing locks on cached data. DBMS Server: handles query optimization, data access and transaction management. Typical scenarios: multiple clients/single server; multiple client/multiple servers (dedicated home-server or any server)

26. Client/Server Reference Architecture User Interface Application Program Operating System Client DBMS Communication software SQL Queries Result Relation Communication software Semantic Data Controller Query Optimizer Operating Transaction Manager Recovery Manager Runtime Support Processor System Database

27. GCS ES1 ES2 ESn LCS1 LCS2 LCSn LIS1 LIS2 LISn Distributed Database Reference Architecture

28. User Global Conceptual Schema Local Internal Schema External Schema Local Conceptual Schema Global Execution Monitor Runtime Support Processor Global Query Optimizer Local Query Processor User Interface Handler Semantic Data Controller Local Recovery Manager Components of Distributed DBMS System Responses User Requests User Processor GD/D Data Processor System Log Database

29. GCS GES1 GES2 GES3 LCS1 LCSn LIS1 LISn LES11 LES12 LES13 LESn1 LESn2 LESn3 MDBS Architecture With Global Schema

30. ES1 ES2 ESn LCS1 LCS2 LCSn LIS1 LIS2 LISn MDBS Architecture without Global Schema Multidatabase Layer Local Database System Layer

31. User Runtime Support Processor Runtime Support Processor Transaction Manager Transaction Manager Recovery Manager Recovery Manager Query Processor Query Processor Scheduler Scheduler Components of MDBS System Responses User Requests Multi-DBMS Layer Database Database

32. Global Directory Issues Directory is itself a database that contains meat-data about the actual data stored in the database. It includes the support for fragmentation transparency for the classical DDBMS architecture. Directory can be local or distributed. Directory can be replicated and/or partitioned. Directory issues are very important for large multi-database applications, such as digital libraries.

33. Impact of new technologies Internet and WWW • Semi-structured data, multimedia data • Keyword based search - browsing versus querying • What does integration mean? Applied technologies • Workflow systems • Data warehousing & Data mining • What is the role of distributed database technology?

34. Research Issues - DDBMS Technology Evaluation of state of the art data replication strategies. On-line distributed relational database redesign. Distributed object-oriented database systems - design (fragmentation, allocation), query processing (methods execution, transformation), transaction processing WWW and Internet - transparency issues, implementation strategies (architecture, scalability), On-line transaction processing, On-line analytical processing (data warehousing , data mining), query processing (STRUDEL, WebSQL), commit protocols

35. Research Issues - Applications Workflow systems - High throughput (supply chain, Amazon,..) short, sweet, and robust versus ad-hoc (office automation) problem solving. Electronic commerce - reliable high throughput, distributed transactions. Distributed multimedia - QoS, real-time delivery, design and data allocation, MPEG-4 aspects.