Distributed Database Management Systems Chapter 12 Distributed Database Management Systems Database Systems: Design, Implementation, and Management, Seventh Edition, Rob and Coronel

In this chapter, you will learn: What a distributed database management system (DDBMS) is and what its components are How database implementation is affected by different levels of data and process distribution How transactions are managed in a distributed database environment How database design is affected by the distributed database environment Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

The Evolution of Distributed Database Management Systems Distributed database management system (DDBMS) Governs storage and processing of logically related data over interconnected computer systems in which both data and processing functions are distributed among several sites Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

The Evolution of Distributed Database Management Systems (continued) Centralized database required that corporate data be stored in a single central site Dynamic business environment and centralized database’s shortcomings spawned a demand for applications based on data access from different sources at multiple locations Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

The Evolution of Distributed Database Management Systems (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

DDBMS Advantages and Disadvantages Advantages include: Data are located near “greatest demand” site Faster data access Faster data processing Growth facilitation Improved communications Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

DDBMS Advantages and Disadvantages (continued) Advantages include (continued): Reduced operating costs User-friendly interface Less danger of a single-point failure Processor independence Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

DDBMS Advantages and Disadvantages (continued) Disadvantages include: Complexity of management and control Security Lack of standards Increased storage requirements Increased training cost Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

DDBMS Advantages and Disadvantages (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

DDBMS Advantages and Disadvantages (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

DDBMS Advantages and Disadvantages (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Characteristics of Distributed Management Systems Application interface Validation Transformation Query optimization Mapping I/O interface Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Characteristics of Distributed Management Systems (continued) Formatting Security Backup and recovery DB administration Concurrency control Transaction management Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Characteristics of Distributed Management Systems (continued) Must perform all the functions of centralized DBMS Must handle all necessary functions imposed by distribution of data and processing Must perform these additional functions transparently to the end user Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Characteristics of Distributed Management Systems (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

DDBMS Components Must include (at least) the following components: Computer workstations Network hardware and software Communications media Transaction processor (application processor, transaction manager) Software component found in each computer that requests data Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

DDBMS Components (continued) Must include (at least) the following components (continued): Data processor or data manager Software component residing on each computer that stores and retrieves data located at the site May be a centralized DBMS Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

DDBMS Components (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Levels of Data and Process Distribution Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Single-Site Processing, Single-Site Data (SPSD) All processing is done on single CPU or host computer (mainframe, midrange, or PC) All data are stored on host computer’s local disk Processing cannot be done on end user’s side of system Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Single-Site Processing, Single-Site Data (SPSD) (continued) Typical of most mainframe and midrange computer DBMSs DBMS is located on host computer, which is accessed by dumb terminals connected to it Also typical of first generation of single-user microcomputer databases Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Single-Site Processing, Single-Site Data (SPSD) (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Multiple-Site Processing, Single-Site Data (MPSD) Multiple processes run on different computers sharing single data repository MPSD scenario requires network file server running conventional applications that are accessed through LAN Many multiuser accounting applications, running under personal computer network, fit such a description Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Multiple-Site Processing, Single-Site Data (MPSD) (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Multiple-Site Processing, Multiple-Site Data (MPMD) Fully distributed database management system with support for multiple data processors and transaction processors at multiple sites Classified as either homogeneous or heterogeneous Homogeneous DDBMSs Integrate only one type of centralized DBMS over a network Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Multiple-Site Processing, Multiple-Site Data (MPMD) (continued) Heterogeneous DDBMSs Integrate different types of centralized DBMSs over a network Fully heterogeneous DDBMS Support different DBMSs that may even support different data models (relational, hierarchical, or network) running under different computer systems, such as mainframes and microcomputers Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Multiple-Site Processing, Multiple-Site Data (MPMD) (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Database Transparency Features Allow end user to feel like database’s only user Features include: Distribution transparency Transaction transparency Failure transparency Performance transparency Heterogeneity transparency Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distribution Transparency Allows management of physically dispersed database as though it were a centralized database Following three levels of distribution transparency are recognized: Fragmentation transparency Location transparency Local mapping transparency Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distribution Transparency (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distribution Transparency (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Transaction Transparency Ensures database transactions will maintain distributed database’s integrity and consistency Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Requests and Distributed Transactions Can update or request data from several different remote sites on network Remote request Lets single SQL statement access data to be processed by single remote database processor Remote transaction Accesses data at single remote site Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Requests and Distributed Transactions (continued) Allows transaction to reference several different (local or remote) DP sites Distributed request Lets single SQL statement reference data located at several different local or remote DP sites Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Requests and Distributed Transactions (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Requests and Distributed Transactions (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Requests and Distributed Transactions (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Requests and Distributed Transactions (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Requests and Distributed Transactions (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Concurrency Control Multisite, multiple-process operations are much more likely to create data inconsistencies and deadlocked transactions than are single-site systems Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Concurrency Control (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Two-Phase Commit Protocol Distributed databases make it possible for transaction to access data at several sites Final COMMIT must not be issued until all sites have committed their parts of transaction Two-phase commit protocol requires each individual DP’s transaction log entry be written before database fragment is actually updated Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Performance Transparency and Query Optimization Objective of query optimization routine is to minimize total cost associated with execution of request Costs associated with request are function of: Access time (I/O) cost Communication cost CPU time cost Must provide distribution transparency as well as replica transparency Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Performance Transparency and Query Optimization (continued) Replica transparency DDBMS’s ability to hide existence of multiple copies of data from user Query optimization techniques include: Manual or automatic Static or dynamic Statistically based or rule-based algorithms Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Distributed Database Design Data fragmentation How to partition database into fragments Data replication Which fragments to replicate Data allocation Where to locate those fragments and replicas Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Fragmentation Breaks single object into two or more segments or fragments Each fragment can be stored at any site over computer network Information about data fragmentation is stored in distributed data catalog (DDC), from which it is accessed by TP to process user requests Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Fragmentation (continued) Strategies Horizontal fragmentation Division of a relation into subsets (fragments) of tuples (rows) Vertical fragmentation Division of a relation into attribute (column) subsets Mixed fragmentation Combination of horizontal and vertical strategies Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Fragmentation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Fragmentation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Fragmentation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Fragmentation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Fragmentation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Fragmentation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Fragmentation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Replication Storage of data copies at multiple sites served by computer network Fragment copies can be stored at several sites to serve specific information requirements Can enhance data availability and response time Can help to reduce communication and total query costs Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Replication (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Replication (continued) Replication scenarios Fully replicated database Stores multiple copies of each database fragment at multiple sites Can be impractical due to amount of overhead Partially replicated database Stores multiple copies of some database fragments at multiple sites Most DDBMSs are able to handle the partially replicated database well Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Replication (continued) Replication scenarios (continued) Unreplicated database Stores each database fragment at single site No duplicate database fragments Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Allocation Deciding where to locate data Allocation strategies Centralized data allocation Entire database is stored at one site Partitioned data allocation Database is divided into several disjointed parts (fragments) and stored at several sites Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Data Allocation (continued) Allocation strategies (continued) Replicated data allocation Copies of one or more database fragments are stored at several sites Data distribution over computer network is achieved through data partition, data replication, or combination of both Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Client/Server vs. DDBMS Way in which computers interact to form system Features user of resources, or client, and provider of resources, or server Can be used to implement a DBMS in which client is the TP and server is the DP Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Client/Server vs. DDBMS (continued) Client/server advantages Less expensive than alternate minicomputer or mainframe solutions Allow end user to use microcomputer’s GUI, thereby improving functionality and simplicity More people in job market have PC skills than mainframe skills PC is well established in workplace Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Client/Server vs. DDBMS (continued) Client/server advantages (continued) Numerous data analysis and query tools exist to facilitate interaction with DBMSs available in PC market Considerable cost advantage to offloading applications development from mainframe to powerful PCs Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Client/Server vs. DDBMS (continued) Client/server disadvantages Creates more complex environment Different platforms (LANs, operating systems, and so on) are often difficult to manage An increase in number of users and processing sites often paves the way for security problems Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Client/Server vs. DDBMS (continued) Client/server disadvantages (continued) Possible to spread data access to much wider circle of users Increases demand for people with broad knowledge of computers and software Increases burden of training and cost of maintaining the environment Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

C. J. Date’s Twelve Commandments for Distributed Databases Local site independence Central site independence Failure independence Location transparency Fragmentation transparency Replication transparency Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

C. J. Date’s Twelve Commandments for Distributed Databases (continued) Distributed query processing Distributed transaction processing Hardware independence Operating system independence Network independence Database independence Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Summary Distributed database stores logically related data in two or more physically independent sites connected via computer network Distributed processing is division of logical database processing among two or more network nodes Distributed databases require distributed processing Main components of DDBMS are transaction processor and data processor Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Summary (continued) Current database systems can be classified by extent to which they support processing and data distribution Homogeneous distributed database system integrates only one particular type of DBMS over computer network Heterogeneous distributed database system integrates several different types of DBMSs over computer network Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Summary (continued) DDBMS characteristics are best described as set of transparencies Transaction is formed by one or more database requests Distributed concurrency control is required in network of distributed databases Distributed DBMS evaluates every data request to find optimum access path in distributed database Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

Summary (continued) The design of distributed database must consider fragmentation and replication of data Database can be replicated over several different sites on computer network Client/server architecture refers to way in which two computers interact over computer network to form a system Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel