Introduction to Database Systems A database is a collection of ...

Introduction to Database SystemsA database is a collection of related data. It is a collection of information that exists over along period of time, often many years.The common use of the term database usually refers to a collection of data that is managedby a database management system or DBMS.A database has the following implicit properties:‐A database represents some aspect of the real world, sometimes called a miniworld or theuniverse of discourse (UoD). Changes to the miniworld are reflected in the database.A database is a logically coherent collection of data with some inherent meaning.A database is designed, built, and populated with data for a specific purpose. It has anintended group of users and some applications in which the user is interested.A DBMS is a powerful tool for creating and managing large amount of data efficiently andallowing it to persist over long period of time.A DBMS is a collection of programs that enable users to create and maintain a database.The DBMS is a general purpose software system that facilitates the processes of defining,constructing, manipulating and sharing databases among various users and applications.The DBMS is expected to:Allow users to create new databases and specify their logical structure of data or schemas,using a specialized data definition language.Give users the ability to query the data, (a query is a question about the data) and modifythe data, using an appropriate language called the query language or data manipulationlanguage.Support the storage of very large amount of data over a long period of time, allowingefficient access to the data for queries and data modifications.Enable durability, the recovery of data in the face of failures, data inconsistency orintentional misuse.Control access to data from many users at once, without allowing unexpected interactionbetween users (isolation) or incomplete operations on data (atomicity).Database Catalog or Dictionary or meta­data: Defining a database involves specifyingthe data types, structures, and constraints of the data to be stored in the database. Thedatabase definition is stored by the DBMS in the form of a database catalog or dictionary; itis called meta‐data.Constructing the database is the process of storing the data based on the catalog usingsome storage device controlled by the DBMS.Manipulating a database includes functions such as querying the database to retrievespecific data, updating the database to reflect changes in the miniworld.Sharing a database allows multiple users or programs to access the databasesimultaneously.An application program accesses a database by sending queries or request for informationto the DBMS. A query typically causes some data to be retrieved.

A transaction may cause some data to be read and some data to be written into thedatabase.Data Stored in Other ways:Besides storing data in databases, there are other ways of storing data. For example, worddocuments and excel spreadsheets, etc...Files:Using files such as text files or Word documents to store data has many advantagesespecially when the amount of data is small. Besides, there is no need to get a databasesystem or learn how to use it. It is easy to write applications using standard programminglanguages.Disadvantages of using files:Limited user interfaceAre not efficient in handling large amount of dataDo not provide support to specifying relationships between data itemsDo not support data integrity.Do not provide a query languageSpreadsheets:Advantages:Provide structured storage model (rows and columns)Support data types, Allow fields to be computed (based on other fields) and others.However they also suffer from most disadvantages listed for files.Early Database Management Systems:The first database management systems evolved from file systems and appeared in the late1960’s.File systems store data over a long period of time and they allow for storage of largeamount of data. However, they do not generally guarantee that data will not be lost andthey do not support efficient access to data when their location is not known.The first applications to use DBMS were ones where data was composed of many smallitems and many queries or modifications were required (banking, airline reservations,corporate record keeping, etc...)Nowadays, databases have evolved to be of any size and complexity: a simple address bookof hundreds of records to millions of entries (IRS, Amazon)

Ieee Transactions On Parallel…II. RELATED WORKSFlooding and RW are two typical examples of blind search algorithms by which query messages aresent to neighbors without any knowledge about the possible locations of the queried resources or any preferencefor the directions to send. Some other blind search algorithms include modified BFS (MBFS) [23], directed BFS[6], expanding ring [17], and random periodical flooding (RPF) [24]. These algorithms try to modify theoperation of flooding to improve the efficiency. However, they still generate a large amount of query messages.Jiang et al. propose a LightFlood algorithm, which is a combination of the initial pure flooding and subsequenttree-based flooding [25], [26]. DS and LightFlood operate analogously, but DS avoids the extra cost to constructand maintain the treelike suboverlay.Knowledge-based search algorithms take advantage of the knowledgelearned from previous search results and route query messages with different weights based on the knowl-edge.Thus, each node could relay query messages more intelligently. Some examples are adaptive probabilisticsearch (APS) [27], [28], biased RW [29], routing index (RI) [30], local indices [31], and intelligent search [32].APS builds the knowledge with respect to each file based on the past experiences. RI classifies each documentinto some thematic categories and forwards query messages more intelligently based on the categories. Theoperation of local indices is similar to that of super-peer networks. Each node collects the file indices of peerswithin its predefined radius. If a search request is out of a node’s knowledge, this node would perform aflooding search. The intelligent search uses a function to compute the similarity between a search query andrecently answered requests. Nodes relay query messages based on the similarity. There are some other researchworks that focus on replicating a reference pointer to queried resources in order to improve the search time [33],[34].III. DYNAMIC SEARCH ALGORITHMIn this section, we provide the details of the proposed DS algorithm. Section 3.1 presents the operationof DS algorithm, and Section 3.2 provides the mechanism to combine DS with the knowledge-based searchalgorithms.3.1 Operation of Dynamic Search AlgorithmDS is designed as a generalization of flooding, MBFS, and RW. There are two phases in DS. Eachphase has a different searching strategy. The choice of search strategy at each phase depends on the relationshipbetween the hop count h of query messages and the decision thresh-old n of DS.3.1.1 Phase 1. When h _ nAt this phase, DS acts as flooding or MBFS. The number of neighbors that a query source sends thequery messages to depends on the predefined transmission probability p. If the link degree of this query sourceis d, it would only send the query messages to d_p neighbors. When p is equal to 1, DS resembles flooding.Otherwise, it operates as MBFS with the transmission probability p.3.1.2 Phase 2. When h > nAt this phase, the search strategy switches to RW. Each node that receives the query message wouldsend the query message to one of its neighbors if it does not have the queried resource. Assume that the numberof nodes visited by DS at hop h¼n is the coverage c n , and then the operation of DS at that time can be regardedas RW with c n walkers. However, there are some differences between DS and RW when we consider the wholeoperation. Consider the simple scenario shown in Fig. 1. Assume that the decision threshold n is set as 2. Whenh >2, DS performs the same as RW with c 2 ¼12 walkers. Let us consider an RW search with K¼12 walkers. Atthe first hop, the walkers only visit four nodes, but the cost is 12 messages.656IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS, VOL. 20, NO. 5, MAY2009||Issn 2250-3005 || ||July||2013|| Page 48

Insulation between Programs and Data and Data Abstraction:Data Abstraction:In traditional file processing, the structure of data files is embedded in the applicationprograms, so any changes to the file may require changing the programs that access thatfile. DBMS access programs do not require such changes in most cases. The structure ofdata files is stored in the DBMS catalog separately from access programs. This is calledprogram­data independence (encapsulation).In some types of databases, such as OO (object‐oriented) and OR (object‐relational)systems, users can define functions or methods and only provide their prototype orinterface (name, number and datatype of its arguments) to application programs that usethem without knowing the details of their implementation. This is termed as programoperationindependence (Abstraction).The DBMS provides users with a conceptual representation of the data that does notinclude how the data is stored or how the operations are implemented.Database Views:A database typically has many users, each of whom requires a different perspective or viewof the database. A view may be a subset of the database or it may contain virtual data thatis derived from the database files but is not explicitly stored. A multiuser DBMS whoseusers have a variety of distinct applications must provide the ability for defining multipleviews.Sharing of Data and Multiuser Transaction Processing:A multiuser DBMS must allow multiple users to access the database at the same time. It isessentialif data for multiple applications is to be integrated and maintained in the samedatabase, that the DBMS includes concurrency control. Concurrency control ensures thatseveral users trying to update the same data at the same time do so in a controlled manner,so that the result of the updates is correct.A transaction is an executing program or process that includes one or more databaseaccesses. Each transaction is supposed to execute a logically correct database access ifexecuted in its entirety without interference from other transactions.The isolation property ensures that each transaction appears to execute in isolation fromother transactions, even though many of them could be executing concurrently.The atomicity property ensures that either all the database operations are executed ornone are.Actors:

Database Administrators (DBA):In a database environment, the primary resource is the database, and the secondaryresource is the DBMS and related software. The DBA is responsible for authorizing accessto the database, coordinating and monitoring its use and acquiring software and hardwareresources as needed. The DBA is accountable for problems such as security breach, poorresponse time, etc…Database Designers:Database designers are responsible for identifying the data to be stored in the database andfor choosing the appropriate structures to represent and store the data. These tasks areundertaken before the database is implemented and populated with data.It is the responsibility of database designers to communicate with the prospective users inorder to understand their needs and create a design that meets these requirements.Database designers typically interact with each potential group of users and develop viewsof the database that meet the data and processing requirement of these groups. The finalproduct must support the requirements of all user groups.End users:Casual users: Occasional access, but different needs each time. Middle or high level managers.Naïve or parametric users:Make up a sizable portion of database end users. Main function is to query and update thedatabase using standard types of queries and updates or canned transactions.System Analysts: (Software Engineers)Determine the requirements of end users, especially naïve end users and developspecification for the canned transactions that meet these requirements. Applicationengineers implement these specifications as programs they test, debug and document andthen deploy them.Other workers:• DBMS system designers and implementers: A DBMS is a very complex software systemthat consists of many components or module, including implementation of the catalog,definition and implementation of query languages, interface processors, data access,concurrency control, etc... Systems designers and implementers design and implementthese component, they usually work for a software company that sells the system withsome customization to other non‐software companies.• Tool developers: include persons who design and implement tools‐ that facilitatedatabase system design and use.

• Operators and maintenance personnel: are system administrations personnel who areresponsible for the actual running and maintenance for the hardware and softwareenvironment for the database system.Advantages of Using a DBMS:‐Controlling RedundancyRedundancy is storing the same data multiple times. In traditional software development usingfile processing, each application usually maintains its own files for handling its data‐processingapplications. This can lead to several problems:‐ Duplication of efforts: A single logical update requires entering the same data severaltimes.‐ Storage Waste: when the same data is stored multiple times, there is a waste a space.‐ Data may become inconsistent: since updates can miss one or more repository or datamay be entered differently from one file to the other.In the database approach, the views of different groups are integrated during databasedesign.For consistency, we should have a database design that stores each logical data item,such as a student’s name or birth date, ‐ in only one place in the database. This doessave space and does allow inconsistency.However, controlled redundancy maybe useful for improving the performance ofqueries, in that we do not have to search multiple files to retrieve information. In suchcases, the DBMS should have the capability to control this redundancy so as to preventinconsistencies among files. This can be done by automatically checking that a fieldvalue in a table should have the same value or match that of another field in a differenttable. Such checks are specified to the DBMS during the database definition and designand automatically enforced by the DBMS whenever there is an update.Restricting Unauthorized Access:When multiple users share a database, it is likely that they will have different accessrights and authorization. The DBMS should provide a security and authorizationsubsystem, which the DBA uses to create accounts and to specify account restrictions.Providing Persistent Storage for Program Objects and Data Structure:

An object is said to be persistent if it survives the termination of program execution andcan later be directly retrieved by another program. The persistent storage of programand data is an important functionality of database systems.Providing multiple user interfaces:Because many users have varying levels of technical knowledge use of the database, aDBMS should provide a variety of user interfaces. These include query languages forcasual users; programming language interfaces for applications programmers, forms andcommand code for parametric users and menu driven interfaces for natural languageinterfaces for stand‐alone users.Representing Complex Relationships among Data:A database may include numerous varieties of data. A DBMS must have the capability torepresent a variety of complex relationship among the data as well as retrieve andupdate data easily and efficiently.Enforcing Integrity Constraints:Integrity constraints involve specifying conditions for data correctness. The simplest oneinvolves defining a data type for each data item. Others involve the length of a field or arange of values for a specific field.More complex kind of constraints involve the inter relations between fields in differenttables or uniqueness of data item values.Providing Backup and Recovery:A DBMS must provide facilities for recovering from hardware and software failures. Thebackup and recovery subsystem of the DBMS is responsible for recovery.