DOCSLIB.ORG

(DB): – Collection of Related Data • Has Following Characteristics

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
(DB): – Collection of Related Data • Has Following Characteristics Intro: Databases • Database (DB): { Collection of related data • Has following characteristics: { Logically coherent collection of data with inherent meaning { Designed for specific purpose { Represents some aspect of the real world: the miniworld or universe of discourse • DB is not a random collection of facts • Concept of DB independent of Database Management System (DBMS) • Prior to DBMS concept, DBs maintained as flat file (traditional) systems 1 Intro: Flat File Systems • Flat file system: { One or more data files accessed via dedicated programs • Typical large organization (e.g., university) has many departments, each with specific needs { Each department has own set of data { Each department has own set of apps for processing data { Data stored in one or more data files accessed by app programs which define ∗ Record/field structure (object/attribute structure) { Apps written in high-level language { Generally, significant overlap in data stored in various departments • This approach leads to many problems: 1. Data redundancy { Data stored multiple places { Ramifications: (a) Wasted resources ∗ Wasted disk space ∗ Wasted effort ∗ Both result in wasted money (b) Inconsistent data (as a result of updates) ∗ Doubtful if a datum that occurs in multiple files will be updated simulta- neously · Files will be inconsistent for a time interval ∗ Clerical errors may result in permanent inconsistencies 2. Concurrency control { Have many independent programs with uncontrolled access to same data 2 Intro: Flat File Systems (2) 3. Interdependence of data and programs { Data structure of DB embedded in app programs { Based on structures defined using data types inherent in host language { Ramifications: (a) Difficulty in modifying DB/apps ∗ If modify record structure or field data type, all apps must be modified accordingly (b) Difficulty sharing data (among departments) ∗ Most likely incompatible record formats in different locations, so cannot share files easily (c) Difficulty extracting new info ∗ If want to extract info not anticipated at design time, either i. Extract manually using results from existing software ii. Create new app { The interdependence tends to promote a proliferation of apps ∗ This tends to result in much ad hoc code ∗ This in turn tends to result in problems related to (a) Data integrity · Checks that data meet certain constraints (b) Security · Limiting users to authorized data (c) Concurrency control · Limiting number of users accessing a given piece of data at any one time • These problems motivated the development of DBMSs 3 Intro: DBMS Overview • DBMS: { General purpose software system that facilitates definition, construction, manip- ulation, and maintenance of a DB { Definition: ∗ Specifying objects/entities, logical structure of data, data types, constraints, relationships, ... { Construction: ∗ Creating the DB - installing data { Manipulation: ∗ Insertion, deletion, retrieval, update of data { Maintenance: ∗ Monitoring performance, changing storage structures, changing access paths • DBMS Characteristics { General purpose ∗ Independent of any world of discourse or app ∗ Can represent any type of data (within restrictions of DBMS) { Single, central repository for data ∗ Eliminates redundancy problem (and associated problems, e.g., Cost, consistency) ∗ Data logically related ∗ Note: Some duplication required to create associations among data Referred to as controlled redundancy { Data independent of apps ∗ Data accessed by queries that are independent of physical record structure ∗ User deals with conceptual representation of data, not implementational de- tails ∗ Achieved via data abstraction ∗ Implemented in terms of data structure/module called catalog (dictionary/data dictionary/data directory) 4 Intro: DBMS Overview (2) { Access controlled by a module called the DB Manager ∗ Manager responsible for · Concurrency control · Security · Backup and recovery · Integrity constraints ∗ Since all data access controlled by manager, problems associated with above eliminated { Multiple interfaces (usually provided) ∗ Each presents data in format specific to type of user ∗ Limits access to what is needed by that user 5 Intro: DBMS Architecture - History • Conference on Data Systems Languages (CODASYL) { Purpose was to establish standards for DBs { 1967 - created Database Task Group (DBTG) { DBTG charged with generating standards for environment for creation of DB and manipulation of data { 1969 - DBTG initial report • Codd { 1970 - seminal paper proposing relational model { Proposed 8 services that should be supported by any full DBMS: 1. Data storage, retrieval, and update ∗ Primary purpose of DBMS ∗ Physical details of data storage should be hidden from user 2. User-accessible catalog ∗ Stores metadata - data about data ∗ Stores info pertaining to all aspects of DB design, usage, and maintenance 3. Transaction support ∗ Transaction: an atomic operation on the DB · Consists of a read and/or write ∗ Transaction executes in its entirety, or not at all ∗ Insures consistency of DB in case of DBMS failure 4. Concurrency control 5. Recovery ∗ Enables DB to be returned to a consistent state in case of failure 6. Restriction of unauthorized access 7. Support for data communication ∗ Provide for remote access of DB 8. Integrity support ∗ Insure data is correct and consistent 6 Intro: DBMS Architecture - History (2) • DBTG { 1971 - formal proposal { DBMS should consist of 3 components: 1. Network schema ∗ Describes logical organization of DB ∗ Includes (a) DB name (b) Structure of each record type (c) Data types of each record field 2. Subschema ∗ Describes DB as seen by users 3. Data management language ∗ Used to define structure of data ∗ Used to manipulate data ∗ Proposed 3 sub-languages: (a) Schema data definition language (DDL) · Defines schema · Used by DBA (b) Subschema DDL · Defines parts of DB required by apps (c) Data manipulation language · For manipulation of data · Used by anyone querying DB { Not adopted by ANSI 7 Intro: DBMS Architecture - History (3) • ANSI Standards Planning and Requirements Committee (SPARC) { 1975 - proposed 3-level architecture with data dictionary { Based on IBM (Codd) proposals { Reflected need for independent layer between implementational and application levels { Purpose of 3-level architecture: ∗ Users should be able to access same data, but with customized view · Should be able to change one view without affecting others ∗ Physical data storage should be invisible to user ∗ Changes to physical storage should not affect user view ∗ Changes to physical storage should not affect internal structure of DB ∗ Changes to internal structure should not affect user view • ANSI-SPARC proposal did not become formal standard { Is basis for modern DBMS architectures 8 Intro: ANSI-SPARC Architecture • 3-level ANSI-SPARC Architecture: 1. External level { Presents subsets (views) of the DB { Each view customized for a particular user ∗ Limits what is accessible to the user ∗ Display of data may be different for same data in different views { May containing data not actually stored in DB (derived data) 2. Conceptual level { "Community" representation of entire DB { Data represented logically (structure in terms of components, data types, and sizes), independent of physical storage (not in terms of bytes) { Types of information represented: ∗ Entities, attributes, relations ∗ Constraints ∗ Semantic info ∗ Security and integrity info 9 Intro: ANSI-SPARC Architecture (2) 3. Internal level { Physical (implementational) representation of data ∗ Storage allocation ∗ Access mechanisms ∗ Record disk address ∗ Record structure ∗ Data compression and encryption 4. Physical level { Machine level 10 Intro: Schemas • Schema: { Logical structure of data • Need schema for each level of ANSI-SPARC architecture { Schema for a level describes data at that level only { One internal schema that describes 1. Data fields of records (physical) 2. Indices 3. Access methods 4. ... { One conceptual schema that describes 1. Data fields of records (logical) 2. Relationships among data 3. Constraints 4. ... { Multiple external schemas (subschemas) that describe 1. Same aspects as subset of conceptual level • Schema diagram used to represent structure graphically • Schemas created by DB designers for a particular domain • Schema is relatively static • Often referred to as an intension of the DB • A populated DB is often referred to as a(n) { Instance { State { Extension { Snapshot 11 Intro: Data Models • Data model: { Integrated collection of concepts for describing data, relationships between data, and constraints on data in an organization (Connelly and Begg) { Schemas represented in terms of a data model • Data models are used at each level of the DBMS • Models do not need to be the same across DBMS levels • Data models are categorized in terms of their degree of abstraction 1. High-level/conceptual/object-based models { Highest degree of abstraction { Describe DB in terms of (a) Entities ∗ Concepts to be represented (b) Attributes ∗ Characteristics of the entities (c) Relationships ∗ Associations among entities { Example paradigms: (a) Entity-relation (ER) model (b) Object-oriented (c) Semantic (d) Functional 12 Intro: Data Models (2) 2. Representational/Implementational/record-based models { Describe DB in terms of logical records { Correspond closely to way data represented physically, while hiding the details { Applicable to external and conceptual DBMS levels { Better than OO for representing structure { Poorer than OO for representing constraints {
Load more

Recommended publications
  • Failures in DBMS
    Chapter 11 Database Recovery 1 Failures in DBMS Two common kinds of failures StSystem filfailure (t)(e.g. power outage) ‒ affects all transactions currently in progress but does not physically damage the data (soft crash) Media failures (e.g. Head crash on the disk) ‒ damagg()e to the database (hard crash) ‒ need backup data Recoveryyp scheme responsible for handling failures and restoring database to consistent state 2 Recovery Recovering the database itself Recovery algorithm has two parts ‒ Actions taken during normal operation to ensure system can recover from failure (e.g., backup, log file) ‒ Actions taken after a failure to restore database to consistent state We will discuss (briefly) ‒ Transactions/Transaction recovery ‒ System Recovery 3 Transactions A database is updated by processing transactions that result in changes to one or more records. A user’s program may carry out many operations on the data retrieved from the database, but the DBMS is only concerned with data read/written from/to the database. The DBMS’s abstract view of a user program is a sequence of transactions (reads and writes). To understand database recovery, we must first understand the concept of transaction integrity. 4 Transactions A transaction is considered a logical unit of work ‒ START Statement: BEGIN TRANSACTION ‒ END Statement: COMMIT ‒ Execution errors: ROLLBACK Assume we want to transfer $100 from one bank (A) account to another (B): UPDATE Account_A SET Balance= Balance -100; UPDATE Account_B SET Balance= Balance +100; We want these two operations to appear as a single atomic action 5 Transactions We want these two operations to appear as a single atomic action ‒ To avoid inconsistent states of the database in-between the two updates ‒ And obviously we cannot allow the first UPDATE to be executed and the second not or vice versa.
    [Show full text]
  • Concurrency Control
    Concurrency Control Instructor: Matei Zaharia cs245.stanford.edu Outline What makes a schedule serializable? Conflict serializability Precedence graphs Enforcing serializability via 2-phase locking » Shared and exclusive locks » Lock tables and multi-level locking Optimistic concurrency with validation Concurrency control + recovery CS 245 2 Lock Modes Beyond S/X Examples: (1) increment lock (2) update lock CS 245 3 Example 1: Increment Lock Atomic addition action: INi(A) {Read(A); A ¬ A+k; Write(A)} INi(A), INj(A) do not conflict, because addition is commutative! CS 245 4 Compatibility Matrix compat S X I S T F F X F F F I F F T CS 245 5 Update Locks A common deadlock problem with upgrades: T1 T2 l-S1(A) l-S2(A) l-X1(A) l-X2(A) --- Deadlock --- CS 245 6 Solution If Ti wants to read A and knows it may later want to write A, it requests an update lock (not shared lock) CS 245 7 Compatibility Matrix New request compat S X U S T F Lock already X F F held in U CS 245 8 Compatibility Matrix New request compat S X U S T F T Lock already X F F F held in U F F F Note: asymmetric table! CS 245 9 How Is Locking Implemented In Practice? Every system is different (e.g., may not even provide conflict serializable schedules) But here is one (simplified) way ... CS 245 10 Sample Locking System 1. Don’t ask transactions to request/release locks: just get the weakest lock for each action they perform 2.
    [Show full text]
  • Sundial: Harmonizing Concurrency Control and Caching in a Distributed OLTP Database Management System
    Sundial: Harmonizing Concurrency Control and Caching in a Distributed OLTP Database Management System Xiangyao Yu, Yu Xia, Andrew Pavlo♠ Daniel Sanchez, Larry Rudolph|, Srinivas Devadas Massachusetts Institute of Technology, ♠Carnegie Mellon University, |Two Sigma Investments, LP [email protected], [email protected], [email protected] [email protected], [email protected], [email protected] ABSTRACT and foremost, long network delays lead to long execution time of Distributed transactions suffer from poor performance due to two distributed transactions, making them slower than single-partition major limiting factors. First, distributed transactions suffer from transactions. Second, long execution time increases the likelihood high latency because each of their accesses to remote data incurs a of contention among transactions, leading to more aborts and per- long network delay. Second, this high latency increases the likeli- formance degradation. hood of contention among distributed transactions, leading to high Recent work on improving distributed concurrency control has abort rates and low performance. focused on protocol-level and hardware-level improvements. Im- We present Sundial, an in-memory distributed optimistic concur- proved protocols can reduce synchronization overhead among trans- rency control protocol that addresses these two limitations. First, to actions [22, 35, 36, 46], but can still limit performance due to high reduce the transaction abort rate, Sundial dynamically determines coordination overhead (e.g., lock blocking). Hardware-level im- the logical order among transactions at runtime, based on their data provements include using special hardware like optimized networks access patterns. Sundial achieves this by applying logical leases to that enable low-latency remote memory accesses [20, 48, 55], which each data element, which allows the database to dynamically calcu- can increase the overall cost of the system.
    [Show full text]
  • Concurrency Control and Recovery ACID • Transactions • Recovery Transaction Model Concurency Control Recovery
    Data Management Systems • Transaction Processing • Concurrency control and recovery ACID • Transactions • Recovery Transaction model Concurency Control Recovery Gustavo Alonso Institute of Computing Platforms Department of Computer Science ETH Zürich Transactions-CC&R 1 A bit of theory • Before discussing implementations, we will cover the theoretical underpinning behind concurrency control and recovery • Discussion at an abstract level, without relation to implementations • No consideration of how the concepts map to real elements (tuples, pages, blocks, buffers, etc.) • Theoretical background important to understand variations in implementations and what is considered to be correct • Theoretical background also key to understand how system have evolved over the years Transactions-CC&R 2 Reference Concurrency Control and Recovery in Database Systems Philip A. Bernstein, Vassos Hadzilacos, Nathan Goodman • https://www.microsoft.com/en- us/research/people/philbe/book/ Transactions-CC&R 3 ACID Transactions-CC&R 4 Conventional notion of database correctness • ACID: • Atomicity: the notion that an operation or a group of operations must take place in their entirety or not at all • Consistency: operations should take the database from a correct state to another correct state • Isolation: concurrent execution of operations should yield results that are predictable and correct • Durability: the database needs to remember the state it is in at all moments, even when failures occur • Like all acronyms, more effort in making it sound cute than in
    [Show full text]
  • CAS CS 460/660 Introduction to Database Systems Transactions
    CAS CS 460/660 Introduction to Database Systems Transactions and Concurrency Control 1.1 Recall: Structure of a DBMS Query in: e.g. “Select min(account balance)” Data out: Database app e.g. 2000 Query Optimization and Execution Relational Operators These layers Access Methods must consider concurrency Buffer Management control and recovery Disk Space Management Customer accounts stored on disk1.2 = File System vs. DBMS? ■ Thought Experiment 1: ➹ You and your project partner are editing the same file. ➹ You both save it at the same time. ➹ Whose changes survive? A) Yours B) Partner’s C) Both D) Neither E) ??? • Thought Experiment 2: Q: How do you write programs over a – You’re updating a file. subsystem when it – The power goes out. promises you only “???” ? – Which of your changes survive? A: Very, very carefully!! A) All B) None C) All Since last save D ) ??? 1.3 Concurrent Execution ■ Concurrent execution essential for good performance. ➹ Because disk accesses are frequent, and relatively slow, it is important to keep the CPU humming by working on several user programs concurrently. ➹ Trends are towards lots of cores and lots of disks. § e.g., IBM Watson has 2880 processing cores ■ A program may carry out many operations, but the DBMS is only concerned about what data is read/written from/to the database. 1.4 Key concept: Transaction ■ an atomic sequence of database actions (reads/writes) ■ takes DB from one consistent state to another ■ transaction - DBMS’s abstract view of a user program: ➹ a sequence of reads and writes. transaction
    [Show full text]
  • Concurrency Control in Groupware Systems
    Concurrency Control in Groupware Systems C.A. Ellis S.J. Gibbs MCC, Austin, Texas Abstract. Groupware systems are computer-based systems an hour or two in length but may be much shorter or much that support two or more users engaged in a common task, longer. At any point in time, a session consists of a group of and that provide an interface to a shared environment. These users called the participants. The session provides each par- systems frequently require fine-granularity sharing of data ticipant with an interface to a shared context, for instance and fast response times. This paper distinguishes real-time participants may see synchronized views of evolving data. groupware systems from other multi-user systems and dis- The system’s responsetime is the time necessary for the ac- cusses their concurrency control requirements. An algorithm tions of one user to be reflected by their own interface; the for concurrency control in real-time groupware systems is notification time is the time necessary for one user’s actions to then presented. The advantages of this algorithm are its sim- be propagated to the remaining users’ interfaces. plicity of use and its responsiveness: users can operate di- Real-time groupware systems are characterized by the fol- rectly on the data without obtaining locks. The algorithm lowing: must know some semantics of the operations. However the algorithm’s overall structure is independent of the semantic * highly interactive - response times must be short. information, allowing the algorithm to be adapted to many * real-time - notification times must be comparable to situations.
    [Show full text]
  • An Evaluation of Distributed Concurrency Control
    An Evaluation of Distributed Concurrency Control Rachael Harding Dana Van Aken MIT CSAIL Carnegie Mellon University [email protected] [email protected] Andrew Pavlo Michael Stonebraker Carnegie Mellon University MIT CSAIL [email protected] [email protected] ABSTRACT there is little understanding of the trade-offs in a modern cloud Increasing transaction volumes have led to a resurgence of interest computing environment offering high scalability and elasticity. Few in distributed transaction processing. In particular, partitioning data of the recent publications that propose new distributed protocols across several servers can improve throughput by allowing servers compare more than one other approach. For example, none of the to process transactions in parallel. But executing transactions across papers published since 2012 in Table 1 compare against timestamp- servers limits the scalability and performance of these systems. based or multi-version protocols, and seven of them do not compare In this paper, we quantify the effects of distribution on concur- to any other serializable protocol. As a result, it is difficult to rency control protocols in a distributed environment. We evaluate six compare proposed protocols, especially as hardware and workload classic and modern protocols in an in-memory distributed database configurations vary across publications. evaluation framework called Deneva, providing an apples-to-apples Our aim is to quantify and compare existing distributed concur- comparison between each. Our results expose severe limitations of rency control protocols for in-memory DBMSs. We develop an distributed transaction processing engines. Moreover, in our anal- empirical understanding of the behavior of distributed transactions ysis, we identify several protocol-specific scalability bottlenecks.
    [Show full text]
  • Constant Time Recovery in SQL Server
    Constant Time Recovery in Azure SQL Database Panagiotis Antonopoulos, Peter Byrne, Wayne Chen, Cristian Diaconu, Raghavendra Thallam Kodandaramaih, Hanuma Kodavalla, Prashanth Purnananda, Adrian-Leonard Radu, Chaitanya Sreenivas Ravella, Girish Mittur Venkataramanappa Microsoft One Microsoft Way Redmond, WA 98052 USA {panant, peterbyr, waync, cdiaconu, raghavt, hanumak, praspu, aradu, ravellas, girishmv}@microsoft.com ABSTRACT leverages Write-Ahead Logging (WAL) and defines distinct Azure SQL Database and the upcoming release of SQL Server recovery phases to eliminate ad-hoc recovery techniques. introduce a novel database recovery mechanism that combines Even though ARIES simplifies the recovery process and allows it traditional ARIES recovery with multi-version concurrency to be generic for all transactional operations, recovering the control to achieve database recovery in constant time, regardless database to a consistent state requires undoing all operations of the size of user transactions. Additionally, our algorithm performed by uncommitted transactions which makes the cost of enables continuous transaction log truncation, even in the recovery proportional to the work performed by these presence of long running transactions, thereby allowing large data transactions. This significantly impacts database availability since modifications using only a small, constant amount of log space. recovering a long running transaction can take several hours. A These capabilities are particularly important for any Cloud recent example that demonstrates the severity of this problem is a database service given a) the constantly increasing database sizes, case where an unexpected failure occurred while one of our b) the frequent failures of commodity hardware, c) the strict customers was attempting to load hundreds of millions of rows in availability requirements of modern, global applications and d) a single transaction.
    [Show full text]
  • The Worlds of Database Systems
    Chapter 1 The Worlds of Database Systems Databases to day are essential to every business. They are used to maintain internal records, to present data to customers and clients on the World-Wide- Web, and to supp ort many other commercial pro cesses. Databases are likewise found at the core of many scienti c investigations. They represent the data gathered by astronomers, byinvestigators of the human genome, and by bio- chemists exploring the medicinal prop erties of proteins, along with many other scientists. The p ower of databases comes from a b o dy of knowledge and technology that has develop ed over several decades and is emb o died in sp ecialized soft- ware called a database management system,orDBMS, or more collo quially a \database system." A DBMS is a p owerful to ol for creating and managing large amounts of data eciently and allowing it to p ersist over long p erio ds of time, safely. These systems are among the most complex typ es of software available. The capabilities that a DBMS provides the user are: 1. Persistent storage. Like a le system, a DBMS supp orts the storage of very large amounts of data that exists indep endently of any pro cesses that are using the data. However, the DBMS go es far b eyond the le system in providing exibility, such as data structures that supp ort ecient access to very large amounts of data. 2. Programming interface. A DBMS allows the user or an application pro- gram to access and mo dify data through a p owerful query language.
    [Show full text]
  • Exam 70-464: Developing Microsoft SQL Server Databases – Skills Measured
    Exam 70-464: Developing Microsoft SQL Server Databases – Skills Measured Audience Profile This exam is intended for database professionals who build and implement databases across organizations and who ensure high levels of data availability. Their responsibilities include creating database files, data types, and tables; planning, creating, and optimizing indexes; ensuring data integrity; implementing views, stored procedures, and functions; and managing transactions and locks. Skills Measured NOTE: The bullets that appear below each of the skills measured are intended to illustrate how we are assessing that skill. This list is not definitive or exhaustive. NOTE: In most cases, exams do NOT cover preview features, and some features will only be added to an exam when they are GA (General Availability). Implement database objects (30–35%) Create and alter tables develop an optimal strategy for using temporary objects, including table variables and temporary tables; define alternatives to triggers; define data version control and management; implement @Table and #table appropriately; create calculated columns; implement partitioned tables, schemas, and functions; implement column collation; implement online transaction processing (OLTP); implement columnstore and sparse columns Design, implement, and troubleshoot security implement data control language statements appropriately, troubleshoot connection issues, implement execute as statements, implement certificate-based security, create loginless users, define appropriate database roles
    [Show full text]
  • Revisiting Concurrency in High-Performance Nosql Databases
    Revisiting Concurrency in High-Performance NoSQL Databases Yuvraj Patel*, Mohit Verma+0, Andrea C. Arpaci-Dusseau*, Remzi H. Arpaci-Dusseau* *Computer Sciences Department, University of Wisconsin-Madison, +NVIDIA Abstract of five popular NoSQL databases – MongoDB [36], Cas- We measure the performance of five popular databases sandra [1], CouchDB [2], Oracle NoSQL DB [21], and and show that single-node performance does not scale ArangoDB [5]. Our first and most important finding: on a while hosting data on high-performance storage systems single node with many cores, all five systems do not scale (e.g., Flash-based SSDs). We then analyze each system, well as the client load increases; as a result, despite the unveiling techniques each system uses to increase concur- presence of a fast storage device (in this case, a modern rent performance; our taxonomy places said approaches SSD [11]), system throughput is notably lower than what into six different categories (thread architecture, batch- could be realized, sometimes by 3× or more. ing, granularity, partitioning, scheduling and low-level ef- To understand the concurrency control techniques uti- ficiency) and thus points towards possible remedies that lized in these systems, we classify each popular technique can scale the system. Finally, we introduce Xyza, a modi- into six categories: thread architecture, batching, granu- fied version of MongoDB that uses a wide range of classic larity, partitioning, scheduling, and low-level efficiency. and novel techniques to improve performance under con- Classifying these techniques helps us understand weak- current, write-heavy workloads. Empirical analysis re- nesses and identify why the databases are not scaling.
    [Show full text]
  • Chapter 1 Introduction to Databases
    Learning Objectives Database Systems: • The Three-Level ANSI-SPARC Architecture • Database Languages Thomas Connolly, Carolyn Begg, Database System, A Practical Approach to Design Implementation and Management, 4th Edition, Addison Wesley • Data Models and Conceptual Modeling • Function & Components of DBMS 1-1 Chapter 2 Part One: Database Environment | 1 Chapter 2 Part One: Database Environment | 2 The Three-Level ANSI-SPARC Architecture The Three-Level ANSI-SPARC Architecture • An early standard terminology and general architecture for • In 1975, The American National Standards Institute (ANSI) database systems produced in 1971 by Data Base Task Group Standards Planning and Requirements Committee (SPARC), (DBTG) and Conference on Data Systems and Languages ANSI/SPARC produced a similar terminology and and (CODASYL, 1971). recognized the need for a three-level approach with a system • The DBTG recognized the need for a two-level approach with a catalog: system view called the schema and user views called • External Level subschemas. • Conceptual Level • Internal Level Chapter 2 Part One: Database Environment | 3 Chapter 2 Part One: Database Environment | 4 The Three-Level ANSI-SPARC Architecture The Three-Level ANSI-SPARC Architecture Objectives • The users’ view of the External database. Level – All user should be able to access the same data. • Describes that part of database that is relevant to – A user’s view is immune to changes made in other views. each user. Conceptual – Users should not need to know physical database storage details. • Community view of the Level database. – The Database Administrator (DBA) should be able to change the • Describes what data is stored in database and database storage structures without affecting the users’ views.
    [Show full text]