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Databases are a set of data that is which is formatted in a standard manner that is designed to be shared by
Multiple users. A database can be described by definition as “A collection of interconnected data items that may be
is processed in one of the applications software”.
A database may be described by “A collection of data that is persistent that is utilized by the program
the systems of a particular company”. A company could be one person (with an incredibly small
personal database) or a complete corporatist or similar large body (with the benefit of a massive database shared),
or any other thing in between Denmark quality email lists.
Data is the base material from which valuable information is obtained. The word “data” is plural
of Datum. Data is often utilized in singular and plural form. It’s defined as uncooked facts or
observations. It comes in a various forms, such as numbers, text, images as well as voice. Data
is a set of facts which are unorganized, but is able to be to be organized and useful.
The words Data and Information come across in everyday life and are often used interchangeably.
Examples: Weights, Prices and costs, quantity of products sold etc.
The data that is processed in such a manner to enhance the understanding of the person who is using the data.
The information. Data and information are inextricably linked. Data are the raw materials that
They are then processed into final information products. The data is processed
so that it enhances the understanding of the person who is using to increase the knowledge of the person using it.
In real life, the database is today able to contain either data or other information.
The process of changing data into useful information is referred to as data processing.
Data processing can also be referred to by the name of information processing.
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Data that define the characteristics or properties of other types of data.
Data only becomes useful when it is used in a certain context. The main mechanism for providing
the context for data Metadata is the term used to describe data. Metadata are the data that define the characteristics, or properties of
Other data. These properties may include the definition of data or data structures, rules and data or
constraints. The Metadata provides the characteristics of data, but doesn’t include data.
It helps the database designer as well as users to know what data Exitmeans, what the data means and
What are the subtle distinctions between data items that appear to be similar. The managing of Metadata
is at a minimum as vital as understanding the data , since data with no clear significance could be
Incorrect, unclear, or confusing.
SUPER PROFESSIONAL UNIVERSITY 3
Unit 1: Database Fundamentals
1.2 Database System Applications Notes
Databases are extensively used. Here are a few examples of database applications:
1. Banking: To provide information about customers account information, loan details, and accounts as well as banking transactions.
2. Airlines for reservations as well as schedules. Airline companies were among the first to utilize
databases distributed geographically way – terminals located around the globe
Access to through the database central system via telephone lines and other networks for data.
3. Universities: For information about students courses, registrations for classes, and grades.
4. Transactions with credit cards: for purchases made with credit cards as well as the creation of monthly
5. Telecommunication: To keep records of calls, creating monthly bills
keeping balances on prepaid call cards, and keeping records regarding the
6. Financial: To keep track of information on sales, holdings and purchase of financial
instruments, such as bonds and stocks.
7. Sales for customer, product and purchase details.
8. Manufacturing: To manage the supply chain and to track the production of products in
factories, inventory of items in stores and warehouses, and requests for items.
9. Human Resources: To get details regarding salaries, employees benefits, payroll taxes and other benefits,
and also for the generation of pay checks.
1.3 Specifications of Database Approach
A common collection of data that is logically connected together with the information that is suitable to
the demands of large companies.
This section explains the fundamental difference between the conventional method for processing. It is also referred to as
processing of files, in addition to the Database method for processing data. Every operating system has
users can open, save and close files. The user can save the appropriate information within these files.
Have a look at figure 1.1 which depicts the conventional process of processing files that store the
Program and data description within an file. The information related to an application is
Files are stored in different files, such in various files such as File1, File2, etc. The files can be altered by using Program1,
Program2, etc. This was the method that was employed in the beginning.
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4 LOVELY PROFESSIONALS UNIVERSITY
Database Management Systems/Managing Database
Notes This means that, without the use of a DBMS the data would be stored in several files. Any
update, the files have to be opened, and then manually search for the line , or record, then update, and finally
Save the file. This will help you understand the challenges involved in making this kind of
Application program 1
with Data semantics with data semantics
Application program 2
with semantics of data
Application program 3
with semantics of data
Since the advent of databases The file processing method has been abandoned. Today, you
We can see in Figure 1.2 in Figure 1.2 that the data is on the disk, which controls the
DBMS. This is the way to approach Application Program-1 along with its data semantics the Application Program-1, also known as the
Program-2, along with its semantics for data and data semantics. Connect to the database that holds the actual data is stored.
And constraints can be stored in and constraints are stored through DBMS. and constraints are stored through the DBMS. DBMS is the primary control and
manipulative software modules in these applications to gain access to the data that is stored in
The applications will be completely free of the program-specific code that is dependent on the system, and also get data-driven programs.
Task: Find the different sources of databases management system.
Negatives of File Processing System
1. Catalog catalog: Catalog: DBMS the structure of the database is saved in a catalog. It also includes the
storage information with restrictions.
The DMBS software should work with any of the database software applications that are offered
The catalog contains the structure and other specifics of this application. In file processing
The definition of data forms part of an application software.
Example Record declaration in Pascal.
Structure declaration or class for structure declaration in C++.
2. Program-data independence: When processing files when changes are made to structures of
Then we could need to change the design of the program that connects to the file. If the file is in DBMS the access
Programs are written without regard to specific files. This is referred to as program-data.
The DBMS records the data in an approach that the user does not need to be aware of these particulars.
The concept is known as data abstraction. It could also be referred to as conceptual
Figure 1.2: Database Approach
AWESOME PROFESSIONAL UNIVERSITY 5
Unit 1: Database Fundamentals
3. The database could contain many users, and each user may be interested in specific Notes
View of the view of the. Views are conceptually a table. However, the table’s records do not exist.
that are stored in the database.
Take a look at the Student database, in which we have two options:
1. View: Student’ Grade in different classes. To find this information, consult the Tables Course
and Grade_Report should be created and joined as an view
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View 2. If we are looking to know about the prerequisite courses that students must learn, there are three
Tables are to be joined. These tables are nothing more than sections, student and prerequisite.
4. Processing transactions and sharing Processing and Sharing DBMS must ensure control over different users
trying to connect to the database.
Examples: Railway Reservation System with many counters.
When multiple users attempt to connect to the same app simultaneously This is what we refer to as
the situation is referred to as concurrent processing of transactions. Generally, simultaneous access is accomplished
using a basic Local Area Network (LAN). You can also purchase railway tickets on the internet
i.e. through the Internet.
1.4 Benefits of DBMS
One of the primary benefits of having a database management system is that it allows the company can
through the DBA control, centralization of control and management of the database. The administrator of the database
is the main focus of central control. Any application that requires an alteration to the organization of the structure of
Data record The DBA performs the necessary changes that do not impact other applications.
or the users of or users of.
The following are the main benefits of the use of the Database Management System (DBMS):
1. Redundancies are reduced Data Control Centralized by the DBA eliminates redundant
Duplication of data effectively reduces the total amount storage for data required. It reduces the need for storage of data.
It also removes the extra processing required to locate the needed data within the large volume of
data. Another benefit of not having duplicates is the elimination of inconsistent data
which are often found that are often found in that are often found in redundant that are often found in redundant data that are often found in redundant data.
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2. Data Independence and efficient access Application programs for databases are completely independent
of the specifics of storage and auto representation. In addition a DBMS provides efficient
storage and retrieval methods, which include support for extremely large files and index structures
Optimization of queries and.
3. Control of Data Integrity: Centralized controls will ensure that appropriate checks are carried out
within the DBMS to guarantee integrity of data. This means that the data stored in the database are secure.
It is accurate and reliable. Thus, the data values recorded for storage may not be
Verified to make sure they’re within a specific interval and conform to the proper format. Checked to ensure that they fall within the specified range and are in the correct format.
For instance, the amount for an employee’s age could be within the range between 16 and 75. For instance, the age of an employee could be between 16 and. It is also possible that it
It is important to ensure that, when there is a reference to an object, the object must be present. In the
In the case of an automated machine to teller, for instance the user isn’t allowed to transfer money
from a savings account with no existence from a savings account that is not in existence to a checking account.
4. Data Security: Data that is confidential should not be accessible to unauthorised persons. Different
Security levels could be implemented for different kinds of operations and data.
6 – LOVELY PROFESSIONAL UNIVERSITY
Database Management Systems and Managing Database
Notes 5. Reducing the time to develop an application The DBMS has a number of key
Applications require certain functions that are required by applications, like functions that are required by applications, such as functions, such as crash recovery and concurrency control.
Level of query facilities level query facilities. level query facilities, etc. must be written.
6. Conflict Resolution: As the database is under supervision of the DBA the DBA should be able to resolve conflicts.
the contradicting needs of various application and user.
The DBA determines the best access method and file structure to ensure optimal performance
the critical applications that require response, while permitting applications that are less critical to continue
to access the database, although with a slower response.
7. Data Administration by providing a an unifying base for a vast amount of data, it allows for
It is used by many users. that are shared with multiple users, DBMS assists in maintenance and data administration tasks. Agood DBA is able to efficiently ensure the correctness of the database representation, the regular backups
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8. The concept of concurrent access and crash recovery A DBMS is a good fit for the concept of a transaction as well as
performs the transactions’ actions in an interleaved way to achieve high performance.
But schedules them in such so as to make sure that no conflicting operations are allowed
to operate simultaneously. Furthermore to allow for concurrent processing, the DBMS keeps a permanent record of changes made to
the database, and in the event that there’s a system malfunction it will be able to restore the database to consistency in transactions
state. In other words, the actions of incomplete transactions will be unfinished. Therefore, if each transaction is completed
A single transaction keeps the same criteria in place in the database, and then maintains the same criteria.
Following a recovery from a crash is consistent after recovery from a crash.
Task Discuss What are the benefits of using an oracle instead of access.
1.5 Negatives of DBMS
The downside that comes with this DBMS system is its overhead costs. The processing overhead that is introduced by
the DBMS to provide security and integrity as well as sharing of data can cause an impairment of the
responses and throughput times. A further cost is that of the migration process from a conventional
Separate application environments in conjunction with an integrated application.
Although centralization can reduce duplicates, the absence of duplication demands that the database is maintained.
Be adequately backup to ensure that in the event in the event of a failure, the data will be recovered in the event of failure.
Backup and recovery processes are complicated within an DBMS environment. It is an increase
in a concurrent multi-user database system. A database system requires certain amount of
monitored redundancies and duplication for easy access to the related data items.
Centralization also means that data is accessible through one source, which is the database.
This can increase the severity of security breaches, and can cause disruption of the operations of the
Organizations suffer because of downtimes and failures.
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1.6 Database Architecture
The functions of a database can be broadly classified into query processors
components and components for storage management. The query processor consists of:
1. DML Compiler transforms DML statements from an inquiry language into instructions at a low level
that the engine for query evaluation can comprehend.
AWESOME PROFESSIONAL UNIVERSITY 7
Unit 1: Database Fundamentals
2. embedded DML Pre-compiler This converts DML statements embedded in application Notes
The program must be able to make normal procedure calls within the language of host. The pre-compiler should interact
using and the DML compiler to create the correct code.
3. DDL Interpreter: It interprets DDL Stateline and stores the results in a set of tables that contain
4. The Transaction Manager ensures the database is in a stable (correct) state.
despite any system malfunctions, and concurrent transactions are executed without
5. File Manager: Controls the allotment of disk storage and also the data structures that are used
to represent the information that is stored to represent information stored on disk.
6. Buffer Manager is responsible to fetch data from disk storage and transferring it to main memory, and
deciding which data to store to memory.
Additionally, certain data structures are needed in connection with the physical system’s implementation:
1. Data Files: These files contain the database itself.
2. Data Dictionary: It stores data regarding the database’s structure in the manner it is utilized
3. Indices: It gives quick accessibility to items of data which hold certain values.
4. Statistics Data: It is statistics about the data contained in the database. It is
information that is used in the process of query processing to ch
Figure 1.3 the structure of DBMS
8 LOVELY PROFESSIONALS UNIVERSITY
Database Management Systems and Managing Database
Case Study Requirements Analysis
B&N’s owner B&N has considered what he would like to achieve and has provided an easy outline:
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“I would like my clients to have access to my book collection and also place an order
via using the Internet. At present, I accept orders by phone. I mostly work for corporate
Customers who contact me and provide me with my ISBN numbers for the book as well as the quantity. I will then
make a shipment of the books they’ve purchased. If I do not have enough
copies available I place an order for more copies and hold off the delivery until I receive I receive the new copies
Arrive; I’d like to send a customer’s entire order in one. My catalog contains all of the books
I also sell. Each book’s catalog will include the ISBN number the title, author, and ISBN number. purchase
cost, sales price as well as the year in which the book was released. My customers are mostly regulars,
as well as my own records that include their name, address as well as their credit card numbers. Customers who are new have
To call me first, to establish an account prior to they can access my web site. I would advise them to call me first and set up an account.
On my brand new website the customers must first identify themselves with their individual customer
identification number. After that, they will be able to browse through my catalog and place orders
The DBDudes’s experts are amazed at how fast the requirements phase went
Once they’re done, it can take several weeks of discussions (and numerous meals and lunches) to
finish the task, but then get back to their office to review the information.
The term database refers to a repository of data that can be persistent and is used by an software system that runs some
enterprise. It could be an Bank or a Hospital or an Educational Institution, a Library,
etc. The word”persistence” means that once the data in the database can be recognized by the DBMS and is subsequently able to be accessed by the DBMS, it will
Then it can be removed from the database only upon the specific request. It cannot be erased or altered because of a adverse effect, just as the program.
Language variables for language. There are many advantages to keeping the data in a the database instead of storing it in
Operating system files. A good example is the university database to illustrate this idea was used to illustrate this concept.
discussed. In the DBMS context, we talk about several individuals. For instance, the principal users are
There are DBA, Database Designers, and a variety of other users include DBA, Database Designers, and various types of. While database approaches do have some disadvantages, it is not without its limitations however, it is not the only option.
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the use of databases is vital. The most significant implications of the using databases are:
Potential for enforcing standards
Reducing the time to develop applications
Security and integrity of data
AWESOME PROFESSIONAL UNIVERSITY 9
Unit 1: Database Fundamentals
The database should not be used all the time. There are instances when you can manage your data using Notes
Access an information database.
Data Abstraction The database management software is set of files that are interconnected and an array of data
of software that allows users to modify and access the of software that allows users to modify and access these. One of the main functions of databases is to allow users to access and modify them.
System is designed to give users an abstract understanding of data. This is known as data abstraction.
Data processing is the process of changing the facts into useful data is referred to as
Data processing. The process of processing data can also be referred to by the name of information processing.
Data The term “data” refers to the raw substance from which information is obtained.
Database: A collection of data that is logically connected together with descriptions of data
is suited to the requirements of large companies.
Metadata: Data that describes the characteristics or properties of data.
1.9 Self Assessment
Select the correct answer:
1. DBMS is the abbreviation for:
(a) Database Managerial System
(b) Database Management System
(c) Database Management Source
(d) Development Management System
2. Data processing can also be referred to as
(a) Programming data
(b) Data access
(c) Processing of information
(d) Database sourcing
3. DDL is the abbreviation for
(a) Data Development Language
(b) Document Language Document Language
(c) document definition language
(d) Data Definition Language
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Database Management System
9. In DBMS the access programs are written independent of any specific ……………………………..
10. In file processing the data definition is part of the …………………………….. program.
1.10 Review Questions
1. Define database. Define the terms used in the an environment of databases.
2. Discuss and list different Database System Applications.
3. What are the main differences between the File Processing Systems and DBMS?
4. Write about the benefits of DBMS.
5. Make short notes about the disadvantages that come with Database Management System.
6. What is Data Independence? Define the different kinds of Data Independence.
7. What are the databases languages? Define the different languages.
8. What are the duties of the role of a DBA? Give them a list and explanation.
9. What is the function of a Data user? Discuss the various kinds of users.
10. Discuss the design of DBMS.
11. Explain the various components of DBMS.
12. The history of writing for Database Systems.
Answers for Self-assessment
1. (b) 2. (c)
3. (d) 4. Indices
5. plural 6. information
7. Metadata 8. view
9. files 10. application
1.11 Additional Readings
Books C.J. Date introduction to Database Systems, Pearson Education.
Elmasri Navrate Fundamentals of Database Systems, Pearson Education.
Martin Gruber, Understanding SQL, BPB Publication, New Delhi
Peter Rob & Carlos Coronel, Database Systems Design, Implementation and
Management, 7th Edition.
Raghurama Krishnan, Johannes Gehrke, Database Management Systems, 3rd Edition,
Tata McGraw Hill.
Silberschatz, Korth, Database System Concepts, 5th Edition, McGraw Hill.
AWESOME PROFESSIONAL UNIVERSITY 11
Unit 1: Database Fundamentals
SIlberschatz-Korth-Sudarshan, Database System Concepts, 4th Edition, Tata Notes
Vai Occardi Relational Database: Theory & Practice, BPB Publication, New Delhi
Online Links www.en.wikipedia.org
12 LOVELY PROFESSIONALS UNIVERSITY
Database Management Systems/Managing Database
Notes Unit 2 Notes Unit 2: Database Model of Relationships
2.1 Relational Model
2.1.1 Relational Model Concepts
2.1.2 Solutions in the Relational Model
2.1.4 Applicability to Databases
2.1.5 SQL and the Relational Model
2.1.6 Set-theoretic Formulation
2.2 Advanced Relationsal and Extensive Algebra Algebra Operations
2.2.1 Relational Algebra Expression
2.2.2 The Set Operation Relational Algebra
2.6 Review Questions
2.7 Additional Readings
After completing this unit, you’ll be able identify a relational model. the additional and extended algebraic operations
A relational database is an array of tables that are used to store specific kinds of information. The
The invention that is a database has standardized the method the data is processed and stored.
The idea behind the relational database comes from the fundamentals of relational algebra. It is implemented as
an entire system developed by the founder of the relational database, E. F. Codd. A majority of databases are currently used
Today, we are built in the relationship system.
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2.1 Relational Model
The relational model used for managing databases is a database model that is based on a the first-order predicate
logic was first developed and first proposed in 1969 in the year 1969 by Edgar Codd.
Pooja Gupta Lovely Professional University
SUPER PROFESSIONAL UNIVERSITY 13
Unit 2 Modeling Database Model. Relational Model
The basic idea behind it is to define the database as a set of predicates that are based on an infinite set of predicate
variables, which describe restrictions on the possibilities of values and combinations. The contents
of the data at any time , is a finite representation (logic) for the data, i.e. the set of relationships,
one per predicate variable so that all predicates satisfy. Request for details to
The databases (a queries to the database) is also an example of a predicate.
2.1.1 Relational Model Concepts
The primary purpose of the model is that it provides a formal method of describing data, and
inquiries: we explicitly state what data the database holds and the details we
What you want to get it to get from it. Let the database management software manage the process of describing the data
storage structures for the data as well as retrieval processes to answer queries.
IBM adopted Codd’s ideas using the DB2 database management system. it also introduced the
SQL Data definition, query and language. Other relational management systems for databases
They are all used SQL and SQL. A table within an SQL schema for databases is akin to the concept of a
Predicate variable; the content of a table in the relation key constraints, additional constraints and
SQL queries can be compared to predicates. However, it is to be remembered it is true that SQL databases, which includes
DB2 is a deviation from the relational model in a number of specifics; Codd strongly fought against the deviations
that undermine the basic principles.
2.1.2 Alternate models for the Relational Model
Other models include the hierarchical model and the network model. Certain systems employing these models.
These architectures are still in use currently in data centers that have massive data volumes, or in areas where they are not currently used.
Systems are so complex and abstract that it would be prohibitive to switch to systems that use
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the relational model. Also to be noted are the newer object-oriented databases. However, they are not all
They are DBMS-construction kits instead of actual DBMSs. The most recent innovation is an improvement in the ObjectRelation type-Object model. This relies on the idea that any truth can be expressed using
The model is composed in which one of the binary relationships. The model is employed to model objects in Object Role Modeling
(ORM) (ORM), Notation/RDF 3 (N3) (ORM), RDF/Notation 3 in Gellish English.
Relational models were the initial model of a database. Following its definition informal models were developed.
They were developed to define the hierarchical database (the hierarchy model) as well as network databases.
(the network model (the network). Network and hierarchical databases existed prior to relational databases
Figure 2.1 2. Relational Model
14 LOVELY PROFESSIONALS UNIVERSITY
Database Management Systems and Managing Database
Notes were models described after the model of relational was established so as to create
A basis to compare.
There have been numerous attempts to develop a complete application of the relational database
model, as first described by Codd and further explained model as originally defined by Codd and explained Date, Darwen and others however none of them have
So far, the songs have received a lot of attention. Rel is among the most recent attempts at this.
It was developed by E.F. (Ted) Codd as a generalized model of data. It was later reformulated by Codd as a model of
created and maintained created and maintained by Chris Date and Hugh Darwen along with others. in The Third Manifesto
(first released (first published in the year 1995) Date and Darwen show how the model of relation can be used to support
certain desirable object-oriented features.
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Codd himself, shortly after the release of his model in 1970 He proposed the concept of a three-valued logic
(True False, False, Missing or NULL) version to handle the absence of information, as well as in his
In the Relational Model for Database Management Version 2 (1990) He took it one step further by introducing an
Four-valued logic (True False but but not applicable) version. However,
These have not been implemented, probably due to their high complexity. SQL’s NULL
This was as a part of an underlying three-valued logic system however, it fell short due to
Logical errors in the standard as well as in its implementations.
The principle base assumption behind the relationshipal model is every data is represented mathematically
relations n-ary, an n-ary relationship being a subset of that of Cartesian product of two domains. In the
mathematical models, the reasoning the data is performed using two-valued predicate logic which is a reference to
there are two possible assessments for each claim one of which is either true or false (and specifically, there is there is no
Third value, such as unidentified third value, such as unknown, or not applicable both of which are connected with the
Concept of notion of NULL). Two-valued logic, as some believe, is an essential to the model of relational relationships,
Where others believe an algorithm that is based on a type of three-valued logic is still a valid option
Data is operated on using the relational calculus or algebra. These are
equivalent in terms of expressive power.
A relational database model allows developers of database to construct an logical, consistent
representation of data. Consistency is achieved through the inclusion of explicit limitations in the
Database design, often referred to as the logical schema.
The theory encapsulates a method of normalization of databases that allows for designs that have certain desirable characteristics are normalized.
The properties are available from a list of logically similar options. Access plans and
other details of operation and implementation are managed through the DBMS engine other details of operation and implementation are handled by the DBMS engine, and not
The logical model is reflected in the. This is different from the standard practices for SQL DBMSs, which the logical model is reflected in
Performance tuning usually requires modifications in the model.
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The fundamental building block of a relational system is the data type or domain which is often abbreviated to
type. Tuples are an unordered array of attributes. An attribute is a paired order of attributes.
Name and type. The value of an attribute is a valid and specific value for the kind that the attribute is. This
It could be the scalar type or a more complicated type.
AWESOME PROFESSIONAL UNIVERSITY 15
Unit 2 Modeling Database Model. Relational Model
A relation is made up of a heading and body. Headings are a collection of characteristics. A body (of notes) is a body of
relationship) is an n-tuple set. The head of the relation is the heading of every one it’s tuples.
A relation can be defined as an n-tuple set. Both in mathematics and using the database relational model
A set is an unorderly collection of items, though some DBMSs require an order on their data. A set is a collection of items that are ordered.
math, a tuple is a mathematical unit that has an order and is able to allow for duplicates. E.F. Codd originally defined
Tuples with these mathematical terms. Then the idea was considered to be one of E.F. Codd’s great insights that
using attribute names in lieu of an order is much more efficient (in the general sense) in
Computer language is built on relations. This idea is still employed in the present. However, it’s not the same,
Conceptualization has changed, but however, the name “tuple” is not. The immediate and crucial result of
This distinctive characteristic is that within the relational model, the Cartesian product is transformed into
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A table is a commonly accepted graphic representation for a relationship and a tuple has a similar concept to the notion of row
But take note that in data-driven language SQL the columns and rows of tables are arranged.
The term “relvar” refers to a name variable that is a specific type, which is every time a particular relationship
This type of relation is assigned, even though the relation could have zero of tuples.
The principle that drives this model of relation is called the Information Principle All information is
The data is represented as values in the form of data values in. According to this Principle it is a relational database
is a collection of relvars. The results of each query is shown as an underlying relationship.
The reliability of a relational database is enforced by rules embedded into the software that
It is not used, but rather by constraint definitions, which are an element of the schema logical, and controlled by DBMS
for all kinds of applications. Constraints are generally defined using relational comparators,
of which of which one “is subsets of” that is theoretically enough. In actual use, there are a variety of helpful shorthands are available,
Are expected to be in stock and the most important ones are candidate key (really superkey)
and foreign key limitations.
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To fully comprehend the relationship model of data it is crucial to know the intent behind the model.
Interpretation of a relation.
A body relationship is often referred to as its extension. This is because it’s to be understood as
A representation that extends a predicate, such as the set of truthful statements that
It is possible to form a predicate by substituting each free variable of that predicate with the name (a term that denotes
There is a one-to-1 connection between the parameters of the predicate as well as the attribute
names of the heading of the relation. Each tuple in the relation body gives attribute values to
Instantiate the predicate by substituting all of its variables that are free. The outcome is a predicate that
It is believed, because by the appearance of the word tuple within the relationship body to be the case. Contrariwise,
every tuple with a heading that is identical to the heading of the relation, but is not in the
Body is believed to be an illusion. This is known by the term “closed world.
For a detailed explanation of these concepts, refer to this section Set Theory Formulation.
Task Write the total amount of rules outlined to you by E.F. Codd.
Recovery System The database system has to be prepared to ensure integrity and atomicity of the database are maintained.
the properties of transactions on computers, as every other gadget, can be susceptible to failure due to
There are many reasons for this that could be the cause: disk failure or power loss or software glitch, a an explosion inside the machine room and even
sabotage. In the event of a failure, information can be lost. They are also saved. A fundamental part of the database
The system is a recovery plan that allows you to restore the database to its original state was in place
prior to the before the. The recovery strategy must also be highly reliable; that is, it should
limit the period for which the database will not be usable following the crash. This will reduce the time for which it is not usable after.
11.1 The Introduction to Crash Recovery
A transaction could fail due to of hardware or software malfunction. It is the duty of the
Recovery manager is designed to deal with such failures and to ensure durability and ‘atomicity’. It achieves
Atomicity is achieved through the undoing of non-committed transactions. Also, it improves the durability of transactions by keeping the
committed transaction results persist even after the system has crashed.
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In normal operation, the transaction manager handles serializability through the provision of locks
If it is it is. It writes the data onto the disk to ensure that data is not lost in the event of a system crashes.
11.1.1 Involving Frames in Stealing and forcing
1. Steal Approach: Any changes that are made to an object ‘0 through a transaction are recorded on the
disk, even before the transaction has been recorded. This is due to the fact that another transaction requires disk before the transaction is committed.
the page is loaded and the buffer manager determines replacing frame with object “0” as the best option.
2. Forcing Approach: Every one of objects in the buffer pool are moved to disk once the transaction is completed.
The most straightforward way to implement Recovery management would be to adopt a no-steal-force methods. It is possible to achieve this with
There is no theft, the information is not written until a transaction has been committed so there is no need for
Undo operation, and force approach permits users to copy data on the disk following commit,
Therefore, we don’t need to perform the redo operation.
Although these strategies are easy however, they come with a few drawbacks. No steal approach requires
A large buffer pool. Force approach involves expensive I/O costs.
When an object gets regularly modified, then it is necessary to be written to the disk frequently
involve expensive I/O operation.
Therefore, the steal and no-force strategy is followed by the recovery management. Utilizing this
Techniques that ensure that the page is not recorded on disk when the modification process is running. In addition,
it doesn’t make a page written to disk, once a the transaction is committed.
Recovery – Related steps during Normal Execution
The recovery manager saves the changes made to the storage that does not react to
system failures. The storage system is known as stable storage.
The modifications made to information is known as the log. The recovery manager load the log on the stable
storage prior to the changes are changed.
If a transaction is terminated and logged, it allows the recovery manager to undo operation and start again
the operation if it’s performed.
A force-free approach will not write data to the disk following the time the transaction has been completed. If there is
If the transaction was made just prior to a crash, then changes made by transaction not be reflected in the crash.
It is then transferred to the disk. The modified data is loaded by the stabilization storage.
The Steal method lets you write data to the disk prior to making a decision to commit. If there is a crash
Before committing, any data that has been modified on the disk needs to be erased. This is accomplished by
1. Analyse Phase: It examines the buffer pool in order to find the transactions that are in use and dirty transactions.
2. Undo: When changed data has been loaded onto the disk prior to committing a transaction the transaction, then it
You must reverse the modifications in the event of a crashes.
3. The Redo phase has to restore the data was there prior to the crash. This can be done if there is data is damaged.
Modified by a committed transaction is not stored on the disk.
4. Rollback: All log entries are kept in a linked listing and for rollback operations using the linked list
It is accessed in reverse order.
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11.2 Failure Classification
There are many kinds of failure that could happen in a system each one of which has to be addressed
by using a different method. The most straightforward kind in failure doesn’t result in loss of
data within the information within the. The problems that are the most difficult to handle are those that arise from
that result in the data loss. Different types of failure can result:
Failure of a Transaction
There are two kinds of mistakes that could result in a transaction failing:
1. Error in logic: The transaction cannot continue the normal flow of execution due to
an internal issue like poor input, data not found an overflow or the resource limit
2. System error System error: The system is in an unwelcome state (for instance deadlock) in the form of an
consequence of which the transaction is unable to continue the normal course of its execution. The transaction is
However, the execution can take place at a later date.
There’s a hardware problem or a flaw in the software for databases or operating system that
The loss of content of volatile storage and causes transaction processing to come to a standstill. The
The content of nonvolatile storage is in tact and isn’t damaged.
The idea that hardware failures or bugs in the software can bring the system to slow halt, however, they can
that will not damage the nonvolatile contents. This is known as the fail-stop hypothesis. Well-designed
The systems are able to perform numerous internal checks, both at both the hardware and software level, which make sure that the
the system will stop when an error occurs. Thus, the fail-stop assumption is a sensible one.
A disk block is lost contents due to either head crashes or the failure of a transfer
operation. Data copies on different disks or archives on third-party media, like
tapes can be used to recuperate from the damage.
To figure out how the system can be able to recover from failures, we must identify the cause of failure.
methods of the devices that are used to store the data. We must then think about the failure
influence the contents of the impact the content of the database. Then, we can propose methods to ensure consistency of the database.
and transaction atomicity in the event of failure. These algorithms, also referred to as recovery algorithms
Two parts are present:
1. In normal processing of transactions to ensure that there is enough information
allows for the recovery of failures.
2. The actions taken following failing to retrieve the database’s contents to a state that is secures the integrity of the database
the consistency of transactions, atomicity of transactions, and long-term durability.
Task If your system isn’t functioning properly, it could be a the result of a system or logical error.
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11.3 Storage Structure
The different data items contained in the database could be saved and accessed through a variety of ways.
storage media. To comprehend how to maintain the durability and atomicity of storage media.
transaction, we should be able to better understand the storage media used and the methods of accessing them.
Storage media are differentiated by their capacity, speed and the ability to withstand failure
and classified as nonvolatile storage and volatile storage. We go over these terms and then introduce
Another type of storage is also known as stable storage.
Stable Storage Implementation
To ensure that storage is stable it is necessary to duplicate the required information into various non-volatile
Storage media (usually disk) with failure modes that are independent and the ability to change the information stored on
A controlled method to ensure that any failure in data transfer doesn’t damage the data that is needed
RAID systems are guaranteed that failure of one disk (even when data is transferred) will not be a problem.
This can lead to data loss. The most simple and fastest type is RAID can be the mirror disk. This ensures that data is not lost.
two copies of each block two copies of each block, on different disks. Other types of RAID provide lower cost however at the cost of
cost of performance that is lower.
RAID systems, however are not able to protect against data loss caused by catastrophes like floods or fires.
Many systems save archives of tapes off-site to protect against disasters like this. However,
Since tapes are not able to be transported off-site indefinitely, any updates made from the last time that tapes were taken off-site may be lost in the event of a catastrophe. Secure systems will keep copies of every
Block of storage that is stable at an unidentified location, and writing it out on the internet and in addition to
saving the block to the local disk system. Because these blocks can be output on a distant system and
when they are being output to local storage, after an operation to output is complete when the output operation is completed, it will not be available for download.
The data is not lost even during a catastrophe like a flooding or fire. We analyze remote backup systems
This section we’ll discuss the ways data storage devices can be safeguarded from damage during data transfer.
Block transfer between disk storage and memory can lead to:
1. Successfully completed Transfer of information successfully completed. The data was received in good order at its destination.
2. Partially failed: A malfunction occurred during the course of transfer as the target block was damaged.
3. Failure total The failure was triggered so late in the transfer process that the destination was not reached.
Block remains in good condition.
If there is a failure in the transfer of data the system is alerted and initiates the recovery process.
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process to return the block back to its normal condition. To achieve this it is necessary for the system to maintain two
Physical blocks are used for every database block in a logical manner; for two disks that are mirrored the two blocks are both at
the same place; when it comes to remote backup the block is local, while the other one is
on a remote location. A output operation is carried out in the following manner:
1. Then, write the information onto the first block of physical.
2. Once the first write has been completed successfully, you can copy exactly the same details onto the second
3. The output is finished only when the second write has completed successfully.
When recovering, the computer checks every block of the physical. If they are identical and there is no
If an error can be detected, the next steps are not required. (Recall that errors can occur within the disk block,
like partial writes to the block discovered by storing checksums for every block.) If the checksum is not present, it will be detected.
The system detects an inconsistency in one of the blocks, and it replaces its content by what’s in the other block.
block. If both blocks are free of discernable error, however they differ in content then the system
Replaces the contents from the initial block by the value of the second block. This recovery process
makes sure that a write to a stable storage device either is successful completely (that means, it ensures that all copies are updated) or
result in no change.
The necessity of comparing each block with its corresponding counterpart during recuperation is costly
to be met. It is possible to reduce the costs significantly by keeping track of the block write-ups that are currently in process,
by using a small amount nonvolatile RAM. In recovery there were only blocks for which write-ins were present were restored
progress must be evaluated.
The protocol for writing out an entire block to a remote server are the same as those for writing
blocks onto the system of mirrored disks
This procedure can be extended easily to permit for the usage of an unintentionally huge number of copies
each storage block is stable. While a lot of copies can reduce the chance of the loss of
failure that is less than two copies normally allow to try to replicate stable storage using
Access to Data
The database system is always within nonvolatile storage (usually disks) and
They are divided into fixed-length storage units , referred to as blocks. Blocks are the primary units for transfer of data
as well as from disks and disks. It may also contain multiple and from disk, and may contain several. We’ll assume that each data item is not spread across two
There are only two copies.
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Database Management Systems and Managing Database
Notes or blocks. This assumption is valid for the majority of data processing applications such as our
Transactions load information from the disk into the main memory. They then out the data
and then back to the disk. Input and out functions are carried out by block units. Blocks that are residing
The disks are known as physical blocks. blocks that reside temporarily in the main memory
These are known as buffer blocks. They are also referred to as buffer. The memory space in which blocks are stored temporarily is referred to as
The disk buffer.
Block movement between the disk and main memory occur by two of the steps that follow.
1. input(B) transfer the block of physical memory B into main memory.
2. output(B) moves the buffered block B onto the disk and replaces the physical
Each transaction Ti is assigned an exclusive work area which there are copies of the data items that were accessed are kept.
The Ti updates are maintained. Ti creates this work space at the time of the transaction and the
The system eliminates it once the transaction is either committed or terminates. Every data item X is stored in the
The work area of the Transaction Ti is indicated by Xi. The transaction Ti communicates to the database system through
Transferring data from its work area to and from the buffer of the system. We transfer data using these two ways
A buffer block is written to the disk, or because the buffer manager is in need of the
memory space used for other reasons, or due to the fact that the database system wants for the database system to be aware of the modification
The disk has a buffer B. We can declare that the system for database is performing the force-output of buffer in the event that it
produces output(B) output(B).
When a transaction must access an item of data X for the first time it has to run reading (X). The
The system will then make every update to X system, which then performs all updates to X Xi. When the transaction has accessed X for the last time it
It is necessary to execute write(X) to reflect the change in X within the database.
In the output(B x) operation performed for the B buffer, on where X is located, doesn’t require any action to take effect.
immediately following write(X) is completed after write(X) is executed, as the Bx block could contain additional data items that
It is still accessible. Therefore, the actual output might be delayed. Note that, if the system is operating, it will produce output.
The crash occurs when it was discovered that the write(X) process was completed but prior to output(Bx) was completed The new
The value of X is not written down to disk and, consequently, it is lost.
Recovery and Atomicity
Think about our simple banking system, and consider the transaction Ti that transfer $50 from our account
A B to account A with the initial values for A and B of $1000 and $2000 respectively. Imagine that
an error in the system has occurred in the course of Ti after the output (B A) was completed however,
prior to when output(B before output (B) was run, where B A as well as B B are the buffer blocks that were executed prior to output (B B), on which B
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Because the contents of the memory were lost, we don’t know what will happen to the transaction, and therefore we do not know where the memory contents
It is possible to trigger one of two possible methods of recovery:
1. Apply Ti. This can result in amount of A changing to $900, instead of $950.
This causes the system to enter an inconsistency condition.
2. Do not perform Ti. The current state of the system contains value of $900 and $2000 in A and B.
respectively. This causes the system to enter an inconsistency state.
In any case the database will be left in a state of confusion which is why this easy plan of recovery
The database isn’t working. The reason we are having this issue is that we’ve modified the database, but without
with the assurance having confidence that this transaction can be able to. We aim to complete either in all or none
changes to databases made by Ti. However, if Ti performed multiple database modifications,
various output operations could be needed, and a malfunction could happen after one of these
Changes have been made however, before they changes are made, they must be completed.
To reach our goal of atomicity we first need to output information that describes the modifications
to secure storage for a long time, without altering the database in any way. As we will see the procedure will
Allow us to output all changes that are made during a transaction, even if it fails.
Recovery algorithms help assure consistency of databases and to ensure atomicity
and endurance despite and endurance despite. The recovery algorithms are made up of two components:
1. What is done in normal transaction processing are to make sure that there is enough information
exists to recuperate from failures
2. What actions are taken following an inability to restore the database’s contents back in a condition that guarantees an atomicity
Consistency and endurance.
In the process of altering the database without making sure that the transaction will actually be committed, could result in
The database is in an inconsistency condition. Let’s look at an example transaction called T1 which transfers
1000/- of funds from account X into account Y. The goal is to execute all database changes that are made
by T1 or no by T1 or none. By T1, x is modified by subtracting 1000+/-, and then modifies Y by adding
` 1000/-. The failure could be observed after one of these changes was implemented, but prior to all of them have been made.
These are the products that are produced. To ensure consistency even in the event of failings, we’ve put in place multiple ways to recover.
Logs are maintained by a durable storage media. It is a collection of log records and keeps track of
A record of updates in the database. When the transaction Ti begins the process, it will register itself
1. Log record records that Ti has written on the data item X. The value of X was V1 prior to the write.
write. It will also be of value after the write.
2. When Ti completes its last sentence when it is done, the log record is written. We take it as
Now that log entries are recorded directly onto a secure memory media (that also means they do not
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Two ways to recover with logs are:
1. Deferred database modifications.
2. Immediate database modification.
Deferred Modifications to the Database
The deferred modification scheme for databases tracks all modifications made to the log, however it delays the log.
all writes all writes that occur after a partial commit. Let’s assume that transactions run in serial order, to make it easier for
The transaction begins with a written document. record into record to log. Write(X) operation results in a log. write(X) operation produces the creation of a log
Record the data is being written, and V is the value that will be used for the value X. The write does not take place on
X at the moment however, it is not deferred. If Ti is committed in part, The log is then written to.
The log files are read and utilized to perform the previously deferred write.
After a crash, when recovering the transaction has to be re-done in the event that both and
There are log entries in the. The process of redoing the transaction Ti (redoTi) alters each data item’s value which have been updated through
the transaction to changes in value. It is possible for crashes to occur during
1. The transaction is running the original updates,
2. As recovery actions are being carried out.
Immediate Modification of Database
The scheme for immediate modification of databases lets database modifications be made immediately on the database that is stored
even for transactions that are not committed. The updates are issued as the write-ups are issued (since
It is possible that reversing the process is required to undo the process, so update logs should contain both the previous value, as well as an updated value).
Log records that have been updated must be created prior to when the an item in the database is created (assume that the log record is updated).
can be directly output to a secure storage device and is extended to delay log record output, so long as
prior to the performing the input (Y) operation on the data block Y, all log records that are corresponding
Items Y have to be flushed into stable storage).
The output of updated blocks may occur at any point either prior to or after the transaction has been committed. Make an order
The order in which the blocks are output may differ from the order the blocks are put in.
The process of recovery in this case is comprised of two parts instead of one
1. undo(Ti) resets all the data elements that were updated by Ti to their previous values, and then moves them to the new value.
reversed from the previous log record for Ti,
2. redo(Ti) changes each data item that have been updated by Ti to new values and moves forward
from the log entry from the first log record Ti.
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Both operations are irreversible which means that they are idempotent even if the same operation is repeated several times.
Effect is the same when the effect is the same if it’s executed only the same way if it is executed once. (This is because some operations could need to be repeatedperformed.
executed during recovery).
In each of the situations as follows:
1. Undo (T1) 1. undo (T1): You are restored to 8000, and the X value is raised to 10000.
2. Undo (T2) as well as redo (T1) 2. Undo (T2) and redo (T1): Z is set to 20000, and then X and are reset to 9000 and 9000
3. Redo (T1) (T1) and redo (T2) 3. redo (T1) and redo (T2): Redo (T2) and X have been set at 9000, 9000 each. Then Z is set to 19000.
The following issues arise in the recovery process:
It takes a long time since we don’t know the reliability of the database.
following restart. So, we may needlessly redo transactions that have already completed their
Updates on the database.
This will allow us to streamline recovery process by performing periodic check-pointing. Make sure to check
1. Output of all log records that are currently stored in the memory that is non-volatile onto solid memory
2. Output all modified buffer blocks onto the disk.
3. Write a log file checkpoint> on a reliable storage.
During recovery , we should take into account only the most recent transactions that were initiated prior to the
checkpoint , and it isn’t completed until checkpoint and transactions began after the checkpoint. Scan
reverses backwards from the end of the log to determine the most recent record. Continue scanning
backwards till a record The log is discovered. You only need to look at a portion of the log that follows the previous paragraph.
Start record. The initial portion of the log could be lost during recovery and could be deleted
Any time you want. All transactions (starting at Ti and onwards) without Execute,
undo (Ti ). (Done only in the event that an immediate modification scheme is employed). Forward scanning in the
Log, which includes any transactions that begin at Ti or later, with an Perform a redo(Ti).
Recovery of Concurrent Transactions
We can alter log-based recovery systems to permit multiple transactions to run
concurrently. Each transaction shares one disk buffer as well as one log. The buffer block can contain
data items are updated as a result of the result of one or several transactions. We will assume that concurrency control is achieved through strict
Two-phase locking is performed as explained earlier. The checkpointing method and actions
Recovery transactions must be modified since multiple transactions could be in effect at the time of the checkpoint is in use.
The work is carried out.
Checkpoints are carried out like before, with the exception that the log entry for checkpoints has changed to the format
< checkpoint L>
which is L, the transaction list that were active at the moment at the point of checkpoint. We assume that there are no changes.
is in process while the checkpoint is being completed. After the system has recovered from an accident, it will begins by resuming the checkpoint.
Does these things:
1. Initializes the redo-list and undo list to empty
2. The log is scanned backwards starting from the point at which it reaches the end. Stops when the first Record record
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For each log entry found in the backward scan
1. If the record includes Add Ti to your redo-list.
2. If the record is containing If Ti isn’t in redo-list, then add Ti to the undo-list
3. For each Ti in L If Ti is not on redo-list then add Ti to the list of undos.
Undo-list at this point consists of incomplete transactions, that need to be rectified, and redo-list must be reconstructed.
comprises of completed transactions which need to be redone.
Recovery continues now in the following manner:
Scan logs backwards, starting with the latest record, and stop at the point The records have been
There is a record for every Ti that is found in the Undo-list. During the scan, you will perform undo for every log record that you
is a transaction on the the undo-list. Find the most current record.
forwards from the Keep track until the end the journal until the end of the. During the scan process, the scan again
for each log entry which is a part of an activity on the redo-list.
SQL doesn’t have any specific commands for recovery , however it does allow explicit COMMIT,
ROLLBACK and the other commands.
11.7 Buffer Management
When the database is upgraded there are a number of records that are altered in the buffers assigned to the log
records databases, and records and database. While buffer management is the responsibility that the system
However, there are times when there are times when DBMS have buffer management policies that are their own.
Buffering of Log Record
Log entries are buffered in the main memory instead of being sent directly to an external storage device.
storage media. Log records are written to a permanent storage the log record block is within the
buffer is fully filled when an operation to log force is completed. Log force operations are executed to make a transaction permanent.
by forcing all log files (including records for commit) to be stored in stable storage. Several log records
could be output with one output operation, which reduces the cost of I/O.
The following rules must be observed when log records are buffered
1. Log records are written to storage that is stable in the order they are made.
2. The transaction Ti is in the commit state only after the log record is created. Has been
output to storage that is stable.
3. Before an entire block of data in the main memory transferred onto the database it is logged. log entries must be recorded.
related to the data in that block, must be transferred to a secure storage.
These rules are also referred to as the write-ahead log scheme.
The database keeps an in-memory storage buffer of data blocks. It is updated whenever a new block is required, or if
The buffer is filled, and the block that is in use must remove from buffer. If the block selected for
Removal has been upgraded however, it still has to be written onto the hard disk. But, according to the write-ahead
log method, block of uncommitted updates are output to disk log records that have undo
Information for updates should be uploaded to the log file on an archive that is stable. The log should not contain any updates.
being processed on a block, when it is being output to disk. This can be done by following:
1. Before writing data items the transaction obtains an exclusive lock on block that holds the
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2. Lock is released after the write process has been completed. (Such locks with a limited durations are
known as known as latches).
3. When a block is sent on disk, it obtains an exclusive lock on that block
(ensures that no updates are taking place for the block).
A buffer for databases can be constructed in the area of main memory reserved for the
databases, or in virtual memory. Implementing buffers in the reserved main memory can be done in the main memory, or in the
drawbacks. Memory is partitioned prior to hand between applications and database buffers thus,
limiting flexibility. The operating system does know how to allocate memory at
Any time, it’s impossible to alter the partitioning of memory.
Database buffers are usually implemented as virtual memory, despite the disadvantages. When a
operating system has to remove the page that was altered, in order to make room for a different page
The page will be written to exchange the disk space. If the database decides that it wants to write the buffer page to disk
The buffer page might be stored in swap space and could need been read by disk swap space and output
to the database stored on the disk, leading to additional I/O, also known as a dual paging. Ideally , when
When you swap out a buffer page when swapping out a buffer page in a database, the operating system must give the control over to the
database, which then outputs the page into the database instead of exchange space (making sure that the database outputs
Log records initially) dual paging could thus be avoided. However, most operating systems don’t
This functionality is available to support.
Failure with loss of Non-volatile Storages
It is important to remember that the ACID transaction has responsibility for the atomicity as well as the durability characteristics of the ACID transaction
is in the recovery component that is part of DBMS. To achieve this, it is important to know the difference
Between two kinds of storage:
1. Storage that is volatile
2. Non-volatile storage
Volatile Storage: A type of storage like main memory where the state of the storage is lost during an
Power outage or system crash.
Non-volatile storage: Non-volatile storage , such as magnetic disks and tapes, whose content
It is the same throughout these events.
This subsystem can be rely upon to operate correctly when there are three
Different kinds of failures of different types.
1. The failure of a procedure that is currently in progress is in a state that it fails, it
If it fails to commit, any updates it has made have to be removed from the database. If it fails to commit, all updates made must be removed from the
in order to protect the in order to protect the. This is referred to as rollback of transactions.
2. System Failure: When it fails, the computer system will fail in a manner that results in the loss of memories that are volatile.
Recovery should ensure that:
(a) The changes of all transactions been completed before the crash are recorded in
Purchase Denmark email lists
(b) Any updates made to other transactions are deleted from the database.
3. Media Failure in the event that data is lost or corrupted on the storage that is not volatile (e.g. due to
in the event of a disk head crash) and the version on-line of the data will be deleted. In this scenario, the database has to
The database can be restored using an archive edition of the database, and kept up to date by the operation
The 192 Lovely Professionals UNIVERSITY
Database Management Systems/Managing Database
Case Study Remote Backup Systems
Remote backup systems ensure high reliability by allowing for the processing of transactions
to continue to operate even to continue, even if the main site to continue even if the primary site is to continue even if the primary site is destroyed.
Detection of Failure: The backup site should be able to determine failure of the main site.
To differentiate the primary website failure and link failure we’d have to keep various
connections between the primary backup and the remote backup. Communication links between the primary and remote.
Transfer of Control To assume control, the backup site does recovery with the help of its
A copy of the database as well as all log entries it received from the main site.
So, the completed transactions are redone while incomplete transactions are then rolled back.
If the backup site is able to take control of processing will become the primary site, allowing it to
Transfer control to original primary control back to the original primary. Once it restores the database, it’ll be able to restore control back to the original
Primary site receives redo logs from the previous backup, and then apply any updates locally.
Recovery time: In order to decrease delay in takingover, the backup site regularly processes the redo
log records (in actuality, performing recovery from a previous database state) is performed.
checkpointand then remove earlier entries in the log.
Hot-Spare configuration enables extremely quick takeover, and backup processes continuously redo
Log record when they are made, and apply the update locally. If the primary site occurs, the log record is updated.
was detected and backed up, the backup roll over any incomplete transactions, and is now ready to process new transactions.
Alternatives to remote backup are distributed databases that have replicated data. Remote
Backups are faster and more affordable however it has a lower threshold to detect failure.
The hot-spare configuration assures the longevity of updates through delaying transactions
The update will not be logged until it is until backup is logged. It is possible to avoid this delay by allowing lower
levels of endurance.
One-safe: A transaction is committed in this manner when the the commit log of the transaction is recorded
The content is published on the primary website. The issue in such a scenario is that updates might not be available at
Backup site to be used before taking over.
Two-very-safe: A transaction is committed when the commit log record of the transaction is recorded at
both backup and primary sites. However, this can reduce the availability of database base
because transactions aren’t able to be committed in the event that either website is not functioning.