Crash recovery

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Transaction is a collection of actions that transform a database from a consistent state to another consistent state; during the execution the database might be inconsistent.

Properties of a transaction

The ACID properties of a transaction are:

Atomicity: a transaction is treated as a single/atomic unit of operation and is either executed completely or not at all.

Consistency: a transaction preserves DB consistency. It transforms DB from its consistent state to consistent state.

Isolation: Execution of one transaction is isolated from that of other transactions.

Durability: if a transaction commits, its effect persists. If a transaction has been reported back to the user as complete, the resulting changes to the database survive subsequent hardware and software failures.

A transaction can be interrupted before running to completion for variety of reasons e.g. system crashes, disk failure, logical errors, system errors, etc. A DBMS must ensure that the changes made by such incomplete transactions are removed from the DB. For example, if the DBMS is in the middle of transferring money from account A to account B, and has debited the first account but not yet credited the second account when the crash occurs, the money debited from account A must be restored when the system comes back up after the crash.

The recovery manager of a DBMS is responsible for ensuring atomicity by undoing (rolling back) the actions of transactions that do not commit and durability by making sure that all actions of committed transactions survive system crashes and media failures i.e. corrupt disk.

The aim of crash recovery is to restore the database to the most recent consistent state which existed prior to the failure/crash.

Introduction to ARIES

The ARIES (Algorithm for Recovery and Isolation Exploiting Semantics) is a database recovery algorithm and has three phases:

Analysis phase: It scans the log (history of executions by the DBMS) forward (from the most recent checkpoint) to identify all active transactions and dirty pages in the buffer pool at the time of the crash.

Redo phase: This phase returns the database to the state at the time the DB crash occurred. Database history is repeated to reconstruct state at crash.

Undo phase: When the system restarts after the crash, the list of active transactions (uncommitted at the time database crashed) identified in phase 1 is rolled back (reversed) individually and restores the database to the consistent state that existed before the start of transaction.

Other recovery related data structure

In addition to the log, the following two tables contain important recovery related information:

Transaction table: this table contains one entry for each active transaction. The entry contains (among other things) the transaction id, status, etc. The status can be that it is in progress, committed or is aborted.

Dirty page table: contains one entry for each dirty page in the buffer pool, i.e. pages with changes that are not yet reflected on disk.

Write Ahead Logging (WAL) protocol

WAL is one of the principles behind ARIES recovery algorithm. In a system using WAL, all modifications to the database object  are first recorded in the log, the record in the log must be written to stable storage before the change to the database object is written to disk( before they are applied on the database).

The purpose of this can be illustrated by an example. Imagine a program that is in the middle of performing some operation when the machine it is running on loses power. Upon restart, that program might well need to know whether the operation it was performing, succeeded or failed. If a write-ahead-log was used, the program could check this log and compare what it was supposed to be doing when it unexpectedly lost power to what was actually done. On the basis of this comparison, the program could decide to undo what it had started, complete what it had started, or keep things as they are.

Check pointing

Keeping and maintaining the logs in real time may fill out all the memory space available in the system. As time passes log file may be too big to be handled at all. Checkpoint is a mechanism where all the previous logs are removed from the system and stored permanently in storage disk. Checkpoint declares a point before which the DBMS was in a consistent state and all transactions were committed. Periodic check pointing can reduce the time needed to recover from a crash.

Lock: A mechanism used to control access to a database objects.

Shared lock on an object can be held by two transactions at the same time. If a transaction needs to read a database object, a shared lock is required.

Exclusive lock on an object ensures that no other transaction hold any lock on this object. If a transaction needs to write (modify or change) a database object, an exclusive lock is required.

Media recovery: this is the process of restoring the entire disk files/folders after a crash. Disk failure can occur as a result of formation of bad sectors, disk head crash, or any other failures which destroys all or parts of disk storage.

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SS3 COMPUTER STUDIES FIRST TERM 2020/21 SESSION

•  SCHEME OF WORK FOR FIRST TERM

WEEK	TOPIC
1	·      REVIEW OF LAST TERM EXAM QUESTION ·      WRITING THE SCHEME OF WORK
2-3	SYSTEM DEVELOPMENT CYCLE (SDC) ·         Definition of System Development Cycle ·         Description of the stages in the SDC
4-6	PROGRAM DEVELOPMENT ·         Definition of program ·         Characteristics of a good program ·         Steps in program development ·         Description of the steps in program development ·         Compilation and interpretation of program ·         How compiler and interpreter works
7-8	ALGORITHM AND FLOWCHART ·         Definitions of algorithm and flowchart ·         Functions of algorithm ·         Characteristics of algorithm ·         Writing algorithm for problem solving ·         Flowchart symbols and their uses ·         Flowchart examples
9-10	INTERNET ·         Definition of internet ·         Definition of terms – browse, browser, chat room, cyber café, download, home page, e-mail, html, http, internet service provider etc. ·         Internet services – e-mail, search engine, telnet, ftp, www etc. ·         Benefits of the internet ·         Problems of the internet

 

SYSTEM DEVELOPMENT CYCLE (SDC)

Software Development Cycle (SDC) or Software Development Life Cycle (SDLC) is a process that consists of a series of planned activities to develop or alter an Information system.

System development Cycle can be thought of as a set of activities that analysts, designers and users carry out to develop and implement an information system.

Stages in the SDC

The diagram below shows the stages involved in the SDC, starting from the initial stage i.e. the preliminary study to the study review stage. Note that since this is a cycle, the event may be repeated.

The stages in the SDC include the following:

1. Preliminary
2. Feasibility study
3. Investigative
4. Analysis
5. Design
6. Implementation
7. Maintenance
8. Study review

Description of the stages in SDC

1.       Preliminary: The intended goal (i.e. what the system will do) of the project (information system) is established in this stage.

2.       Feasibility study: This is the study carried out before development of the system to ascertain if the proposed system is possible, practical and can serve a purpose.

3.       Investigative: The project goals are restated into specific functions and operation of the intended system. Whatever the end-user want, how he/she wants it and when he/she wants it is considered and analysed here.

4.       Analysis: The goal of the system analysis is to examine the type of the system on the basis of user requirements (what the user needs in the system).

5.       Design:  Designing the system in terms of user interface, data storage and data processing functions on the basis of the analysis phase by developing system flowcharts, system and data flow diagrams, screen layouts and reports.

6.       Implementation: programming the system as designed and conducts the continuous testing and debugging. The user accepts it at this stage before migration to live environment.

7.       Maintenance: This stage involves making changes, corrections, additions to the software while in use.

8.       Study review: The system is continuously evaluated as it functions in the live environment; this is done to see if there is need for upgrade or replacement of the system.

PROGRAM DEVELOPMENT

Meaning of a Program

A computer program is a general term used to describe the set of instructions that the computer use to performs a specific task. A computer program is usually written by a computer programmer using a programming language.

Characteristics of a Good Program

A good computer program should have following characteristics:                    

• Portability: Portability refers to the ability of an application to run on different platforms (operating systems) with or without minimal changes. 
• Readability: The program should be written in such a way that it makes other programmers or users to follow the logic of the program without much effort. 
• Efficiency: Every program requires certain processing time and memory to process the instructions and data. A program should be laid out in such a manner that it utilizes the least amount of memory and processing time. 
• Structural: To develop a program, the task must be broken down into a number of subtasks which are developed independently, and each subtask is able to perform the assigned job without the help of any other subtask. If a program is developed structurally, it becomes more readable, and the testing and documentation process also gets easier.
• Flexibility: A good program should allow changes without having to rewrite the entire program. Most programs are developed for a certain period and they require modifications from time to time.
• Accuracy/Correctness: Accuracy ensures that the program does what it is meant to do according to the design specification.
• Generality: Generality means that if a program is developed for a particular task, then it should also be used for all similar tasks of the same domain. For example, if a program is developed for a particular organization, then it should suit all the other similar organizations.
• Maintainability: Maintainability is the ease with which changes can be made to the program to satisfy new requirements. Maintainability allows the program to be upgraded or revised when the need arises.
• Reusability: Reusability is the ease with which a program can be reused in developing other program. By reusing existing program, developers can create more complex program in a shorter amount of time.
• Security: This ensures that the software protects the information it is responsible for.
• User-friendliness: It ensures that the program is easy to use by the intended users. It should have a sensitive user interface that will enable the user to interact with the program without any difficulty.
• Robustness: The program should be provided with the mechanism to respond to unexpected conditions, e.g., if wrong input is entered into it.

Steps In Program Development

Before coding of a good program, there must be a proper planning and steps on how the problem or program will be stored.

1. Problem definition
2. Problem analysis
3. Algorithm/Flowcharting
4. Desk checking
5. Program coding
6. Program compilation
7. Program testing and debugging
8. Program documentation.

• Problem Definition: Entails defining and understanding the problem statement and deciding the boundaries of the problem. In this phase we need to understand the problem statement, what is our requirement, what should be the output of the problem solution? These are defined in this first phase of the program development life cycle.
• Problem Analysis: The requirements like variables, functions, or resources to solve the problem are gathered.
• Algorithm/Flowcharting: During this phase, a step by step procedure to solve the problem using the specification given in the previous phase is developed.
• Desk checking: This involves manual checking of the logic of a program for correctness
• Coding: This phase involves using a programming language to write or implement actual programming instructions for the steps defined in the previous phase. In this phase, we construct actual program. That means we write the program to solve the given problem using programming languages like C, C++, Java etc.,
• Program compilation: Compilation is the process whereby the source program (program written in other language different from machine language), is translated into machine readable code. A compiler does the job of compilation.
• Testing & Debugging: During this phase, we check whether the code written in previous step is solving the specified problem or not, i.e. check whether input data will provide desired output or not.
• Maintenance: During this phase, the program is actively used by the users. If the user encounters any problem or wants any enhancement, then we need to repeat all the phases from the starting, so that the encountered problem is solved or enhancement is added.
• Documentation: Program documentation includes hard-copy or electronic manuals that enable users, program developers, and operators to interact successfully with a program, e.g. user manual (a description of software commands and troubleshooting guide). 

Compilation and interpretation of program

A compiler translates the entire program (source code/program) written in a high level language into an intermediate form called (object code/program), which can be directly executed by the machine.  On the other hand, an interpreter does not produce an object code but translate the source program line by line directly into machine language.

Examples of compiled programs languages are: C, C++, C#, Ada, ALGOL, Java, COBOL, Visual Basic, Lisp, FORTRAN, Pascal

Examples of interpreted languages are: QBASIC, JavaScript, PHP, ASP, APL, Python

How a compiler works

The brain of the compiler is the parser. It knows the syntax (rule) of the source language or the grammatical rules that determine how the source statements are written. Whenever the parser needs more of the source program statements to work on, it calls upon the scanner. The scanner reads in the source program statement and breaks it into a sequence of tokens – words, numbers, identifiers, operators etc. It hands them one at a time to the parser whenever the parser calls for the next one.

The parser also knows the semantics (meaning) of the source language. The parser’s knowledge of the semantics enables it to call the code generator to produce the object code, which performs the operation specified by the instruction. This continues until the entire program has been read in and translated.

How an interpreter works

Just like the compiler, an interpreter also has a parser that controls it. Its scanner does the same job as that of the compiler. However, an interpreter has an executor instead of the code generator.

ALGORITHM AND FLOWCHART

Algorithm can also be defined as a procedure consisting of a series of steps which specify a sequence of operations that provides the solution to a problem.

An algorithm is a finite set of an instruction that specifies a sequence of operations to be carried out in order to solve a specific problem. It is a detailed sequence of simple steps that are needed to solve a problem.

Pseudocode is a detailed yet readable description of an algorithm, expressed in a formally-styled natural language rather than in a programming language. It is very similar to everyday English.

A flowchart is a pictorial representation of an algorithm in which the steps are drawn in the form of different shapes of boxes and the logical flow is indicated by interconnecting arrows. The boxes represent operations and the arrows represent the sequence in which the operations are implemented.

A flowchart is a diagrammatic representation of an algorithm

Functions of algorithm

• It provides a step-by-step solution to a problem
• It is used to prove that a problem has a solution
• It is used to give a sense of direction to the steps of solving a problem
• It may also be used to show that a problem does not have a solution

Characteristics of an algorithm

1. Finiteness: it specifies an exact number of steps to be taken and has an end
2. Absence of ambiguity: it shows that every instruction is clearly and precisely specified
3. Sequence of execution: it shows how the instructions are performed from top to bottom
4. Effectiveness: it ensures that the solution prescribed is guaranteed to give a correct answer and that the specified process is faithfully carried out.
5. Input and output: these defines the unknowns of the problem specified and the expected output
6. Scope definition: it specifies precisely the problem or class of problem
7. Independence: algorithm is language independent

ALGORITHM Examples

Example 1: Calculate the Interest of a Bank Deposit

Algorithm:

Step 1: Start

Step 2: Read amount,

Step 3: Read years,

Step 4: Read rate,

Step 5: Calculate the interest with formula “Interest=Amount*Years*Rate/100

Step 6: Print Interest

Step 7: Stop

Example 2: Algorithm to Determine and Output Whether Number N is Even or Odd

Algorithm:

Step 1: Start
Step 2: Read number N,

Step 3: Set remainder as N modulo (divide) 2,

Step 4: If remainder is equal to 0 then number N is even, else number N is odd,

Step 5: Print output.

Step 6: Stop

Example 3: Algorithm to Print 1 to 20:

Algorithm:

Step 1: Start

Step 2: Initialize X as 0,

Step 3: Increment X by 1,

Step 4: Print X,

Step 5: If X is less than 20 then go back to step 2.

Step 6: Stop 

Flowchart symbols, names and uses

Below are the corresponding flowcharts

Flowchart for example 1: Calculate the Interest of a Bank Deposit

Flowchart for example 2: Determine and Output Whether Number N is Even or Odd

Flowchart for example3: Print 1 to 20

Assignment:

1. Write the algorithm to output the Even numbers between 20 and 40.
2. Write an algorithm to output the smaller of two numbers
3. Write an algorithm to show the process involved in login into your gmail account.

INTERNET

The Internet is the global system of interconnected computer networks that use the Internet protocol suite (TCP/IP) to link billions of devices worldwide. It is an international network for communication, where millions of computers are connected together. It is the largest computer network in the world.

Definition of some basic terms

Home page: is an introductory page of a website, typically serving as a table of contents for the site. It is an initial page or point of entry to all information stored within.

Web page: is a single page of information on a website.

Website: is a collection of related web pages (document that are accessed through the internet), typically identified with a common web address or domain name or URL (Uniform Resource Locator), published on at least one web server. Examples are https://google.com, https://isi.ui.edu.ng, https://facebook.com etc.

HyperText Markup Language, commonly referred to as HTML, is the standard markup language used to create web pages. Web browsers can read HTML files and render them into visible and audible web pages.

A protocol can be defined as set of rules and regulations that determine how data is transmitted in telecommunications and computer networking

HyperText Transfer Protocol, HTTP is the underlying protocol used by the World Wide Web. HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands.

Upload: this is the process of transferring files from a local computer to another (remote) computer on the World Wide Web.

Cyber cafe: this is a place to use computer to access the internet, create document, chat with friends using voice and video as well as a number of other computer related tasks.

Browser: A browser or a web browser is an application program with a graphical user interface for displaying HTML files, used to navigate the World Wide Web. Technically, a Web browser is a client program that uses HTTP (Hypertext Transfer Protocol) to make requests of Web servers throughout the Internet on behalf of the browser user.

Examples include Mozilla Firefox, Apple safari, Opera Mini, Lynx , Flock, Microsoft Internet Explorer, Netscape Navigator, UC browser, Google Chrome, Microsoft Edge  etc.

Internet Service Provider (ISP): an organization that provides individuals and other companies’ access to the internet. Examples include MTN, Airtel, 9mobile, Smile etc.

Internet services

• E-mail (Electronic mail): this is a message sent from one person to another, or many others through the internet connected computers.
• Telnet is a user command and an underlying TCP/IP protocol for accessing remote computers.
• Usenet (Users network) is an early non-centralized computer network for the discussion of particular topics and the sharing of files via newsgroups. Newsgroups are Internet discussion forums where groups of users with common interests gather to talk about everything from software to comic books to politics.
• File Transfer Protocol (FTP) is a standard network protocol used to transfer computer files from one host to another host over a TCP-based network, such as the Internet.
• World Wide Web (WWW) is a network of online content that is formatted in HTML (connected by hyperlinks and URL) and accessed via HTTP. A system of Internet servers that support specially formatted documents. The documents are formatted in a markup language called HTML.
• Search engine:  this is a computer program that retrieves documents or data from a database or from a computer network (especially from the internet). It is an utility for finding information on the internet. 

Benefits of the internet

• It provides quick access to almost any kind of information
• It is a fast, cheap and effective means of communicating and exchanging information all over the world
• E-commerce: buying and selling has been made possible through the internet
• Job opportunities such as graphic designing, blogging, freelance writing etc. have been made possible
• It is an avenue for advertisement of goods and services
• Online training and seminar called WEBINARS are made possible
• Online degree/E-learning: the internet provides us the opportunity to learn and earn degree/certificates by taking online classes.
• Online banking

Abuse of the internet

• Hacking: using the internet to gain unauthorized access to people’s computers
• Fraud: fraudulent activity involves any attempt to unlawfully obtain money from unsuspecting users
• Copyright violation: someone’s electronic content can be copied without due reference or permission
• Virus distribution: internet users are prone to downloading virus and other malicious program when downloading software/files from illegitimate website on the internet.
• Pornography: the internet is flooded with websites that are pornographic in nature
• Cyber bullying: using the internet to threaten and bully someone
• Spamming: sending unsolicited messages to individual or network 
• Fake news dissemination